structural geology and rock mechanics

1. Structural geology &
Rock mechanics
Module-3

2.  Structural geology is the study of factors such as
origin, occurrence, classification, type and effects of
various secondary structures like folds, faults, joints,
rock cleavage and are different from those primary
structures such as bedding and vesicular structure,
which develop in rocks at the time of their formation.

3. Outcrop
 When weathering and erosion expose part of a
rock layer or formation, an outcrop appears. An
outcrop is the exposed rock, so named because
the exposed rock "crops out.

4. Dip and strike
 Dip literally means slope or inclination.
 In structural geology dip is expressed both as
direction and amount.
 The dip direction is the direction along which the
inclination of the bedding plane occurs.

6. Dip and strike
 Strike refers to the direction in which a geological
structure is present.
 The strike direction may be defined as the direction
of the trace of the intersection between the bedding
plane

7.  The true dip is the maximum angle, which an
inclined bed makes with the horizontal. It is
measured at right angles to the strike.
 If the angle measured in any direction, it will have
value less than the true dip, which is known as
apparent dip.
 The inclination of bed to the horizontal in any
other direction than the direction of true dip-
apparent dip.

8. Folds
 Folds are one of the most common geological
structures found in rocks.
 When a set of horizontal layers are subjected to
compressive forces, they bend either upward or
downward.
 This bend noticed in rocks are called folds.

9.  In terms of their nature too, folds may occur as
single local bends or may occur repeatedly and
intricately folded to the tectonic history of the region.

11. Classification and Types of
Folds
Usually, folds are classified on the basis of
• Symmetrical Character
• Upward or Downward Bend
• Occurrence of Plunge
• Uniformity of Bed Thickness
• Behavior of the Fold Pattern with Depth.

12.  Anticline and Syncline
 Symmetrical and Asymmetrical Folds
 Plunging and Non-Plunging Folds
 Open and Closed Folds
 Similar and Parallel Folds
 Miscellaneous folds- Overturned Fold,
fan folds, cheveron folds, isoclinal folds,
drag folds

13. Anticline and Syncline
 When the beds are bent upwards, the resulting fold is
called anticline. This fold is convex upwards.
 Naturally, in such a fold, the older beds occur towards
the concave side, In a simple case, the limbs of
anticline slope in opposite directions with reference to
its axial plane.
 But when the anticline is refolded, the inclined
character of limbs will be complicated.

14. Anticline and Syncline
 Syncline is just opposite to anticline in its nature, i.e.
when the beds are bent downwards the resulting fold
is called syncline.
 This fold is convex downwards. In this the younger
beds occur towards the concave side and, in a
simple type of syncline, its limbs dip towards each
other with reference to the axial plane.

17. Symmetrical and Asymmetrical
Folds
 When the axial plane divides a fold into two equal
halves in such a way that one half is the mirror
image, then the fold is called as symmetrical fold.
 If the compressive forces responsible for folding are
not of the same magnitude, asymmetrical folds are
formed.

18. Open and closed folds
 Depending on the intensity of deformation, the beds of the
fold may or may not have uniform thickness.
 If the thickness of beds is uniform throughout the folds, it
is called an open fold.
 On the other hand, in a fold, if the beds are thinner in the
limb portions and thicker at crest and trough, such a fold
is called closed fold.

19. Similar and Parallel Folds
 Based on whether the shape of folds remain the same
or altered with depth, folds are grouped as similar or
parallel folds.
 In the case of similar folds, the shape or pattern of
folds remain the same at depths also.
 But in the case of parallel folds, the crest and trough
become pointed or angular.

21. Plunging and Non-Plunging Folds
 The plunge of a fold has already been described as the
inclination of the fold axis to the horizontal plane. Based on
this, i.e. whether the axis of a fold is inclined or horizontal, the
folds are grouped as plunging folds or non-plunging folds.
 In geological maps, when strike lines are drawn for both the
limbs, for a non-plunging fold, they will be mutually parallel
and for a plunging fold they will be either converging or
diverging but not parallel.

