Principal Investigator Co-Principal Investigator Co-Principal Investigator Prof. Talat Ahmad Vice-Chancellor Jamia Millia Islamia Delhi

Transcription

1 Subject Paper No and Title Module No and Title Module Tag Geology Hydrogeology and Engineering Geology Influence of Different Geological Structures on Civil Engineering Constructions HG & EG XI Principal Investigator Co-Principal Investigator Co-Principal Investigator Prof. Talat Ahmad Vice-Chancellor Jamia Millia Islamia Delhi Prof. Devesh K Sinha Department of Geology University of Delhi Delhi Paper Coordinator Content Writer Reviewer Prof. P.P. Chakraborty Department of Geology University of Delhi Delhi Dr. Shashank Shekhar Department of Geology University of Delhi Delhi Dr. M. Masroor Alam Department of Civil Engineering Aligarh Muslim University Aligarh Prof. Vinay Jhingran Department of Geology University of Delhi Delhi

2 Table of Content 1. Learning outcomes 2. Introduction 3. Objectives of Structural Geology and its Application in Civil Engineering 4. Factors Controlling the Deformation of Rocks 4.1 Lithology 4.2 Lithostatic Pressure 4.3 Pore Fluid Pressure 4.4 Temperature 4.5 Strain Rate and Time 5. The Basic Structures 6. Contacts 7. Joint 7.1 Genetic Classification Tectonic Joints Non-Tectonic Joints 7.2 Geometric Classification 7.3 Joint Parameters and their influence on Rock Mass Properties Number of Joints per unit area/volume Joint Spacings Orientation or Attitude Block Size and Shape Length and Depth Persistence Aperture or Openness Asperities or Roughness Joint Filling Material Presence of Water

3 8. Shear Zone 9. Fault 9.1 Genetic and Geometric Classification 9.2 Identification and Effect of Faulting Fault Plane Criteria Topographic Criteria Geological Criteria Types of Faults and its relation to Major Stress Directions 9.3 Influence of Faults on Major Geo-Engineering Projects 10. Fold 10.1 Genetic and Geometric Classification 10.2 Plunging Fold 10.3 Effect of folding 10.4 Fold and its relation to Major Stress Directions 10.5 Influence of Folds on major Geo-Engineering Projects 11. Unconformity 11.1 Influence of Unconformity on Major Geo-Engineering Projects 12. Some other structures 12.1 Diapirs 12.2 Nappe 12.3 Klippe 12.4 Window 12.5 Outlier and Inlier 13. Summary

4 1. Learning outcomes After studying this module, you shall be able to: Know different structures in rocks important in context of civil engineering. Learn about different aspects of joints and its importance in rock mass characterization. Understand different aspects of shear zones and rock mass strength. Identify effect of folding and faulting on ground conditions. Relate deformation structures with residual stresses. 2. Introduction All the rocks, on or near the earth surface are deformed to some level or degree, a testimony of ever presence of forces in and around the rock masses. The movement of plates can be cited as one of the manifestation of extremely high magnitude of these forces termed as tectonic forces. Tectonics a word from Greek word tektos, meaning builder and the word structure is from Latin word struere, meaning to build goes hand in hand and are responsible for earths geological architecture or in other words geological structures. The word deformation refers to the changes that take place in the original location, shape and volume of a body in response to some force. A rock body too, no matter how hard, provided right conditions would undergo deformation. The features forming due to negotiation and accommodation of forces by rocks are called as structure. Structural Geology, which deals with the identification, classification and genesis of these geoarchitecture, plays an important role in deciding site, size and types of different civil structures. As for as civil engineering is concerned, the deformation of rocks results into different kinds and magnitude of heterogeneities. A very common result of deformation is generation of discontinuity surfaces other than the primary ones resulting into separation of rock mass into smaller units or blocks. Pulverization of

5 rocks to various degrees along linear zones are common effects of rock deformation. Large-scale deformation also results into disruption, dislocation, repetition and omission of rocks. In a nutshell, deformation of rocks bring in chaos and unpredictability, which should be identified, classified to get some order by engineering geologist. So that the problem can be rectified and minimized by civil engineer while designing for any mega construction project. 3. Objectives of Structural Geology and its Application in Civil Engineering Structural geology primarily deals with solid materials, present in nature in form of minerals and rocks. Solid mechanics are an integral part of this discipline dealing with dynamics and kinematics of deformation forces and resulting deformation structures. The structural geology is concerned with three major objectives: (1) what type of the structure (deformation)? (2) When did it develop (time)? (3) Under what physical conditions did it formed (forces, temperature)? To answer these questions geological field work becomes an essential and indispensable tool for structural geology involving mapping of rocks exposed as outcrops, relationship amongst the rocks present, identification and measurements of structures in rocks, present either as primary (genetic) structures or as secondary (deformation) structures for analyzing it to work out its genesis i.e. stress strain analysis. Exposed outcrops, open pits, mines, road, rail, river cuttings and excavations for civil engineering works are important locales for observing abovementioned features. The structures or strain features developed in response to stresses come in many numbers, varied sizes and shapes. Some of them may represent latest stressing event some past events covering a large geological time span. Here lies the second objective of structural geologists that is to resolve the chronological order in which these structures came into being thereby judging the number and magnitude of the forces the rocks were subjected to, from time to time and are being stressed even presently. To resolve the third objective knowledge of intrinsic properties of material undergoing deformation, residual stresses locked in

