Dams Literally speaking Dam means a body of water confined by a barrier.

Dams, structure that blocks the flow of river, stream, or other water way. Some dams divert the flow of river into a pipeline, canal, or channel. Others raise the level of inland water ways to make them navigable by ships and barrages. Many dams harness the energy of falling water to generate electric power. Dams also hold water for drinking and crop irrigation, and provide flood control. The oldest known human made dams were built more than 5000 years ago in arid parts of the Middle East to divert river water to irrigate crops. Today there are more than 500000 dams world wide. The wast majority of these are small structures less than 3m (10ft) high. Engineers regard dams that measure more than 15m (50ft) high as large dams. About 40000 large dams exist in the world today. Dams are of two types,

1. Concrete dams 2. Embankment dams

1. Concrete dams Concrete dams are those which are constructed of concrete and they are mainly of three types,

a. Gravity dams b. Arc dams c. Buttress dams

a. Gravity dams

Gravity dams are the concrete dams which stayed due to their weight. A concrete gravity dam has a cross section such that with a flat bottom, the dam is free standing. That is, the dam has a center of gravity low enough that the dam will not topple if unsupported at the abutments. Gravity dams require maximum amounts of concrete for their construction as compared with other kinds of concrete dams, and resist dislocation by the hydrostatic pressure of reservoir water by sheer weight. A favourable site usually is one in a constriction in a valley where the sound bedrock is reasonably close to the surface both in the floor and abutments of the dam. The availability of

suitable aggregate for manufacture of concrete is also an important consideration.

Masonry dams that relied upon their weight for stability against sliding and overturning date back 3000 to 4000 years, both upstream and downstream faces were sloped and the base thickness was many times the height. In 1872 Rankine proposed that there should be no tensile stress in a gravity dam. In 1895 Levy proposed that the compressive stress in the material of the dam at the upstream face should be greater than the water pressure at the corresponding depth in the reservoir.

The danger from uplift had been recognized in 1882, and the danger of sliding was highlighted by the failure of the Austin Dam, USA. The most recent advance has been in the application of the finite element method of analysis.

Typical Section Example

Hoover Dam, Nevada-Arizona (221m)

Grand Coulee Dam, Washington State (168m)

Fontana Dam, Tennessee (137m)

Studen Kladenetz, Bulgaria (67.5m)

Design Concepts and Criteria

A gravity dam shall be:

Safe against overturning at any horizontal plane within the dam. Safe against sliding at any horizontal place within the dam. So proportioned that the allowable stresses in both the concrete and the

foundation shall not be exceeded.

Loading Criteria

See Loading and Factor of Safety Page

In 1940 Houk and Keener, listed twenty five basic assumptions that should be considered relative to the design of important masonry dams.

1. The rock that constitutes the foundation and abutments at the site is strong enough to carry the forces imposed by the dam with stresses well below the elastic limit at all places along the contact planes.

2. The bearing power of the geologic structure along the foundation and abutments is great enough to carry the total loads imposed by the dam without rock movements of detrimental magnitude.

3. The rock formations are homogeneous and uniformly elastic in all directions, so that their deformations may be predicted satisfactorily by calculations based on the theory of elasticiy, by laboratory measurements on models constructed of elastic materials, or by combinations of both methods.

4. The flow of the foundation rock under the sustained loads that result from the construction of the dam and the filling of the reservoir may be adequately allowed for by using a somewhat lower modulus of elasticity than would otherwise be adopted for use in the technical analyses.

5. The base of the dam is thoroughly keyed into the rock formations along the foundations and abutments.

6. Construction operations are conducted so as to secure a satisfactory bond between the concrete and rock materials at all areas of contact along the foundation and abutments.

7. The concrete in the dam is homogeneous in all parts of the structure. 8. The concrete is uniformly elastic in all parts of the structure, so that

deformations due to applied loads may be calculated by formulae derived on the basis of the theory of elasticity or may be estimated from laboratory measurements on models constructed of elastic materials.

9. Effects of flow of concrete may be adequately allowed for by using a somewhat lower modulus of elasticity under sustained loads than would otherwise be adopted for use in technical analyses.

10. Contractions joints are properly grouted under adequate pressures, or open slots are properly filled with concrete, so that the dam may be considered to act as a monolith.

11. Sufficient drains are installed in the dam to reduce such uplift pressures as may develop along areas of contact between the concrete and rock materials.

12. Effects of increases in horizontal pressures caused by silt contents of flood waters usually may be ignored in designing high storage dams, but may require consideration in designing relatively low diversion structures.

13. Uplift forces adequate for analysing conditions at the base of the dam are adequate for analysing conditions at horizontal concrete cross sections above the base.

14. Internal stresses caused by natural shrinkage and by artificial cooling operations may be adequately controlled by proper spacing of contraction joints.

15. Internal stresses caused by increases in concrete temperature after grouting are beneficial.

16. Maximum pressures used in contraction joint grouting operations should be limited to such values as may be shown to the safe by appropriate stress analyses.

17. No section of the United Sates may be assumed to be entirely free from the occurrence of earthquake shocks.

18. Assumptions of maximum earthquake accelerations equal to one tenth of gravity are adequate for the design of important masonry dams without including additional allowances for resonance effects.

19. Vertical as well as horizontal accelerations should be considered, especially in designing gravity dams.

20. During the occurrence of temporary abnormal loads, such as those produced by earthquake shocks, some increases in stress magnitudes and some encroachments on usual factors of safety are permissible.

21. Effects of foundation and abutment deformations should be included in the technical analyses.

22. In monolithic straight gravity dams, some proportions of the loads may be carried by twist action and beam action at locations along the sloping abutments, as well as by the more usually considered gravity action.

23. Detrimental effects of twist and beam action in straight gravity dams, such as cracking caused by the development of tension stresses, may be prevented by suitable construction procedure.

24. In monolithic curved gravity and arch dams, some proportions of the loads may be carried by tangential shear and twist effects, as well as by the more usually considered arch and cantilever actions.

25. The distribution of loads in masonry dams may be determined by bringing the calculated deflections of the different systems of load transference into agreement at all conjugate points in the structure.

Uplift

Two factors directly affect the design of a dam, the intensity of hydrostatic pressure at various points within or under the dam and the area upon which pressure acts.

It is now accepted for design purposes that uplift pressures act on the full area of the section. The intensity of pressure may be represented by the diagram showing the ideal case of underflow conditions for an impermeable dam with a straight base on a homogeneous isotropic foundation of unlimited depth and horizontal extent.

Headwater and tailwater depth are represented by H and h, respectively. The concentric semi-ellipses represent lines of flow of water passing through the foundation. The hyperbolas, drawn normal to the lines of flow at all points, represent lines of equal hydrostatic pressure within the foundation and at the base of the dam. This network of flow lines and pressure lines is called a flow net. The diagram indicates an almost linear distribution of pressure on the base, and this is the distribution for which the stability of the dam should be checked if no drainage is provided. (or all drainage is blocked.)

Drainage is in the form of curtain of cored vertical holes 150mm or more in diameter at 3-5m spacing and located 304m from the upstream face. A gallery runs from one end to the other of the dam, above the tailwater level. Drainage from the holes is led away via open gutters, with measuring weirs installed to record the flow.

It is now general to adopt a distribution of uplift pressure as above, the value of factor k being decided having due regard to the porosity of foundation rock and the existence of joints and cracks therein. It is important to expend effort and money on a drainage system to ensure satisfactory function over the entire life of the dam.

Stresses

Considering vertical cantilever sectionsm of unit width it is a simple matter to compute vertical stresses on horizontal planes for the cases of reservoir empty and reservoir full. In general, efforts are made to avoid tensile stresses in the concrete for normal loadings. Compressive stresses are not usually high in gravity dams.

The usual analysis stresses normal to horizontal planes are

assumed to have a linear trapezoidal distribution. Finite element methods show the stress distribution to be as in the figure.

It is significant that the maximum stresses do not occur at the downstream toe, and there may be tension rather than compression at the upstream heel. However, there is similarity between the two methods. It is important to check the distribution and intensity of stress around galleries and other openings in the dam and to provide adequate reinforcement to prevent propagation of cracks from points of high stress concentration.

Contraction joints

Transverse Joints

It is good practice for normal methods of construction to provide contraction joints in gravity dams. They are usually spaced about 15m apart, experience having shown that cracks are likely to develop in monoliths much wider than this. It is however, essential to locate the joints to best advantage relative to the shape of the abutments.

Longitudinal Joints

For large structures the problems of cooling large masses of concrete are enormous. Resulting in limiting the dimensions of monoliths to 15m squares, keyed on all sides. There is now a tendency to decrease the number of longitudional joints or even omit them all, since there are doubts of the final behaviour of dams built in columns.

Galleries

The normal function of a gallery is to provide access for inspection purposes, to monitor the behaviour of the dam, and to carry out remedial work if required. It must therefore be of sufficient height to permit easy movement of personnel and minor equipment, commonly 2.13m but varied to suit construction methods. The width is usually 1.5m but should be related to the function of the gallery. Wide opennings induce quite high local stresses with consequent cracking of the concrete. Spiral staircases can link other galleries, ventillation and pipes in quite a small shaft.

Circular shafts are the most desirable, with a removable floor covering drainage, but it is harder and more expensive to form. Rectangular galleries require greater amounts of reinforcement. Galleries also should be well lit and ventillated.

Appurtenant structures

These are the power station and spillway.

Prestressed gravity dams

Strengths of rocks in foundation and elements within concrete dams are increased by installation of steel rocks or steel cables which are injected to tensioning. The procedure that is followed is called prestressing.

The reluctance to use cables has been related to a lack of knowledge of steel cables when embedded in concrete. It is however generally agreed that steel does not rust when embedded in high quality concrete or cement grout in which there are no cracks or interconnected voids.

If prestressing is accepted in the design then it is prudent to make provision for retensioning if required, replacement of bars or cables, or the installation of new cables. Serious corrosion of cables can be detected by the regular measurement of their electrical resistance.

The actual behaviour of the dam will depend upon the nature of the foundation rock, any initial stress in the rock and the effect of saturation of the rock mass.

For reasonable stress distribution the depth of the anchorage should be not less than the width of the base of the dam. The advantages of wires over bars are:

The allowable working stress in high tensile wires is usually greater than in bars

Wire cables can be fabricated on site in one length, avoiding the use of couplers that are necessary with bars and are a source of trouble

Cables can be accomodated in drilled holes whereas bars with couplers usually require larger pits.

b. Arc Dams

The ultimate complexity of design and analysis of stresses is attained in arch and dome dams. These dams are thin, curved structures commonly containing reinforcement, either steel rods or prestressed steel cables the volume of concrete required is much less than for gravity and gravity arch dams, but the competency of bedrock in foundations and abutments to sustain or resist loads must be of a high order.

Arch dams are usually built in narrow, deep gorges in mountainous regions where access and availability of construction materials pose especially acute problems.

Arch dams are of two kinds.

Constant radius arch dams - commonly have a vertical upstream face with a constant radius of curvature

Variable radius dams - have upstream and downstream curves (extrados and intrados curves) of systematically decreasing radii with depth below the crest.

When a dam is also doubly curved, that is, it is curved in both horizontal and vertical planes, it is sometimes called a dome dam. Some dams are constructed with two or several contiguous arches or planes and are described as multiple arch or multiple dome dams.

Analysis assumes that two major kinds of deflections or dislocations affect the dam and its abutments. Pressure of water on the upstream face of the dam and uplift pressures from seepage beneath the dam tend to rotate the dam about its base by cantilever action. In addition the pressure of reservoir water tends to flatten the arch and push it downstream.

Design Concepts and Criteria

An arch dam transfers loads to the abutments and foundations both by cantilever action and through horizontal arches, and a method of distribution was developed by Stucky in Switzerland and the USBoR.

The assumptions made are not strictly true so the effect of each must be understood before accepting the design.

The concrete in the dam and the rock foundations are homogeneous and isotropic;

Stresses within the elastic limit for both concrete and the rock formations and that stress will be proportional to strain;

That plane sections before bending remain plane after bending; That direct stresses vary linearly between the upstream and downstream

faces, in both arch and cantilever elements; That the modulus of elasticity of concrete and the modulus of deformation

of the foundation are the same in tension as in compression; That temperature stresses and strains are proportional to temperature

changes; That water load on the reservoir walls does not cause differential

movements at the damsite;

That foundation deformations are independent of the shape of the foundation;

That tensions are relieved by cracking so that all loads are carried by compression and shear in the uncracked portions;

That the dam acts as a monlith, i.e. that contraction joints or slots have been tightly grouted and that all shrinkage of the concrete has taken place before this.

The parameters controlling design, other than actual geometry include:

The loads on the dam; Loading and Factor of Safety The degree of fixity to foundation and abutments; The properties of the component materials of the dam and the

foundations.

Steel reinforcement can reduce the thickness of the dam but at a cost. If reinforcement was not used then cracking in the faces of an arch dam may result from:

Excessive tensile stress due to dam geometry; Secondary tension resulting from high compressive forces in thin

members; Secondary tensile stresses at the arch haunches and parallel to the

abutments; 'Hang up' of concrete adjacent to a near vertical abutment; Temperature effects - either due to hydration of the cement or climatic

conditions.

Definition of different arch dams based on base thickness (h is height of the dam):

Thin arch <0.2h Medium arch 0.2h - 0.3h Thick arch >0.3h Arch-gravity >0.5h

Reinforcement is not generally required in arch-gravity dams or thick arch dams. Its use in thin arch dams is favoured, however for a 90m high dam the cost of reinforcement will be many millions of dollars, which could mitigate the adoption of such a dam.

Uplift - is not usually of importance in thin arch dams, but in thick arch dams provision is made for internal drainage, as for gravity dams. If the design assumes that the concrete will crack if tensions exceed say 0.4MPa, then it is consistent to assume that full hydrostatic pressure can act in such cracks.

Tensile stresses - the aim of the designer is to eliminate tensile stresses, although this is not always possible since an irregular cross-section can generate local stress concentrations, and necessary excavation of abutments beyond the design limits will alter the geometry of the dam, and possibly affect the degree of fixity.

Abutment Stability

In the rock body the following are involved:

The weight of the rock; Static tectonic and dynamic seismic stresses; Hydrostatic thrusts and buoyancy after filling of the reservoir; Forces transmitted from the dam.

Minimum safety is usually found in the upper part of the double curvature dams because:

The upper zones of the valley are less tight and earthquake forces here cause stronger reactions;

The rock overburden is less - providing less normal loading on possible sliding planes;

The direction of the resultant forces from the dam often meet the abutments at less favourable angles.

Percolation of water under pressure may affect the strength of a rock abutment:

Saturation frequently decreases the strength of rocks, probably due to infiltration of microcracks;

Natural rock stresses will be modified by the water pressure, and Shearing resistance may be decreased

Shell Geometry

Constant-Radius Arch Dam The simplest form of arch dam with a vertical cylindrical upstream face and a uniformly inclined downstream face. Used in wide valleys with the possibility of slip forming construction methods.

Constant-Angle Arch Dam

Variable-Radius Arch Dam

Double curvature - Cupola Dam

Vertical curvature introduced so that the weight of the dam will offset vertical tensions due to water load. Cupola dams are ideal for narrow valleys and are similar to the thin arch dams in regard to foundation requirements.

Cross Sections of typical arch dams

Contraction Joints It has been normal practice to provide radial contraction joints in arch dams at approximately 15 meter spacing. This dimension has evolved from experience since cracks often appeared in monoliths of 20 meter or more in length, where full control of concrete temperature was either impractical or uneconomical; cracking occurred particularly at sides subject to sudden and large falls in ambient temperature. For constant radius arch dams the joints are radial and plane, whereas for double curvature dams they are frequently warped; in some cases they are formed to leave the rock almost normal to the contact surface.

Since monolithic action is required in the arch, provision is made for the injection of cement grout into the joints after the concrete has cooled to mean temperature, or has been artificially cooled to a little below mean temperature in order to introduce some compression into the arches.

Each joint is usually divided by horizontal grout stops so that zones from ten to fifteen meters high may be grouted progressively to ensure stability of the

completed sections against inadvertent overtopping by floods.

Arch dams are usually sufficiently flexible to defect measurably under the forces exerted by joint grouting; the effectiveness of the grouting can therefore be assessed by comparing measured with calculated deformations. To prevent harmful overstress regular observations should be made during grouting on joint meters embedded in the concrete across the joints, on dial gauges fixed to be upstream and downstream faces of the joint, on clinometers on faces of the dam and galleries and on plumbobs and survey targets as convenient.

Prestressing

In seeking further economies in the construction of arch dams it appears to be necessary to consider means of applying external loads to the dam to counteract undesirable tensile stresses that would otherwise develop. Many dams have now been built with compressive stresses up to 8.5 MPa but to increase these stresses would most likely not be possible without prestressing to counteract the

higher tensions. Potential application of prestressing to arch dams

Prestressing induces vertical compressive stresses upstream at the heel of the dam and downstream near the crest. This can be achieved by two processes, firstly by the information of flat jacks to force open the end joints of the shell which would defect the dam upstream, secondly by applying a radial load at the crest by means of a horizontal cable

to defect the upper part of the dam downstream.

c. Buttress dams

Buttress dams were first developed to conserve water in regions where materials were scarce or expensive but labour was cheap. Dams were used for irrigation and mining purposes. As designs have become more sophisticated, the virtues and weaknesses of the buttress type dams have become apparent.

The pressure of water on the inclined upstream at face adds to the stability of the dam, both by its magnitude and direction.

With free drainage of the foundations between the buttresses, uplift on their bases is considerably reduced.

The general flexibility of the dam can accommodate differential movement of the foundations.

Unless the foundation material was erodible minor leakage should not endanger the dam.

A minimum of materials is required but its accurate placement involves skilled tradesmen and higher unit costs.

Whilst construction is at low levels, the work can be overtopped by floods without serious damage - with considerable saving in river diversion works.

For large dams the stress distribution in the buttresses [from water load, own weight, thermal effects and foundation movements] is complex and does not conform to linear distribution on horizontal planes. Models show tensile stresses near the foundation of buttress heads in the case of good foundations - though such stresses are not evident from conventional analytical analysis. Preliminary designs should therefore be supplemented by detailed studies using finite elements or photoelastic methods.

The buttress type of dam finds particular application in wide valleys where sound rock would be the exception rather than the rule. Thorough investigations are therefore essential particularly if the dam is to be rigid.

If a buttress dam is of slender dimensions, especially a multiple arch, and flood waters are to pass over it, a very careful examination is necessary of possible modes of vibration. What may not be serious for a gravity dam could be disastrous for a buttress dam.

Lateral stability of buttresses is not now considered to be serious except for high dams, but it should be checked, especially in areas of known seismicity.

There appears to be a case for studying large span multiple arches in wide valleys, i.e. the arches would be thick, unreinforced, and constructed by mass concrete methods.

There is considerable scope for the application for prestressing to modify stresses within buttress dams as well as to improve their stability.

Concrete Slab Deck

The flat slab is simply supported on the buttress heads to avoid negative bending and cracking on the upstream face of the slabs. Flexible seals should be installed to prevent water loss around the ends of the slab as they defect. Some buttress dams have been constructed with the slab continuous over one or more buttresses.

1. Simple slab deck

2. Continuous slab deck

Massive Head Buttress To avoid tensile stresses in a thin slab, and hence the need for reinforcement, the massive head buttresses were developed.

1. Massive Head (flat head)

2. Massive Head (round head)

The relative economy of buttress dams will depend on the foundations, the cost of the materials, and the cost and reliability of the skilled tradesmen at the particular site. However, for a height of 20m a flat head buttress would require 40% of the concrete used in a gravity dam.

For dams up to 150 meters high it should be possible to dimension a buttress type of dam so that the first principal stress does not exceed 7 MPa, i.e. a stress comparable with that in a thin arch dam.

Multiple Arch Dam

Multiple arch dams evolved at approximately the same time as the slab and buttress dam, but at a slower rate. The factors influencing the selection of multiple arch dams as a preferred type are similar to those for slab and buttress structure relative to reduction in materials, low uplift forces, and adaptability to a wide variety of canyon configurations.

Multiple arches are continuous monolithic structures where loss of an important structure component could lead to loss of the entire dam. Thus these structures require better foundations.

The majority of multiple arch dam where constructed before 1935, and although state of the art at the time, by today's standards are deficient relative to seismic and hydrologic conditions.

Buttresses

For small dams the buttresses are usually analysed as gravity blocks subject to the inclined water load, their own weight and small uplift. A buttress can also be considered as composed of a system of curved beams, each of which trasmits part of the water load and its own weight to the foundations.

The columns can be proportioned to develop uniform compressive stress and curved to avoid eccentricity of loading. In order to avoid secondary tensile stresses the buttresses of many large dams have been built with contraction joints following the directions of the principle stresses

Uplift and Sliding A major advantage claimed for buttress dams, including the hollow gravity dam is that uplift forces acring on the dam are minimal. It is usual to adopt a distribution of uplift pressure, acting on 100% of the area, as shown in the figure.

