influence of geological conditions on design and construction of tunnels2-libre

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INFLUENCE OF GEOLOGICAL CONDITIONS ON DESIGN AND CONSTRUCTION OF TUNNELS

Introduction

There are several particular geological features, however, which are commonly encounter

in the tunneling operations. These can gives rise to difficulties especially in impending progress and\or increasing the hazardous nature of the operation.

Changes in geological conditions which decreased competence of the rocks surrounding the excavation often results in increasing tunneling costs in addition to affecting operational and safety aspects. Consequently, in depth appreciation of the geological conditions plays an important role for design and planning, through to construction and eventual commissioning and operation of the tunnel.

The first of the obvious geological conditions that are directly related to the tunnel is :

A. The type of the rock and their strength and deformation behavior B. Geological discontinuities and associated strength and deformation

behavior C. Groundwater conditions D. Squeezing and swelling rock conditions E. Running Ground F. Gases in rocks

G. Rock temperature

H. Topographic conditions.

A. The type of the rock and their strength and deformation behavior

The first basis of classification of rock is the geological classification according to which rocks are divide into three classes i.e.

a) Igneous rock b) Sedimentary rock c) Metamorphic rock

Prepared By : Arvind Kumar Jha ( 065/MsG/R/801)

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a) Igneous rock The crystalline nature of the igneous rocks signifies high compressive strength

with potential difficulties in rock excavations process, but can also indicate the mark competence with the advantages of decreased support needs to achieve an acceptable degree of stability. Localized and relatively thin intrusives are usually fine-grained and often possess high strength and significant resistance to weathering by comparison to the coarser grained igneous types with similar mineral constituents. Igneous rocks consisting of volcanic tuff and pumice, can be particularly weak and porous and whilst usually exhibiting low strength values with ease of excavation, they can be subjected to rapid weathering with accompanying loss of competency and can also give rise to the significant ground water problems.

b) Sedimentary rocks The effects of stress and advanced weathering, and weakening by the action of water can give rise to the problems especially where such rock type contains appreciable clay minerals. The banded characteristics are sometimes responsible for marked variation in strength, deformation and permeability in different directions.

c) Metamorphic rocks Rock types such as quartzite, marble, dolomite marble, and hornfels generally exhibits random distribution of minerals and display minor foliation and are relatively more competent. Rock containing micaceous minerals have well defined planes of weakness and can easily split along these planes of weakness and show very rather properties in terms of both strength and deformation properties.

Rock alteration The natural processes of weathering produce rock alteration which can be of major importance to tunneling.


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B) Geological discontinuities and associated strength and deformation behavior There are many discontinuities but among them followings are the major discontinuities which affects the tunnel design are : a) Folds b) Faults c) Joints. a) Folds Although folds occur in all types of rock but it is more conspicuous in

layered rock due to deformation of rock mass. This of course depends on the degree of fracturing which is almost associated with folding fractured and folded rock, therefore present more serious problem in tunnels; particularly during excavation. To overcome from this problem, it needs stronger supports and besides, fractures rock pieces immediately surrounding excavation tend to dislodge creating hazards. The degree of fracturing is more in stronger rocks although this depends on the depth of excavation and/or in situ stress condition. Folds are sometimes the natural traps of natural gases, which might be harmful to the persons working in tunnels. Trough of fold accumulates water during excavations causing pumping problems. Figure below Presents example of the influence of folded rock masses on the location of tunnel whilst excavation is in progress.

b) Faults Are associated displacements along the plane of the rapture caused by tectonic stresses.

The following points are based on the Wahlstrom who has discussed faults in relation to tunneling.


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1 Repeated intermittent movements occur at several sites, particularly where tectonic and igneous activities are still present.

2. Faults are frequently preferred paths for groundwater movement but may act as hydrological barriers of 7 below. Consequently internal erosion can occur and is particularly pronounced with certain rock types, eg. Limestone, whilst significant wall- rock alteration is likely with other rock

types, e.g. igneous, feldspathic, sandstones etc.

3. Frictional effects of movement along the fault plane can induce wall- rock alteration in addition to chemical reaction from water circulation.

4. The width of the fault zone is related to the geological and tectonic history and rock types. Fault zones can be tens of meters in width even where relatively minor displacement has occurred between strata and is possibly indicative of several reversals in movement over long periods of time.