6 between mineral grains, involvement and role of already existing structures are to be taken into consideration. As we know that rocks are being used in civil engineering as a construction, material and as founding ground. In case of rocks used as construction material all the concept of solid mechanics can be applied and only structures at the scale of mineral or grain size (tectonites) will come into play. Pulverization of rocks to various degrees along linear zones are common effects of rock deformation. Largescale deformation also results into disruption, dislocation, repetition and omission of rocks. The rock strength, elasticity, Poison s ratio, rigidity and deformability etc. are of prime concern as far as their common uses are concerned and have been dealt in module 2, to the extent needed at this level. In this module, the emphasis will primarily be on rocks being used as founding ground or as site rocks. The plains or surfaces of discontinuity are of utmost importance hence need to be scrutinized in context of built structure to come up. Here the contacts apart from deformation structures such as joints, shear zones, faults, folds and unconformity at a scale of outcrop to the base map (1:50,000), are considered, as they have profound control on the stability civil engineering structures. There are three stages of deformation, initial elastic, intermediate plastic and final brittle. In the elastic stage, theoretically, the strain vanishes as and when stresses are withdrawn. When deformation goes beyond elastic limit, the body does not return to its original shape or size then it is called as plastic deformation. The plastic deformation is a permanent deformation. When strain is such that fractures develop then it is brittle deformation, it is also a permanent deformation (Fig. 1).

7 Fig. 1 Stress-strain diagram showing different kinds of deformation. 4. Factors Controlling the Deformation of Rocks Igneous and sedimentary rocks undergo deformation after they form by processes of solidification and lithification respectively, while metamorphic rocks can undergo deformation during and also after their formation. The mechanical behavior of rocks to stresses are controlled by its internal properties such as mineral composition, texture, primary structures as well as by some external factors such as lithological association, lithostatic pressure, pore fluid pressure, temperature, strain rate and time. Hence, similar rocks in different external conditions may give rise to different deformation structures.

8 4.1 Lithology: All other things being equal strong rocks undergo readily elastic and brittle deformation as compared to weak rocks, which undergo elastic and plastic deformation before brittle deformation. Sometime weak, incompetent rocks in intimate association with strong competent rocks deform differently to a force condition giving rise to different structures. In general, igneous rocks are very strong and sedimentary rocks are weak while metamorphic rocks will come in between. Some metamorphic rocks such as granulite and quartzite are as strong as any igneous rock and schist may be weaker than many sedimentary rocks (Table 1). Table 1: Ranking of some common rocks based on Strength tests in laboratory. Strong Moderate Weak Weakest Basalt Dolerite Granite Gabbro Quartzite Granulite Hornfels Gneiss Siliceous sandstone Limestone Slate Laterite Calcareous sandstone Marble Phyllite Slate Schist Tuff Serpetinite Marl Shale Mudstone Chalk Evaporites 4.2 Lithostatic Pressure: For any given rock, its strength parameters will have higher and higher values with increasing lithostatic pressure. This has been documented in lab tests that with increasing confining pressure yield strength, rupture strength, ductility etc. increases. Hence, other parameters remaining same the rocks at near earth surface conditions will undergo brittle deformation while the rocks at greater depth in sub surface conditions will tend to deform plastically due to high lithostatic pressure. 4.3 Pore Fluid Pressure: The rocks with intergranular and fracture porosity may have water, gases, oil as fluids. The presence of these fluids causes pressure, which works against lithostatic pressure and may result into lowering the strength parameters of the rock. The difference in lithostatic

9 pressure and pore fluid pressure is called as effective stress. If this difference is high, the rocks will have higher strength and ductility while lesser difference will make rock weak with low ductility. 4.4 Temperature: Increased temperature of rocks generally lowers yield and ultimate strength and helps in increasing the ductility. The rocks near or closer to the earth surface will more likely undergo brittle deformation while rocks deeper and deeper in subsurface conditions will be subjected to ductile deformation due to high-temperature regime caused by geothermal gradient (25 0 C/km). Most of the metamorphic reactions and changes are brought in by an increase in temperature. As such, igneous rocks are resilient to increase in temperature to some extent as compared to sedimentary rocks. Many of the deformation features from microscopic to macroscopic levels depicting ductile deformation, found in metamorphic rocks are important examples to show that sufficient heating can deform rocks. 4.5 Strain Rate and Time: The rate of application of stress is an important factor in deciding the nature of deformation. A rock may deform in brittle fashion if the rate of loading is fast but in the case of the slow rate of loading, deformation may tend to be ductile. Creeping glaciers, slopes, salt and clay diapirism etc. show rheid behaviour, ascribed to the development of fatigue in response to a long and sustained presence of stresses. These deformations can be cited as examples of deformations effected in a very large time span or very slow strain rates. As we have seen that the importance of time in strain rate. For geologist there is no dearth of time, he has 4500 million of years to his command. Geological processes have a great length of time to operate and inflict change. The dead slow epeirogenic processes built continents in 1000 million years with many inbuilt variations. Similarly, the fast orogenic movements built mountains like the Himalayas within 25 million years with equal variations.