For this to be factual there must be release to atmosphere, or tailwater pressure, around the buttress footing. Should the foundation be horizontally stratified then uplift could act on a layer of rock only a little distance below the dam; drainage of such a foundation is therefore essential.

Example, Muda Dam, Malaysia. Using post tensioned restressible cables fixed to the foot of the buttress to prevent uplift and sliding.

Spill-over Buttress Dams When flood waters are to be passed over buttress dams the following factors deserve attention:

The nappe must be adequately aerated to avoid vibrations or pulsations that could be transferred into the dam to cause overstressing or into the foundations to weaken their shearing resistance.

The nappe should impact on to reinforced concrete slabs that are adequately anchored into the foundations. Erosion behind the buttress heads or arches should be prevented by provision of a concrete turbulence control wall or suitable paving.

It must be possible to destroy most of the energy of the surcharge withiut rupture of the river bed downstream from the dam. Should excesscive erosion occur the shearing resistance of the foundation could be lost.

Prestressing

Prestessing is used to minimise the quantity of concrete and counteract tensions that would otherwise exist. It is usually used as an extra factor of safety on an otherwise adequate structure, for example to cope under extreme flooding or earthquake conditions.

Prestressing can be applied in at least three manners to a buttress dam,

1. To 'pull down' the upstream face 2. To 'jack up' the downstream face 3. To compact the buttress on to the foundation rock to improve the

resistance to sliding of dam onrock, at the same time tightening seams to improve the resistance to sliding within the foundations.

2. Embankment dams

ICOLD defined an embankment dam as, "any dam constructed of excavated materials placed without addition of binding materials other than those inherent in the natural material. The materials are usually obtained at or near the damsite" The materials available locally control the size and configuration of the dam. Many small embankment dams are built entirely of a single type of material such as stream alluvium, weathered bedrock, or glacial till. These are homogeneous dams, constructed more or less of uniform natural material.

Larger embankment dams are zoned and constructed of a variety of materials, either extracted from different local sources or prepared by mechanical or hydraulic separation of source material into fractions with different properties. An important element in a zoned dam is an impermeable blanket or core which usually consists of clayey materials obtained locally. In locations where naturally impermeable materials are unavailable the dams are built of rock or earth-rock aggregates, and the impermeable layers of reinforced concrete, asphaltic concrete, or riveted sheet steel are placed on the upstream face of the dam.

Embankment dams have been built on a variety of foundations, ranging from weak glacial deposits to strong rock. An advantage compared with concrete dams is that the bearing strength requirements of the foundation are much less. Minor settlement during and after construction is generally not serious because of the adjustability of the material.

There are two types of embankment dams

a. Earth fill dams b. Rock fill dams

a. Earth fill dams

An earthfill dam is an embankment dam, constructed primarily of compacted earth, either homogeneous or zoned, and containing more than 50% of earth.

A rockfill dam where all the voids have been filled by finer materials by hydraulic sluicing is usually regarded as an earthfill dam.

Terminology of Earthfill dams

Types of Earthfill Dam

1. Homogenous 2. Central Impervious Core 3. Sloping Impervious Core 4. Hydraulic Dams

Slopes of 1 in 1.33 are suitable for concrete faced rockfill dams, but for effective placing and stability of an asphaltic concrete facing, the upstream slope must be about 1 in 1.7. It is significant that men can walk

on this slope without ropes, but on a slope of 1 in 1.33 safety ropes are essential. An asphaltic concrete allows for more movement due to settlement that for a rigid concrete deck.

Homogeneous Earthfill Dams

Such embankments are made of a single type of material or material from the same source. This may be small particles placed by hydraulic means, or compacted earth or gravels that are handled and compacted mechanically.

Basic properties required in the material for an homogeneous embankment or for the core of a rockfill dam are:

It must be sufficiently impervious to prevent excessive loss of water through the dam, the acceptable loss being determined by the safety of the structure and the value of the lost water;

It must be capable of being placed and consolidated to give a practically homogeneous mass, free from potential paths of percolation, either through the fill or along its contact with the foundation;

The soil should develop a maximum practical shear strength under compaction and maintain most of it after the filling of the reservoir;

It must not consolidate, soften or liquify upon saturation.

The stability of an embankment dam is enhanced if the downstream portion can be maintained free from seepage. Internal drains are therefore put within the

dam. See figure, leaving the 'dry' compacted fill as support. The section A-A represents the filter, drainage, filter divisions.

The location and inclination will depend on the materials used. It has been suggested that maximum stability would result from locating it nearer the upstream face with the angle ø less than a right angle. Central Impervious Core Earthfill Dams Where there is only limited supply of soil for the impervious core but plenty of pervious material for the embankment, the designer has no option but to decide on a thin core dam. However, where there are plentiful supplies of pervious and impervious material, a thin core dam may be more economically or easily constructed for a number of reasons:

1. The unit cost of placing impervious materials may be more than the unit cost of placing pervious materials.

2. The amount of embankment volume can be reduced in a thin core dam more effectively than in any other type of dam.

3. The construction time available and the weather conditions may not allow the use of an impervious core of large thickness.

The minimum thickness of core is dependent on a number of factors:

1. tolerable seepage loss; 2. minimum width which will allow proper construction; 3. the type of material chosen for the core and shoulders of the dam; 4. design of proposed filter layers; 5. past experience on similar projects.

Core Stability - The core material usually has less shear strength than the rest of the embankment, therefore from a stability standpoint, a thinner core is better. However, a thicker core has increased resistance to differential cracking; which may lead to piping. Therefore, piping resistance is dependent upon the soil properties such as plasticity and gradation of the core material.

Advantages of vertical cores:

1. One advantage of the vertical core is that higher pressures will exist on the contact between the core and the foundation, and will provide more protection against the possibility of leakage along the contact.

2. The vertical core tends to be slightly thicker for a given quantity of impervious soil than the thickness of a sloping core.

The following criteria represent a rough cross-section of opinion among experienced earth dam engineers:

Cores with a width 30% to 50% of the head of water have proved satisfactory on many dams under diverse conditions. Cores of this width are adequate for any soil type and dam height.

Cores with a width of 15% to 20% of the head of water are considered thin. However, when adequately designed and constructed filter layers are used, then the core is satisfactory under most circumstances.

Core widths of less than 10% of the head of water are not used widely and should only be used when a large leak through the core would not lead to failure of the dam.

Sloping Impervious Core Earthfill Dams Advantages of sloping cores:

1. The principal advantage of the upstream sloping core is that the downstream portion of the embankment can be constructed first and the core placed later. This a distinct advantage when there is only a short season of dry weather suitable to allow construction of a core from fine-grained soils.

2. Another advantage is that the foundation grouting can take place whilst the embankment is being placed.

3. Filter zones between the upstream and downstream pervious zones can be constructed more thinly and are easier to install than in vertical core dams.

4. The sloping core dam is advantageous with the speed and economy of foundation grouting which can be achieved. The advantage comes from the fact that grouting can be performed while the main downstream pervious embankment is being constructed.

Disadvantages of sloping cores:

1. At some sites the area of contact between the core and the foundation depends on the depth of the foundation excavation: i.e. when the excavation is carried deeper, the contact area moves upstream. However, in some cases the depth of excavation required to provide a suitable contact between the earth core and foundation cannot be determined reliably in advance of construction.

2. Due to the reason above it may be difficult to locate the grout curtain in the desired position relative to the core contact area.

3. If it is anticipated that additional grouting is required through the embankment after the dam is completed then a central core design is preferred, because the work can be done from the crest of the dam without lowering the reservoir.

Hydraulic Fill Earthfill Dams

A hydraulic fill dam is one in which the material is transported in suspension in water to the embankment where it gets placed by sedimentation. The sorting effect of flowing water is utilised in creating a fine-grained core at the centre of the embankment with coarse shells on the sides. In a semi-hydraulic fill dam the material is transported by hauling units and dumped at the edge of the embankment. It is then washed to its final position by water jets. The use of this type of dam is rare, because;

The cost of rolled earth has droped rapidly with the development of larger more economical earth moving equipment.

It is difficult to control the quality which makes them less dependable than other types of dam.

Drainage of the core takes place in two ways, some of the water percolates horizontally into the more pervious shell. The remainder moves upward to the surface, allowing the centre of the dam to subside. The downward movement eventually develops arching in the core and prevents its full consolidation.

Materials The thouroughness with which borrow areas are investigated can have a major effect on the cost of the dam. The best information is derived from trenches cut by bulldozer. Two questions must be asked;

1. Is the material acceptable? 2. How will it be excavated?

The materials must be tested in the laboratory and must be representative of what would be used in the final dam.

When selecting earth for a core or for a homogeneous dam, one must consider its permeability, resistance to piping, shear strength, flexibility and resistance to cracking. The water content will effect each of these differently; testing and judgement are required to determine the optimum mositure content for the particular soil in the particular part of the dam.

Earthfill Design

An earth dam is basically a trapezoidal embankment built in a valley to form a water reservoir. The design has to ensure:

1. It is impermeable enough to prevent excessive loss of water from the reservoir.

2. The design must ensure stable slopes. 3. Settlement of the dam must not be excessive so as to reduce the

freeboard of the dam. 4. The upstream slope of the dam must be protected from the destructive

action of waves, and the downstream slope must withstand rainfall erosion.

5. A sufficient bond between the embankment and its foundation must exist to prevent the development of seepage paths; excessive hydrostatic uplift must be controlled by proper drainage.

Freeboard

A homogeneous embankment dam should never be overtopped and for preference no permanent embankment dam should be overtopped. However, provision for freeboard can be expensive because it requires enlargement of the dam section and hence much more materials.

It may be convenient to pave the crest and downstream face. The level of the crest is then determined to allow for only spray to pass over, or for the peak flood discharge to pass over or even more frequent overtopping. However this is only used for dams under 30m high.

An alternative method of reducing the quantity of fill is to provide a wave wall along the crest of the embankment. See figure.

Crest Width

This is often governed by construction procedure and the access required either during construction or as a permanent feature. The Japanese Code 1957 specifies crest width (W) in terms of the height of the dam, as

W=3.6H1/3-3(m) which would give crest widths as in the table.

Height of dam (m) Crest Width 30 8 50 10 70 11 100 13 200 18

Culverts under Embankments At some locations it is necessary to construct a large culvert under the dam, although this should be avoided where possible.

The conventional culvert is one of reinforced concrete designed to withstand both the internal water pressure and external embankment loading. It is important that leakage does not occur within the core area or upstream from it, or anywhere within an homogeneous bank. To prevent this cut-off collars usually encircle the pipe, their location and dimensions being governed by the head from the reservoir.

b. Rockfill dams ICOLD defined a rockfill dam as, "an embankment type of dam, dependent for its stability primarily on rock. As rockfill dams must contain an impervious zone - now usually selected earth with filter zones, comprising a substantial volume of the dam - the term Rockfill dam usually represents a dam that contains more than 50% of compacted or dumped pervious fill. The dam is dependent for watertightness on an impervious upstream blanket or an impervious core."

Like an earth dam it is composed of fragmental materials, with each particle independent of the others. The mass stability is developed by the friction and inter-reaction of one particle on another rather than by any cementing agent that binds the particles together.

Types of Rockfill dams Composite Earth and Rockfill Central earth core Sloping earth core Upstream core Rock with a thin membrane or diaphragm to hold water Central thin membrane

Upstream thin membrane or deck Unbonded or dry masonry Dam with rubble retaining zone

Advantages of Rockfill Construction

Economical - due to the use of cheap local materials. Suitable where the foundation conditions are not good, especially where

high hydrostatic uplift is likely to be a factor in design.

Rockfill is particularly suitable when there is no satisfactory earth available and when a plentiful supply of sound rock is at hand. The rockfilling is especially adapted to construction during wet and cold weather and permits continuous work under weather conditions that would not permit earth or concrete construction.

Very rapid constructions are possible with rockfill because of its adaptability to bad weather and because the process of filling does not have to be interrupted for rolling or other separate compaction operations.

The rockfill dam with an upstream diaphragm is very well adapted to stage construction. The dam height can be increased merely by dumping more rock behind the impervious diaphragm without interfering with or encroaching on the reservoir. The dam is then made water-tight by continuing the impervious face upward. The stage construction concept is also suitable for cofferdamming, as the first part of the dam serves as a cofferdam which protects the remainder of the foundation for further construction.

Rockfill Materials

The quality of the rock is a major factor in the choice of a rockfill dam and in the design of the structure. Extensive testing is necessary to judge whether the rock is suitable for construction.

Quarrying - The cost of drilling and blasting constitutes a large part of the unit price of rockfill. Quartzite for example has excellent qualities for rockfill but is extremely expensive to drill. The way the rock breaks up is also important, sandstone produces a lot of fines, others produce flat slate pieces which do not lend themselves to dumped rockfilling.

Rock Durability - There is no entirely satisfactory test to determine durability of rock over centuries, and hence good judgement has to be used. Examining old structures such as walls and bridge piers built of the same material is helpful. Accelerated durability tests do exist, where the samples are subjected to alternate cycles of wetting and drying or freezing and thawing. Compressive strength tests can be made after each series of wet-dry and freeze-thaw testing if there are sufficient samples.

Strength - In high dams where crushing of the corners of the rock pieces will result in settlement, the strength is important. In general strengths of over 35MPa or more are desirable for dams over 40m, while strengths as low as 14MPa are more suitable for dams less than 15m in height. Friability, the tendency to become a powder during crushing is important because too many fines can seriously interfere with construction.

Petrography - The study of the rock under chemical reaction and under a microscope to establish rock breakage.

Likely to be satisfactory Likely to be unsatisfactory Granite, diorite Shale Gneiss Slate Basalt Schist Sandstone Siltstone Dense limestone Porous limestone Dolomite Chalky limestone Quartzite Massive Schist

Shear Strength - Large size triaxial or direct shear tests are the best method for determining strength.

Earth Core Rockfill Dams The rockfill dam consists of a number of components:

1. The main rockfill 2. The impervious zone 3. Auxiliary supporting members

The main rockfill provides the structural support for the dam by its weight and internal stability. The impervious zone holds back the water. It is made up of the membrane which holds the water and transition zone which transfers the water load to the rockfill. The membrane may be a thick blanket or core of earth or a thin diaphragm or deck of wood, concrete, steel, asphalt, dry rubble masonry or stone masonry. The auxiliary support members help to sustain the membrane or parts of the main rockfill. These components are similar to the shell, core, and appurtenances of the earth-fill dam and are analysed in a similar way.

Cores

The core may be defined as a membrane built within an embankment dam to form the impermeable barrier, the balance of the dam being provided to ensure stability. It may be of natural materials, clay, gravels etc. or prepared materials such as cement or asphaltic concrete, or of metal, plastic, rubber, etc.

The thickness of the core will depend primarily on the material available, i.e. if a good clay is available at low cost one would tend to be liberal with the core. The core width will often be related to the type of foundation, the permissible hydraulic gradient along the contact zone.

A core of natural materials may be central, inclined and close under the upstream face or in some intermediate position. A general core thickness is one half of the height of the dam, depending on materials available. Permeability of the compacted core should not exceed 10-5 cm/s.

The hydraulic gradient relative to the core is the ratio of maximum head of water to the thickness of the core. Thin cores may be adequate for impermeability but it is essential to provide well designed filters on either side. The greatest danger with thin filters is the possibility that a 'blow through' may occur in a segregated zone.

The principal factors considered in determining core dimensions and embankment zoning are:

The type and volume of core materials available; The relative economics of earthfill and rockfill; The plasticity of the available core material and its effect on the risk of

core cracking; The extent and rate of reservoir draw-down; The nature of the foundation rock under the core.

Cracking of Core - cracks frequently occur in earthfill dams and in cores of rockfill dams. Care must be taken to prevent such cracking and the Engineer must decide whether the cracks are likely to extend and become serious or whether they are stable and can be backfilled.

Influence of Post Construction Settlement at Crest on Cracking

Crest Settlement (mm) Kind of cracking

Less than 50 No cracking of dams Equal or greater than 50 Transverse cracking of dams compacted dry may appear

Greater than 100 Reinforced concrete facing without perimetral joint may crack Equal or greater than 130 Longitudinal cracking between core and shell may appear

Greater than 160 Longitudinal cracking of core compacted dry may appear Greater than 180 Hydraulic fracturing may appear

Equal or greater than 220

Transverse cracking of core compacted wet may appear. Longitudinal cracking between core compacted wet and shell may appear.

Equal or greater than 350

Asphaltic concrete facing may crack (self healed for settlement of 350mm)

Greater than 400 Longintudinal cracking of core compacted wet may appear. Reinforced concrete facing with perimetral joint will crack

Greater than 1000 No uncracked dam in those studied

Greater than 1200 All dams exhibit transverse cracking

Equal or greater than 1400 Serious cracking of asphaltic concrete facing

Equal or greater tahn 3800 Cracking needing substitution of reinforced concrete facing

Decked Rockfill Dams

Timber Face - used mostly for mining purposes, relying upon dumped rock for stability with a facing of timber for

watertightness. Leakage under and around the dam could not be prevented, but usually did no harm to a free-draining rockfill. Although not used in present day construction, the value of timber should not be overlooked.

Steel Face - consists of large welded panels, connected by flexible joints to allow for expansion and contraction and any displacement of the plates relative to the face of the rockfill. To help reduce corrosion, coats of coal-tar epoxy resin preparation and supplementary cathodic protection are provided below water level, giving about a 50 year life.

Cement Concrete Face - since cement has a very long life, it is an obvious watertight membrane on rockfill dams. Details of typical facing are shown in the figure.

The facing can be tied to the dam in two ways, either poured directly onto the rubble transition zone. A mortar bed is initially placed which penetrates into the rubble a few centimeters. This is immediately covered with the concrete to form a monolithic mass which extends into the rubble and is thus bonded to the dam.

Or, ribs are placed in the bottom of the slab by forcing grooves in the facing. The ribbed support, however, is unnecessary if the bonding with the backing is effective.

Two types of facing have been used.

One is a thin monolithic slab of concrete with no joints. It is sufficiently flexible to conform to movements in the backing without failure and the tensile forces are distributed by the reinforcements so that numerous small cracks develop rather than any major failures.

The second type, used in most concrete faced dams, has a facing which consists of monolithic slabs, 10 to 30 sqm each. The concrete thickness is largely a matter if experience. Only nominal reinforcement is required, about 0.5% concrete area in each direction. Water tightness is ensured by copper water stops.

Asphaltic Concrete Face - two types of asphaltic facings have been used, a laminated facing consisting of:

1. Rubble concrete masonry transition 2. 10cm thick porous concrete 3. 15cm bituminous concrete, placed in two layers and rolled 4. Reinforced concrete protective and insulating layer 12cm thick.

The outer layer of concrete only serves to protect the bituminous side from sunshine and physical damage. It is sprayed with water during very hot weather to keep the bituminous concrete from sagging in plastic flow. The second form employs an asphaltic concrete paving similar to that used in highways. It is placed in layers and rolled as for paving. The advantage of a purely asphaltic paving is that it tends to adjust itself to movement by plastic flow. As with other thin facings, a transition zone is required to equalise settlement movements and to provide uniform support.

Rubble Retaining Zone - used in the upstream face to permit a slope that is steeper than the angle of repose of the rockfill. This makes it possible to reduce the volume of rockfill, but at the expense of construction of the retaining zone. There are two types for this zone, a wedge of compacted rockfill, where the steeper slope is merely the angle of repose of dense rather than loose rock. Slope angles of 45 degrees can be obtained this way.

The second form is an unbonded rubble masonry retaining wall. It is essentially a retaining wall, and should be designed as such. The typical width of the base of the supporting zone is 0.25 the height of the dam.

Impermeable Zone Location

The location of the impermeable zone in a rockfill dam involves the same factors as it does in the case of an earth dam.

The upstream deck has a number of advantages:

1. It is more stable under the water load, because the downward force of the water produces frictional resistance to sliding

2. The permeable rock embankment develops no uplift, since the embankment permits no movement of water upward from the foundation.

3. The impermeable deck can easily be inspected and repaired if necessary. 4. During construction the height of the dam can be increased by dumping

only on the downstream side and extending the membrane upward on the sloping surface.

The disadvantages of an upstream deck are:

1. The deck is vulnerable to weather and wave attack. 2. If constructed of earth, sudden drawdown greatly reduces its stability and

may cause it to slide. 3. Settlement of the rock embankment tends to produce tensile cracks in the

membrane.

The central core location has a number of advantages:

1. The core is equally supported and is more stable during a sudden drawdown (if constructed from earth).

2. Settlement of the rockfill induces compressive stresses in the core, tending to make it more compact.

3. There is less core volume and less cross sectional area for leakage for a given height of dam and thickness of core.

The choice for dams with impermeable zones depends largely on the stability of the core material. If it is strong, a near upstream location is often the most economical. However, if the core material is weak a central location is better.