5. Fault filling and gouge characteristics differ quite markedly and often reflect the degree of influence of groundwater movement .

6. Brescia filling is characterized by its fragmented nature derived from relatively competent rocks,. Such filling can exhibit voids but often contains fine materials. The breccias and in-filling materials can originate from upper or lower horizons depending upon the motive force causing movement of the fault filling.

7. Rock in a crushed and comminuted state caused by the grinding action of relative movement along the fault plane is commonly referred to as gouge. Water assists in the breakdown of some rocks and the fault gouge can often contain clay minerals which can give rise to plastic deformation in to underground excavations by virtue of time dependent behavioral properties and swelling pressure effects. Consequently humid conditions and groundwater contact with fault gouge can initiate progressive collapse


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in to tunnel excavations. Fault gouge has minor or insignificant bonding strength and exhibits poor stand up times. The fine- grained character of fault gouge often gives it an essentially impermeable property, although what might be regarded as insignificant seepage from gouge in to a tunnel can in time lead to creating a flow path for water and water borne debris in quantities of concern to the project . A tunnel intercepting a wide zone of fault gouge beneath and in hydraulic contact with the water table can experience inrush conditions. The width of fault gouge zones is difficult to predict and calls for careful observation, investigation and monitoring whilst tunneling is in progress. Effective support measures are called for to achieve early control over the deterioration potential of fault gouge properties.

8. Relative movement of rock masses produces scratches, grooves and polished interfaces. These can indicate movement direction, but they are of special significance to tunneling in representing planes of very low

friction with ability to readily encourage detachment and sliding of rock into tunnel excavations.

9. Faults and interconnecting structures allow circulation of groundwater to penetrate deep below the surface and laterally into the side walls. This can be produce wall rock alteration and result in deep seated weathering with consequent loss of competency of rocks appreciably below the surface.

10. The orientation of the faults in relation to the tunnel line is vitally

important since this govern the length of tunnel affected by the faults and its accompanying faults zone.

c) Joints and their relevance to tunneling Are structural plane of weakness and greatly affect shear strength of properties of rock and rock masses. The presence of joints is responsible for a number of instability in the underground excavations.


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The spacing and orientation of which with respect to excavation and tunnel surface is of paramount importance. Joints pattern considerably affects the mode of rock failure or collapse potential and the degree of over break during the tunnel excavation. Therefore the joints properties and their patterns studied with care and detail.

C) Groundwater aspects The presence of groundwater is recognized as a major hazard in addition to causing operational difficulties in respect of tunnel construction works. Predicting with accuracy the likely water inflow qualities is however, difficult, and detailed monitoring and regular reviews of conditions together with adoption of special measures such as dewatering or injection programmes need consideration. Encountering large quantities of water in weak ground conditions can lead to rapid formation of cavities around the tunnel excavation and can be produce the potential for the significant quantities of wet and loose ground to flow the tunnel. Some tunnel projects have experienced problems from the relatively warm( greater than 30-350c) groundwater which can impair the environmental conditions within the tunnel. So ground occurrence should be assessed during the site investigation stage. There are some example of problem due to groundwater excavation during tunnel design. X The Thames tunnel :- flooding occurred five times and the tunnel

was reclaimed each time by dumping clay and gravel in the riverbed and pumping out the water and digging out the dedris . what happened was that water softened the silt and clay came in ever-increasing volume and ultimately led to the flooding of tunnels. It was accompanied by the methane causing minor explosion causing illness and death of some tunnelers.


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X In Nepal :- removal of gauge material in fault led to serious flooding of the tunnel. X The seikan tunnel :- it experienced very large inflows, which resulted worsening of the ground conditions and finally the TBM method had to abandoned in favour of drill and blast method. The inflows had to be controlled by grouting which took considerable time. This reduced the rate of progress of tunneling. During mid 1981, grout consumption attained 98.2m3/m and this consumed 67% of the total time. Total drivage amounted to 131m. In 1978 the corresponding values were 5.7m3/m, 53% and the total tunnel drivage was 1490m. X Li (1987), China :- corrosive groundwater badly attacked concrete linings in at least three tunnels i.e. Lousoi, Luduohu, and Gajialen tunnels. He also mentions a number of cases where heavy groundwater inflows through fault zones consisting of mud and fault breccias destructed the support and seriously damaged the invert. X St. Gotthard tunnel :- it faced problems associated with high temperature and hot springs up to 60oc.