10 5. The Basic Structures There are three kinds of basic structures: contacts, primary structures and secondary structures. Contacts are the surface along which two different rocks are juxtaposed for example normal depositional contacts, intrusive contacts, erosional contacts (unconformity) etc. Primary structures are the features develop during the formation of rock itself such as bedding planes in sedimentary, foliation in metamorphic and flow bands in volcanic igneous rocks. Secondary structures also called as deformation structures are incorporated in all kinds of rocks in response to stresses as strain features such as: Brittle Deformation: Ductile Deformation: Ductile - Brittle Deformation: Joints, Faults Folds Shear Zones, Rock Cleavage, Foliation 6. Contacts The presence of contacts will offer first level of discontinuity in rock mass may not always be discrete and clean, but will have different rock character on its two sides or for that matter altogether different rock (Fig. 2). The contact being depositional, intrusive and erosional may have different length and geometry. If marked change is observed, then they should be taken into consideration and accordingly design parameters may be changed. Some times more prevalent structures such as joints and shear zones may mask them. Their occurrence in relation to proposed structure may be treated as the case may be. Fig. 2 Contact between Limestone (below hammer) and sandstone, Lalitpur. Joint

11 7. Joints Joint is a misnomer and actually is a fracture. Defined as discrete fractures along which there has been almost no or imperceptible movement. The rocks host innumerable such discrete fractures and is known to be the most common deformation structure or joints are ubiquitous. The joints impart discontinuity in rock mass or in other words a rock mass is separated into different shapes and sizes of rock blocks along these joints and this property is very important from the geoengineering point of view. Most joints are planar but curvilinear surface are not uncommon. The length persistence of joints can be measured as less than a meter to tens of kilometers while depth persistence may vary from less than a centimeter to thousands of meters. The spacing between them can be from a centimeter to tens of meters. The joints of regional dimensions (1-10 km) are called as master joints. Most of the joints show running lengths smaller than a kilometer. Joints, which are parallel to each other and show a regular pattern of distribution are said to form joint sets and are called as systematic joints. The haphazardly oriented joints are called as random or nonsystematic joints (Fig 3). Some very small sized random joints are present in between the systematic joint sets. The joints can be classified on the basis of their origin and geometrical distribution. Fig. 3 Regularly disposed systematic joints, short, discontinuous and randomly oriented non-systematic joints. Also see inclined, vertical and horizontal joint sets (Bundelkhand Granite Lalitpur, UP).

12 7.1 Genetic Classification: Joints may be classified on the basis of their origin. The ultimate cause of large scale jointing in rocks are tectonic stresses. Residual stresses, contraction, desiccation etc. may also cause development of joints though of smaller sizes Tectonic Joints: Due to regional or large magnitude stresses- Compressive stresses- Diagonally criss-crossing, tight and closed joints with rough surface showing plumose markings (Fig. 4a). Fig. 4 (a) Bird s feather like plumose marking, seen on compressive joint surface, (b) Shearing joints in limestone showing en-echelon pattern. Also, see small s shaped shear cracks filled with filled with secondary calcite (white). Tensile stresses- Open joints, sharp edged and smooth surfaced mostly found as three mutually perpendicular fractures, especially in rocks with deep-seated origin (Fig. 4b). Shearing stresses- Form by ever so slight sliding parallel to joint surface, partly open, discontinuous, sometimes in en-echelon fashion with rough surface Non-Tectonic Joints: Due to local or small magnitude stresses- Columnar joints- Form mostly in volcanic rocks due to cooling and contraction of lava during solidification (Fig. 5a).

13 Fig. 5 (a) Geometry of columnar joints, (b) Ground surface parallel, sheet joints. Sheet joints- Ground surface parallel joints, found almost in all rocks especially of plutonic origin or those, which have undergone deep burial. When exposed to the earth surface, the rocks undergo de-stressing due to removal of overburden material causing development of fractures parallel to the ground surface (Fig. 5b). The joints are more in numbers and are closely spaced near earth surface and their numbers decrease and spacing increases with depth. 7.2 Geometric Classification: Geometric classification is a descriptive one, simple and is easy to apply specially from geo-engineering point of view. It uses attitude of the fractures to identify and differentiate one joint from the other. The attitude of any surface or plane can be defined by the strike and dip. The joints are found in large numbers with varying orientations. Hence, for their meaningful interpretation joints can be classified by taking into account the strike and dip of joints as well as some recognizable rock features, such as bedding or foliation especially in layered rocks. In figure 6a a block of rocks showing few rock beds and joints which can be recognized and classified geometrically as:

14 Fig. 6 (a) Block diagram showing joints, classified with reference to strike and dip of the rock bed. (b) Block diagram showing different sets of joints. Strike Joints- Joints parallel to the strike direction of the rocks, STUV, S T U Dip Joints- Joints parallel to the dip direction of the rocks MNO, PQR Bedding Joints- Joints parallel to the strike as well as dip direction of the rocks, JKL Diagonal Joints- Joints neither parallel to the strike, nor to the dip of the rocks, WXY, W X Y Only the bedding joints will have unique attitude. Strike, dip, or diagonal joints may be many, if parallel to each other will form set for example in figure 6b, joints with notation I, II, III will form one set, joints with notation, ab, cde, fgh, ijk will form second set, horizontal joints with notations, opq, o p q will form third set and two vertical joints with notations RST, R S T and UVW, U V W will form fourth and fifth set respectively. In non-layered rocks such as non-foliated metamorphic and igneous rocks, the dip of joints is use for the classification (Fig. 7). Horizontal Joints a-a Dip of the joints less than 0 0 to 5 0 Gently Inclined Joints b-b Dip of the joints from 6 0 to 15 0 Moderately Inclined Joints c-c Dip of the joints from 26 0 to 45 0 Inclined Joints d-d Dip of the joints from 46 0 to 75 0 Steeply Inclined Joints e-e Dip of the joints from 66 0 to 85 0 Vertical Joints f-f Dip of the joints from 86 0 to 90 0

