Structural Geology 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
• 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, road
s 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 Dams
Case-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
• 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
Case-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
Case: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 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.

• 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 Tunnels
Location 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
• 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
• 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.

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.

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.

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.

Miscellaneous
Ore 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
Marbles, 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.

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
• 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.

• 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.

• 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
• 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
• 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.

• 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.

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.

• 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/

Structural Geology II

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