Not always possible, potential groundwater problems can be predicted using deep bore holes. It is difficult to predict the likely water inflow rates. The problem becomes more serious as the thickness of overburden increases.

D) Squeezing and swelling rock conditions Squeezing rocks Squeezing is in the effect a type of displacement into an excavation due to

stress gradient created around the tunnel by excavation. Squeezing the phenomenon that starts to show up, theoretically, as soon as excavation is made. But it is time dependent phenomenon. Stress released due to excavation may cause shear displacement to take place of hundreds of meters beyond the excavation surfaces.


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Swelling rocks Swelling is a time dependent process and involves physico-chemical reactions with water. Swelling is assisted by stress release or increase in water content or a combination of both giving rise to time dependent process of swelling. Swelling ground displaces into the tunnel opening as result of volume change due to water adsorption and absorption effects. Swelling behaviour denotes the response of rock to the presence of water. A simple test is that of immersing a piece of rock in a vessel of water and observing its response and whether it disintegrates.

Einstein and Bischoff (1975) suggest the following steps to contain or reduce swelling effects in tunnels. X Invert arch :- An arch invert to reduce the main zone of swelling. The

arched shaped invert was more effective than the invert slab. X Invert slab anchored or bolted :- if the invert slab is to be used it should be bolted down or anchored to contain inward heave. X Trimming :- it is applicable only when the sidewall lining is not affected by heaving. X Backfilling with weak material or compressible support:- absorb swelling pressure. X Grouting :- effective in controlling the passage of water to swelling prone materials. X Drainage :- protect the swelling prone minerals ( iron) getting access to water.

Example

The Belchen tunnel:- passes through swelling prone marl, anhydrite and clay shale. Original design incorporated a 0.45 m thick concrete arch invert, which was supposed to cope with the expected swelling. Two abutment drifts( 2.9 * 3.0 m) were driven to install the concrete invert so that full face excavation could be carried out afterwards under a shield. Heaving occur after the excavation. Rock bolts of 2.5m length was subsequently installed to shave the


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invert, but this measure could not contain swelling and the invert did finally fail close to the abutments. On the basis of continuous monitoring of the swelling pressure, reinforced concrete lining of thickness 0.60 to 0.85 m was found suitable.

E) Running Ground Is often saturated and the presence of water can encourage liquefaction when

disturbed by tunneling activities. Can arise at a later stage due to the progressive collapse and formation of a significant cavity tapping a major aquifer and overlying unconsolidated saturated deposits.

F) Gases in rocks Gases are frequently in the sedimentary rock. Methane can move great distances laterally and vertically to accumulate in unsuspected locations. It was released from shale during driving of the great Apennine tunnel between italy and Switzerland which resulted in an explosion by fire. Carbon dioxide arise from carbonaceous materials. Various gases encountered in tunnels are: carbon dioxide, methane, sulpher dioxide, hydrogen sulphide and rarely hydrogen. Sulpher dioxide is a product of oxidation of sulpher or sulphides and can occur in sedimentary rock and hydrothermal deposits containing significant sulphide concentrations. Hydrogen sulphide is a product of decay of sulpher bearing compounds and can be released from the thermal water to deep seated origins.

G) Rock temperature Increase with depth. Temperature increases about 10c for every 60-80metres in geologically stable areas and 10c for 10-15 meters in volcanically active areas.


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The simplon tunnel experienced very high temperature i.e. 560c at a depth of 2134 meters. Alpline tunnels, a sudden increase in temperature from 270c to 450c and even 630c occurred due to the sudden release of methane gases. Effective ventilations is perhaps the only means which can alleviate the problem.

H) Topographical conditions Is responsible for modifying stress conditions at the surface. One principal stress is zero at surface and other is parallel to it which is directly related to the slope of the ground. One obvious changes that tunnel construction may bring about is the change in the situ stress conditions as well as ground water conditions.