15 Fig. 7 Block diagram showing different joint sets in non-layered rocks. Another important classification of joints is related to the fold geometry, which we will learn later on in this chapter. 7.3 Joint Parameters and their Influence on Rock Mass Properties Joints are ubiquitous and can be readily observed and identified in the rock outcrops. Knowledge of spatial distribution of joints is very important in engineering geology, geo-engineering and rock mechanics. Even in quarrying operations and ground water explorations joints play a pivotal role. In almost all, the engineering classifications of rock masses joint parameters play the key role. As joints are numerous, varied in orientation and with differing sizes therefore, it is important to have thorough knowledge of joints and related parameters to be observed and measured, as they are the single most important

16 parameter controlling geotechnical behavior of rock mass. These joints need to be plotted on the plan of civil engineering structure for example dam foundation site. Due to their small dimensions, plotting joints on a map is bit difficult. As most of joints are smaller than 1000 m in length with less than 1 mm to 10 mm of opening, it is impossible to show them on maps of above mentioned scales or even on Survey of India topographic maps with scales 1: 50,000 (1 mm = 50 m or 1cm = 500 m). Most of the maps and plans for execution of civil engineering works are made on the scales of 1: 1,000 to 1: 5,000 (1mm = 1m to 1mm = 5m). To understand the importance of maps showing joints, let s take example of a dam-reservoir setup. Maps of 1: 50,000 may be chosen for analyzing very large catchment area for its geomorphic and hydrologic setup, the scale for reservoir site comparatively smaller in area but important for water storage, the scale of the order of 1: 5,000 can be taken, but for the most important dam site area or the foundation of site the scale will be of the order of 1: 1000 or of still smaller scale of 1: 500 (1cm = 5 m). On the above mentioned scales, the map of catchment area will not be able to show joints, unless specifically made for, the map of reservoir site will show almost all major joints, while the dam site map will show almost all the systematic and non-systematic joints which are important for rock mass evaluation. There are different methods of showing joints on the map for example simply by plotting their length and orientation on a map of suitable scale or by making rose diagrams by measuring and incorporating all the joints and showing their strength (Fig. 8a). The observation and measurements of joints can be easily done on naturally exposed rocks and on vertical cuts along roads, rail tracks, stream sections in hilly areas or in deep excavations. The most important joint parameters which have significance in civil engineering projects involving rock mass are: (1) Number of joints per unit area (2) Length and depth persistence, (3) Orientation or attitude, (4) Aperture or openness, (5) Joint spacing, (6) Asperities or roughness, (7) Joint filling material (8) Presence of water etc. (Fig. 8b).

17 Fig. 8 (a) Rose diagram showing three joint sets with direction and strength (each circle represent 20%). The most prominent one is with direction N /S with strength 60%; (b) Block diagram showing different joint parameters used for rock mass characterization Number of joints per unit area/volume: Total number of systematic and non-systematic joints per unit of area or volume is an important factor in deciding the level of discontinuity in a rock mass. These joints may form regular sets with different orientations. The small randomly oriented joints connecting these major joint sets are also important Joint spacings: The distance between joints varies from less than a centimeter to more than 5m. Within one rock exposure, the joint spacing may show random distribution, similarly some rock mass show very regular spacing. The number of joints per unit volume as well as joint spacing will decide the size of the blocks in rock mass Orientation or Attitude: The strike, amount and direction of dip not only help in identifying the individual joints and joint sets but also control the shape of the rock blocks by virtue of their intersection. In geoengineering, the joint attitude decides the most favorable and unfavorable joints with respect to slope, dam foundation and tunnel alignment.

18 7.3.4 Block Size and Shape: Depending upon the spacing between the joints and their orientation, rock mass is divided into different sizes and shapes of blocks. The smaller is the block weaker will be the rock mass while regularly shaped blocks will have higher tendency of slippage towards an opening rather than the irregular and randomly shaped blocks Length and Depth Persistence: The length of joints which may be discontinuous or continuous may vary from < 1 m to >1 km, on the surface as well as in sub surface. Normally with increasing depth, the joints are reduced in length and numbers Aperture or Openness: Depending upon the origin, present state of stresses in rocks, weathering and exposure of rocks the joints may have varying degree of openness. Some joints are seen as hair cracks; few may have opening ranging from 1 mm to 5mm and sometimes can go up to opening more than a meter, termed as fissures. Joints owe their origin to tensile stresses have maximum aperture followed by joints originated under shearing stresses. Joints formed under compressive stresses are comparatively tighter. The openness of joints is more in sedimentary rocks especially limestone, which may have aperture, more than one meter due to weathering and erosion caused by moving/flowing water. The openness of joints is generally more on the surface and diminishes with depth Asperities or Roughness: It is related to the irregularity or roughness of the joint surface. The joint surface may be planar, wavy and stepped. Within these three types, the surface may be polished or slicken-sided, uneven or irregular. The surface of joints form under tensile stresses have comparatively even surface with feather like plumose markings, while joint surfaces developed under shear and compressive stresses are irregular to highly irregular. The joint roughness is a very important factor for the strength of rock mass. For example on rocky slopes,

