structural geology ii

Structural Geology

Unit -II

Effects of Folds• Folds as we know, mainly occurs due to the

tectonic forces and as a result, the affected rocks get deformed, distorted or disturbed.

• This results in the occurrence of great strain in rocks which when occur released, say in the form of human interference, say in the form of tunneling, may cause bulging, caving, etc.,

• Thus affected rocks are bent upward or downward, which means the sedimentary strata, which were originally horizontal get inclined in some direction. The upward and downward bends are created in rocks.

Effects of Folds

Effects of Folds• Thus a formationed physical effects

produced in rocks due to folding are very important from the civil engineering point of view, particularly in the location of dams, reservoirs, tunneling, quarrying, roads and railways etc. these effects are also important in the occurrence of ground water, oil and gas and some economically important ore deposits.

Location of Dams• Lithological being the same, the

inclination of limbs in the dam site produce a geological setting which may be either more favorable or unfavorable at the dam site.

Location of DamsCase-I • At the dam site, if the beds of limbs the dip gently in

the upstream direction it is more favorable and advantageous because of following reasons: at the dam site, the weight of the dam (W) acts vertically downwards. In addition to this, there exists a great lateral thrust (T) due to reservoir water.

• The resultant force (R) of these two will be always inclined in the downstream direction.

• Depending on the quantum of reservoir water, its (R) inclination may vary from 10 0 to 30 0 from the vertical. This means the beds which have a gentle upstream dip will perpendicular to this resultant force and hence can offer their best resistance to withstand the stresses or loads acting in the area.

Location of Dams

Location of Dams• Further, this geological setting

caused by folding also indirectly contributes to the stability of the dam by completely eliminating the possible uplift pressure. This is so because any possible leakage of reservoir water is directed to the upstream side by virtue of the inclination of beds.

Location of Dams

Location of DamsCase-2 Resultant force in the dam which is inclined slightly

in the downstream direction is not perpendicular to the bedding plane of strata. Hence this geological setting is not ideal, though not bad. Further, the reservoir water, which is under great pressure, shall attempt to leak, beneath the dam along the horizontal bedding planes, thereby causing uplift pressure; of course, the heavy weight causing uplift pressure over the dam is minimum, though not absent. Thus the comparison of these two cases clearly shows that the folding may sometimes provide favorable geological condition from the civil engineering point of view.

Location of Dams

Location of DamsCase:3• Suppose the dam is located over the limb of a fold

which dips along the downstream direction in this case the resultant force of the dam will be parallel or nearly parallel to the bedding plane. This means that the sedimentary beds there are less competent. Therefore, such a geological condition is unfavorable. It also causes leakage of reservoir water along bedding planes. This leads to inevitable and considerable uplift pressure which means a reduction in the stability of the dam structure.

Location of Dams

Location of ReservoirLocation of Reservoir• The forgoing also illustrates that if the three

different geological setting occurs at the reservoir site, there will also be a significant difference in terms of leakage of reservoir water.

• In the first case, irrespective of whether the rocks are aquifer or not and whether the local ground water table is at a shallow a shallow depth or not, there will not be any effective leakage of water from the reservoir.

• This is so because all percolated water will be directed in the upstream direction only, along the bedding plane.

Location of Reservoir


• In the second case, there may be a little seepage of water of the reservoir in the downstream side along the horizontal bedding plane.

• In the third case, even if the rocks are not aquifers there shall be considerable leakage of reservoir water along the bedding planes which are dipping in the downstream direction.

Location of TunnelsLocation of Tunnels• For tunneling purpose, folded rocks

are in general unsuitable because the affected rocks are under great strain and the subsurface removal of material i.e. creation of tunnel in such rocks may cause the release of the contained strain which may appear as collapse of the roof, or as caving or bulging of sides, or floor etc.

Location of Tunnels

Location of Tunnels

• Comparatively speaking, it is better to take up tunneling work along the thick bed of limbs, parallel to the axis of the fold, because the disadvantage associated with crest and troughs do not occur.

• Along the crest of folds the bed contains numerous tension and other fractures. Therefore, if tunneling is made through them, frequent falling of rocks from the roof may occur.

Location of Tunnels

Location of Tunnels

• Along the troughs on the other hand, rocks will be highly compressed. Therefore they will be tough and offer greater resistance for excavation. This means tunneling work will be difficult and progress will be slow.