Filter and Transition Zones

Since the core is established with rock or gravel zones, it is necessary to prevent the fine core material being sucked into the upstream shell material during rapid drawdown of the reservoir, or forced into the downstream shell by seepage water under reservoir head. Transition or filter zones must therefore be provided on each side of the core.

The upstream filter, if non-cohesive and of proper grading, can serve a valuable service by providing material for induced self-healing should a transverse crack appear in the core. Selection of the best material for this purpose is well justified. Although its prime function is to retain the core material against movement into the rockfill, the downstream transition material should be selected and placed so as to inhibit the propagation of a core crack into the compacted rockfill. It is good practice to widen the transition zones towards each abutment, i.e. where tension and oblique cracking may occur.

To prevent migration of fines from the core:

D15/D85 < 4-5 (filter)/(zone being filtered)

D50/D50 < 25 (filter)/(zone being filtered)

For sufficient permeability: D15/D15 > 4-5 (filter)/(zone being filtered) To prevent segregation of the filter: D60/D10 < 20 (filter)/(filter)

Single filter between core and rockfill

Double filter to core

Settlement Settlement is a problem for embankment dams. It begins during construction and continues for many years after the dam is complete. The two main causes are:

1. The migration or working of fines from between the points of contact between the larger rock allows the particles to re-orient themselves into a more dense structure

2. The crushing of the contact points between the larger rocks under the extreme stress developed by the embankment weight causes the rocks to develop new points of contact which in turn crush again.

The problem can be avoided by proper compaction during construction. In earthfill dams it may be possible to overbuild the dam, to make a, say 50% higher dam which will settle to the correct height. Multi-stage construction also helps.

a. Settlement in section b. Settlement - elevation c. Irregular abutment d. Overhanging abutment Slope Stability

Introduction

Failure of an embankment dam can result from instability of either the upstream or downstream slopes. The failure surface may lie within the embankment or may pass through the embankment and the foundation soil. The critical stages in an upstream slope are at the end of construction and during rapid drawdown. The critical stages for the downstream slope are at the end of construction and during steady seepage when the reservoir is full.

It is common to install piezometers to measure pore water pressures and compare data with the predicted values used in design. Since pore water pressures are a dominant influence on the factor of safety of slopes, remedial action should be taken if the factor of safety, based on the measured values, is considered to be too low.

To ensure stability a number of conditions must be investigated:

1. The slopes must be safe against surface slipping. To ensure this the slopes must be no steeper than the angle of repose

2. The dam must be safe against sliding on the foundation 3. The mass of the embankment must be safe against a circular arc failure or

composite linear failure. This is likely to occur within an earth core or weak foundation

The safety against failure can be increased by reducing the gradient of the slopes.

Homogeneous Embankment

1. Slip within embankment 2. Slip circle through foundation

Zoned Embankment

1. Within rockfill 2. Through rockfill and foundation 3. Through core and foundation

End of construction

Most slope failures occur either during, or at the end of construction. Pore water pressures depend on the placement water content of the fill and on the rate of construction. A commitment to achieve rapid completion will result in high pore water pressures at the end of construction. However, the construction period of an embankment dam is likely to be long enough to allow partial dissipation of excess pore water pressure, especially for a dam with internal drainage. Dissipation of excess pore water pressures can be accelerated by installing horizontal drainage layers within the dam. However, a total stress analysis would result in an over conservative design. An effective stress analysis is therefore preferred. A factor of safety as low as 1.3 may be acceptable at the end of construction provided there is reasonable confidence in the design data.

Steady seepage

When the reservoir has been full for some time, conditions of steady seepage become established through the dam with the soil below the top flow line in the fully saturated state. This condition must be analysed in terms of effective stress with values of pore pressure being determined from the flow net. The factor of

safety for this condition should be at least 1.5. Internal erosion is a particular danger when the reservoir is full because it can arise and develop within a relatively short time, seriously impairing the safety of the dam.

Rapid drawdown in low permeability soils

Rapid drawdown of the reservoir after a condition of steady seepage will result in a change in the pore water pressure distribution. If the permeability of the soil is low, a drawdown period measured in weeks may be 'rapid' in relation to the dissipation time and the change in pore water pressure.

Rapid Drawdown in high permeability soils

The pore water pressure distribution after drawdown in soils of high permeability decreases as pore water drains out of the soil above the drawdown level. The saturation line moves downwards at a rate dependant upon the permeability of the soil. A series of flow nets can be drawn for different positions of the saturation line and values of pore water pressure obtained. The factor of safety can then be determined, using an effective stress analysis, for any position of the saturation line.

Slope Protection Both faces of an embankment dam must be protected against structural damage. In normal circumstances the downstream will only be subject to the forces of nature. The upstream face must be protected against erosion or disturbance by wave action, ice or by impact of floating debris. Various methods of protection include large rocks (rip-rap), precast concrete forms, soil cement or the waterproofing membrane of the dam. Protection must be well above and below the operating range of the reservoir.

Soil Cement Slope Protection

Rip-rap size : Mass of individual rock = 1000 x (Wave Height Hs)3 (kg)

The rip-rap must be durable, weatherproof and of good quality sound rock to enable it to withstand the changing harsh conditions.

Seepage Paths

Piping

Internal erosion of the foundation or embankment caused by seepage is known as piping. Generally, erosion starts at the downstream toe and works back toward the reservoir, forming channels or pipes under the dam. The channels or pipes follow paths of maximum permeability and may not develop until many years after construction.

Resistance of the embankment or foundation to piping depends on:

1. plasticity of the soil 2. the gradation 3. the degree of compactness

Plastic clays with a plasticity index >15, for both well and poorly compacted are the materials which are most resistant to piping. Minimum piping resistance is found in poorly compacted, through to well-graded cohesionless soils with practically no binder. It is also found in uniform, fine, cohesionless sand, even

when well compacted. Settlement cracks in resistant materials may also produce piping.

Piping can be avoided by lengthening the flowpaths of water within the dam and its foundations. This decreases the hydraulic gradient of the water flow and hence its velocity. The flowpaths can be increased by:

Cutoff walls

Impermeable cores

Impermeable blankets extending upstream from the upstream face

Seepage control

Seepage is the continuous movement of water from the upstream face of the dam toward its downstream face. The upper surface of this stream of percolating water is known as the phreatic surface. The phreatic surface should be kept at or below the downstream toe.

The phreatic surface within a dam can be controlled by properly designed cores or walls.

Internal drain systems

Purpose

A homogeneous dam with a height of more than about 6 m to 8 m should have some type of downstream drain. The purpose of a drain is:

1. to reduce the pore water pressures in the downstream portion of the dam therefore increasing the stability of the downstream slope against sliding.

2. to control any seepage that exits the downstream portion of the dam and prevent erosion of the downstream slope: i.e. to prevent 'piping'.

The effectiveness of the drain in reducing pore pressures depends on its location and extent. However, piping is controlled by ensuring that the grading of the pervious material from which the drain is constructed meets the filter requirements for the embankment material.

Toe drains

The design of a downstream drainage system is controlled by the height of the dam, the cost and availability of permeable material, and the permeability of the foundation.

For low dams, a simple toe drain can be used successfully. Toe drains have been installed in some of the oldest homogeneous dams in an effort to prevent softening and erosion of the downstream toe.

For reservoir depths greater than 15 m, most engineers would place a drainage system further inside the embankment where it will be more effective in reducing pore pressures and controlling seepage.

Horizontal drainage blanket

Horizontal drainage blankets are often used for dams of moderate height.

Drainage blankets are frequently used over the downstream one-half or one-third of the foundation area. The Bureau of Reclamation's 45 m Vega Dam is a homogeneous dam which has been constructed with a horizontal downstream drain. Where pervious material is scarce, the internal strip drains can be placed instead since these give the same general effect.

Disadvantages of horizontal drainage blankets

An earth dam embankment tends to be more pervious in the horizontal direction than in the vertical. Occasionally, horizontal layers tend to be much more impervious than the average material constructed into the embankment, so the water will flow horizontally on a relatively impervious layer and discharge on the downstream face despite the horizontal drain.,p> Where this has occurred the downstream slope is prone to slipping and piping. Repairs can be made by installing pervious blankets on the downstream slopes or constructing vertical drains to connect with the horizontal blanket. Such vertical drains are normally composed of sand and gravel.

Chimney drains

Chimney drains are an attempt to prevent horizontal flow along relatively impervious stratified layers, and to intercept seepage water before it reaches the downstream slope. Chimney drains are often incorporated in high homogeneous dams which have been constructed with inclined or vertical chimney drains.

In some major dam projects, chimney drains have been inclined at a considerable slope, both upstream and sometimes downstream. An upstream inclined drain can act as a relatively thin core. In addition to controlling seepage through the dam and increasing the stability of the downstream slope, the chimney drain is also useful in reducing pore water pressures both during construction and following rapid reservoir drawdown.

Dimensions and permeability of drains

The dimensions and permeability of permeable drains must be adequate to carry away the anticipated flow with an ample margin of safety for unexpected leaks. If the dam and the foundations are relatively impermeable, then the expected leakage would be low. A drain should be constructed of material with a coefficient of permeability of at least 10 to 100 times greater than the average embankment material.

Thin upstream sloping core

In an earth dam with an upstream sloping core of low permeability, the foundation is assumed to be impermeable and in a steady state. Under steady state conditions the small amount of water that seeps through the core flows vertically downward in a partially saturated zone and then more or less horizontally in a thin saturated layer along the impermeable foundation. For this type of dam the downstream shell must be several hundred times more permeable than the core.

Partial cutoffs

An earth dam constructed without a cutoff on permeable or semi-permeable foundations of earth or rock may lead to seepage beneath the dam creating unacceptable uplift pressures and causing instability. If an impermeable cutoff is installed to 60 % of the depth of the permeable foundation, the flow net and downstream slope gradient is only slightly modified to a lower level. A theoretical line of seepage for several depths is given here.

For an effective cutoff the positioning and depth of cutoff must be essentially 'perfect'. Since this is impossible to achieve, other methods of seepage control should be used in conjunction with cutoffs.

Spillways

The provision of adequate spillway facilities can pose more problems than the design of the dam. Complete protection against the greatest flood that might occur would in almost all circumstances be unjustifiable. The existing or possible future habitation in the valley below the dam must influence decisions to be made regarding the spillway. Four standards for dam design have been suggested;

Freeboard and still capacity sufficiently to ensure that the dam will not be overtopped by floods up to probable maximum categories;

Such that the dam can be overtopped without failing, and in so far as practicable, without suffering serious damage;

Such as to ensure that breaching of the structure would occur at a relatively gradual rate; and

The height of the dam and storage are small enough that no serious hazard exists downstream in the event of breaching.

Handling of Flood Waters

Retention in Storage - In rare cases it is economically possible to store the entire volume of the design flood within the reservoir without overtopping the dam. The occurence of a subsequent storm shortly after the first must also be considered. In some cases an auxillary spillway or fuse plug spillway may be built in for emergencies.

Auxiliary Spillway to another valley. At certain locations it is possible to build one or more spillway outlets on the rim of the storage basin and to divert flood waters into adjacent valleys. Impact on the total environment must be considered before floods can be by-passed in this manner. Generally the main valley will have carried floods of maybe half the design flood and it is simple to assess the damage likely to be done by larger floods. The owner of the dam will be responsible for damage resulting from diversion of a major flood into a valley not normally subject to large floods.

Fuse-Plug Spillways are structures built instead of an auxiliary spillway. They may be simple earth banks, flash boards, or other devices designed to fail when overtopped. Such plugs should only be used when the sudden release of water is both safe and not over-destructive to the environment. For preference fuse plugs should be so constructed as to make their intensional destruction. This is much more positive than endeavouring to design a structure to fail at a predetermined overload.

Spillway Location Options - The sites indicated by the red areas are the most favourable locations for spillway positioning. The order of preference for rapid concrete construction is indicated by the numbering.

Passage over or through the dam - Many dams are designed for the safe passage of controlled and uncontrolled flood waters over the crest. Radial or sector gates are also used in large diversion weirs, however spilling over the crest is the cheaper method.

Bottom spillway:Advantage - provision can usually be made for its use for the passage of floods during construction. Disadvantage once built its capacity is finite wheras the forecasting is indefinite. a single outlet can be blocked by flood debris.

Siphon Spillway Disadvantage - construction is expensive - sudden appearance of flood water downstream - large flood debris can block outlet.

Gates or no gates - many Engineers are not inclined to place full reliance on effective operation of gates at the time of a major flood. The provision of gated spillways is usually economic, whatever height of dam the cost remains roughly the same and is only dependent on the magnitude of the flood provision. The possibility of maloperation can not be overlooked and their accessibility is important.

If proper gate operation can not be guaranteed then the effect of flood water passing over the top of gates must be investigated. Hydraulic gates are most reliable, followed by mechanically and electrically operated gates.

Spillway gates may therefore be installed:

Purely on economic considerations of total cost of dam and spillway, or In order to protect upstream property or installations, or In order to exercise control over the magnitude and duration of flood flows

below the dam - having due regard to flow in downstream tributaries, or In order to derive some economic benefit from water stored above the

fixed crest level.

Uncontrolled Spillways

The discharge over a spillway crest is given by the formula: Q=C.L.H3/2

where Q=discharge, C=coefficient, L=length of the crest, H=effective head of water.

Crest Profile - the crest of an overfall spillway is usually dimensioned to conform to the underside of the nappe of the free-falling jet. Greater efficiency is obtained by operating a spillway at greater than design head, as can be seen in the figure showing the effect of nappe profile on coefficient.

It is common practice to choose the design head for the nappe as 75%-80% of the maximum expected head. When the spillway so designed does pass the greater flows, pressures lower than atmosphere will occur over the crest, causing problems associated with cavitation.

The flow over a spillway gives rise to self-excited vibration, in which three coupled elements are involved; the jet, the overflow crest and the air cushion between dam and jet. This can be avoided by using splitters on the crest.

The cross section of a dam is normally determined to meet stability requirements. Optimum nappe can be obtained by the provision of an upstream overhang, as can be seen in the figure on the left, with the overhang not less than 0.3 times the height of the dam.

With the reservoir at a particular level the discharge over the spillway will be proportional to its length. It is possible to introduce variations in the plan shape of the spillway crest so that the effective length is increased, for example, rectangular 'duckbill' spillway or triangular sections.

In narrow gorges it is often expedient to adopt the glory-hole spillway. The design of a glory hole spillway is involved since it includes flow over the weir, free or forced flow in the shaft, flow around the bend and flow in the discharge tunnel. Since velocities are very high at the bottom of the shaft damage to lining is likely to occur. The main disadvantage with the glory-hole spillway is that beyond a

certain surcharge the discharge only increases slowly with increased head. It does not provide any substantial margin for underestimation of the maximum flood.

Foundation

The foundations of a dam must be able to withstand without unacceptable deformation the loads imposed upon it by the structure, both immediately after filling the reservoir and in the long term.

With time, deteriotation by saturation and percolation of water can occur, whilst soft rocks and clays usually exhibit lower residual strengths under sustained loading than under rapid testing. It is the 10-20m of rock immediately below the dam that is of greatest importance.

Terzaghi's advice might well apply to foundation testing - "...because of unavoidable uncertainties involved in the fundamental assumptions of the theories and the numerical values of the soil constants, simplicity is of much greater importance than accuracy." The Engineer must use all the available resources, concentrating on the zones of foundation that appear weak and that will be subject to stresses once loaded.

Foundation Preparation

Introduction

If it is economically feasible, all material under the base of a proposed dam which could cause excessive settlement and leakage should be removed. If this cannot be done, the dam design should be modified to take account of such material. Sometimes it may be necessary to remove material to considerable depths in isolated areas of the foundation. This is known as dental work. The general overall removal of material is termed stripping, whereas the removal of loose masses of rocks on the abutments is termed scaling. The engineering geologist has to determine the expected depth of weathered or unsound rock or overburden that must be removed in advance of construction.

Foundation programme

A planned programme of foundation excavation should be initiated with the view that the volume of excavation and configuration of the excavation will approximate reasonably to the plans and specifications established. It is the responsibility of the construction engineer to ensure slopes for excavations will be permanently stable or will not fail during construction. In earth materials slopes of 1.5:1 to 2:1 are excavated in permanent cuts and slopes of 1:1 are established in temporary cuts, except where unusual conditions are anticipated. In bedrock that is not closely fractured or does not contain inclined planes of

potential slippage, such as bedding planes in weak rocks, slopes are excavated at angles up to the vertical.

Problematic foundation materials

In foundations in unconsolidated material excavation of natural deposits may reveal inadequate localised or widespread foundation materials that require special treatment or total removal. Unacceptable or inadequate materials rich in organic substances such as topsoil, swamp muck or peat, loose deposits of sand or silt, talus accumulations and plastic, active, sensitive, or swelling clays.

Poor foundation conditions in rocks are associated with close fracturing, weathering or hydrothermal alteration, or poorly indurated sedimentary rocks.

Excavation in bedrock

The objective of excavation is the preparation of a clean surface that will provide optimum contact with the dam materials, whether earth or concrete is to be placed on that surface. Therefore excavations in bedrock should extend into firm, fresh rock. Any closely fractured zones extending downward, especially if containing soft altered materials such as clay gouge or other products of weathering, should be removed if feasible.

Prolonged exposure of both earth and rock foundations to the atmosphere or to water frequently results in deterioration by hydration, dehydration, frost action, shrinkage, and expansion with changes in temperature. It is good practice to protect reactive surfaces that will be exposed for long periods of time with bituminous materials. Alternatively, original cover is not removed until final cleanup and just prior to placement of the dam.

Construction on unconsolidated deposits

At an ideal site, excavations in unconsolidated deposits should extend to solid bedrock for the full width of the dam, whether it is constructed of concrete or earth/rock fill. However, there are many locations where the depth of the valley fill is so great that dams must be constructed in part or entirely on unconsolidated deposits. Where this is the case steps must be taken to improve the engineering properties of the foundation materials and to reduce subsurface seepage to allowable levels.

Except for low dams of small gross weight, concrete dams are not built on unconsolidated deposits because of their generally low bearing strength. Larger dams constructed in whole or in part on unconsolidated deposits should without exception, be earth or rockfill dams with the capacity to adjust to settlement in the foundation materials.

For concrete dams

Preparation of foundations - the extent of the work that will be necessary in the foundations for a concrete dam will be determined by two main factors, their strength to sustain the loads that will be imposed by dam and the reservoir water, and the effect of water entering the foundations under pressure from the reservoir.

Generally the quality of foundations for a gravity dam will improve with depth of excavation however the abutments for an arch dam often do not improve with distance excavated into the sides of the valley. Deterioration of clay could endanger the dam and/or lead to collapse of abutments downstream from the dam.

Frequently the course of the river has been determined by geological faults or weaknesses; proving of the river bed is therefore of first importance in the investigation stage. The depth to be excavated will depend upon the nature of the infilling material, the shape of the excavated zone, and the depth of cutoff necessary to ensure an acceptable hydraulic gradient after the reservoir is filled.

Concrete dams may be constructed on foundations other than massive rock, i.e. shales, glacial deposits or even sand for river works. Each case must be examined relative to permeability, settlement, and load-carrying capacity (vertical and horizontal).

The final preparation of the foundation should be undertaken just prior to the placement of concrete. It should include the removal of loose rock and all debris, roughening of smooth rock surfaces, washing down of all surfaces, and the removal of excess water from pools to leave a clean damp surface to receive the concrete.

Foundation Design

1. Pressures associated with dams and reservoirs 2. Mechanisms of Foundation Failure 3. Geologic Conditions Promoting Foundation Failure 4. Bearing capacity 5. Seepage 6. Settlement

1. Pressures Associated with Dams and Reservoirs

Construction of a dam and filling of the reservoir behind it create load stresses on the floor and sides of a valley that did not exist previously.

The kinds and distributions of imposed stresses created by a dam on its foundation depend on the shape of the dam and the materials used in its construction.

Dams built of masonry or concrete can be considered to behave as cohesive, rigid, monolithic structures. The stresses acting on the foundation is a function of the gross weight of the dam as distributed over the total area of the foundation on which the dam rests.

Earth and rock fill dams exhibit gross semi plastic behavior, and the pressure on the foundation at any point depends on the thickness of the dam above the point.

Pressure due to water in reservoir

The pressures exerted by earth and rock-fill dams resemble in some respects those exerted by the water in a reservoir, but pressure distribution is modified by the fact that the materials of construction have some inherent strength, and fail only after some threshold stress has been exceeded. Pressures exerted by water in the reservoir behind a dam are hydrostatic and increase linearly with depth.

The pressures are hydrostatic and increase with depth. On the assumption that the pressures are directed normal to the floor and sides, they are shown as vectors of increasing magnitude with depth.

Pressures from the weight of a rigid concrete dam

The deadweight load of a concrete dam is distributed over the total area of the foundation and is shown by vectors normal to the surface beneath the dam. The figures are essentially static, and depend only on the weight of the dam and the area of the foundation.