KARST PROBLEMS Are associated with limestone and carbon rocks and including crystalline limestone and to lesser extent, dolomitic limestone dolomite conglomerates and marl. Accessibility of water to these rock is the major factor leading to the karst.


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GROUTING

Definition


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3. Categories of Grouting

Grouting for ground engineering can be subdivided into: permeation grouting, displacement grouting, compaction grouting, grouting of voids, jet grouting, Special grouting applications and techniques, including electro grouting.

Penetration Displacement Penetration Jet grouting (Intrusion) (Compaction grouting) (permeation) (displacement, replacement)

I. Permeation grouting

This method describes the process of filling joints or fractures in rock or pore spaces in soil with a grout without disturbing the formation. More specifically, permeation grouting refers to the replacement of water in voids between soil particles with a grout fluid at low injection pressure so as to prevent fracturing. It is a technique that is generally used to reduce ground permeability and control ground water flow, but it also can be used to strengthen and stiffen the ground.

II. Displacement Grouting

Displacement grouting is the injection of grout into a formation in such a manner as to move the formation; it may be controlled, as in compaction grouting, or uncontrolled, as in high-pressure soil or rock grouting which leads to splitting of the ground, also called hydrofracture.


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III. Hydro fracture grouting

Hydro fracture grouting is the deliberate fracturing of the ground (soil or rock) using grout under pressure. Typically it is used to compact and stiffen the ground or to access otherwise inaccessible voids, thus reducing the mass permeability of the ground and produced the controlled uplift of structures. If the grouting pressure is increased sufficiently, a soil mass may split and artificial grout-filled fissures are formed; in rock, existing fissures may enlarge and new breaks may occur. This also referred to as claquage by French engineers.

IV. Compaction grouting

In this method, grout mix is specifically designed so as not to permeate the soil voids or mix with the soil. Instead, it displaces the soil into which it is injected. In granular deposits not at their maximum density, the volumes of voids are reduced and the deposit is locally densified. In Compaction grouting a very stiff (say 25-mm slump) mortar is injected into loose soils, forming grout bulbs which displace and densify the surrounding ground, without penetrating the soil pores.

V. Grouting of voids

Grout may also be used simply to fill voids, such as may develop below the joints in a concrete pavement through pumping. Special terms have evolved for the grouting behind the lining of a tunnel due to over break: Backpack grouting, contact grouting, or more especially crown grouting are found in the relevant literature, in addition to interface and gap grouting.

VI. Jet grouting

The high-pressure water or grout is used to physically disrupt the ground, in the process modifying it and thereby improving it. In normal operation the drill string is advanced


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to the required depth and then the high-pressure water or grout is introduced while withdrawing the rods.

VII. Deep Mixing Methods (DMM)

The Deep Mixing Method (DMM) is today accepted world-wide as a soil improvement method which is performed to improve the strength, deformation properties and permeability of the soil. It is based on mixing binders, such as cement, lime, fly ash and other additives, with the soil by the use of rotating mixing tools in order to form columns of a hardening material since pozzolanic reactions between the binder and the soil grains are developed. In Sweden and Finland, deep stabilization techniques are quite popularly used for stabilization of soft soil.

Based on design requirements, site conditions, soil and rock layers, restraints and economic, the use of deep mixing methods (DMM) is increasingly spreading. These methods have been suggested and applied for soil and rock stabilizing, slope stability, liquefaction mitigation, vibration reduction (along the railway), road and railroad and bridge foundations and embankments, construction of excavation support systems or protection of structure close to excavation sites, solidification and stabilization of contaminated soils etc. The demand for improving and stabilizing land for different purposes is expected to increase in the future and the best way to fulfill it is by using deep mixing methods (DMM). It is strongly suggested that, where sufficient space is unavailable, sliding and overturning stability be augmented using soil anchors. The main advantage of these methods is long term increasing in strength especially for some of the binders used. Pozzolanic reaction can continue for months or even years after mixing, resulting in the increase in strength of cement stabilized clay with the increase in curing time.

VIII. Electro grouting


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Electro grouting is a term used for promoting electrochemical hardening during electro osmosis by adding chemicals, such as sodium silicate or calcium chloride, at the anode. Under the influence of the electric field, these chemicals permeate the ground, flowing in the direction of the cathode, while the anode becomes a grout injection pipe.