19 inclined joints along which rocks may slide, roughness provides resistance to such movements Joint Filling Material: The open joints are amenable to filling by sediments brought in by wind and water. The filling sediments may wary from pure gravel, granule, sand, silt and clays to mineral such as calcite, hematite and quartz precipitated out of flowing water and other solutions. The precipitated minerals may completely fill or heal the opening and provide strength. The wet clays as filling material pose problem, as they act as lubricant and decrease the frictional resistance along joints. If clay or gouge is continuous and has more than 5 mm thickness in a fracture, it will make the surface roughness redundant Presence of water: The joints are the avenues for natural ground water in rocks. The rainwater seeps in, stored and moves through interconnecting joints forming secondary porosity and permeability. However, the presence of ground water in joints has negative affect on the strength of rock mass due to its weathering effect and due to fracture water pressure. 8. Shear Zone Next to contacts and joints, shear zone is the most prevalent deformation structure. It is also most unpredictable and is most problematic in the field of geo-engineering. A shear zone represents partly brittle to partly ductile deformation in form of tabular planar to curviplanar zone of highly strained rock within a largely non deformed rock block. It is a common deformation feature after joints varying from microscopic size (Fig. 3.9a) to outcrop size (Fig. 3.9b) or as large as tens of kilometers with large length and depth persistence as compared to its thickness.

20 Fig. 9 (a) Microscopic outcrop sized shearing in micaceous schist, (b) An outcrop size folded shear zone in phyllite. The fault and shear zone both accommodate offset but the former one shows discrete displacement while later one distribute the total offset along its thickness (Fig. 10a & b). Fig. 10 (a) A fault showing discrete displacement of a bed, (b) A shear zone showing accommodation of displacement. The effectiveness of deformation can be gauged from the fact that the rocks get pulverized to form cataclastic rocks as breccia, mylonite and as gouge. Even a granite or basalt may get sheared to schist and phyllite like rocks along the shear zone.

21 Fig. 11 Different types of shear zones. (a) Horizontal H, Vertical V, Inclined; (b) Diverging D, Converging C, Parallel; (c) Anatomizing; (d) Conjugate; (e) Folded; (f) Faulted. The overall effect is weakening of rock mass along horizontal, inclined, vertical, parallel, diverging, converging, anatomizing, conjugate, folded or faulted shear zones (Fig. 11 a, b, c, d, e & f). The most problematic material found along the shear zones are gouge, whose thickness and continuity has profound effect on the behavior of rock mass as they are the locales of least fraction along the rock wall surfaces.

22 9. Fault Fracture along which at least some perceptible movement or offset has taken place is called as fault. The fault may be a single discrete fracture or it may have multiple fractures forming fault zones. The movement and displacement along a fault surface may vary from few centimeters to 100s of kilometers. The faults are not as common as joints but are very important deformation structure and their importance can be gauged by the fact that all most 90% of the earthquakes are generated due to faulting or renewed movement along already existing faults. The faults can be seen in a hand specimen, in an outcrop, in geological maps aerial photographs and imageries depending up on its size. The basic cause of faulting is the brittle deformation of the earth crust, which is subjected to the tectonic loading, which not only breaks the rocks, but also bring them into best fit after some displacement. The movement long the faults are mostly translational but rotational movements are not uncommon. Faulting brings in lot of changes starting from movement of rocks upward and downward and displacement of rocks laterally depending upon the nature of fault. The orientation of the fault plane can be identified by its running direction (strike) and by the direction and amount of its inclination (dip) if measured from the horizontal surface. Hade is the inclination measured from the vertical plane. Identify in figure 12, other elements of fault such as vertical displacement, termed as throw and horizontal displacement, termed as heave, of the two previously adjacent points before faulting. These parameters are measured in vertical sections perpendicular to strike of the fault plain. Fig. 12 Different elements of fault, see dip, hade, throw, heave, slip etc.

23 9.1 Genetic and Geometric Classification: The faults are classified as per their origin and geometry. The faults originate due to the natural application of stresses on rock masses. Different kind of stresses will produce following types of faults: Tensile Stresses- Gravity Fault- The movement is along the direction of gravity Shearing Stresses- Tear Fault- The movement is lateral and along the earth surface Compressive Stresses- Thrust Fault- The movement is against the direction of gravity Geometrically faults are classified as Normal Fault, Reverse Fault and Translational Fault, depending upon relative movement of foot and hanging blocks along the fault plane. A low angle (< 300) reverse fault is called as Thrust fault. The relative movement along the dip and strike of the fault plane faults are also classified as Dip Slip Faults (normal and reverse), Strike Slip Faults (right-handed lateral or Dextral Fault and left-handed Sinistral Fault) and Oblique Slip Faults (normal and reverse). See, in the figure 13, displacement along the strike as ps, termed as strike slip, pd along the dip, termed as dip slip and pn as oblique or net slip. Fig. 13 Normal oblique slip Dextral and Sinistral Fault. See the displacement along strike (ps), along dip (pd) and oblique or net displacement (pn).

24 The other kinds of faults are based on their mode of occurrence seen on map, aerial photographs or satellite imageries and regional stresses can be deduced, an important aspect of geo engineering. For example normal parallel faults are common in the regions of tensile (divergent plate boundary) stress regions while in compressive stress regions (convergent plate boundary), reverse parallel faults are common. En-echelon faults are common in the regions of shearing stresses (translational plate boundary). 9.2 Identification and Effect of Faulting: Field identification of faults is important for mega geo-engineering projects. One has to identify if a fault is live, dormant or dead by geological and geophysical methods. Most of the major faults are already identified and mapped. It is the small scale and local faults encountered during excavations need to be identified and mended if need be. There are different methods to identify faults but only few can be applied in any particular case. The selection of identification method depends on the size of the faulting and area of observation. Out crop, small scale faults can be identified in natural exposures, road and stream cuttings, mines, tunnel etc. applying fault plane criteria. However, large-scale faults can only be visualized and identified after applying topographic and stratigraphic criteria Fault Plane Criteria: The fault planes themselves have some features, which become conclusive proof of faulting. Presence of Striations, or longitudinal scratches few millimeters deep or few centimeters deep Grooves and Casts, on the fault plane indicative of movement of one rock block over the other. The movement along fault plane also results into development of polished, striated surface with transverse sharp steps and some precipitation of silicified material, together called as slickensides (Fig. 14a).