Water exerts hydrostatic pressures not only on the floor and walls of a reservoir but also on the upstream face of a dam. D is the depth of water in a reservoir, P is the hydrostatic pressure per unit area acting on the vertical face of a concrete dam assumed to behave as a rigid body. The change in pressure with depth (in

the y direction) is given by dP/dY=þg in which þ is the density of water and g is the acceleration due to gravity.

Torque about 0 = þgD³/6

Resultant Pressure = þgD²/2

In calculations of the stability of the dam the torque tending to rotate the dam about 0 should be added to the tendency of the dam to be rotated in the same direction about the same point by uplift forces related to seepage beneath the dam.

Forces acting on a rigid dam owing to hydrostatic pressures Figure 1 illustrates an earth dam, a non rigidstructure that under stress behaves semiplastically. Because of relatively easy internal adjustments to loads, the pressure exerted on the foundation are approximately equal to the weight of overlying prisms of material of different heights. Pressures exerted on the dam by water in the reservoir tend to cause greater adjustments near the base of the dam than at shallower depths. A cross section of a concrete gravity dam, presumed to behave as a rigid body. When the reservoir is empty, the weight of the dam is directed vertically

downward. When the reservoir is full, a combination of hydrostatic pressure on the upstream face of the dam and the weight of the dam produces a force vector inclined downstream away from the vertical force vector, and there is a tendency for the dam not only to be displaced downstream but also to rotate about the downstream toe of the dam because of a torque.

These figures show force vectors for empty and filled reservoirs behind concrete arch dams. Unlike gravity dams, arch dams because of the egg-shell effect tend to resist downstream dislocation and the displacing forces, instead, are transmitted laterally through the dam and toward the abutments.

Geologic Conditions Promoting Foundation Failure The geologic conditions in foundations for concrete dams that should be avoided

are indicated below.

Brittle, fractured sandstones rest on a weak shale layer dipping upstream.

Horizontally layered limestones rest on a weak shale layer which extends

downstream to a steep slope in the valley floor.

Fractured crystalline rocks lie above a flat fault containing sheared, gougy materials of

very low strength.

Intersecting strong conjugate joints have attitudes that promote easy mass shear

dislocations.

Sedimentary rocks dipping downstream are intersected by a fault dipping upstream and

containing materials of low strength.

Folded rocks containing thin, weak layers of shale present a potential for foundation

failure.

Slope failures toward abutments (in direction of the dam axis) which disturb or dislocate the abutments are rare. In concrete dams in which slopes in the abutment areas maintain themselves during excavation for the foundation, the possibility of downslope movement along surfaces that intersect the foundation of the dam is remote because of the added stability provided by the weight and strength of the dam. However, the possibility that slopes

above the dam, especially in deep valleys, may fail and bury surface structures with rock and/or soil debris. Figure - Conditions promoting possible slope failure beneath abutments of an earth or rock-fill dam along curves shear surfaces. Shale beneath a sandstone layer has been weakened by infiltration of water from the reservoir. Bearing capacity To avoid shear failure, the foundation pressures used in design should have an adequate factor of safety when compared with the ultimate bearing capacity of the foundation. If failure is to be avoided, then a factor of safety must be applied to the ultimate bearing capacity, the value being obtained being the safe bearing capacity. The ultimate bearing capacity is defined as the least pressure which would cause shear failure of the supporting soil immediately below and adjacent to a foundation. However, this value still may mean risk of excessive settlement or differential settlement. Thus the allowable bearing capacity which is used in design will take into account all possibilities of ground movement and so its value will normally be less than the safe bearing capacity. Seepage Seepage under an embankment is much more dangerous than that for a concrete dam, since embankments are usually built on soft material which is liable to be scoured out and it is also vulnerable to influx of water; whereas a concrete dam is usually built on rock which is not worn away so rapidly by the scouring action of water; and even then a defective dam will not necessarily be endangered by passage of water through it or even under it.

Basic seepage problems

Stored water behind dams, gives rise to three basic seepage problems, which can lead to difficulties and in serious cases to total failure:

1. Piping occurs when water picks up soil particles and moves them through unprotected exits, developing unseen channels or pipes through a dam or its foundation.

2. Heave or slope failures caused by seepage forces. 3. Excessive loss of water.

Three basic methods for controlling seepage are:

1. Use of filters to prevent piping and heave 2. Seepage reduction 3. Drainage

Settlement

All structures undergo some settlement, regardless of their construction or of the quality of their foundations. Structures made of soil or founded on soil settle so much that their performance is affected and their safety is compromised.

Concrete dams are almost always based on strong rock foundations where settlement of the dam is kept to a minimum otherwise the dams would crack leading to serious structural faults. Embankment dams can be founded on soft compressible materials and are able to withstand large settlements.

Causes of settlement Measurement of settlement Effects of settlement on structures Settlement due to changes in environment

Causes of settlement

1. Bearing capaicty failure or instability, including landslides. 2. Failure or deflection of the foundation structure. 3. Elastic or distortion of the soil or rock. 4. Consolidation (compression) of the soil or rock. 5. Shrinkage due to desiccation. 6. Change in density due to shock or vibration. 7. Chemical alteration of constituents, including decay. 8. Underground erosion. 9. Collapse of underground openings such as caves or mines. 10. Structural collapse due to weakening of cementation upon saturation.

Measurement of settlement

Measurement of settlement within a dam should illustrate the progress of consolidation in the dam and point out whether addition of height will be necessary to maintain freeboard. In embankment dams settlement measurements are helpful in computing the volume of material placed in the dam from the dimensions of the completed structure and provide a check on original design specifications.

Measurement of Crest Settlement

Crest settlement is measured by bench marks placed at intervals along the top of the dam. Obviously these are tied to a reference bench mark on the abutment which is immovable.

Measurement of Internal Settlement

Measurement of internal settlement is made using settlement plates embedded in the dam or the foundation.

Diagram of settlement plates -

Effects of settlement on structures The settlement configuration of a uniform load on a thick deposit of compressible soil is a saucer shaped depression which extends beyond the limits of the structure. If the loading is irregular or the soil uniform, the saucer shaped curve is distorted. If the deposit is thin, the 'saucer' is flattened at the centre. The effect that the settlement has on a structure depends on where the structure is located in the depression and on how the movements at that location influence the performance of the structure.

Total settlement Tilting Distortion

Total Settlement

The total amount of settlement a structure can undergo without damage is large provided it is relatively uniform. However, with large amounts of settlement several forms of trouble develop. In embankments and dams on earth foundations the result will be a lowering of the crest. This is an insidious form of trouble since it usually develops slowly, often without the operators of maintenance personnel being aware of the loss of height and free board.

Allowances must be made for settlement in the design height, and periodic measurements should be taken to be sure that the proper crest level is maintained. A considerable part of the settlement, both of the foundation and of the embankment, occurs during the construction period. This can result in

discrepancies in the computed volume of the structure unless it is anticipated and careful records of the settlement are kept. With proper allowances, embankment settlements of a few metres can be tolerated. Total settlement is not a serious matter if it is anticapted and provisions made before hand.

Tilting

Tilting occurs in the parts of the structure that are outside the centre of the saucer-like depression. It also takes place when the structure is unevenly loaded, or when the soils are non-uniform. It is of importance mainly with tall structures such as large retaining walls, transmission towers, water tanks, and smoke stacks. It is particularly serious in structures that are inter-connected. The amount of tilt which can be tolerated depends on the height-width ratio of the structure. Distortion Differential settlement which produces relative movement is known as distortion. The load of an embankment on a uniform soil produces a settlement profile as shown below. There is also a tendency for cracks to develop as indicated. These cracks may lead to accelerated seepage, erosion, and even failure.

Figure 1 - The load of an embankment on a uniform soil produces a settlement profile as shown and a tendency to develop cracks at the points indicated. Such cracks can possibly lead to accelerated seepage, erosion and even seepage failure.

Figure 2 - The non uniform foundation thickness and the greater loading at the centre than at the abutments brings about a sagging profile along the axis. Shear cracks tend to form as shown. These are far more serious because they extend from upstream to downstream and several dam failures have been attributed to such cracking.

Figure 3 - When a small portion of the embankment extends beyond the main section shear cracks sometimes develop.

Figure 4 - Similar settlement adjacent to an overhanging abutment can create cracks at their juncture.

Settlement due to changes in Environment

Changes in environment can bring about a reduction in void ratio in certain soils and a corresponding settlement. Shock and vibration from earthquakes, blasting, and construction machinery can cause loose cohesionless soils to densify. In addition, flow failure may accompany the settlement if the soils are saturates. Detioration of cementing agents from physical and chemical changes brought on by exposure and inundation can cause the collapse of loose skeleton soil

structures and settlement. Bacteriological decay of organic materials can produce settlement accompanied by formation of gas pockets. Such decay is inhibited by permanent submergence.

Exposure to soils to hot dry weather during construction can cause both settlement and shrinkage cracking. A desiccated clay that is subsequently inundated may swell and damage a superimposed structure or embankment by heave. Moreover, the cracked, swollen soil is weakened and can be a cause of foundation failure.

Foundations Improvement

1. Pre-Consolidation 2. Densification of cohesionless soils 3. Dynamic Compaction 4. Grouting

Pre-Consolidation

Pre-consolidation is a useful foundation treatment method in compressible soils, depending on the rate of consolidation. If the rate is rapid (one to two months for 50%) it will be possible to pile up the soils removed from stripping and scaling of the abutments to form an artificial surcharge.

If the rate is slower (one to two years for 50%) the dam weight can be used to consolidate the soil and increase it's strength. It would be necessary to control the rate of construction so that the weight applied does not exceed the ability of the foundation to support the structure safely. It may, however, be necessary to increase the length of the construction period to obtain a sufficient gain in strength. Drainage of the foundation can also help to accelerate consolidation.

Densification of cohesionless soils

Densification of cohesionless soils is carried out using shock and vibration. Vibroflotation is used to improve poor foundations. The process may reduce settlement by more than 50% and the shearing strength of treated soils is increased substantially. Vibrations can convert loosely packed soils into a denser state.

A vibroflot can be used to penetrate the soil and can operate efficiently below the water table. The best results are obtained in coarse sands which contain little or no silt or clay, since both reduce the effectiveness of the vibroflot.

Dynamic compaction

Dynamic compaction improves the mechanical properties of the soil by repeated application of very high intensity impacts to the surface. This is achieved by dropping a weight, typically 10 to 20 tonnes, from crawler cranes, from heights of 10 to 20 metres at regular intervals across the surface. Passes should be repeated over a site, although several tampings may be made at each imprint during a pass. Each imprint is back-filled after tamping. The first pass at widely spaced centres improves the bottom layer of the treatment zone and subsequent passes consolidate the upper layers. In finer materials the increased pore water pressures must be allowed to dissipate between passes, which may take several weeks.

Grouting

1. Grouting Operations in Bedrock 2. Dam Construction on Unconsolidated Deposits 3. Grout 4. Types of Grouting 5. Pattern Grouting 6. Blanket Grouting 7. Curtain Grouting 8. Off-pattern, Special Purpose Grouting 9. Grouting Consistency and GroLuting Pressure

Grouting Operations in Bedrock

The goal of foundation and abutment grouting in bedrock is improvement of strength and bearing capacity and the filling with grout of underground channelways that have a potential for impermissible seepage. The most general technique uses drilling and pressure grouting, either with water-cement mixtures or with other types of sealants.

Preliminary geological and geophysical investigations usually reveal only the general characteristics of the bedrock, it is not until the keyway for the dam has been excavated and the bare rock can be examined. This is a critical time because the constructor is eager to proceed with the dam construction, however this is the last chance to ensure that all the fissures are sealed to prevent water loss at a later stage and must be extensively treated to assure ultimate safety of the dam.

Although grouting of a rock foundation may be conducted with meticulous care, the possibiliy always exists that some channelways of underground water circulation remain and that flow through these chanelways will accelerate as the reservoir is filled. If the volumes are excessive then remedial steps must be

taken, otherwise the flows may be intercepted and diverted by drain holes or porous prisms.

Dam Construction on Unconsolidated Deposits

There are many locations where the depth of valley fill is too great to remove and so the dam or parts of it may have to be built on unconsolidated deposits. Cross sections of several earth and/or rockfill dams constructed at least in part on unconsolidated subsurface deposits are shown below. It is clear that considerable information as to the distribution and permeabilities of subsurface materials is required prior to the design and construction of cut-off features.

Rockfill dam. Impervious membrane (asphaltic concrete) extends to a grout cap on bedrock.

Cut-off trench extends to bedrock

Cut-off trench penetrates impervious layer in unconsolidated valley fill.

Cut-off extends to layer of impervious material in unconsolidated valley fill. Grout holes extend through a limestone layer in bedrock below valley fill.

A cut-off is provided by sheet piling driven into an impervious layer in valley fill.

Flow beneath dam is reduced by a leyer of impervious material placed upstream from the dam.

Grout

Grout is a liquid, either a uniform chemical substance or an aqueous suspension of solids that is injected into rocks or unconsolidated materials through specially drilled boreholes to improve bulk physical properties and/or to eliminate seepage of groudwater.

There are three basic types;

1. Portland cement-base slurries 2. Chemical Grouting solutions 3. Organic resins, including epoxy resins.

Portland based are the most widely used.

Types of Grouting

In dam foundations three kinds of grouting programs are identified:

1. Shallow blanket or consolidation grouting over critical portions 2. Curtain grouting from a gallery or concrete grout cap 3. Off pattern, special purpose grouting to improve strength

Some cross sections of dams with rock foundations showing locations of drilled holes for foundation treatment.

A - Curtain grout holes B - Blanket grout holes C - Special purpose, off-pattern grout holes D - Drain holes

Rockfill dam with impermeable concrete face

Zoned earth and rock-fill dam

Zoned earth and rock-fill dam

Earth dam

Concrete gravity dam with 'C' holes intersecting a fault zone

Concrete gravity dam with double grout curtain and 'C' holes intersecting a permeable fault zone

Concrete gravity dam with special purpose 'C' holes

Concrete gravity dam wiht a porous filter to collect seepage water

Hollow conrete gravity dam

Concrete buttress dam

Concrete arch-dam

Concrete arch-dam

Pattern Grouting

Plans for dams commonly include broad specifications for a systematic program of blanket and/or curtain grouting. Grouting is an uncertain process, it is impossible to accurately estimate the amount of grout required, and usually the 'take' amounts moderately to greatly in excess of the estimate. 'Grouting is an art and not a science.'

Pattern grouting is grouting included in the plans and specifications for a dam and commonly is the basis for estimation prior to construction of the total footage of grout holes and the expected amount of grout consumption. It is general practice to lay out locations of grout holes in the plans with a definite, systematic pattern, spacing and assumed depths.

Blanket Grouting

Are intended to remedy flaws in the foundation, such as fractured rock, by reducing permeability and increasing bulk strength. Although holes may be routinely drilled normal to the foundation surface, there is considerable merit in directing the holes to intersect specific local features identified in the dam

foundation during excavation. Blanket grouting must be completed before construction of a dam.

Locations of blanket grout holes may usefully be indicated on plans and specifications for several types of dams. The drawings are entirely schematic, and no scale is shown, on the assumption that the actual number of holes will be determined by the area and the cross-sectional configuration of the excavation for the dam foundation.

Curtain Grouting

In earth / rockfill dams, curtain grouting is usually completed before a dam is constructed and involves filling a narrow excavated trench in the foundation with concrete. The exception to the timing of the grouting operation is grouting after construction for a grouting cap at the upstream heel of a dam.

Curtain grouting of the foundations of concrete dams is most effective after completion of the dam, at a time when the full load is being applied to the foundation. Under such circumstances higher pressures may be used in grouting so as to assure maximum travel of grout in all directions along flow paths intersected by grout holes.

In gravity and gravity arch dams of moderate to large size it is common practice to construct a gallery inside the dam for drilling curtain grout holes and drainage holes. Foundations of small gravity and thin arch dams are efficiently grouted from grout caps along the contact of the upstream face of the dam with rock.

Where there are no geological controls the depths of curtain pattern grout holes are determined by a formula. A frequently used formula is: the vertical depth of grout holes shall be a third of the dam height at the location of the hole plus (15 - 20m).

Off-pattern, Special Purpose Grouting

During investigations prior to dam construction, or as unanticipated geological conditions are exposed in foundation excavations, the need for 'off-pattern' grout holes may be required. These holes are drilled and grouted to improve the strength and / or reduce the permeability of rock masses that are not intersected by blanket or curtain grout holes. The depths, directions and inclinations of the grout holes are determined by the three-dimensional geometry of zones of incompetent and / or permeable rocks as revealed by field examination of bedrock exposures in foundation and abutment excavations

Inclined holes from the surface and horizontal holes from a shaft intersect steep faults and associated fractures at depth.

Inclined holes are drilled to intersect sheeted zones in crystalline rocks.

Dipping sedimentary layers present a potential for seepage under a dam. Inclined holes are drilled to intersect a limestone layer and a brittle sandstone layer.

Inclined holes are drilled into jointed and sheared rock in crests and troughs of folds.

Holes are drilled to intersect a closely jointed igneous dyke at depth.

Off-pattern grout holes intersect a jointed, weathered zone in crystalline rocks below an unconformity.

Grouting Consistency and Grouting Pressure The ability of cement grout to penetrate interconnected open spaces is limited by the dimensions of the open spaces and the amount and size of the cement particles suspended in the water base. Openings of slightly greater than capillary size that may permit free circulation of groundwater are quickly filled and obstructed by cement particles and lateral and/or vertical travel of the grout suspension is greatly impeded or brought to a halt. In larger openings, presupposing interconnecting avenues of circulation, grout suspensions move with ease and in some instances travel surprisingly large distances.

If easy grout circulation continues with the progress of the grouting operation, the suspension is gradually thickened and, if necessary, the pressure correspondingly increased until filling of available openings is indicated by refusal of the grout hole to accept additional grout. Grout leaks at the surface should be calked or otherwise sealed to promote confined subsurface movement of grout suspensions.

The definitions of thin and thick are not precise, but generally thin mixtures are construed to mean mixtures prepared by mixing 8-10 volumes water with one volume of cement. Thick mixtures have volume proportions of cement to water of approximately 1:1, or thicknesses that are not so great that the grout can not be pumped with reasonable ease. In highly permeable materials thick grout mixtures are indicated with inert additives such as clay or sand may be added to grout suspensions as inexpensive fillers.

If grout pressures exceed certain limits there is the possibility of foundation dislocation and new channelways being created. Because of a wide range in complexity of patterns of underground circulation it is not possible to establish a rigid formula for controlling grout pressures at the top of a grout hole. For curtain grouting, a rule that is sometimes followed states that pressure in an initially thin grout suspension is increased to a level which establishes a free circulation (assuming channelways for circulation are present) but not in excess of the calculated hydrostatic pressure of the filled reservoir at the elevation of the collar of the grout hole plus 0.7-3.5 bar.

Premature thickening of grout or reduction of pressures to cause grout refusal in a grout hole should be avoided unless it can be demonstrated that grout is escaping to the surface well outside of the foundation area. So long as grout is circulating somewhere in the foundation of a dam or in the near proximity of the foundation, it must be assumed that it is contributing to an improvement of the engineering properties of foundation materials and to a reduction in permeability to groundwater seepage.

Prevention of Piping Failures

Piping failures

Water that percolates through earth dams and their foundations can carry soil particles that are free to migrate. The seepage forces tend to cause the erodible soil or soft rock to move towards the downstream face of the dam. That is if the seepage forces are large enough and the pore spaces in the material are large enough. Along the unprotected discharge face AB, the soil will heave if the gradients are large enough.

Every seepage discharge surface, both internal and external, which could be susceptible to piping or heave must be covered with filters that permit water to pass but will hold the soil particles firmly in place.

Filter criteria

Since the core is stabilised with rock or gravel zones, it is necessary to prevent the fine core material being sucked into the upstream shell material during rapid drawdown of the reservoir, or forced into the downstream shell by seepage water under reservoir head. Transition or filter zones must therefore be provided on each side of the core.

The upstream filter, if non-cohesive and of proper grading, can serve a valuable service by providing material for induced self-healing should a transverse crack appear in the core. Selection of the best material for this purpose is well justified. Although its prime function is to retain the core material against movement into the rockfill, the downstream transition material should be selected and placed so as to inhibit the propagation of a core crack into the compacted rockfill. It is good practice to widen the transition zones towards each abutment, i.e. where tension and oblique cracking may occur.

To prevent migration of fines from the core:

D15/D85 < 4-5 (filter)/(zone being filtered)

D50/D50 < 25 (filter)/(zone being filtered)

For sufficient permeability: D15/D15 > 4-5

(filter)/(zone being filtered) To prevent segregation of the filter:

D60/D10 < 20 (filter)/(filter)

Problems associated with natural formations

The foundations and abutments of dams are usually stable under the influence of the natural groundwater flow. However, reservoir filling greatly changes the groundwater regime and may lead to piping and internal erosion. The potential for internal erosion and piping may occur at joints in rock, beds of gravel and in cavities left by rotting roots, animals’ burrows or other buried organic matter.