Grouting as an art The largest quantities of grout (usually cement-based) are used in creating more or less impervious curtains below dams in order to reduce water losses, uplift pressures, and reduce the potential for hydraulic fracturing( leading to piping failures) under operating conditions. Foundation grouting for increased stability and reduced compressibility is probably next in importance. It may serve a permanent or temporary function: only a temporary increase in strength may be required for surface or underground excavations. In some cases, grouting may provide an elegant, if not the only way, of repairing existing structures or making up for inaccurate or imperfect construction procedures.

For many engineers, grouting is still considered an art rather than science. Its successful application requires a great deal of experience, thorough knowledge of geological conditions, and an awareness of equipment capabilities and limitations. Houlsby (1982) went even further in saying that grouting requires an intuitive perception of just what the liquid grout does as it flows through the open joints and cracks hidden away down there underground.

4 GROUT MATERIALS Classification of Grout materials

Three basic types of grout are differentiated according to composition: Suspensions: Small particles of solids are distributed in a liquid dispersion medium. Examples: cement and clay in water. Emulsions: A two phase system containing minute (colloidal) droplets of liquid in a disperse phase. Example: bitumen and water. Also in this category are foams, organic chemical. Foaming agents, such as additives which increase surface tension, assist in


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forming bubbles by agitation; alternatively, they may induce gas- forming chemical reactions.

Solutions: Liquid homogeneous molecular mixtures of two or more substances. Examples: sodium silicate, organic resigns, and a wide variety of other so called chemical grouts. A difference may be made between colloidal solutions (e.g. silica or lignochrome gels) and pure solutions(e.g., phenolic and acrylic resins, aminoplasts). (Note: A cement grout is not commonly classified as a chemical grout; it is usually classified as a suspension grout.) Principal types of grouts as distinguished by cambefort( 1987) are listed in following table which also gives an indication of appropriate uses, construction controls, and relative costs.

Cambefort (1977) characterizes foam grouts by the following parameters:

Expansion coefficient, eg =

Bulking coefficient, Cb = = 1 + eg

Air Ratio, ng = =

State Suspensions Liquids

Unstable Stable Chemical Products Aerated emulsions

Grout Type Cement

Bentonite + cement

Deflocculated bentonite

Sodium silicate

hard gels

Sodium silicate diluted

gels

Organic resins

Cement foams

Organic foams

Sands and gravels, k m/s Range of uses

Fissures

> 5 X 10 -4 > 10-4 > 10-4 > 10-5 > 10-6

Cavities High water flows

Grouting Control

Refusal Pressure Limited quantities

Relative cost for the products to fill 1 - m3

voids

4.2 (deposit with =1.5)

1 (cement 200kg;

bentonite 30 kg)

0.8-1 6 2-4 10-500 1.2 10


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The relationship between the expansion coefficient eg and the air ratio ng is analogous to that of the void ratio and porosity in a soil. The rhelogical properties and ground penetrability of foams are not only related to the expansion coefficient, which could vary from less than 3 for cement based foams up to 50 for organic foams, but also on the bubble size distribution. The latter can be controlled, to a degree, by the choice of foaming agent and the method of bubble formation.

5 GROUTING TECHNIQUE AND CONTROLS Cement clay and cement started to be used to repair masonry walls, fill cracks in load bearing structures and seal off water flow in rock fissures around 300 years ago. Initially grout consisted only of single or multi component material mixed with a reactant before injection (one slot technique) to set with in few minutes or hours. Improvement in the grouting

methods depends to a large extent on development of an efficient mixing and pumping technology. A two set grouting technique suitable for stabilizing soil as small grained as fine sans was invented around 1925 by HJ Joosten. In this two shot grouting sodium silicate and calcium chloride in to the ground from two separate pipes, these chemicals reacted instantly

in the soil mixed. Fig: Equipment employed in modern grouting

Grout could be injected as drilling proceeds but it is preferable to organize drilling and grouting as separate phases e.g. with grouting commencing

once the borehole is compacted, or alternating with drilling in stages.