23. Overturned Fold
 Also known as recumbent fold.
 The fold in which the axial vertical plane is almost
horizontal in position is termed as overturned fold.
 In this, limbs lie vertically above the other limb.

24. Cheveron folds
 Usually the crest and troughs of beds are smoothly
curved.
 But some folds have sharply bent, angular crest and
troughs, such folds are known as “ Chevron folds”.

26. Isoclinal Folds
 Usually the folds have inclined limbs, i.e. The limbs
will be mutually diverging or converging with
reference to axial planes.
 But in some folds, the limbs will be mutually parallel
to a great extent. Such folds are called isoclinals
folds. These folds may be vertical inclined or
horizontal.

27. Fan Folds
 Usually in simple anticlines, the limbs dip away from one
another and in simple synclines they dip towards each
other.
 But in the case of fan folds,this trend is just the opposite,
i.e. in anticlines of fan folds, the limbs dip towards each
other with reference to their axial plane.
 In synclines of this kind, the limbs dip away from each
other. As the term suggests, these folds are fan shaped.

29. Domes and Basins
 Usually, a fold will have two distinct limbs. But some folds
do not have any such specific limbs and appear as beds
locally pushed up or down, i.e. their shapes appear as dome
or basin.
 In a dome, which resembles an upper hemisphere, the dips
are found in all sides from the common central top point.
 In the basin, which is like a bowl, the slopes are just
opposite

31. Causes and Effects of Folding
 Most of the important folds, as already pointed out, are
due to tectonic causes. But a few folds of a minor type are
due to non-tectonic causes,
 Mainly, the compressive and shear type of tectonic forces
are responsible for the folding phenomenon.
 Non-tectonic causes like landslides, creeping, differential
compaction, isostatic setting and glaciations too are
responsible for some folds.

32. Causes and Effects of Folding
 When a folded area is affected by weathering and erosion,
interesting topographic features are produced as follows,
immediately after folding,
 Anticlines by virtue of their upward bending appear as hills and
synclines due to downward warping appear as valley.
 During folding in the crest portions, the geological formation are
subjected to tensional forces and hence numerous fractures
appear there.
 Because of these fractures, crest portions are eroded quickly.

33. Causes and Effects of Folding
 On the other hand, trough portion are highly compressed
and hence offer a greater resistance to erosion.
 Thus, they stand out in the long run at a greater elevation,
while the adjacent parts degrade fast.
 The net result of this response to erosion is that the
anticlines will change over to hills, while synclines
change 'over to valleys.

35.  Project like a dam,tunnel,railway station should
be avoided in a highly folded site.
 Syclinal fold- may yield hard and tough quallity
stones.
 Anticlinal folds – yield weaker stones.
 Anticlinal folds – good prospects for stored
petroleum.

36. Faults
 Planes along which rocks are dislocated and displaced.
 Occurs due to the movement of strata in opposite direction.
 The displacement may be along the vertical plane, inclined plane,
horizontal plane or diagonal plane.
 This movement may vary from a few centimetres to many kilometres
depending on the nature and magnitude of stresses and resistance
offered by the rocks.
 faults considerably weaken the rocks and render the sites in which they
occur as unfavorable places for all constructional purposes.

38. Parts of a fault:
 Fault plane: surface along which the fracture
occurs in the rock body.
 Hade: angle between the inclined fault plane
and the vertical axis is termed as hade. Hade is
calculated as 90-dipping angle.
 Hanging wall or Foot wall : upper block or
block above the fault plane-hanging wall.
Block below the fault plane- foot wall.
 Throw: vertical displacement between hanging
wall and foot wall.
 Heave: vertical displacement between hanging
wall and foot wall.

39. Hanging
wall Foot wall
Parts of a fault

40. Types of faults
 Normal fault
 Thrust fault
 Transverse fault
 Vertical fault
 Strike fault
 Dip fault
 Oblique fault
 Parallel fault
 Horst fault
 Graben fault

41.  Normal fault: Hanging wall is moved down with
respect to foot wall. The effect of an normal fault on
the bed is to pull apart or extend the strata.
 Thrust fault: Hanging wall is moved up with
respect to foot wall. The effect of an thrust fault on
the bed is to shorten the rock strata as against their
extension.