Recommendations for preventing piping in natural formations

Field exploration and geological mapping for dam projects should identify the important soil and rock formations that could cause failure by internal piping or heave. The geotechnical properties of these materials should be thoroughly investigated. If the materials are proven to be unsuitable then remedial action should be taken to improve their geotechnical properties. All new dams and reservoirs should be carefully observed and monitored once in service to detect the development of unsafe conditions. If seepage quantities increase or if there is an unexplained change in seepage conditions then protective measures should be put into action. Such actions should include lowering the reservoir and placing weighted filters over areas where seepage discharges occur.

Seepage Reduction Basic considerations

Seepage-reduction methods make use of impermeable cutoffs, grout curtains, and upstream blankets, which consume energy at locations within cross sections

where large water pressures and seepage forces have no detrimental effects. The net result of these methods is that water pressures and seepage forces are reduced in the downstream region. These seepage-reducing features are usually

combined with properly designed filters and drainage features, since seepage

reduction can only be partially effective by itself.

Drainage Methods

Permeable downstream shells

At dam sites where there is an abundance of at least two different materials with significantly different permeabilities, a zoned dam may be constructed. In such cases permeable material is placed downstream of less permeable material, often with a transition zone between. For example, in a zoned dam which has a thick impermeable core and rests on an impermeable foundation, the flowpaths

within the downstream portion of the dam will be low. Thus seepage has a negligible effect on the stability of the downstream slope, which is the ideal condition in zoned earth dams.

Internal drain systems

Purpose

A homogeneous dam with a height of more than about 6 m to 8 m should have some type of downstream drain. The purpose of a drain is:

1. to reduce the pore water pressures in the downstream portion of the dam therefore increasing the stability of the downstream slope against sliding.

2. to control any seepage that exits the downstream portion of the dam and prevent erosion of the downstream slope: i.e. to prevent 'piping'.

The effectiveness of the drain in reducing pore pressures depends on its location and extent. However, piping is controlled by ensuring that the grading of the pervious material from which the drain is constructed meets the filter requirements for the embankment material.

Toe drains

The design of a downstream drainage system is controlled by the height of the dam, the cost and availability of permeable material, and the permeability of the foundation.

For low dams, a simple toe drain can be used successfully. Toe drains have been installed in some of the oldest homogeneous dams in an effort to prevent softening and erosion of the downstream toe.

For reservoir depths greater than 15 m, most engineers would place a drainage system further inside the embankment where it will be more effective in reducing pore pressures and controlling seepage.

Horizontal drainage blanket

Horizontal drainage blankets are often used for dams of moderate height.

Drainage blankets are frequently used over the downstream one-half or one-third of the foundation area. The Bureau of Reclamation's 45 m Vega Dam is a homogeneous dam which has been constructed with a horizontal downstream drain. Where pervious material is scarce, the internal strip drains can be placed instead since these give the same general effect.

Disadvantages of horizontal drainage blankets

An earth dam embankment tends to be more pervious in the horizontal direction than in the vertical. Occasionally, horizontal layers tend to be much more

impervious than the average material constructed into the embankment, so the water will flow horizontally on a relatively impervious layer and discharge on the downstream face despite the horizontal drain.,p> Where this has occurred the downstream slope is prone to slipping and piping. Repairs can be made by installing pervious blankets on the downstream slopes or constructing vertical drains to connect with the horizontal blanket. Such vertical drains are normally composed of sand and gravel.

Chimney drains

Chimney drains are an attempt to prevent horizontal flow along relatively impervious stratified layers, and to intercept seepage water before it reaches the downstream slope. Chimney drains are often incorporated in high homogeneous dams which have been constructed with inclined or vertical chimney drains.

In some major dam projects, chimney drains have been inclined at a considerable slope, both upstream and sometimes downstream. An upstream inclined drain can act as a relatively thin core. In addition to controlling seepage through the dam and increasing the stability of the downstream slope, the chimney drain is also useful in reducing pore water pressures both during construction and following rapid reservoir drawdown.

Dimensions and permeability of drains

The dimensions and permeability of permeable drains must be adequate to carry away the anticipated flow with an ample margin of safety for unexpected leaks. If the dam and the foundations are relatively impermeable, then the expected leakage would be low. A drain should be constructed of material with a coefficient of permeability of at least 10 to 100 times greater than the average embankment material.

Geology of dams

1. INTRODUCTION 2. TERMINOLOGY

3. CLASSIFICATION OF ROCKS 4. TYPES OF ROCK 5. ROCK PROPERTIES 6. SURFACE FEATURES OF VALLEYS 7. EXCAVATION AND FILLING OF VALLEYS 8. TOPOGRAPHICAL AND GEOLOGICAL CONDITIONS 9. SEISMIC ACTIVITY 10. GEOLOGICAL HAZARDS

Introduction

The geological services required for the engineering of a large dam are in the following areas;

The safety of the dam on its foundations; The watertightness of the reservoir basin; The availability of natural materials for its construction.

The engineering geologist is a key member of an engineering team, since he will ensure the feasibility of the project, continuing through the design stage and terminating only when construction has either proved that geological conditions revealed are in conformity with the premises adopted in design, or he has made possible proper evaluation of any conditions not foreseen in the earlier stages.

The safety, viability and cost of a dam are all dependent upon geology. Most rocks have adequate strength but their weakness is in the orientation and dip of discontinuities relative to the loading from the dam, as well as the infilling material in, and depth of, weathering in such discontinuities.

It is necessary to investigate both the regional geology and the specific local geology to ensure a global picture is developed.

Terminology

1. Bedding planes - The planes marking the termination of one sedimentary deposit and the beginning of another; they usually constitute a weakness along which the rock tends to break.

2. Foliation - In rocks that have been subjected to heat and deforming pressures during regional metamorphism, some new materials such as muscovite and biotite mica, talc and chlorite may be formed by recrystallisation. These new minerals are arranged in parallel layers of flat or elongated crystals - the property of foliation.

3. Joints - These are fractures along which no movement has occured. All rocks are jointed to some extent and weathering occurs in these joints. They offer pathways for water, any clay infilling offering little resistance to sliding.

4. Faults - These are fractures along which movement has occured. They may range from rather inconspicious zones hundreds of metres wide and many kilometres long. The movement may have formed a zone that is so crushed and chemically altered as to be unable to support any weight. The presence of faults may be recognised from such physical features as;

Offset of beds, dykes or veins; Slickensides; Gouge; Brecciation or crushing; Topographic features like escarpments, linear trenches or sag valleys.

5. Weathering - The following definitions appeared in the Quarterly Journal of Engineering Geology, UK, 1970.

Fresh Rock No visible signs of weathering Slightly Weathered

Penetrative weathering developed in open discontinuity surfaces but only slight weathering of rock material.

Moderately Weathered

Weathering extends throughout the rock mass, but the rock is not friable.

Highly Weathered

Weathering extends throughout the rock mass, but the rock material is partly friable.

Completely Weathered

Rock is wholly decomposed, and in a friable condition but rock texture and structure are preserved.

Soil A soil material with the original texture, structure and mineralogy of the rock completely destroyed.

Classification of rocks

1. Uniaxial Compressive Strength

Weak - less than 35MPa Strong 35-115MPa Very Strong - greater than 115MPa

2. Prefailure Deformation

Elastic Viscous

3. Failure Characteristics

Brittle Plastic

4. Gross Homogeneity

Massive Layered

5. Continuity in Formation

Solid - joint spacing greater than 2m Blocky - joint spacing 1-2m Broken - fragmented

Types of Rock

1. Granite 2. Gabbros, Andesites, Dolerite and Basalt 3. Amphibolites 4. Metamorphic Rocks 5. Limestone 6. Sandstones 7. Clays 8. Gravel, Sands and Boulder Clay

Granite

Can bear great pressures Generally watertight

Investigation must be made for

Fissures Disintegration due to weathering China clay

Caution must be taken when large masses of china clay appear, for it is not feasible to anchor pre-stressed cables in china clay.

Example: The Sarrans dam had a broad foundation of 11,000 sqm of decomposed granite. To improve the strength, and bearing capacity and to reduce seepage of the foundations and abutments a grouting programme was undertaken. This involved 691 tonnes of cement in 81 boreholes which had an aggregate length of 2800 m or 240 kg per m.

Gabbros, Andesites, Dolerite and Basalt

These types of rock cannot be trusted for dams and reservoirs. Porphyritic rocks need careful grouting.

Example: The Rieutord dam which is on a tributary of the Loire, necessitated a considerable amount of grouting. Conversely, the Tirso multiple arch dam in Sardinia is founded on trachytes and volcanic tuffs with little grout.

Amphibolites

Gneiss, mica schists and associated rocks are considered to be satisfactory for sustaining bearing pressure and for water-tightness. However, gneiss and particularly mica schist are less favourable due to the mica which may facilitate slipping.

Where gneiss and mica schists are associated, a very weak zone of disintegrated rock may be found at the junction of these two rocks.

Example: the Forks dam, California, founded on gneiss and mica schists, had to be abandoned in 1929 because of bad foundations which occurred at the junction of these two rocks which, in themselves, were quite sound. Metamorphic Rocks

Metamorphic and intrusive igneous rocks are to an extent unpredictable. However, many satisfactory dams have been constructed on them

particularly in Scotland (for example Pitlochry, Errochty, Shira), but grouting of the foundations is generally essential. The usual types of dams constructed are gravity, buttress and rockfill.

Where the rock is weathered at the surface, an investigation is usually required since weathered formations may prove exceptionally difficult when the foundations are dug out.

Example: The Lavaude-Gelade dam in the Central Massif, Creuse, France was founded on altered granulite. The alteration in the granulite was found to persist to a depth of 20m, in addition to being broken and fissured. The site required an extensive grouting injection with cement, clay and bentonite.

Limestone

Limestone dam sites vary widely in their suitability. Thickly bedded horizontally lying limestones which are relatively free from solution cavities afford excellent dam sites. On the other hand, thin bedded, highly folded, or cavernous limestones are likely to present serious foundation or abutment problems involving bearing capacity and water tightness.

Concrete dams have been constructed on Jurassic limestone at Castillon, where slips and leakage problems have occurred. These have been surmounted by an extensive grouting scheme.

Sandstones Sandstones have a wide range of strength depending largely upon the amount and type of cement matrix material occupying the voids of the rock. Generally sandstones do not deteriorate rapidly on exposure to the surface with the exception of shaly sandstone. As a foundation rock sandstone is not susceptible to plastic deformation, even with poorly cemented sandstones. However, sandstones are susceptible to erosion due to the scouring and plucking action from the overflow of dams and so have to be adequately protected by suitable hydraulic structures.

Sandstones are frequently interbedded with shales. The sandstone-shale contact may allow seepage of water and may cause potential sliding. Severe uplift pressures may also develop beneath beds of shale in a dam due to the swelling characteristics of shales.

Many dams in the English Pennines have been constructed on Carboniferous sandstones interbedded with shales, most of them as earth embankments.

Example : Longdendale, Langsett, Scar House reservoirs and Ladybower reservoir on the Sabden shales in the Derwent valley.

Clays

Clay formations are often thick and massive and are frequently associated with thin seams of sandstone or limestone. Earth dams or rockfill dams are usually constructed on clay foundations because clays lack the load bearing properties necessary to support concrete dams.

Example : The embankments of the Staines, Chingford, and other reservoirs in the Thames and Lee valleys may be cited as reservoirs wholly in London Clay ,whilst the Cheddar reservoir near Bristol lies on Keuper Marl.

Gravel, Sands and Boulder Clays

Gravels, sands and boulder clay of glacial origin are notoriously variable in composition both laterally and vertically.

As a result dam sites in glaciated areas are among the most difficult to appraise on the basis of surface evidence. Generally, earth dams are constructed in areas of glacial deposits.

Example : Selset reservoir in the North East of England is founded on Boulder Clay.

Rock Properties

The following properties must be examined to ensure the dam will be stable and the reservoir watertight -

Crushing strength Shearing strength Elasticity of rock Deformability of the rock mass Tectonic stresses

Crushing Strength

In general the compressive load from a dam on to its foundations will not exceed 10 MPa.

The strength of a rock will depend upon its -

Quality The degree of weathering Presence of micro-cracks

The strength of a rock mass will depend upon -

The number of cracks and joints The nature of their infilling material Whether there are any rock-to-rock contacts across the joints Planarity and continuity of seams and foliations

Table - Unconfined compressive strength of rocks

Rock type Strength (MPa) Siltstone 24-120

Greywacke 20-30 Shale 35-110 Sandstone 40-200 Limestone 50-240 Dolomite 50-150 Granite 90-230 Basalt 200-350 Dolerite 240-320 Gneiss 80-330

Shearing Strength

The minimum angle of friction for sound rock is 55°.

Table - Angle of internal friction of rock

Rock Tangent of Angle of Internal Friction Tuff 0.9 Schist Biotite 0.5 Limestone 0.6 Limestone (med. grained) 0.5 Granite (weathered) 0.8

The shear strength of a rock mass may be seriously affected by saturation since both the cohesion and angle of friction will decrease. Elasticity of Rock It is not appropriate to classify rocks by elastic constants alone, since many rocks are nonelastic. Elasticity refers to the property of reversibility of deformation when subjected to a load. Many fresh, hard rocks are elastic when considered as laboratory specimens. But on the field scale rocks can be expected to contain fractures, fissures, bedding planes, contacts, zones of altered rock and clays with plastic properties.

Therefore, most rocks do not exhibit perfect elasticity. The extent of irrecoverability of strain in response to load cycles may be important for the design and can be determined by the slope of the load/deformation curve.

The concave upward curvature of this load/deflection path is typical for fractured rocks on first loading because the fractures close and stiffen at low loads. When

the reservoir is lowered, the rock unloads along path 2, with a permanent deflection.

The dam will try to follow the loading, but since it is often more elastic than the rock, it will move away from the rock on unloading. This could open joints in the rock or concrete or simply lower the compressive stress flowing through the structure. Repeated cycles of loading and unloading in response to cyclic operation of the reservoir would produce the series of loops ('hysteresis').

Some sites have been considered unacceptable for concrete dams because of large hysteresis even though the modulus of elasticity of the rock itself was considered reasonable.

Table of Modulus of Elasticity Rock Type Modulus of Elasticity

- (MPa x 1000)

Limestone 3-27

Dolomite 7-15

Limestone (very hard) 70

Sandstone 10-20

Quartz-sandstone 60-120

Greywacke 10-14

Siltstone 3-14

Gneiss - fine 9-13

Gneiss - coarse 13-23

Schist - Micaceous 21

Schist - Biotite 40

Schist - Granitic 10

Schist - Quartz 14

Granite - very altered 2

Granite - slightly altered 10-20

Granite - good 20-50

Quartzite - Micaceous 28

Quartzite - sound 50-80

Dolerite 70-100

Basalt 50

Andesite 20-50

Amphibolite 90

The large ranges emphasize the need for testing at each site.

Deformability

The modulus of elasticity of rock is normally adequate, but due to the existence of joints, faults amd seams in the rock mass - sometimes open and sometimes filled with products of decompostion, the modulus of deformation may be inadequate.

The capacity of a rock to strain under applied loads or in response to unloading on excavation is known as deformability. The strains present in rock concern engineers even when there is little risk of rock failure, because large rock displacements can raise stresses within structures.

For example a dam founded on varying rock types of different deformability properties will develop shear and diagonal tension stresses due to the unequal deflections of the foundations. The deflections can be handled by structuring the dam correctly, if the rock properties are known and the variation of properties within the foundations are determined.

Tectonic Stresses

The fact that rock may be in a state of high internal stress is often overlooked. It is common to assume a vertical stress field due to the weight of overlying rock. The corresponding horizontal stress will vary with the rock and the rock formation. Frequently one horizontal principal stress will equal or exceed the vertical stress, the other being much lower - indicating the existence of large shearing stresses.

Crustal horizontal stress increases with depth. As excavation proceeds and loading on the strata is reduced, there will be upward changes in level. As a result of the reduction in vertical restraint the strata can no longer transfer the horizontal forces, but buckle upwards with horizontal cracking. This deformation

reduces the horizontal load on the layer so that the underlying strata tend to carry the horizontal tectonic stress. As a result the strata down to considerable depths suffer disturbance to their equilibrium. f horizontal cracks are caused then erosion can occur and resistance to sliding will be decreased.

Testing for rock properties

Laboratory testing

Compressive strength

Unconfined compression Triaxial compression Splitting tension (Brazilian) Four-point flexure Ring shear

Shear tests

Direct shear Triaxial shear

Field Testing

In-situ shear tests

Commonly carried out on 'undisturbed' specimens in the galleries of the dam. Disturbance of the specimen should be kept to a minimum as the specimen is exposed from the parent rock. The specimen is then protected and loaded in two directions. It is important that the axes of the jacks pass through the centre of the zone under test. A normal load is applied and held until any displacements have stabilised, the tangential load is then applied in steps and displacements measured. By repeating the test with different normal loads, values of cohesion and angle of friction can be derived.

Techniques for measurement

Hydraulic Fracturing Flat Jack Method Overcoring

Surface Features of Valleys

The shape of a valley and the rock with which it is formed affect the type of dam and its dimensions.

Type Chord-height ratio Gorges Under 3 Narrow Valleys 3-6 Wide Valleys Above 6 or 7 Flat country plains -

Dams in Gorges - Cupola or Dome

When the crest chord-height ratio is under 3 and the rock is capable of withstanding high pressures, not being able to fail by shearing, thin arch or thin cupola dams are the most successful and the most economical.

Soundness of the foundation is of paramount importance for all arch and cupola dams.

Dams in narrow valleys

Narrow valleys have a chord-height ratio of between 3 and 6. Gravity arch dams are normally constructed in narrow valleys providing that the foundations are suitable.

Example : Piave di Cadore dam (Italy) was constructed as a thick arch dam with a chord-height ratio of 5.5. It's thickness was less than a gravity dam but more than a thin arch dam.

If the narrow valley is filled with permeable and compressible material, for example from a glacial origin, the dam engineer has two choices:

To increase the depth of excavation to bedrock If the depth of material is economically unfeasible to remove, then

redesigning the dam to an earthfill or rockfill design may be the only option.

More and more thick arch dams with a thickness of less than the gravity section will be constructed in the future as more confidence is gained in:

� The reliability of new models confirm and even supplant the mathematical analyses. � The experience of strengthening weak foundations to carry heavier unit pressures which are to be sustained compared with the gravity section.

Dams in wide valleys

Wide valleys may be defined where the chord-height ratio of the dam is above 6 or 7. In a wide valley nearly every type of dam can be constructed, except a thick or thin arch dam. The most influential factors in a wide valley in determining the type of dam are:

The geology of the site. The proximity of materials from which the dam is to be made.

Many different types of dams have been built in wide valleys;

Gravity Dams : there are many examples of masonry and concrete gravity dams in wide valleys, especially where the bedrock is close to the surface. The earliest large example in Great Britain is the Vyrnwy dam (masonry), which supplies water to the city of Liverpool. The chord-height ratio of the dam is 7.

Earthfill Dams : because there are a great many wide valleys in England, there are a number of examples of earthfill dams. These dams are most suitable if the foundation is soft compressible sedimentary strata.

Rockfill Dams : the wide valley is suitable for all forms of rockfill dams.

Buttress Dams : with suitable foundations capable of withstanding direct pressures and resistance to sliding, the buttress dam can usefully be adopted in a wide valley.

Example : Scotland, the Errochty and Shira dams are situated in wide valleys and have chord-height ratios of 10 and 15 respectively.

Dams in Country Plains

Normally, dams are associated with valleys and are not built on level ground in the middle of sandy plains. However, examples of dams on plains are to be found on the Rhone diversion canals, the Rhone being diverted by means of gate-control barrages, into canals. These canals are some 30 miles in length and lead the water from the Rhone to normal gravity section dams, built several miles away on alluvial permeable strata.

Other types of dams constructed on flat country and which may certainly be considered dams, are the embankments of the large reservoirs of the Metropolitan Water Board, and the large reservoirs at Cheddar, Bristol. There are also many instances of what might be considered to be dams; the embankments of which are measurable in terms of kilometres in length and which retain water well above ground, such as the man-made levees on the Mississppi River.

Excavation and Filling of Valleys

Introduction

Valleys have been formed or have been modified by downward and lateral erosion of running water and/or ice, and commonly contain unconsolidated deposits transported by water, ice, or wind. The individual characteristics of a valley are a function of the topography, climate, rock type and geologic structure.

Artificial reservoirs are usually created by construction of a dam or dams in a large or small valley, commonly in a constriction. Correct interpretation of the various physical aspects of a valley reveals much concerning the characteristics of bedrock beneath a dam site and beneath the floor and sides of the reservoir basin above the dam site.

Erosion, transportation, and deposition by running water

Running water erodes the materials in the bottom and sides of the channel by corrosion, corrasion and cavitation.

Corrosion - is a chemical process whereby materials are taken into solution so as to become part of the dissolved load of a stream. Limestone is very susceptible to this process.