25 Fig. 14 Features common along fault and fault zone. (a) Striations or slickensides on a rock surface over which another rock block has moved. (b) Fault Breccia found along fault zone with coarse quartzite clasts embedded in iron oxide cement/matrix. The breaking of rocks caught along the fault plane or fault zone is very common. The rocks may get pulverized into powder called as gouge, sand sized foliated coherent material called as mylonites or into gravel sized angular material called as breccia (Fig. 14b), similar to what we find along shear zones but here in much wider, lengthier and thicker Topographic Criteria: Faulting leaves some distinct imprint on the topography. The offsetting of ridges and valleys across the fault line, which can be viewed in topographic or aerial photographs, are common. The sharp linear bends of rivers and linearly distributed natural springs and lakes are also probable sites representing fault lines. Topographic features, which are typical of faulted areas, include Scarp hills or linear array of such hills forming Fault Line Scarp along which ridges with triangular facets are common. Regional scale faults can develop hills and valleys by combination of up and down thrown fault blocks known as horst and graben topography Geological Criteria: The most important effect of faulting is displacement or disruption of rocks, which can be seen only if displacement is more than the thickness of the individual rock beds.

26 The apparent displacement of the rocks may be very different from the net slip. To work out geologically correct displacement of rocks different variables such as strike and dip of the fault plane, strike and dip of the disrupted rocks, orientation and slope of the surface on which observations are carried out and the erosion level of rocks should be properly known. Depending upon the magnitude of the displacement, the younger rocks of a sequence may come in contact with the older rocks of the same sequence across the fault plane. Similarly, a geologically younger sequence may get juxtaposed to older or much older sequence of rocks. When a large area is affected by, the large scale faulting then it can be only observed through geological mapping of the area. It has been normally found that due to faulting there happens to be repetition of all the rocks with omission of one or few rocks Types of Faults and its Relation to Major Stress Directions: Normal, reverse and strike slip faults can form respectively in the regions of tensile, compressive and shearing stresses. If the stresses are resolved into three mutually perpendicular principal stress directions as greatest principal axis (σ1), intermediate principal axis (σ2) and least principal axis (σ3) then the faults can be used to ascertain the orientation of these principle axes. In the case of normal faults the greatest principal axis will be vertical, intermediate principal axis will lie along the fault plane and will be horizontal, while the least principal axis will be horizontal and perpendicular to both the axes (Fig. 15a). In reverse faults the least principal axis will be vertical, intermediate principal axis will lie along the fault plane and will be horizontal, while the greatest principal axis will be horizontal and perpendicular to both the axes (Fig. 15b). For the strike slip faults the intermediate principal axis will be vertical and will lie along the fault plane, the greatest principal axis will lie along the fault plane and will be horizontal,

27 while the least principal axis will be horizontal and perpendicular to both the axes (Fig. 15c). Fig. 15 Regional scale stresses and different type of faults. (a) Normal fault; (b) Reverse Fault; (c) Strike Slip Fault. σ1- Maximum Principal Stress, σ2- Intermediate Principal Stress, σ3- Minimum Principal Stress. 9.3 Influence of Faults on Major Geo-Engineering Projects: Faults with known history of activity should be avoided on or near the major geoengineering projects. Along the fault plane, it is common to have pulverized rocks sometimes with lot of water making rock mass amenable to failure. The location of faults are also a common site of landslides, hence should be taken into consideration for any road or rail project and hill area development. In case of dam and reservoir site, it is not advisable to have dam foundation or abutment on or near a fault plane. If there is a known active fault at a probable site, it should be in downstream of the dam. Because reservoir area use to cover a lot of ground and in geo-tectonically active area such as Himalayan Mountain System, it is not always possible to avoid all the faults. In such cases, faults should be excavated to reasonable depths and back filled by concrete to avoid seepage through fault planes. In case of tunnels, the alignment chosen should be such that if any fault comes in its way it should be negotiated at 90 0, to keep its effect minimum. The strengthening along the fault zone by hacking and back filling by shotcrete and fibercrete is a must.

28 10. Folds Folds are up and down warps form in rocks due to their ductile deformation, especially under compressive forces. The size of fold can be measured with the help of wavelength of the fold i.e. crest-to-crest or trough to trough distance and its amplitude. Folds may have wavelengths from vary from few millimeters to be seen in hand specimens to thousands of meters, can be seen on geological maps, aerial photographs and satellite imageries. Folds of outcrop size can be observed directly and are best seen in layered rocks. Folding in metamorphic rocks are very common and pervasive, due to involvement of high temperature and pressure leading to ductile deformation wherein even minerals can be seen forming microscopic sized folds Genetic and Geometric Classification of Folds: Folds normally form under compressive stresses at high temperature and high confining pressures resulting into ductile deformation of rocks. Such conditions are normally found deep inside the earth. That is why most of the dynamo thermal (regional) metamorphic rocks are folded to some degree. The areas of huge salt deposits or clay deposits experience diapirism i.e. upward movement of low-density salts and mud through high-density sand and/or carbonate deposits. While moving up the salts and mud pierce through overlying sedimentary rocks and molding them into fold like features called as diapiric folds. There are large numbers of folds, which can be identified based on their geometry, starting from Symmetrical, Asymmetrical, Overturned, Isoclinal and Recumbent fold (Fig. 16). Fig. 16 Cross sectional view of types of folds and its relation to formation of thrust and nappe.