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In order to treat a particular ground stratum the corresponding length of bore hole is isolated by expanding rubber Packer built in to the drilling rods grout is then only allowed to flow in to soil or rock from between two packers or, if a single packer is used, between it and the bottom of the whole. Packers in contact with the ground are only feasible in rock grouting, for stage grouting in soils the sleeve tube has been developed.

The procedure is as follows: - The hole is drilled and cased.

- A steel or plastic tube, slotted at regular intervals, is inserted. The vertical slots are covered with rubber sleeve.

- As the casing is withdrawn, the space between sleeve tube and borehole wall is sealed with cement- bentonite groute inserting bentonite pallets or cement -bentonite slurry.

- After the seal has set, the grouting tube is inserted. Grout exists between two packers allowing injection through selected slots. With increasing pressure, the rubber sleeves burst and grout flows in to the soil.

With sleeve tube technique, grouting can be repeated in the same hole using different viscosity grouts or different chemicals in planned sequence. Flexibility is important where permeability of ground vary significantly from point to point. Grouting in stages may proceed in descending (down hole down stage) or ascending upstage direction. In descending method, impregnation of the grounds occurs in advance of the borehole, which could be advantageous in loose soil or rock. In the ascending technique grouting follows drilling as separate phase, a benefit would be that water pressure testing is possible immediately prior to grouting allowing for a choice of most suitable grout type, pressure and quantity of for that particular stratum.

Experience and intuitive judgement are used in deciding which includes:


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- the pattern of boreholes.

- the sequence of grouting holes with in a group

- the stages of injection along a single hole. - the pressures.

- the viscosities.

- quantities to be used in each phase of operation

All above mentioned points are aimed to: - minimum wastage of grout

- least damage to the ground

- maximum gain in strength and

- reduction of seepage.

The successful grouting can only be assessed after job is completed, when seepage measurements or settlement observation for the structure under operating conditions are available. However some controls are possible, if not vital during the actual grouting process or at intervals during construction. This may include:

- monitoring the grout taken as a function of pressure.

- observing ground heave.

- recording piezometre levels.

- digging inspection pits.

- Retrieving core samples for examination and laboratory testing.

- Photographing walls of boreholes for visual inspection (particularly effective if grout has been dyed.)

- conduting pressuremetre tests, possibly penetration tests

- conducting borehole permeability tests

Obviously the better the site investigation before grouting, the better is the chance of selecting a successful grouting strategy.


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6 PERMEATION GROUTING OF SOILS

Grout permeation through soil is generally related to the grouts permeability, measured in terms of the coefficient of permeability k according to Darcys law(Q=k/A). For a particular fluid, k is primarily a function of the void ratio (or corresponding porosity or density), but particle size distribution, soil structure, saturation, and other factors also influence its value.

If soil voids are represented by a system of tubes with equal permeability, then the Hagen- Poiseuille equation for the viscous and visco-plastic flow in pipes could be used to estimate grout take and reach. A spherical or cylindrical flow model for a porous medium is however more appropriate for permeation grouting of souls from boreholes. Using basic well hydraulics , the distance traveled by the grout can be related to the grouting rate and time.

Spherical Flow Model for Porous Media (Newtonian fluid) Imagine that grout permeates soil from a spherical cavity of radius Ro under the influence of a net pressure po (in excess of local hydrostatic pressure). Then, for laminar Newtonian flow, the following relationship holds:

Where, Q = grouting rate, m3/s = unit weight of grout, kN/m3

= permeability of soil to grout, m/s k = permeability of soil to water, m/s

= viscousity of grout, Pa . s

= viscosity of water, Pa . s

During spherical grout permeation in time dt the grout travels a distance dr. The grout taken in time t can be found by integration from

Where, is the porosity of the soil (volume of voids/ total volume)


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The time required to travel a distance R from a spherical cavity with radius R0 can be computed by

QRR

t3

).(.4 303 =

Theoretically, a Newtonian grout will be continue to travel outward,as long as

excess head (pe / ) exists or until it sets or gels.