Corrasion - is a mechanical process that causes materials to wear away and includes abrasion by solid particles carried by the stream, and evorsion, which wears down compact materials by the impact of clear water carrying no suspended load.

Cavitation - requires high velocities in running water and results first from formation of vapour bubbles because of pressure decrease associated with velocity increase in accordance with the Bernoulli theorem, and then explosive collapse of the bubbles where the velocity diminishes.

Deposition of the solid load is a consequence of a decrease in the stream gradient, volume or velocity. Features of deposition in a stream are alluvial flood plains, deltaic deposits and alluvial fans.

When considering the construction of a dam and reservoir in a valley the concern generally is with only a relatively short segment of the total length of a stream, and particular attention is given to whether in the floor of the valley erosional features on the average dominate or are subsidiary to depositional features.

Glaciated Valleys

Streamcut valleys that have been modified by glaciers moving through them are of interest. The figure shows an idealised plan and sections of a stream and glacier eroded valley with two stages of glacier advance and retreat and prior and intervening periods of stream erosion. Morainal ridges formed by deposition of glacial till along the sides of the glacier are called lateral moraines. Stationary moraine is termed end or terminal moraine.

Gravity slips dislocations in steep-walled valleys

The alluvial, glacial, and landslide deposits on the floors and sides of valleys generally have locations, configurations, and physical properties that are identified in the field with relative ease. During planning, design, and construction of a dam and reservoir an assessment of these deposits can be made without difficulty, and appropriate measures can be taken for their removal or stabilisation.

In many steep-walled valleys, stream-cut or glaciated a relatively inconspicuous kind of slope failure is present, especially in highly competent, crystalline igneous and metamorphic bedrocks. Although they may not be easily observed, gravity-slip surfaces may be present in bedrock as indicated in the figure below and contribute to the instability of the foundation and abutments of a dam that might be constructed at the site.

Obstructions in Stream Valleys

The two major consequences of impounding water in a natural or artificial reservoir are -

Deposition of all or much of the suspended and traction load transported by the stream;

Increase in downward and lateral erosion by clear or desilted water downstream from the obstruction.

Attempts to control the rate of filling of reservoirs by sediment may include construction of dams and reservoirs to intercept sediment upstream from a major facility, such as a large dam for electric power generation, and regional programs for soil stabilisation and conservation in upstream drainage basins.

Topographical and Geological Conditions for Different Types of Dams

Gravity Dams

Hard rock at or near the surface. Depth of soft material above the rock should not exceed 7-10m thereby

avoiding excavation. Materials for concrete, i.e. aggregate, stone and sand should easily be

accessible within 5-10 miles. Gravity dams are suited when the length of the crest is five times or more

than the height of the dam.

Buttress Dams

The buttress dam is suitable where the rock is capable of bearing pressures of 2 - 3 MPa.

Buttress dams require between a half and two thirds of the concrete required for a gravity section, hence making it more economical for dams over 14m.

Additional skilled labour is required to create the formwork. Threat of deterioration of concrete from the impounded water is more

likely than from a thick gravity section. There is also an elimination of a good deal of uplift pressure, the pressure

resulting from the water in the reservoir and possibly of water from the

hillside rocks gaining access through or under any grout curtain and exerting upwards underneath the mass concrete dam.

An arch dam utilises the strength of an arch to resist loads placed upon it by 'arch action'. The foundations and abutments must be competent not only to support the dead weight of the dam on the foundation but also the forces that are directed into the abutments because of arch action in response to the forces acting on the dam. Therefore, the strength of the rock mass at the abutments and immediately downvalley of the dam must be unquestionable and its modulus of elasticity must be high enough to ensure deformation under thrust from the arch is not so great as to induce excessive stresses in the arch.

Multiple Arch Dams

The multiple arch concrete dam is a variety of buttress dam. The chief geological criterion is that the rock must be absolutely reliable to

bear 2-3 MPa or more without any appreciable settlement (<8mm) There is some saving in concrete compared with buttress dams. In respect of uplift, corrosion and economy the two types are very similar.

Thick Arch Dams

The thick arch dam can be built where the crest chord-height ratio is between 3 and 5.

The chief geological criterion is that the rock must be absolutely reliable to bear 3.5 MPa or more without any appreciable settlement.

A substantial saving in material compared with that of gravity dams. Thick arch dams are difficult to design on paper but are well determined

from trials on models.

Thin Arch Dams

Thin arch dams require valleys to have a crest chord-height ratio of under 3, with a radius of under 150m.

The pressure exerted on the valley sides is between 5.5 - 8 MPa Where there is a vertical radius of curvature as well as a horizontal, this is

known as a cupola or dome type. Used where cement is expensive and labour is cheap.

Rockfill Dams

Rockfill dams can be built where the following conditions exist -

Uncertain or variable foundation which is unreliable for sustaining the pressure necessary for any form of concrete dam.

Suitable rock in the vicinity which is hard and will stand up to variations of weather.

An adequate amount of clay in the region which may be inserted in the dam either as a vertical core or as a sloping core.

Accessibility of the site and the width of the valley is suitable for the manipulation of heavy earth-moving machinery, caterpillar scrapers, sheepfoot rollers and large bulldozers.

Hydraulic Fill Dams

Suitable in valleys of soft material and are constructed by pumping soft material duly consolidated up to moderated heights up to 30m.

Earthen Embankments

Near the site there must be clay to fill the trench and embanking material capable of standing safely, without slipping, to hold up a clay core.

An advantage of earthen embankments is that troubles due to the deterioration of the structure by peaty waters of low pH do not arise.

Composite Dams

Not only can different types of dam can be built in the same valley, but the same dam can be of different types owing to the varying geological and topographical features of the dam site.

Many buttress dams also join up with gravity mass concrete dams at their haunches at the sides of the valley, and again at the centre have a mass concrete gravity dam to form a suitable overflow or spillway.

Seismic Activity

An engineer is interested in two aspects of seismic activity:

1. Whether natural earthquakes are likely to occur in close proximity to the dam and would they be of an intensity to cause damage to the dam or appurtenant structures. Natural Events

2. Whether filling of the reservoir might induce earthquake activity, with the possibility of damage to the dam or liability for damage to other structures or persons. Although the magnitude of the shocks maybe low, the proximity of the epicenters could make the effects more serious. Triggered Events

Natural Events

Preliminary investigations should include researching the earthquake history of the region. This should involve investigating official records and local newspapers which often reveal shocks felt by people in centres remote from any seismographs. If no evidence of earthquake activity in the region is apparent, it would be unwise to assume nothing could happen in the future. Field surveys should include the recording of all faults in the region and the installation of seismographs in the region.

The scope of the seismic investigation is decided by the engineer. The engineer must consider the probable cost in comparison with the cost of conservative assumptions in design, the effect of such extra cost on the viability of the project, and the damage that might occur by neglect of such investigations.

Appraisal of the seismicity of the site should be undertaken at the earliest possible date. Seismographs should be installed to establish the magnitude of all natural events, their epicentres and depths of focus. Background noise, such as quarry blasting should be filtered out of the records. Records should be continued for at least 5 years after filling of the reservoir, and preferably to cover periods of large drawdown and refilling of the reservoir.

Seismographs - For large dams the installation of seismographs is not expensive. These seismographs will be triggered to record major events of a predetermined magnitude. It is usual to install such instruments on rock at the base of the dam, on the crest of the dam and preferably on rock at a short distance from the dam.

The magnitude of an earthquake is an indication of its absolute size, or total energy release. It is measured by the Richter Scale which is an arbitrary logarithmic scale. It defines the magnitude in terms of the maximum amplitude of a standard seismometer at a distance of 100 km from the epicentre.

Triggered Events

In this type of seismic activity there is a big seismic trigger system is blasting. Blasting in nearly situated quarry or any excavation.

Geological Hazards

Valley wall stability Valley bulging Mining

Valley Wall Stability

A gorge wherein the side slopes are equal to or steeper than the angle of repose of loose rock is attractive as a damsite, however, in such a gorge instability of the slopes can pose serious problems.

Landslips are a common feature of valleys in mountainous areas and large slips often cause narrowing of a valley which may then look topographically suitable for a dam. Unless they are shallow seated and can be removed or effectively drained, it is prudent to avoid land slip areas in dam location, because their unstable nature may result in movement during construction or subsequently on drawdown.

Valley Bulging

Valley bulges consist of folds formed by mass movement of argillaceous material in valley bottoms, the argillaceous material being overlain by thick, competent strata. These features cause stress-relief, that is, as stream erosion occurs within the valley the excess loading on the sides causes the argillaceous material to squeeze out towards the area of minimum loading. This causes the rocks in the valley to bulge upwards.

The valley movement of argillaceous material results in cambering of the overlying competent strata, blocks of which become detached, and move down the hillside. Fracturing of cambered strata produces deep debris-filled cracks which run parallel to the trend of the valley.

Mining

The existence of a mine either under a reservoir or a dam will present many problems such as:

Possible subsidence of the foundation of the dam Loss of water from the reservoir Flooding of the mine Excessive hydrostatic pressure at faces in the mine

When the mine is under a reservoir there is the possibility that sufficient water could pass through the intervening rocks to flood the mine, or at least increase drainage problems. Even if the rock series were sufficiently impermeable to impede the flow of water, there is the possibility that excessive interstical pressure could build up - with the danger to mine faces. If the mine is above and adjacent to the reservoir, saturation of the hillside and change in the water table could lead to potential instability. Seismic effects from blasting within the mine might then be sufficient to trigger a landslide.

Any site investigation must include both existing mines and potential mines, for matters of liability.

Hydrology

1. INTRODUCTION 2. HYDROLOGICAL CYCLE 3. STORAGE CAPACITY 4. FREEBOARD 5. FLOODS 6. HYDRODYNAMIC FLOW NETS 7. ANISOTROPIC BEDROCKS 8. FILLED RESERVOIRS

Introduction

Hydrology is a science of prediction - the likelihood of recurrence of natural events. Mathematicians may try to predict events based on past history but Nature is unpredictable as to time and magnitude of occurence.

Based on past information the low flow characteristics of the river will control the storage required and hence the normal full supply level of the reservoir. High flow records and flood forecasting techniques provide the basis for design of the spillway, and hence the flood storage required above normal full supply level.

Meteorology - Weather forecasting is important to the dam engineer because future seasonal weather could influence the decision as to which type of dam is built. For example, too short a dry season may preclude the economical construction of an earthfill dam. A weather station should be established at a proposed dam site at the earliest possible date. Records of temperature, humidity, rainfall, wind and air pressures can materially assist the meteorologists in synthesising storm patterns and is one step in the process of maximum flood estimation.

Whatever dimensions the Engineer selects for the dam and spillway there will always be some risk, assessment of the acceptable risk is the art of dam engineering.

Hydrological Cycle

The cyclic movement of water from the sea to the atmosphere and thence by precipitation to the Earth, where it collects in streams and runs back to the sea, is referred to as the hydrological cycle. The cycle is not as simple as that, firstly, precipitation may fall at all stages, secondly, there is no uniformity in the time a cycle takes, thirdly, the intensity and frequency of the cycle depend on geography and climate.

Water in the sea evapourates under solar radiation, and clouds of water vapour

move over land areas.

Precipitation occurs as snow, hail, rain and condensate in the form of dew, over land and sea. Snow and ice on land are water in

temporary storage. Rain falling over land surfaces may be

intercepted by vegetation and evaporate back to the atmosphere. Some of it infiltrates into the soil and moves down or percolates into the saturated ground zone beneath the water table, or phreatic surface. The water in this zone flows slowly through aquifers to river channels or sometimes directly to the sea. The

water that infiltrates also feeds the surface plant life and some gets drawn up into this vegetation where transpiration takes place from leafy plant surfaces.

The water remaining on the surface partially evapourates back to vapour, but the bulk of it coalesces into streamlets and runs as surface runoff to the river channels. The river and lake surfaces also evapourate, so still more is removed here. Finally, the remaining water that has not infiltrated or evapourated arrives back at the sea via the river channels. The groundwater, moving much more slowly, either emerges into the stream channels or arrives at the coastline and seeps into the sea, and the whole cycle starts again.

Man can exercise some control only when the rain has fallen on the land and is making its way back to the sea.

Storage Capacity

The storage capacity required in a reservoir may be determined in a number of ways. In tropical regions it may be decided to store the whole runoff from precipitation in one season. Whether this would ensure continuity of flow would depend upon the season selected and the seasons occurring later. It may be decided to provide sufficient storage to ensure continuity based upon a repetition of past history.

In evaluating storage requirements a hydrologist would use various hydrological tools such as cumulative mass curves, runoff, estimation of flood design, flood routing and other factors.

The storage capacity of a reservoir is defined as the volume of water which can be stored. Initial estimates of storage capacity can be made from topographic maps or aerial photographs.

The reservoir volume can be estimated by planimetering areas upstream of the proposed dam site up to the proposed top water level. The mean of the two successive contour areas is multiplied by the contour interval to give the interval volume, the summation of the interval volumes provides the total volume of the reservoir site.

The figure shows a typical mass curve.

Freeboard

Freeboard - 'The vertical distance between the top of the dam and the full supply level on the reservoir.'

The top of the dam is the level of watertightness of the structure and may be the top of a parapet that is watertight throughout its length. Full supply level is the level adopted in design for the maximum operation of the reservoir.

To determine a value for freeboard the following must be considered;

Flood Surcharge Seiche effects Wind set-up of the water surface Wave action Run-up of waves on the dam. Inaccuracy of data; Large risks if breached; Type of dam

Floods

Estimation of design flood

There are two methods now commonly used;

The statistical analysis of past floods with extrapolation to estimate the magnitude and probability of occurence of future floods, and;

The estimation of probable maximum precipitation on to the particular catchment under the worst meteorological conditions likely to occur over the catchment, followed by an estimation of the run-off that would result from such a storm.

The determination of probable maximum precipitation for a particular drainage basin requires comprehensive study of major storms on record and is a job for experts. One is limited by the lack of data, records usually do not go back more than 50 years, which makes prediction of more than the 100 year flood impossible. As it is, 50 years of data will predict a 100 year flood to within 25%, and 115 years will predict it to 10%.

The Engineer is faced with conflicting requirements in terms of safety and economy, he is therefore obliged to use to the best advantage the data and procedures that are available;

Statistical analysis of past flow records at the site - and extrapolation; As above, but with extension of the flow records by correlation with flows

from adjacent catchments; Statistical analysis of rainfall records and extrapolation; As preceeding, but with extension of data by correlation with other

stations; Correlation studies including both rainfall and flow records; Estimation of 'maximum possible' rainfall by Meteorological Services and

application of such data to the estimation of 'probable maximum flood' from catchment;

Comparison with known events and other designs adopted for the region by the use of such means as the Creager coefficients.

When a flood enters a reservoir it will cause the water level to rise, with consequent discharge over the spillway. The reservoir level will continue to rise until the free discharge over the spillway equals the inflow at time 'X' on the figure. Spillway discharge will then exceed inflow until the reservoir level falls to spillway crest level.

If spillway gates are installed they can be opened in advance of peak of the flood. The rate at which they can be opened will usually be governed by permissible river rise conditions downstream. There is the danger that the flood inflow will not

reach the volume anticipated and the water will be wasted or a flood of unjustifable size will be created downstream of the dam.

Hydrodynamic Flow Nets

Flow of water through permeable materials is directional and is in response to head differentials. Flow can be graphically portrayed by hydrodynamic flow nets, which are drawn in vertical section parallel to the general direction of flow. A flow net consists of two sets of lines, flowlines and equipotential lines. Flowlines or streamlines are the loci of the paths of flow of individual water particles. Equipotential lines pass through points of equal pressure. All intersections between the streamlines and equipotential lines are at right angles.

Symmetrical hydrodynamic flow net beneath a dam with its base at ground level

When the base of the dam figure 2, is set below ground elevation and a cut-off is constructed there is a change in flow net compared to figure 1, that results in the following advantages:

1. The uplift pressure at the heel of the dam is reduced and the total uplift pressure downstream from the cut-off has been diminished. Accordingly, the moment of uplift forces tending to lift the dam has been reduced.

2. The danger of piping and erosion at the toe of the dam has been reduced or eliminated.

3. The longer flow paths along the streamlines below the cut-off causes a notable decrease in the exit velocities downstream in the proximity of the dam and reduces total seepage under the dam.

Hydrodynamic flow net beneath a dam with its base below ground level and with

an impermeable cut-off near the heel of the dam Hydrology of Anisotropic Bedrocks Almost an infinite number of possibilities exists with respect to the magnitude and space distribution of zones of potential seepage in bedrock in the vicinities of dams and in the reservoirs behind the dams. A few of the many possible configurations of zones of potential are shown below;

Idealised cross sections of dams showing various kinds of zones of potential seepage in bedrock.

Brittle, fractured sandstones in horizontal sedimentary sequence beneath dam present a potential seepage.

Dam is situated on basltic lava flows and interlayered pyroclastic deposits. Lava flows are fractured, brecciated at their tops, and contain lava caves.

A brittle layer of quartzite in tightly folded metamorphic rocks is likely to contain numerous intersecting fractures.

Sandstone layers alternating with shale layers in a syncline contain fractures associated with development of a syncline.

A fault provides access of water to brittle sandstone layers which dip upstream.

A fault provides egress for water moving through inclined, brittle sandstone layers.

Brittle sandstone layers have been extensively fractured during development of an anticline.

Fractured sandstones in folded rocks are intersected by a fault zone which expedites groundwater movement to the surface beneath the dam.

Faults in brittle crystalline rock provide channelways for groundwater circulation.

Fractured sandstones, a weathered zone on granite beneath the sediments, and a fault zone create channelways for subsurface circulation of water.

Extensively jointed crystalline rocks are permeable to groundwater flow.

A closely jointed igneous dike intersects a sedimentary sequence and provides a channel for groundwater movement.

Idealised cross section of valleys at dam and/or reservoir sites.

Jointed sandstones present potential for seepage around dam abutments when reservoir is full.

Basaltic lava flows and layers of pyroclastic rocks create potential for seepage. Lava flows are jointed, have breccia tops, and contain lava caves.

Fractured sandstones in a strike valley are prone to seepage.

A subsided block (arrow) has created an open channelway in a massive horizontal sandstone layer.

Gravity-slip faults and fractures formed by elastic rebound produce potential zones for groundwater movement in a glaciated valley.

A strong fault system renders crystalline rocks permeable on one side of a valley.

A wide fault zone promotes deep circulation of water beneath dam.

Fractured sandstones and a weathered zone beneath an angular unconformity enable easy circulation of groundwater.

Fractured sandstones in an anticline create a permeable zone parallel to the valley.

Folded, jointed rocks and a strong fault create a potential for groundwater flow.

A sheeted, jointed zone in crystalline rocks create a permeable zone.

Joints in a brittle quartzite layer and a fault produce channelways for underground water circulation.

Hydrology of Filled Reservoirs Failure of slopes on sides of reservoirs frequently during drawndown when the reservoir is nearly empty doesn't produce unmanageable problems. However, of more concern is when rock and earth slides into a full reservoir causing sudden destructive overflow of the dam.

Filling of a reservoir causes adjustments in the groundwater table in adjacent materials. Over a period of time when the reservoir is full a new groundwater table is established with coincides with the elevation of the water surface. The groundwater surface is a free surface in contact through unfilled pore spaces with the atmosphere, changes in atmospheric pressure are accompanied by changes in pore pressure in the saturated zone. Wave action also undercuts the slopes and oversteepens them.

Figure 1 - Glacial till deposits in a glaciated canyon in crystalline rocks

A reservoir is located in an extensively glaciated valley in crystalline rocks. Lateral morraines, consisting of a jumbled mixture of large and small boulders, gravel, and rock flour have been deposited by the glacier that occupied the valley high on its sides. Filling of the reservoir causes an elevation of the water table in the rill in the moraines, and because of the cross-sectional configuration of the valley, considerably increases the possibility for sudden downslope movement of the moraine material. Failure of the slopes might occur at any time. The bedrock profile of a canyon eroded by a stream in a tilted succession of sandstones and clay-rich shales. Unconsolidated materials in rock slides and stream deposits are not shown. Water from the reservoir, by seepage through the sandstones, comes into contact with the shale layers for a considerable distance into the canyon walls, and by slow penetration of the shales, greatly reduces their strength. Under these circumstances, a highly unstable condition is created, especially where the sedimentary layers dip into the reservoir.

LOADING AND FACTOR OF SAFETY

1. INTRODUCTION 2. STATIC LOADING 3. DYNAMIC LOADING 4. FACTOR OF SAFETY - Gravity Dams 5. FACTOR OF SAFETY - Concrete Arch Dams 6. FACTOR OF SAFETY - Embankment Dams 7. FACTOR OF SAFETY - Abutments and Foundations

Introduction

A dam is a three dimensional structure and despite assumptions, it is not homogeneous and its integrity is in the hands of the constructors.

The foundations are neither isotropic or truly elastic. Concrete and rock are brittle, although elastic theories are applied in

stress calculations. The dam and foundation will become saturated with varying effects on the

materials. The dam will be subjected to water load, daily and seasonal temperature

cycles. It will be subjected to random events such as : Floods, Waves, Seiche

Effects, Earthquakes, Ice Formation and other natural phenomena.