29 The folds can also be classified on the basis of inter limb angle (Fig. 17). This classification is important because the wavelength and amplitude of the fold will come into play, which will have direct implication on its recurrence with reference to width and length of the civil engineering structure. The open folds signify lesser compressive forces as compared to tight folds during their formation. Broad fold to Open fold to Closed fold to 90 0 Tight fold 90 0 to 60 0 Very tight fold 60 0 to 30 0 Extremely tight fold < 30 0 Fig. 17 Type of fold based on inter limb angle.

30 10.2 Plunging Fold: Most of the regional or large-scale folds are plunging folds. The term plunging is used for inclination of a line, similar to the term dip, used for the inclination of a plane. In plunging folds, the fold axis is always inclined and strike lines drawn on the fold limbs will converge in one and diverge in other direction. In the case of non-plunging folds, the strike lines drawn on the fold limbs will be parallel to each other and to the fold axis and will remain horizontal. The map pattern of plunging and doubly plunging folds are shown in figure 18, 19 and 20 (After Billings, 2001). Fig. 18 (a) Plunging fold with about 100 plunge towards left. See two plunging anticlines with one plunging syncline in its mid. Non-plunging fold with parallel limbs/strike lines. Fig. 19 Map pattern of plunging fold.

31 Fig. 20 Map pattern of doubly plunging fold Effect of Folding: Folds can easily be observed in rail, road and stream cuts if they are of sizes, which can be glanced by human eyes. Aerial photographs and satellite imageries are also helpful in identification of folds especially the plunging and doubly plunging folds due to their typical topographic impressions. Most of the topographic edifices are made by folding of rocks. In fact small scale folds seen on outcrops are features actually associated with much larger fold systems. When a folded rock sequence gets exposed through uplift and erosion, the anticlinal folds form hills and synclinal folds form valleys. However, with time as more of the anticlinal zone gets exposed it experiences tensile stresses resulting into development of hundreds of closely spaced fractures called as rock cleavages well as numerous tensile joints in anticlinal zone. Contrary to this, the synclinal zone experiences compressive stresses being confined by rocks all around thereby has fewer rock cleavages and joints. The erosive agencies such as rivers will find it easy to erode easily along the anticlinal zones as compared to synclinal zones. This results into differential erosion of rocks i.e. more of rocks around anticlines and less around synclines resulting into pene-planation and then formation of valleys at anticlinal zones and hill at synclinal zones (Fig. 21 a, b, c & d). It is important for the engineering geologist to know the evolution of topographic features, such as of a river valley over which a bridge is to be made or the hill through which a tunnel is to be carved out because rock mass behavior and residual stresses will be guided by the intrinsic deformation structure.

32 Fig. 21 Different stages involve in the reversal of topography. (a) Folds as underground; (b) Exposure of rocks due to erosion and isostatic upheaval. See anticlines forming hills and syncline forms a valley; (c) High rate of erosion along the anticlinal zone due to presence of myriads of fractures makes the ground leveled; (d) Still rising anticlines will be subjected to more erosion as compared to synclinal zone resulting into development of valleys along anticline and high grounds along synclines. R- River This phenomenon is called as Paradox of Folding or Reversal of Topography. Most of the hills in today s world of folded regions are actually synclines i.e. antiformal synclines, while most of the valleys are actually anticlines i.e. synformal anticlines. For example, hills of geologically very old Aravalli and Vindhyan ranges comprising metamorphic and sedimentary rocks are synclinal in nature. Even many high peaks of Himalayas, geologically youngest mountain system too are synclinal in nature. Folds bigger than outcrops can be discerned by some direct and indirect methods. Identification of present day exposed folded sequences, left after an age of weathering and erosion are based on direct observation of changing

33 dip direction while making traverse perpendicular to strike line of the rocks. The dip direction will change as one crosses the anticlinal or synclinal axis. The repetition of rocks is another effect of folding which can be observed directly by traversing across the strike direction and from proper geological maps. Identification of youngest and oldest rocks for locating synclinal and anticlinal fold axes is another way out provided stratigraphy of the area is known. There are some other methods based on drilling, geophysical methods and stereo net plotting (beta and pie diagrams) to identify and classify folds, but are beyond the scope of this book and course Folds and its Relation to Major Stress Directions: Folds are the result of compressive stresses. The maximum stresses are perpendicular to the axis or axial plane of the fold. In response to compressive stresses there use to be development of feeble tensile stresses along the axis of the fold (Fig. 22). The presence of these stresses is reflected when folded rocks undergo brittle deformation and develop joints. Fig. 22 The folds are manifestation of compressive deformation hence σ1 (maximum) is perpendicular to the axial plane, σ2 (intermediate) is horizontal and runs along axial plane while σ1 is vertical perpendicular to the σ3 (minimum) in normal symmetrical fold.