42.  Transverse fault: blocks have moved against each
other in an essentially horizontal direction.
 Vertical fault: blocks have moved against each other
in an essentially vertical direction.

43.  Strike fault: in which the
fault plane is parallel to
the strike direction.
 Dip fault: in which fault
plane is normal to the
strike direction.

44.  Oblique fault: two
direction of
displacement
occurs.combination of
dip-slip and strike slip
motion.
 Parallel faults: series of
faults running more or
less parallel to one
another and hadings in
the same direction.

45.  Horst fault: one in
which wedge shaped
block has gone up
with respect to the
side blocks.
 Graben/Trench fault:
one in which wedge
shaped block has
come down with
respect to the side
blocks.

46. Impact of faults
 From the Civil engineering point of view, faults are the most unfavorable and
undesirable geological structures at the site for any given purpose
 Passage for percolation of water, and are responsible for the formation of
lakes, swamps, spring heads.
 Unfavourable for dam and reservoir sites.
 Fault movement causes earthquakes and landslides.
 Further, as long as the faults are active, the site is unstable and susceptible to
upward, downward or sideward movement along the fault plane, thereby
making the places highly hazardous for foundation purposes

47. Impact of faults
 Thus, by virtue of the harm they are capable of causing,
faults are necessarily investigated with special care in dealing
with any major construction.

48. Joints
 Cracks or fracture present in the rocks along
which there has been no displacement.
 Joints occur in all types of rocks.
 They may be vertical, inclined or horizontal.
 They are formed as a result of contraction or
consolidation of rocks.
 Commonly a large number of joints lie parallel to
one another. These parallel joints together form a
joint set.

51. Types of joints – based on origin
1. Compression joint: occurs due to compressive
force. Usually seen in core part of the folds
2. Tension joints: formed as a result of tension
forces. These joints are relatively open and
have rough and irregular surfaces.
3. Shear joints: arises due to shear stresses
involved in folding and faulting of rocks. These
joints are rather clear cut and tightly closed.
occur in two sets and intersect at a high angle
to form conjugate joint system.

52. Types of joints- based on
geometry
 Strike joint: joints parallel to the strike of rocks
 Dip joint: joints parallele to the dip of rocks.
 Oblique joints: which run in a direction that lies between
the strike and dip direction of the rock
 Bedding joints: parallel to the bedding plane
(sedimentary rocks).
 Master joints: joints running in two direction at nearly
right angles, one set-parallel to dip direction and other
parallel to strike direction. One set more strongly
developed than other andextends for long distance

53. Types of joints- based on
geometry
 Mural joints: three sets of joints mutually at right
angles which divide rock mass into more or less
cubical blocks.
 Columnar joints: divides the rock into hexagonal
columns, arranged at right angles to chief cooling
surface.

55. Engineering Consideration of
Joints:
 In quarry operation joints in rocks are helpful for
easier detachment of the rocks.
 Well-cleaved rocks with many systems of joints are
broken at much less expenses.
 Joints provide passage for the percolation of water
and help weathering and formation of soil.
 Joints control the natural ground water drainage
system in rocks and underground.
 Joints are useful in exploration of water and in
location of well sites

56. Unconfirmity
 Defined as a surface of ersoion or non deposition
of a stratified rocks which separates the
underlying older formation from the overlying new
formation.

57. Types of unconformities
 Angular unconformity
 Disconformity
 Non conformity
 Regional confirmity

58. Angular unconformity
 Charcaterised by the different inclination and
structural features above and below surface of
unconfirmity.
 Sequence below unconfirmity may be steeply
inclined and the sequence above unconfirmity
may be horizontal or gently inclined.

59. Disconfirmity
 Bed lying below and above the surface of erosion
is almost parallel, there is no angular variation in
deposity of the rocks of entire sequence.

60. Nonconfirmity
 Sequence of rocks composed of platonic igneous
rocks as underlying rocks and sedimentary or
volcanic rocks as the overlying younger or newer
rocks.