The factor of safety must relate to the strength, stability and durability with consideration to magnitude of economic and personal loss that would result from its failure.

The aim of the Engineer must be to reduce the number of uncertainties, both as regards loading on the dam and in the means by which the dam and the foundations withstand such loads. The Engineer must also be satisfied that there is no feasible mechanism that could result in failure.

Static Loading

Horizontal Loads

Headwater (H1) - For the basic calculation of stability the level in the reservoir will be assumed at or above the level required for the passage of the design flood. In many instances the dam is designed for the highest level of watertightness, e.g. a concrete parapet.

Silt (H2) - A changed land usage as a result of a dam may well result in increased erosion, causing a deposition of silt. Unless very deep deposits of silt are likely it is adequate to assume a triangular load allotting an appropiate relative

density to the fluid. This would have a maximum value of 1.4

Reservoir Behaviour (H3) - Wind and other natural causes will induce movement in the reservoir as waves, reservoir set-up or seiche effect.

Ice Loading (H4) - It is assumed that ice will not form and exert pressure on the dam at times of maximum flood. The slope of the upstream face of the dam as well as the slope and roughness of the valley walls will influence the magnitude of ice loading. Even wind blowing down the reservoir at 50km/hr may increase the ice loading by 4-5 t/m of exposed face.

Tailwater (H5) - In some cases water is ponded downstream from the dam. Assistance from this may be assumed but it must not be overlooked that, in the case of an overflow dam, flood waters passing over the dam might well evacuate such water from the face of the dam.

Seismic Force (H6) - Force acting on dam in horizontal plane.

Seiche effect (H7) - Is an undulation in the reservoir water due to natural causes, intermittent wind, variation in atmospheric pressure, earthquake and motion of the Earth. Usually less than 0.5m, though levels of 2m have been reported in Lake Geneva

Vertical Loads

Weight of Dam (V1)- The unit weight of material in the dam should be determined as accurately as possible. An underestimation by 1% will represented a considerable additional cost on the dam.

Vertical Water Loading (V2) - Imposed on any sloping surface of the dam, usually the upstream

face, but also on the downstream for overflow dams.

Uplift (V3)- Hydrostatic forces acting within a dam and its foundations including interstitial or pore pressures. Some Engineers rely on drainage to prevent occurence of uplift, assuming the drainage will be effective for the entire life on the dam, therefore some inclusion for uplift must be included in the design. See diagram for distribution of pressure. [k values vary between 0.25 to 0.50 depending on conditions.]

Seismic Force (V4) - Force acting on dam in vertical plane.

Other loads on the dam

Water Density - Some rivers carry very heavy silt load in seasons which changes the density of the reservoir.

Reservoir set-up - The result of continuing wind causing one end of the reservoir to be at a higher level. Calculations for a large reservoir in which the fetch is 38km would indicate the following values:

Return Period Wind Speed Set-up (years) (km/h) (m) 1000 160 0.75 100 125 0.45 10 95 0.26 5 88 0.22 2 77 0.17

Thermal Effects - Concrete dams will be subject to loading from temperature variation within the dam caused by hydration of the cement and due to seasonal variations. Water as depth doesn't vary, but towards the surface it varies with season. A skew loading is used to describe solar and air temperature effects.

Construction Loads - Concrete dams of cupola and buttress shape offer good resistance to water loading when complete but during construction it is necessary to control the rate of construction and to include reinforcement in overhanging sections.

Direction of Forces - At certain locations it may be appropiate to increase the radius of an arch dam and accept higher stresses within the dam in order to ensure better angle of incidence of the resultant thrust with the abutment. The direction of resultant forces is important for gravity and buttress dams - especially on stratified rock.

Hydrostatic Loading within the Foundation or Abutment - Faults, cracks and joints are present in most damsites. Forces due to a large dam may cause cracks to appear in the rock upstream from the dam, this may cause jacking loads that could cause failure. To avoid this, careful surveys should be made of the orientation and inclination of faults, joints and cracks.

Tectonic Forces - Besides seismic effects, there may be significant tectonic forces on the Earth's crust at the site and these may be upset by deep excavation or saturation due to the filling of the reservoir.

Dynamic Loading

The Earth's crust is in a state of stress. When the stress is great enough, and the crust is weak enough, adjustments may occur. These adjustments will release energy in the form of shock waves, propagated from an epicentre. These waves will vary in wavelength and frequency. Short-period waves have predominant frequencies within the range of natural frequencies for dams, they are apt to produce conditions of resonance in the dam. Engineers are therefore more concerned with the possibility of moderate earthquakes occurring within 80 to 120 km of the dam than larger earthquakes occurring outside this limit.

For gravity dams a horizontal coefficient is adopted and applied as an additional static load. For arch dams the dynamic effects receive greater attention with both model tests and in situ testing by vibrating the dam. For embankment dams additional horizontal static loads are considered and a dynamic analysis has been developed with close attention being paid to the characteristics of fill material.

In October 1969 the Committee on Earthquakes of the International Commission on Large Dams (ICOLD) summarized 1969 practice :

Design For gravity dams a horizontal coefficient was generally adopted and

applied as an additional static load. Vertical effects were taken into account in very few circumstances and dynamic analysis was used by very few.

For arch dams the dynamic effects received greater attention in model tests and in situ testing by vibration of the dam.

For embankment dams additional horizontal static loads were considered; dynamic analysis was being developed and closer attention was being given to the characteristics of fill material.

Zoning - Many countries were adopting the principle of seismic zoning. Seismic Coefficients - A coefficient of from 0.1 to 0.2 was commonly used. Seismic Waves - Analyses had been made by applying sinusoidal or

modified earthquake records - but actual earthquake records had only been applied in rare cases for dynamic analyses.

Properties of Materials - Different mechanical properties of various materials when subjected to static and dynamic loading.

Loads Considered - For dynamic water pressure the formulae of Westergaard, Zanger and the USBoR were in use.

Allowable Stresses - In many cases the permissible compressive stress under dynamic loading was increased by up to 30% above the permissible static stress. Factors of Safety for arch dams were usually 4, based on compressive stress and 1.2 minimum for fill dams.

Deformations - An embankment dam which employed dynamic analysis was assumed to suffer 5% axial strain.

Models - These were popular for arch dams and were used for some gravity dams. There was a tendency to employ model tests for fill dams.

In Situ Tests - Some arch dams were shaken by vibrating machines to study natural frequeny and modes of vibration.

Seismographs - In a majority of large dams seismographs were used.

Factor of Safety - Gravity Dams

A gravity dam must be designed to safeguard against overturning and sliding. For the former it is usual to design the dam so that the resultant of all forces intersects the base within its middle third. This will provide a factor of

safety in excess of 2.

The ratio of the sum of the horizontal forces to the sum of the vertical forces is referred to as the sliding factor (Fss). This is usually about 0.75 but must not exceed 0.90 under extreme loading. These figures represent the range of the coefficient of static friction normally encountered at the site of a gravity dam.

At or in the foundations, the horizontal loading will be resisted by cohesion and friction. The ratio of the total resistance by cohesion and friction to the horizontal load is termed the shear friction factor (Fsf). Most countries accept 4 as a minimum value. In practice the foundation is usually prepared in steps or is sloped upward in a downstream direction to provide resistance to failure far in excess of the above figure.

Range of shearing resistance parameters.

Location of plane of shearing/sliding Cohesion (c) Friction tan ø mass concrete intact 1.5-3.5 1.0-1.5 mass concrete horizontal construction joint 0.8-2.5 1.0-1.5 conrete/rock interface 1.0-3.0 0.8-1.8 rock mass sound 1.0-3.0 1.0-1.8 rock mass inferior <1.0 <1.0

Recommended shear friction factors, Fsf (USBR 1987)

Load Combination Location of sliding plane Normal Unusual Extreme dam concrete, base interface 3.0 2.0 <1.0 foundation rock 4.0 2.7 1.3

Factor of Safety - Concrete Arch Dams The factor of safety for an arch dam is the ratio of the compressive strength of concrete to the maximum calculated compressive stress in the dam. The compressive strength is usually referred to as the strength of concrete at the age of 91 days when tested in 150 mm x 300 mm cylinders.

The design criterion adopted by the U.S. Bureau of Reclamation is for a factor of safety of 4 based on the strength of concrete at 1 year. Except for extreme loading combinations the maximum compressive stress is usually limited to 6.9 MPa.

Factor of Safety - Embankment Dams

The minimum factors of safety for embankment dams would be:

Upstream Slope

Immediately after completion with full construction pore pressure 1.3-1.5

Following rapid drawndown (slip circles between high and low water levels)

1.2-1.3

Downstream Slope

Earthquake and Reservoir Full 1.2

Reservoir full - steady seepage 1.5

In an area subject to earthquakes the following factors are indicative of acceptable values:

Seismic coefficient 0.1 FoS 1.8 Seismic coefficient 0.3 FoS 1.15

Factor of Safety - Abutments and Foundations

The dam foundations and abutments should be thoroughly investigated for any possible mechanism of failure. This would involve identification of joints, faults and any other forms of weakness.

A reasonable factor of safety is

- the ratio of shearing resistance to the maximum shearing stress predicted; the lowest value of the ratio in the foundation being the factor of safety of the foundation

SITE INVESTIGATION

1. INTRODUCTION 2. TIME AND MONEY FOR INVESTIGATIONS 3. DESK STUDY 4. PRELIMINARY INVESTIGATION 5. GEOPHYSICAL INVESTIGATION 6. EXPLORATORY INVESTIGATION METHODS 7. EVALUATION OF SELECTED SITES 8. DETAILED INVESTIGATION 9. MONITORING

Introduction

Most failures are due to lack of appreciation of how the particular damsite would react to the superposition of the dam and reservoir. It is therefore essential that a detailed site investigation takes place and the results are appropriately used by Engineers.

In the planning stage possible damsites will have been chosen from contour maps and aerial photography, selected primarily on topography. A narrow gorge

is best, hoping for minimum quantities in the dam and a valley opening upstream to provide the required storage. There maybe alternative sites along the length of a river and hence further investigation has to be done to assertain the best possible position.

Time and Money for Investigation

The amount of money required to investigate a damsite will depend upon the site and the type of dam. An experienced department of engineers, hydrologists, geologists and surveyors may produce sufficient information for an outlay of 2-3% of the dam cost. This figure could reach 6% in remote locations where basic information is not available.

It is not unusual to spend 3 years on site investigations, this will depend on the location and size of the dam, but time must not be underestimated. To meet stringent requirements for environmental studies and public opinion polls could add 2 years to the time and several percent to the cost.

If, as a result of the site investigation another site is to be chosen, the same time and money must be spent investigating the new site. Adequate time and money must always be available to all disciplines to give them the opportunity to investigate and report.

Desk Study

Initial desk study can be done by researching from these sources of information -

1. Ordnance Survey Maps or equivalent 2. Admiralty Charts 3. Geological Maps and Memoirs 4. Old OS Maps or equivalent

Past users of site Concealed mines and adits Infilled pits Original topography and drainage conditions Changes in stream and river courses Changes in landslide areas, fence lines, path lines

5. Aerial Photography

Landsat Images Colour and infra red photography

6. Previous Site Investigation Reports 7. Local people and authorities

Preliminary Investigation

Aerial Reconnaissance - An initial comissioned flyover is essential, providing the Engineer with an idea of the topography and enabling him to form an opinion of the probable hydrological characteristics of catchment.

A Geologist will assist the Engineer in the selection of the damsite, and a construction Engineer will study the access and possible sources of materials.

Ground Reconnaissance - Features that should be sought during early reconnaissance include old and potential land slides, geological faults and major joints parallel with the valley.

The joints may be open or infilled with products of decomposition, they present construction hazards and possible leakage paths around the dam.

Examination along the beds of the river and tributary streams will indicate the strike and dip of rock formations.

Any springs or underground water should be identified since they provide leakage paths from the reservoir.

The depth of alluvium or soil should be determined to indicate the excavation required and the probable quantity of material required for the dam.

At this stage, the preliminary geological data should be assessed and enhanced by mapping and modelling. This can help to highlight important considerations about which type of dam may be most appropriate, and any problems which may be encountered, before extensive drilling or exploratory works are performed. Assessment of preliminary data will assist in the choice of exploratory methods and in the design of the exploratory programme as a whole.

Geophysical Investigation

Geophysical methods provide an indirect evaluation of certain underground conditions. Several procedures have been developed, all of which measure some force pattern in the earth.

Advantages of Geophysical Methods - They permit a rapid coverage of large areas at a relatively low cost which is useful in selecting possible dam sites during reconnaissance phases. They are also not hampered by boulders or coarse gravel which interfere greatly with the direct methods such as boring and sampling.

Limitations of Geophysical Methods - There is difficulty in correct interpretation when the strata are not well defined and horizontal. For this reason it is imperative that all geophysical work is confirmed by borings and other direct observations.

Exploratory Investigation

Purpose - to secure accurate information about the soil and rock stratification, the composition of the materials and the location of ground water.

Boring and Sampling

Auger Boring - Generally limited to firm soils, above the watertable. Gravel larger than about a third of the diameter of the hole cannot be drilled but very hard soil and soft rock can often be penetrated if sufficient power is available.

Test Boring - Core Drilling - Diamond Drilling - Short Drilling or Calyx Drilling -

Programme of Exploration Work for Foundations

Boring layout Procedure Laboratory testing Correlation of results

Planning and Conducting Borrow Pit Explorations

Field Work Testing and Correlation

Evaluation of Selected Site At this stage, potential hazards and problems should have been identified. However, it is still necessary to remain alert for indications of hazardous or problematic features which were not identified during the earlier stages of the investigation. The main effort is directed in producing parameters for the final design. This would involve high quality boring and drilling, with particular attention being paid to sample quality and high core recovery, careful logging of trenches, shafts and adits, in situ testing such as plate loading tests and in situ shear tests in adits, trial embankments, grouting trials and so on.

Foundation testing

Undisturbed Sampling Pit Sampling Thin-walled Samplers Foil Samplers Rotary Samplers

Laboratory Testing Correlation of Test Results

Field testing

Test Pit Plate Load Test Seepage Test

Borrow pit investigations

Sampling Laboratory Testing Test Strip

Detailed Investigation The evaluation of preliminary desk and field work should assess the potential for major hazards and qualitatively assess the likelihood of encountering any more hazards. This should allow a ranking of the potential sites in order of their probable suitability.

Following the desk study and preliminary field work, it may be necessary to establish a pattern and base level of seismicity for later evaluation of induced seismicity. If potential active faults are identified, seismic arrays should be installed to monitor these. This will help assess the need for criteria changes should seismic activity occur after the feasibility stage has been completed and the design is well advanced.

The next stage is to produce a detailed investigation of the chosen site.

Monitoring

Monitoring during construction will include the work of an engineering geologist on site, who will examine all excavations to see whether the expectations of the preceeding investigations have been realised. The identification of exceptions may then lead to an early diagnosis and redemption of any problems.

For the post-commissioning stage, monitoring will involve regular reading of installed instrumentation to check performance against design criteria. This should serve as an 'early warning' system which will initiate a contingency programme, thus minimising the delays which would result from the development of an adverse situation.

CONSTRUCTION

1. GENERAL 2. RIVER DIVERSION 3. CONCRETE DAMS 4. EMBANKMENT DAMS

General

1. Safety 2. Specifications 3. Plant 4. Cost and its control

Safety - with the ever increasing height of dams there is greater need for precautions, especially against falling objects or persons. Double curvature structures have made access and movement of personnel difficult. All site personnel must be alert at all time for the accidents that might happen. The insurance is regular meetings of staff and representatives of the work force, where knowledge and experience can be pooled - especially in the planning stage of an unusual operation.

Specifications For all types of dam, the specification should cover the following:

The required date for completion, with a schedule to indicate dates for completion of stages of the work;

The degree of responsibility to be accepted by the Contractor in the dimensioning of diversion works, for losses due to floods, for river pollution and general care of the river;

Clearing of the site and works areas; The extent of foundation preparation required and the sharing of

responsibility for unforeseen conditions; Protection of the environment, disposal of soil, rehabilitation of borrow

areas, beautification, etc.; Premliminary work that will be done by the Owner and the degree of

responsibility accepted by the Owner for consequences of such work.

Plant - The cost of purchasing plant and its operation are major items. On a dam involving 2 million cum of concrete, the purchase and operation might each represent 18-20% of the direct cost of the dam. For an embankment dam this may be of the order of 25-35%. It is therefore important to select the correct plant to achieve optimum cost.

The specification for some major contracts calls for the use of only new plant and the main advantage is that suitable plant can be matched to the particular job. Material transporters can be matched to quarry equipment for example. For compaction of embankments - soil or rock, it is important to select the most appropiate equipment and this can be best determined by means of a trial embankment. It is also necessary to have a supply of spare parts since many sites are remote. Plant should be simple and rugged, and preferably modular to simplify the replacement of parts.

Consistently high quality of materials is the objective of all dam builders and to sacrifice quality for a doubtful saving in cost is poor engineering.

Cost and its Control - Safety, Time and Cost are interrelated and usually conflicting. Safety is always paramount, and hence time and cost directly relate to quality and degree of perfection required. There is an optimum time for any operation and beyond this time will incur extra costs. Cost is made up of direct charges for manpower and materials, plus overheads and interest. Interest is out of the control of the Engineer.

With regard to the direct charges, the selection of the type of dam will be the major decision and this might well be influenced by local conditions rather than mathematical economies.

Is a labour-intensive job required in the interests of the local community? Is skilled labour available? What degree of mechanisation is desirable or possible at the particular

site?

For a concrete dam, for example, the dissection of costs may be; Materials 25%Formwork 20%Plant Purchase 19%Plant Operation 19%Placing and consolidation of concrete 4% Precooling concrete 3% Concrete Treatment 3%

For an embankment dam, for example. the dissection of costs may be;

Quarry Operation 30-40%Haulage 20-30%Spreading and Compaction 25-30%Face rolling, mesh, etc. 15-20%

River Diversion Regardless of the type of dam, it is necessary to de-water the site for final geological inspection, for foundation improvement and prepartation, and for the first stage of dam construction. The magnitude, method and cost of river diversion works will depend upon the cross-section of the valley, the bed material in the river, the type of dam, the expected hydrological conditions during the time required for this phase of the work, and finally upon the consequences of failure of any part of the temporary works.

At most sites it will be necessary to move the river whilst part of the dam is constructed; this part will incorporate either permanent or temporary openings through which the river will be diverted in the second stage. If the first diversion is not large enough the initial stages of construction will be inundated, if the second stage outlets are too small, the whole works will be flooded.

At some sites there is a distinct seasonal pattern of river flows and advantage can be taken of such conditions but noting that Nature is random.

Construction of the Hendrik Verwoerd Dam, South Africa required a sophisticated arrangement of cofferdams. An approach was developed based on the frequency and distribution of floods that could occur over a five year period of construction. The following is an extract of the original detailed specification:

First Stage (A) -

Construction from each bank of the river of groynes a short distance upstream of the dam, to alter the direction of flow and thus to move the low water channel towards the left bank of the river at the dam site.

Construction of a semi circular concrete arch cofferdam on the right bank of the river.

De-watering of this cofferdam and excavation within it for the main dam blocks, the proportion of the overspill apron and the sections of the mid channel cofferdams.

Concreting of the dam blocks, numbers 14 to 28 up to a minimum level of 1200 meters, the portions of the overspill apron and of the mid channel cofferdams within this cofferdam. In blocks of the dam constructed on this stage, temporary openings were formed through which the river was later diverted.

Second stage (B) & (C) -

Construction of a semi circular arch cofferdam on the left bank of the river. Construction of the flanking portions of each of the upstream and

downstream mid channel concrete arch cofferdams which cross the river upstream and downstream of the central section of the dam.

Excavation of a channel along the right bank, leading to the temporary openings through the dam, demolition of portions of the right bank cofferdam to permit the diversion of the river through the temporary

openings and such clearing out of the right bank diversion channel as may be necessary.

The cutting of a channel through the portion of the right bank groyne adjacent to the bank to form an entrance to the diversion channel described above.

The placing of rockfill to connect together the right and left bank groynes so as to divert the river flow into the right bank diversion channel, thereby cutting down the velocity of the water in the vicinity of the mid river cofferdams.

Completion of the upstream mid river cofferdam completion of the downstream mid river cofferdam.

Placing of spoil, excavated from the works, in the flood channel on the left bank upstream of the dam to prevent the river flooding into the area to the protected by the mid river cofferdams.

Third stage (C) & (D) & (E) -

De-watering of the left bank cofferdam and excavation for the dam blocks and the portion of the overspill apron within this cofferdam.

Concreting of the dam blocks numbers 9 to 27 to a minimum level of 1206 meters.