Quarrying• In the case of quarrying also, it is convenient and

desirable to take up such work along the strike direction and along limbs because this provides a good quality of rocks of the same kind in addition to easily breaking the rocks along the bedding plane.

• Further, severe fractures associated with crests and troughs will not occur along the limbs, which means rocks of suitable size can be obtained. Seepage problem of water in crests and troughs also can be avoided partly by taking up quarrying along the limb.

Quarrying

Ground Water Occurrence

• In terms of ground water occurrence, syncline sometimes furnish favorable condition to tap up enormous quantity of ground water. Thus, some artesian springs and wells, which are good source of ground water owe their origin to syncline structures.

• The numerous fractures which occur in folded region act as convenient channel ways for ground water movement. Such fractures also provide additional water bearing capacity to rocks of such a region.

Ground Water Occurrence

Laying roads and railway tracks along hill slopes

• Regarding laying roads or railway tracks along the slope of folded hills, the stability of the ground depends on the mutual relation of the dip of beds and surface slope of the region or the slope of the cutting wall.

• If the surface slope and beds dip coincide i.e.. occur in the same direction, the ground may be unstable and landslide are likely to occur in such places.

Laying roads and railway tracks along hill slopes

Miscellaneous• In addition to some of the aforementioned aspect

of civil engineering importance folds also contribute to some economic deposits such as oil and gas, a few ore deposits and slates.

Oil and Gas Deposits• Occurrence of the important oil and gas deposits

of the world in association with anticline folds is so characteristic that exploration of oil and gas is essentially made to locate anticline of the subsurface which serve as good structural trap of these valuable and strategic deposits.

Oil and Gas Deposits

Miscellaneous• Since oil and gas occurs at very great

depth and also because they are not amenable to direct exploration, their location is detected first by knowing the subsurface anticline structure by suitable geophysical investigations and then by carrying out actual drilling to find out whether such anticline contains oil and gas or not.

Oil and Gas Deposits

MiscellaneousOre Deposits• The crest of folds sometimes offer

convenient place for the occurrence of ore deposits under favorable condition. It so happens that when competent beds like quartzite and incompetent beds like slate occur alternating and get closely folded, the crest of such folds become convenient sites for ore occurrence. Since such deposits resembles the saddle reef in c/s they are called “ The saddle reef deposits” .

Miscellaneous

MiscellaneousMarbles, Slates • Folds of tectonic origin, naturally are

accompanied by metamorphism also. Under favorable condition, due to metamorphism, valuable deposits like marble and slates come into existence Marble and slates belonging to this kind are profitably worked out.

Marbles

Effects of Faults and their Civil Engineering Importance

• Faulted areas are neither safe nor stable for the foundation works because of the various harmful effects produced by faults.

• Faults cause considerable fracturing and shattering of rocks along fault zones. This means that are not compact or massive or strong. Such places are reduced to physically very weak grounds and hence unfit as foundations sites for withstanding heavy loads of structures like dams


• When such porous and fractured zones get saturated with water, their strength comes down further.

• The same fractures act as channel ways for movement of groundwater. This may cause severe groundwater problems in case of tunnels and leakage problems in case of reservoir.

Effects of Faults and their civil Engineering Importance

• The most dangerous features is its possible reoccurrence at the same place. This means that the fault ground are unstable as long as faulting remain active there.

• Generally faults are accompanied by earthquakes. Earthquake cause severe shaking of the groundwater. Such shaking may cause collapse of civil engineering structures.


• The fault plane itself is a very prominent fracture plane in the fault zone and therefore may act as a severe sources of leakage of water, Such weathering further reduces the competence of the rocks.

• Faults influences the movement of surface water also.

• In some cases where the dip direction of the fault plane and the surface slope occur in the same direction, land slides may occur.


Location of Dams• A dam which is invariably a multi-million

project, cannot be allowed to rest over an active fault irrespective of its dip direction, under any circumstances. However, if the faults are dead and if it becomes unavoidable to locate the dam in a fault region only, then adequate precautions have to be taken in improving the competence of the site by giving proper treatment to it.