61. Regional unconfirmity
 A surface of discontinuity in
sedimentary rocks that extends throughout an
extensive region.
 Extends for hundreds of kilometers.

62. Engineering impacts
 Excavation of tunnel through a unconfirmity is
difficult because there are two types of rocks
present in the zone.
 Forms good ground water zones
 Helps to know the geological history of rocks.
 Angular unconfirmity is more dangerous for any
kind of civil enegineering structure.
 Forms a weaker zone.

63. Rock Quality Designation (RQD)
 Rock Quality Designation (RQD) is a measure of quality of
rock core taken from a borehole.
 RQD signifies the degree of jointing or fracture in a rock
mass measured in percentage, where RQD of 75% or
more shows good quality hard rock and less than 50%
show low quality weathered rocks.
 RFQ is calculated by taking a rock core sample from a
borehole and lengths of all sound rock pieces which are
minimum 100 mm long are summed up and are divided by

64. Rock quality designation RQD =((Sum of length of sound
pieces >100mm)/Total core run length)x100

66. Uses of Rock Quality Designation
(RQD) in Construction
 RQD is used to evaluate quality of rocks such as
degree and depth of weathering, zones of rock
weakness and fracturing.
 This information is used for determining the depth
of foundations, bearing capacity of rocks,
settlement and sliding possibilities of foundations.

67. Rock structure rating (RSR)
 method for describing quality of rock mass and then appropriate
ground support.
 This method is quantitative in nature for describing the quality of
rock mass.
 Following parameters are considered.
Parameter A = rock type, folding and discontinuities.
Parameter B = joint pattern (spacing) and direction of tunnel drive.
Parameter C = water inflow and joint openings.overall mass quality
due to parameters A and B obtained.
RSR = A + B + C
 Numerical values are suggested for each parameter such that
combined value can be calculated

69. Geological site characterisation
DAM:
 Foundation: rocks must be strong enough to
withstand weight of dam, resulatant thrust.
 Rocks must be impervious to overcome serious
passage of water beneath the sole of dam.
 rocks must be free from faults, joints and fissures-
no hevay leakage of water.
 No dams across fault plane.
 Granite,quartzites, limestones, fine-grained
sandstones – good foundation material

70.  No high dams are efficiently founded on loose,
unconsolidated strata like sand and loam.
 Inclined strata- foundation of dam on beds having
upstream dips.
 Folded rocks-
 Use of porous materials in the dam construction
leads to leakge and danger. Upstream side floor of
dam should be watertight.
 Weathering reduces strength, durability of rock
hence the extent of weathering sholud be carefully

71. Geological site characterisation
RESERVOIRS:
 Groudwater conditions: If water table is near ground
surface, water in the reservoir does not rise above it,
no serious leakage.
 If water table lies deep below the ground surafce,
water level in reservoir will stand above it. Leakage
will occur.
 Permeability of rocks: hihgly fissured,jointed,faulted
rocks are likely to cause serious lekage.
 Leakage of water from the strata that have
downstream dip will be more than those have
upstream dip.
 Permeable rock outcrops on valley slope- causes

72. Geological site characterisation
TUNNELS:
 Loose ground or unconsolidated rocks : shallow
depth-danger of roof collapse and sides, deeper
tunnels- less risk of collapse, lining of tunnels to be
done.
 Consolidated rocks:
a) Igneous rocks: deeper the tunnel, higher the
temperature-watering, lining not necessary.
b) Sedimentary rocks: heavy lining necessary.
c) Metamorphic rocks: depends on cohesion and
hardness of rocks.
 Driving in igneous and metamorphic rocks is easy
e.g granite and marble.

73.  Anticlinal fold: roof fall may be expected.
 Synclinal fold: ground water problem
 Faulted strata: lot of difficulties and expensive
excavation.

74.  https://www.slideshare.net/ApoorvaS5/geology-
module2

75. http://www.geographynotes.com/geology-
2/structural-geology/joints-definition-
classification-and-consideration-
geology/1375
https://theconstructor.org/geotechnical/rqd-
rock-quality-designation-calculation/20536/
https://www.slideshare.net/jampanihymavat
hi/geology-of-dams-reservoirs