Geology module3
- 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.