Location of Reservoir• Faults causes an enormous leakage of water if they occur in

the reservoir basin. Comparatively, faults which dip in the downstream direction are more harmful. This is so because they not only cause effective and significant loss of water but also endangers the safety of the dam by creating uplift pressure over it. However if the water table occurs at or near the surface of the reservoir site, faults do not contribute to loss of water.

• If severe fault or shear zones occur as outcrops along the upstream course of the river, they get eroded quickly and contribute heavily to the load of the river. This means severe silting problems in the concerned reservoirs. In such cases, to avoid the problems, the fault zones have to be covered or treated suitable.


Location of Tunnels• Knowledge of the effects of fault makes it very clear that a

tunnel alignment should not come in the way of faults, particularly active ones. Otherwise, the possible consequences etc.

• Being heavily fractured, fault zones will be incompetent to provide safety to tunnels.

• Severe ground water problems are likely to occur.• The risk of displacement of the ground as a result of

renewed faulting exists• However if the faults are not active and if they are

minor, they may be treated properly to make tunnel structures safe, when they pass though them.


Quarrying• Since fault zones are highly crushed,

quarrying through them cannot produce blocks of good size. Further, being porous, fault zones provide easy percolation of water which, in turn can cause significant weathering of its material. That means quarrying produces only interior material both qualitatively and quantitatively.


Laying of Roads and railways tracks along hill slopes.

• In a way, fault are similar to bedding planes as far as their bearing on the slope stability is concerned. But faults need more careful attention because any renewed faulting will trigger off landslides, which means an additional problem for the safety of civil engineering works. Saturation of the adversely placed faults zones with water, increases the risk of landslide occurrence. Hence laying of roads and railways along faulted hills should be considered carefully and undertaken with necessary precautions.


Ground Water Occurrence• By virtue of intense fracturing associated

with faults, they significantly improve the ground water potential. This is because these fractures not only provide space for storing ground water but also help in the movement of such water. This fact is particularly important in hard rock areas because, in such places, ground water potentiality depends on the thickness of the weathered zone and structural weak planes like joints and faults.


• Faults and Joints since they extend up to considerable depth, can contribute significantly in improving ground water potentiality in hard work areas, Sheet joints which may occur at a depth are of special ground water importance in such places.


Ore Minerals• Fault zones are economically very

important because they are often richly mineralized zones. This is because they are highly favorable places for the occurrence of a variety of ore minerals formed by different processes.

Ore Minerals


• Precautions and steps to be taken to improve fault sites.

• In view of the numerous problems involved, it is always desirable to avoid the places where faulting has occurred for civil engineering constructions. But if circumstances warrant the necessity of having construction over faults the following precautions should be taken.


• First the tectonic history of the region concerned is carefully studied.

• In the known history of the place if faulting had occurred repeatedly and if the intensity of faulting was severe then there is no alternative but to abandon the site because any amount of treatment cannot improve the conditions.


• Thus a careful study of tectonic history followed by necessary treatment such as grouting or plugging the fault zone with concrete would improve the competence of the site and make it fit to hold any civil engineering structure.

Effects of Joints and their Civil Engineering Importance

• Since Joint are a set of cracks or open fractures, they act as planes of complete breakage or non-cohesion. As a result, such a site is not compact, massive or coherent, which means that it is physically weak, inherently.

• Through these joints water is likely to percolate and saturate the rocks. Further, this may cause decay of rocks along joint plane. Both these facts further reduce the physical strength of rocks considerable. This makes the region unsuitable for tunneling purposes also.


• Further, being open fractures, joint permit ease percolation of water through them. This means they act as effective planes of leakage of water. However, joints are undesirable in case of reservoir, power tunnel, etc. with unfavorable attitude and in association with rocks, joints may case severe landslide along hill slopes, i.e.. the presence of joints is harmful and cause instability along hill slopes.


• Joints are weak planes in rocks just like fault planes and bedding planes. But they are less harmful than faults but more harmful than bedding plane. This is because, when compared with faults, joints do not have brecciation ( i.e. intense fracturing of the fault zone). So joint, unlike faults, cause be treated as simple cracks, and hence can be easily dealth with in improving the sites having faults, can be treated as simple cracks and hence, can be easily dealth with in improving the sites to make them suitable for location of civil engineering structures.