Radical flow from a cylindrical cavity .(Newtonian fluid) Equation equivalent to above but for horizontal cylindrical flow as from a section of a bore hole

w

e kmQ

p ..4=* ln(R/R0)

where,

pe = excess pressure necessary to maintain flow Q when grout has reached distance R from the injection point. R0 = radius of borehole m = thickness of layer being grouted

QRRm

t).(.. 202 =

As for a confined aquifer being recharged the pressure p(R) of the grout diminishes with distance R from the borehole according to

w

er kmQ

pp ..2=* ln(R/R0)

Groutability of soil based on permeability. Groutability of soil/rock is their ability to accept grout. Groutability depends on the properties both of the ground and the grouting

material.


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Granular structure, permeability of the ground, rheological properties of the grout mixture, become more important in the relation to the groutability. Groutability criteria is that the size of grout particles must be small enough so that they can penetrate the soil or rock formations. So

D85 (grout)< 1/3 B ( fissure) D85 (grout) < 1/15 (soil) The ratio of D15 (soil) to D85( grout) is also called groutability ratio. D15s / D85g = Rg ( groutability ratio) The groutability ratio should be exceed 15, but in some cases has been

carried out successfully with this ratio of 5 to 10. Groutability of granular soils Determined by the permeability of the medium and/ or its particle size

distribution. Figure .. Groutability also depends on the injection pressure and the degree of dilution of the grout.

Tubular flow analysis teaches us

that the resistance to

the penetration

decreases with fourth power of the

diameter of the opening. The

permeability of granular soils can

easily be found by the relation to

dewatering by wells.


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Comparison of the permeation of grouts The table above gives a general overview of applications of various suspensions and liquid grouts in soils; the latter rated according to their coefficient of permeability k (m/s)

Relationship between water/cement ratio and viscosity for different types of cement Above figure shows the comparison of the viscosity of microfine cement with that of colloidal and ordinary Portland cement. Increasing the water/cement ratio does decrease the viscosity but also increases the gel time and reduces the strength of the grouted soil.

7 HYDRAULIC FRACTURING OF SOILS AND ROCKS

Hydraulic fracturing or hydro fracture is splitting of soil mass, formation of artificial grout filled fissures (in rock), enlarge of existing fissures and occurrences of new breaks when the grouting pressure is increased sufficiently. Identification of hydraulic fracturing

- ground heave in a granular ground and at shallow depths in rock.

- Surface heave in slab jacking or lifting of building foundation.


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- Backflow of liquid grouts as soon as the injection stops.

For suspension grouts, backflow is inhibited by partial sedimentation. The expansion of fissures during grouting of fractured rock may have a positive result. Amore complete filling of the fissures is achieved after elastic rebound of the enclosing ground mass, in extreme case, the deposited grout is compressed by the closing fissure and some of the grouting pressure is locked in a pre stressing effect. Besides causing ground heave and back flow hydrofracture they also be apparent from a Lugeon bar plot or flow rate/ pressure diagram. In addition, acoustic emissions monitoring appears to have the potential to identify the occurrence of hydraulic fracturing in the ground.

Fig:Interpretation of flow pressure diagrams

In soils, overburden pressure would be a reasonable conservation guide for predicting the danger of hydrofracture. Unless the soil is overconsolidated, initial cracks could well run vertically along boreholes (even to the surface) rather than along weaker layers of soil in a horizontal direction. The existing tectonic stress state, redistribution of stresses due to mining and construction activities could be very important when it comes to predicting or interpreting the effect of high grouting pressures on the ground. Whether the (low pressure) penetration or (high pressure) displacement grouting techniques are to be used on a particular job may not only depend on the prevailing


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geological conditions and the particular type of grout used, but also on the local experience, design philosophy and construction practice. Some experts maintain that penetration grouting is adequate for the treatment of most foundations ;others see the occurrence of hydraulic fracturing of the ground not only as inevitable but necessary for thorough impregnation.

References : 1) Underground excavation in rock by E. HOEK AND ET. BROWN 2) Engineering principle of Ground modification by HAUSSMAN 3) Tunnel engineering hand book by BICKEL, KUESEL AND KING

4) Tunneling and tunnel mechanics by DIMITRIOS KOLYMBAS 5) Class note in underground excavation 2009/10 by Prof. Dr. AKAL BR. SINGH 6) Internet collection.


influence of geological conditions on design and construction of tunnels2-libre

Documents

rock alteration

rock temperature

swelling rock conditions

igneous rocks

rock excavations process

c metamorphic rocks

obvious geological conditions

high strength