Demolition of the left bank cofferdam. Demolition of the remaining portion of the right bank cofferdam within the

areas protected by the mid channel cofferdams. The de-watering of the mid channel cofferdam and excavation within it for

dam block numbers 1 to 7 and 2 to 12 and portion of the overspill apron. Concreting of dam blocks 1 to 7 and 2 to 12 to such levels that the

contraction joints in the lower part of the dam up to gallery can be grouted. Concreting within the mid channel cofferdams of the portion of the

overspill apron downstream of blocks 1 to 7 and 2 to 12. Cooling of the concrete and grouting of the dam construction joints.

Diversion can also be achieved by means of a tunnel, which depends on the nature of the rock and depth of weathering and should be far away from the dam itself to not interferre with the foundations. The tunnel also should be large enough to avoid the possibility of job jams.

Concrete Dam Construction

1. Aggregate Production 2. Concrete Handling, Placing and Consolidation 3. Formwork 4. Built in items 5. Cooling of Concrete 6. Economical Construction

Aggregate Production - The acceptability of natural aggregates is judged upon the physical and chemical properties of the material and the accessibility, proximity to the site and economic workability of the deposit.

Concrete Handling, Placing and Consolidation - The procedure to be adopted for moving concrete from the mixers on to the dam will be governed by site conditions. The problem is to transport it to the dam with the least possible segregation or change in its consistency so it may be compacted uniformly into the dam without unreasonable effort. The cableway is probably the simplest arrangement. The tilting mixers will feed the buckets; these are then moved to a pick up point under the cableway, transported smoothly to the block and emptied quickly through an air operated gate.

Three Tower Cableway

The use of a belt conveyor has also been considered, but problems occur in keeping the belt temperature stable in warm weather and also in windy conditions. The conveyors are usually covered and cold air is blown over the concrete to lower its placing temperature.

The placing of a low-slump concrete, four layers in 2.3m lift

Tractor mounted vibrators at Emosson Dam, Switzerland

Proper consolidation of low-slump concrete is laborious and requires continuous supervision. The most efficient compactor is usually the two man hand-held high-speed vibrator.

Formwork - Probably the most widely used lift is 1.5m, however, on large dams a height of 2.3-3.0m is frequently used. With the larger lifts there are fewer movements of forms and fewer horizontal lift surfaces to be cleaned. The high-lift

formwork is unique and expensive with less prospect for re-use, heavier equipment is required for lifting the forms and the heat problems and risks of cracking in the concrete are accentuated. Modern steel formwork is of cantilever design, see figure. Where possible the use of slip forms will expedite the work and lower the costs. At some locations it may be expedient to use precast concrete slabs for formwork with set-retarding agent on the inner surface.

Built in items - The installation of built in items is always a major source of delay on construction. Advance planning is required with close attention to detail. The complication of installation of reinforcement, prestressing, gate hinges, drainage wells and gate wells are common on spillways. There has been a tendency to use precast concrete units for galleries to save time, however this prevents the inspection of the concrete in the interior of the dam. The simplest method of forming galleries is vertical formwork extending the full height of a lift. When this is removed, precast concrete beams or slabs can be laid over the opening and concreted into the next lift. Reinforcement is usually required above and below rectangular galleries and this is best installed as prefabricated units.

Cooling of Concrete - The method of cooling concrete during the first few days after placing can be of the utmost importance if cracking is to avoided. It is essential to give attention to both internal and external factors that may induce cracking;

Temperature rise, which will depend upon the heat of hydration of the cement, the quantity of cement per cubic metre, the concrete placing temperature and the rate of construction;

Heat dissipation, which will depend upon the conditions of exposure - including the temperature of the underlying concrete and the thermal diffusivity of the concrete. If it is considered necessary to heat the underlying concrete the rate of rise of its temperature should not exceed 2° Celsius per day;

The effects of restraint from a cold surface, i.e. rock or concrete say 14 days old, it will depend upon the temperature gradient which can be reduced by placing concrete in half lifts for a predetermined height, say 3m above the cold surface;

The arrangement of cooling pipes - at 0.25 and 0.75 of the height of the lift may be more efficient than on the top of the old lift and at mid-height of the new lift. The horizontal spacing will depend upon the rate of heat removal required and the temperature of the cooling water (i.e. river water of varying temperature or refrigerated water);

The local weather conditions - humidity, temperature and wind.

Economical Construction - Concrete dams are expensive, however mechanisation over the last 40 years has reduced by a factor of four the number of man hours required to place a cubic metre of concrete in a mass concrete dam. Although every Engineer strives for perfection, consideration must be given to the degree of perfection that is really necessary. Close co-operation between the Owner and Contractor will save time and money. Questions have to be asked at all stages such as;

Is it permissible to design for tensile stress in the concrete? Will arching of the dam result in overall economy? Are longitudinal contraction joints necessary in large gravity dams? Can the transverse contraction joints be omiited, located at wider spacing

etc? What clean up is necessary on horizontal construction joints? Should the height of lift be specified by the designer? Should the cement content of the concrete be specified or only the

properties required in the concrete? Can the dams of intricate shape be justified? Should ancillary works be separated from the dam to minimise

interference with a continuous or cyclic process of dam building? What is the optimum layout and design for galleries?

Looking at the 'Construction - General Section' to see the proportion of costs in a concrete dam, assumuing that the materials have been predetermined, the Contractor should concentrate on formwork, the purchase of plant and its operation.

Embankment Dam Construction

1. General 2. Phases of Construction 3. Quarry development and Haul Roads 4. Material Compaction 5. Earth Dams 6. Hydraulic Fill Dams 7. Rockfill Dams

General - Considerable economy can often accrue if there is 'Progressive Design' - the aim is to provide design flexibility to cover the probability that materials will not be in true accord with samples tested nor will foundations conform to assumptions.

Phases of Construction -

1. Evaluation of plans, specifications, basic requirement, and features of the site.

2. Planning and scheduling of the job 3. Making the site ready 4. Building up the structure 5. Clean Up

Quarry development and Haul Roads - The quarry site should be determined primarily on the basis of rock quality, i.e. the fragmented rock must be sound, hard and clean. If the location is not dictated by rock quality, then it is desirable to separate the quarry and the haul roads from other works such as the intake, spillway or power station construction. Attention should be given to environmental factors such as noise, vibration from explosives and dust. In designing the explosive charge, the form of muck pile must be considered, i.e. for rubber-tyred loaders a wide low pile thrown well out from the face is desirable for minimum loading costs.

Haul roads must be built to suit the required speed of construction, and this involves the size of the haulage units. The roads should be at least 13m wide for two way traffic, for preference, on way traffic should be organised on a large job. The gradients must be such as to give minimum cost for the vehicles involved. The general layout of haul roads and ramps can be greatly facilitated with scale models.

Material Compaction -

Fine - Trial embankments should be constructed to determine relationships between moisture content, layer thickness, type of roller, number of passes of the roller and the resulting density and permeability

of the fill. Additional water during compaction usually improves the impermeability of residual soils by a factor of at least ten compared with compaction on the dry side of optimum moisture content.

Plastic clays - a little wetter than optimum moisture content, can be compacted by rubber-tyred rollers, which are water ballasted. A disadvantage of the rubber-tyred roller is that layer and shear planes tend to form in some materials. Since the fine materials are usually sensitive to moisture the field Engineer must be extremely weather conscious. If heavy rain is expected the surface of the fine material should be rolled smooth with sufficient gradient to shed water from the working area.

Filter or Transition Material - The thickness of the filter and transition zones will depend upon the water pressure to be sustained and the materials economically available. For a large dam the fine filter is often of crushed rock which is expensive. Its width would normally be the narrowest than can be placed and compacted. Setting out of the zone boundaries is important, especially for curved dams with a thin fine filter. The number of passes should be determined such that future settlement of the core and the filter zone will correspond as closely as possible.

Rock Compaction - The steel-faced vibratory roller is normally used for the compaction of rock. On the sloping faces a roller of 1.5t is most useful. The thickness of the layer of rockfill and the maximum acceptable size for rocks should be regarded as factors pertinent to the design of the dam.

Water to aid compaction of rockfill - Wet rock will compact better under rolling than dry rock. Firstly, the friction is less between the rocks and secondly many rocks lose strength when wetted so that crushing occurs at points of contact during the third or fourth pass of the rollers.

Provision for Instrumentation - This will inconvenience construction and failure of the equipment to work will represent a financial loss for the cost of its purchase and installation, and little can be done about it after the dam is built. Vital information about the behaviour of the dam will be lost if care is not taken in its installation.

Earth Dams -

The most important variables affecting construction of earthfill embankments are the distribution of soils, method of placement, water content, and compaction.

Soils may be classified by engineering properties into various groups. These groups fall into two main divisions, the course grains and the fine grains. Course grains are those larger than a number 200 sieve size and include gravels and sands. Fine grains are smaller than a number 200 sieve size and include silts and clays. Course grain material is used for the outer zones of an earthfill embankment, and fine grain material is used for the impervious core or central portion of the dam. A sieve analysis test will determine the percent of material passing a given sieve size.

The soil material must be placed in horizontal layers not more than 15 cm. thick after being compacted. The soil should be homogeneous and free from lenses, pockets, organic material, or other imperfections. Prior to placement, the material should have the optimum moisture content required for the purpose of compaction. The optimum moisture content, or the water content that produces the maximum density, may be obtained by a laboratory Proctor test.

Good compaction of a cohesive soil reduces permeability and increases shear strength and the stability of the dam. Compaction equipment includes sheep-foot rollers, pneumatic rollers, and hand tampers. The dry density of the soil should not be less than 95 percent of standard Proctor test.

Hydraulic Fill Dams -

Excavation - dredging, with hydraulic giants or dry with the aid of a hog box. The choice of the methods depends on the cohesion of the soil and on the topography of the site.

Transport - The materials are transported in suspension in pipelines. Typical mixes are from 10-20% solids by volume or 25-50% solids by weight.

Fill Construction - To start the fill two parallel dikes are constructed at or just inside the embankment toes as shown in the figure. Often these are the permanent rock toes themselves but they can also be made of rolled pervious earth. The pipelines (called beach pipes) are laid on top of these dikes or are carried on low trestles just above them. Outlets are provided to allow full discharge of the pipe. In filling, several adjacent outlets are allowed to discharge into the area between the dikes. The coarse materials settle close to the discharge points while the finer ones are carried to the centre, still in suspension. A pool is created between the 'beaches'. The core level is always below the beach level because the rate of sedimentation there is much slower.

The width of the core is controlled by the percentage of fines in the borrow soil and the level of water in the core pool. At the start of each 1-2m lift, the level in the core pool is raised to provide a width somewhat greater than the maximum limit of core in the shell. Filling commences when the coarse materials settling on the beach above the pool and encroaching on the pool limits. As the beach rises the core pool narrows and becomes deeper. Filling is stopped when the pool

width is close to the minimum permissible core width. A core zone with jagged edges, as shown, is the result.

Re-working the Fill - It is seldom that the beach will conform exactly to the desired dam shape since the deposition will vary with the distance from the outlet. Draglines are placed on the outer edges of the shell to reshape the dam to the proper dimensions. A new pair of dikes, if necessary, is built and the process is repeated.

From time to time fingers of core develop into the shell beyond the established limits. These are removed by digging them out, and replacing them with the shell material. Zones of shell material in the core are likewise equally dangerous. These develop from slides into an excessively deep core pool or as a result of too small a proportion of fines available from the borrow pits. A small hydraulic dredge is used to excavate the core material and discharge the mix back into the core pool so that the coarse materials will be widely dispersed.

Reworking the shell also tends to reduce the loose structure which often develops when fine sands are deposited out of water. Such loose cohesionless soils are potential sources of failures and are real hazards in hydraulic dam construction.

Rockfill Dams -

Pore Pressures - Excessive compactive effort relative to the particular material may induce pore pressures in the earthfill greater than uplift pressures that will result from filling of the reservoir. This would mean a lower factor of safety during construction than when the dam is in operation. If this is not acceptable then extra money must be spent to ensure slope stability during construction. On the other hand, a slope slide during construction would not compare in importance to a slide in a completed dam. It may therefore be a justifiable risk for the short term. If this were to be done then it would be mandatory to monitor pore pressures so that it would be known definitely when the design factor of safety was established. There is justifaction for reduction of construction pore pressures especially when the height reaches 150m for example.

An advantage of the thin core is that construction pore pressures should normally drop by 50% by the end of the construction period. In thick cores, the pressures may remain for years.

Dumped Rockfill - the main body of fill is placed by dumping. The initial part of the fill is dumped from clamshell cranes, cableways, or from ramps on the abutments to form a mound or bank. The remainder of the fill is dumped from the top of this mound, allowing the rock to fall down the sloping surface. The combined effect of sliding, tumbling and impact casue the pieces to become

tightly wedged together. Not more than 15% fines should be in the dumped rockfill, since they prevent good compaction and make drainage of water difficult.

Rolled Rockfill - if the rock is soft and breaks readily into pieces less than a third of a cubic metre, a rolled rockfill can be used. It is placed in layers and then rolled by heavy rubber tyred rollers and heavy vibrating rollers. Four to eight passes are required for compaction.

Reshaping the Fill - the dumped rockfill assumes side slopes of the angle of repose. If a flatter slope is required it can be formed by introducing horizontal berms as required.

GENERAL ABOUT DAMS

Engineering Responsibility

An Engineer's responsibility is to safety. They must act with integrity giving due consideration to the purpose of the project and the ultimate effects of the project on fellow human beings.

At the same time the Engineers are responsible to the community for the cost of the structure. There is always a limit to the finance, so any cut in cost must not sacrifice safety. The Engineers also carries a legal responsibility, and are responsible at all times for both what they do and what they say.

Consequences of Failure

Failure happens with fearful rapidity and usually without little warning, with the potential to cause a national catastrophe.

When the Oros Dam failed in Brazil in March 1960, between 30 and 50 people were lost and 100 000 people were evacuated, some 730 million cubic metres of water were released in 34 hours with a peak flow of 9600 cubic metres per second.

Statistics - Classification of Risk according to Gruner

45% Hydraulic Conditions 30% Type of Structure amd Construction7% Geology6% Environment 6% Consequences

Table based on International Commission on Large Dams 1965 report.

Number of incidents

Arch Buttress Gravity Earthfill Rockfill Misc Total Exploration 9 5 6 49 2 1 72 Material 1 - 2 8 - - 11 Layout - 1 4 17 3 - 25 Design 4 6 13 48 3 2 76 Construction 1 1 2 32 5 - 41 Operation - - - 5 1 - 6 Supervision 1 1 - 3 - - 5 Total 16 14 27 162 14 3 236

Appurtenant Features

Coffer Dams - Coffer dams usually are temporary structures built upstream from a dam to prevent stream flow around the excavation for a dam. In valleys of steep profile diversion commonly is accomplished by a tunnel or tunnels in the walls of the valley. Commonly the diversion tunnels are put to further use to control flow from the reservoir either for drainage of the reservoir or for flow under pressure into a hydroelectric generating plant. In valleys of low profile diversion is by tunnels, canals, or by conduits which subsequently are buried by the dam. It is not unusual in embankment dams to incorporate the coffer dam into the larger embankment structure comprising the designed dam. Hydroelectric power plants - many dams are constructed to generate hydroelectric power. The powerhouse is located at,or in the vicinity of, the toe of a dam or at some distance downstream. Flow of water into the powerhouse is controlled by valves upstream from the dam, within the dam downstream, or in valve vaults excavated in rock outside of the dam. Fish ladders - dams constructed on streams that are the migration paths for spawning fish commonly make provisions for movement of the fish up or in the vicinity of the downstream face of the dam. The facility that permits fish migration is usually called a fish ladder. See figure. Gates - gates are devices installed in the tops of spillways to control the flow of water over the spillway.

Locks - locks are movable dams or portions of dams utilized in navigation along rivers and canals.

Penstocks - a penstock is a sluice or conduit used for control of water flow, especially into a hydroelectric power plant.

Spillways - a spillway is designed to contain and control overflow of reservoir water when the reservoir is full. Spillways are, or should be, designed to accommodate flows during maximum flood stage so as to prevent damage to the dam and appurtenant features. Their size and location with respect to the dam is determined by the size and kind of dam, local topography, geology, and a careful review of the History of stream flow at the site of the dam.

Overflow of embankment dams outside of a spillway can have especially disastrous consequences so that safety usually requires a spillway capable of containing at least a hundred year flood.

Spillways are located within or on the downstream face of a dam, outside of the dam on one side or the other, or within the reservoir, where water spills into a glory hole and passes through a shaft and tunnel or tunnels in the abutment of the dam.

Tunnels - tunnels in bedrock outside of dams serve a variety of purposes. Flow through them is controlled by valves external to the dam or in valve chambers or vaults within the dam or in bedrock outside of the dam. Tunnels for control of the water level in the reservoir are commonly called gravity tunnels and serve a principal function of diverting water to some point downstream from the dam. Tunnels that transmit water under pressure to elevate the water to a higher level than the intake of the tunnel or to generate hydroelectric power are called pressure tunnels and usually require considerable competency in the rock through which they are constructed.

Valves and valve vaults - Valves control the flow of water through tunnels and penstocks. In many large dams the valve are installed in underground vaults or chambers to which access is gained downstream from the dam.

Water Resources - National and International

Water is probably Man's most vital commodity; its optimum utilization will be of prime importance in our expanding civilisation. Planning is therefore essential on a geographically wide scale and over a long period of time. The greatest obstacle is usually the unavailability of finance for comprehensive investigations.

It would be desirable from the engineering viewpoint to start development high up in a river and then progress downstream. This would improve quality and gradually increasing control of the river would facilitate and lower the cost of the

downstream stages. However there is usually less potential, difficult access and hence construction costs are higher, and therefore the benefit to cost ratio is generally lower.

For example the Hoover Dam, used to prevent floods, generate electricty and provide irrigation has two mighty spillways, which due to subsequent development upstream will probably never be used. Such occurences are unavoidable when only part of a river system is developed, i.e. when the economy requires the 'best' damsites be exploited first.

On a larger scale the major rivers of the world often form international boundaries or they rise in one country and flow through several others. International agreements may exist between countries with regards the usage of the river for example. For the satisfactory allocation of costs a basic programme for the full development of the river basin must be evolved and accepted.

Reservoir Utilization

Single Purpose Reservoirs

Mainly for industry, such as mining where the life of the dam depends on the mines resources, town water supply or for beautification and recreation.

Multipurpose Reservoirs

1. Water supply (requires a high reservoir) 2. Irrigation 3. Silt retention 4. Transportation 5. Electricity generation 6. Recreation and beautification (requires a constant reservoir level) 7. Flood mitigation (requires a low reservoir)

Life of a dam

Many dams are in existence that are over 1000 years old. Gravity and rockfill dams must qualify for long structural lives wheras thin arches, multiple arches or buttress dams have more limited lives, especially if they retain aggresive water.

It is usual to finance the building of a dam on the basis of repayment of its cost over 50 or 60 years. After this the only cost will be maintenance. The life of a dam may be prejudiced by the amount of silt carried by the river, since the reservoir loses capacity. It is possible to raise the dam by building up, but at a considerable expense.

Environmental Implications

1. Land inundation - creation of a reservoir will inundate frequently good land, and may cause people to be displaced. These factors lead to loss of productivity and personal hardship.

2. Dislodgment of people. 3. Wildlife - some species being destroyed is almost inevitable. 4. Archaeology - inundation of items of value. 5. Beauty - areas of beauty will be destroyed. 6. Silt - retention of silt from the lower valley which would normally enrich the

land. 7. River Regime - a period of dry river bed below the dam will occur. 8. Flood Warnings - alteration of natural flow can be serious to inhabitants

and wildlife. 9. Effects of Storage on Quality of Water 10. Eutrophication 11. Thermal Stratification 12. Fish - Nitrogen Problem 13. Water-bourne diseases 14. Requirement of fish ladders for fish to continue spawning 15. Induced Earthquakes consequent to filling large reservoirs 16. Climatological Change 17. Access roads during construction destroying the natural environment 18. River pollution from

Waste water from excavations Construction and removal of cofferdams Wash water from concrete and aggregate plants Oil leakage and waster disposal Sewage and stormwater Hot water effluents Soil erosion during reservoir cleaning

19. Fire Risks 20. Aesthetic appearance of final dam 21. Air pollution 22. Noise pollution 23. Dust pollution

Multidisciplinary Approach

We have reached an era when the Engineer must cooperate with members of other disciplines if a project is to be completed for optimum benefits and minimum adverse effects.

An example of the number of disciplines involved, relative to the Auburn Dam project;

Civil Engineering Sanitary Engineering Hydraulic Engineering Structural Engineering Electrical Engineering Illumination Engineering Air Pollution Engineering Acoustic Engineering Demography History Landscape Planning Traffic Landscape Architecture Transportation Ecology Geography Environmental Engineering Geomorphology Geology Hydrology Hydrography Meteorology Soil Agricultural Economics Biology Forestry Range Management Fish Wildlife Legal Photogrammetry Cartography Systems Programming Mathematical Programming

Construction Methods Analysis

Remote Sensor Interpreting