Location of Dams• If the joint are too many, closely

spaced and are of great magnitude, then such a fractured site will be physically too weak to withstand stresses of dams and bridges. Saturation with water along with the accompanying decay of rocks will make the site more incompetent for foundation purposes.

Location of Dams

Location of Dams

• Further, if the joint dip in the downstream direction, their influences will be very bad. Comparatively, joints which dip in the upstream direction are less harmful. This is because the water of the reservoir which is under great hydrostatic pressure percolates forcefully through joints which dip in the downstream direction and cause uplift pressure on dams. This cause instability to the dam structure.

Location of Dams


Location of Reservoir• The first and foremost function of

joints in a reservoir basin is that they act as a venue for serious leakage of water. Of course the prevailing water table position will independently affect the influence of the leakage effect of faults and joints in this regard.


• But a matter of consolidation is that even if joints are contributing to the leakage of water, in course of time, this adverse effect partly disappears slowly, because the fine silt and clay settle in the opening of joints and seal them off. This reduce leakage because the clay which has settled in this way act as impermeable material.

• If joints has occurred acutely in the valley in the upstream side. Such rocks undergo quick erosion and contribution and contribute to the river load heavily. This means the rate of silting will be very high in the reservoir. This in turn, reduce the life, reduce the life of the reservoir. So as precaution, such places have to be covered or grouted suitable.


Occurrence of landslides• Landslides take place when the surface slope

of hill and the dip direction of bed or fault or joint occur in the same direction. Though the preceding statement is true in general, the role played by joints is more important than that of bedding planes or faults. This is because bedding planes are not open fractures like joints so as to facilitate heavy percolate which is mainly responsible for landslide occurrence along hill slopes.


• Likewise, faults too are less important because they are uncommon, where as joints are very frequent. Off course, landslides occur because when the percolated water comes in contact with argillaceous matter below the ground, fine lubricating material is produced which, in turn, causes the slipping or sliding of overlying rocks along the dip direction of joint plane.

• Occurrence of landslides is important from the civil engineering point of view because the safety and stability of civil engineering works like roads, railway tracks and pipelines in hilly region are dependent on slope stability or landslide occurrence.

Occurrence of landslides


Quarrying • The role of joint in quarrying may be easily

may be either helpful or harmful. It should be good if joints occur at suitable interval and they have not contributed to any decay of rocks along joint planes. This is desirable because by virtue of jointing, the insitu rocks of the quarry are already cut into suitable blocks, which may be simple extracted for further use, Such quarrying needs no heavy use of expensive and dangerous explosives. This means quarrying can be done easily, economically and safely.

Quarrying

If joint occur in more number of sets and at very close interval or at great intervals and if they have contributed a lot to decaying of rocks, then the quarry is rendered useless for obvious reasons. Thus careful steady is needed to assess whether occurrence of joints is good or bad in a given quarry.


Tunneling• They cause serious ground water problems ,

unless the water table position is reasonable below the level of tunnel floor.

• If the joint are too many, they may severely hamper the competence of even inherently strong rocks and render them unsuitable for tunneling.

• The opening of joint planes enable the ground to be saturated with water and thereby decrease the strength of the rocks considerably.

• If joint occur unfavorable, they may cause fall of rocks from the roof of the tunnel. This means tunneling will be unsafe and needs lining.

Tunneling


• Joints may act as sites for the development of solution cavities and solution channels in limestone terrain. This is due to the action of percolating carbon dioxide bearing water.

• The only important benefit of joint is with reference to ground water occurrence, particularly in hard rock areas.

Steps to improve the Sites with Joints

• Since joint are gaping fractures, they can be sealed by filling them up in a suitable manner. Such a filling makes the site more compact, massive and coherent. It also simultaneously reduces the porosity and permeability. All this leads to the improvement in the strength of the affected rocks. This shall make the site suitable for foundation purposes. The joint with narrow opening are filled by grouting. If the gap happen to be broader, then they are closed by filling with rich cement mortar or rich concrete. In case of tunnels to avoid ground water problems or possible leakage problems, a reasonable thick lining is given.

• It should be remembered that the need for filling up the joints should be assessed properly.

Engineering and General Geology :by Parbin Singh

Textbook of Engineering Geology :N.Chenna Kesavullu

http://geology.com/

http://geology.com/

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structural geology ii

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