34 10.5 Influence of Folds on Major Geo-Engineering Projects: Folding of rocks can bring in lot of issues in geo-engineering depending upon the wavelength of the folding and orientation of fold axis with respect to the length and the width civil engineering structure and its alignment. If one forgets about the hand specimen scale folding where it is a matter of anisotropy or isotropy of rock material fabric, the outcrop scale folds can pose different kinds of problems. Folds result into contortion of rocks from microscopic to hand specimen to outcrop to regional scales. The level of contortion will control the distribution of stresses exerted on rocks during loading and unloading. The most important issue is of repetition of rocks. If some weak rocks are present in the stratigraphic column of that area then that rock will keep coming after some distance depending upon the wavelength of the folds and dimensions of structure involved. As we know that the anticlinal zones of a fold are under tensile stresses, hence have more fractures or rock cleavages as compared to synclinal zones, which are under compressive forces and are completely less fractured. If a tunnel is derived parallel to fold axis then it will be easy to drive a tunnel along the anticlinal, rather than along synclinal zone, the chances of providing support may be more in first case rather than in second case at the same time the problem of ground water seepage will be less in first case as compared to the second one. It is suggested that tunnel should be aligned in such a way that it goes through the limb lying in between the anticlinal and synclinal axis. However, if the tunnel is being made perpendicular to the fold axis the problems encountered in anticlinal and synclinal zone will keep coming after some distance. Similarly in case of dam, when dam axis runs parallel to the fold axis then the foundation may be kept just before the anticlinal axis to achieve the upstream rock dip conditions. In case of fold axis perpendicular to the dam axis then both anticline and syncline axis may come and as we know that anticlinal zone may have large number of fractures, the must be sealed or

35 grouted to avoid water seepage through them. Fold axis may also have its orientation in between the two extremes discussed above i.e. oblique/diagonal; some new problems may crop up and need solution to specific case. Sometimes incidence of large number of rock cleavages especially in anticlines can inflict extra discontinuity, further weakening a rock mass. 11. Unconformity Unconformity is a surface between two sequences of different geological ages. The rocks below, are much older, more deformed, more lithified and metamorphosed, than the rocks above the unconformity surface. The presence of unconformity represents a time period of large-scale upheavals on the earth surface, tectonic movements and widespread deep erosion. Most of the unconformity surfaces are irregular and represent the paleo topographic surface on which the geomorphological agencies were carrying out the processes of weathering and erosion of the older rock sequence. That is why erosional remnants in form of gravel deposits called as residual conglomerates and soil developed at that geological time span called as palaeosols are found more than often. Depending upon the relationship of overlying and underlying rock sequences unconformities are classified into nonconformity, angular unconformity (Fig. 23a & b),blended unconformity, disconformity (Fig. 24a & b) and paraconformity (Fig. 25). Fig. 23 (a) Nonconformity (U U) between lower igneous and upper metamorphic or sedimentary rocks; (b) Angular Unconformity (U U), lower and upper rocks will have different strike and dip amount and directions. See residual conglomerates lenses along unconformities.

36 Fig. 24 (a) Blended Unconformity with a palaeosols layer; (b) Disconformity, note presence of residual conglomerate. Both lower and upper rocks are sedimentary and are horizontal. (U U) Unconformity. The maximum time of break is involved in non- and angular unconformity followed by blended and disconformity, while the least time break is ascribed to paraconformity. Fig. 25 Paraconformity, both lower and upper rocks are sedimentary and are horizontal, with no irregular surface. (U U) Unconformity Influence of Unconformity on Major Geo-Engineering Projects: The effect of unconformity is almost similar to the fault, as across it there will be found altogether different rocks, less deformed or more having different rock mass characteristics and even groundwater regime. However, the main problem is the presence of very weak, problematic rocks i.e. palaeosols, and residual conglomerates, invariably present along unconformity surfaces. The St. Francis Dam, California, USA, failed in 1928 was founded partly on residual conglomerate and partly on weathered schists.

37 12. Some other Structures There are some other structures, which are directly or indirectly related to deformation, such as diapirs, nappe, klippe, outlier and inlier. These structures are not very common but may be found at some sites of engineering projects hence some basic idea of them is required Diapirs: The Greek word diapir means to pierce. This term is used for rocks, which pierce through some other rocks due to its upward movement owing to less density. Evaporites (rock salt, gypsum, anhydrite), shale, mudstone etc. commonly form most of the diapirs. Mostly rocks move up as solids and form dome, mushroom, umbrella and spindle shapes. The cross sectional diameter and diapiric rise of diapirs may wary from less than a meter to thousands of meters. Such diapirs are very common in sedimentary basins of USA, Iran, Russia, Yemen and Canada. In Indian sub-continent, their presence have been recorded in Salt Range (J & K) and in Bilara-Nagaur area (Rajasthan). Small scale, local folding and faulting are associated with diapirs and have been found to have trapped natural oil and gas. Fig. 26 The formation of Nappes. (a) In a thrust fault rocks are displaced so that older rocks overlay younger rocks; (b) Displacement of folded rocks along a fault bringing older rocks over younger rocks; (c) Thrust fault breaking a crest of a fold and bringing older rocks overlay younger rocks; (d) Over stretching and breaking of recumbent fold bringing older rocks overlay younger rocks.

38 12.2 Nappe: It is a structure associated with thrust fault especially over thrusts, wherein hanging wall moves up relative to footwall along the fault plane, for kilometers. Such thrusts are invariably found in the tectonic regions of convergent plate folded mountain belts. They are also related to recumbent folding and faulting. The nappes are large over thrusted or over folded sheet of older rocks are found to overlie the younger rocks (Fig. 26 a-d) Klippe: The erosional remnants of nappes away from the main body of nappe, present as small outcrops are called as klippe (Fig. 27a). Fig. 27 (a) Mechanism of Klippe and (b) Window formation Window: When a nappe is subjected to erosion in such a way that the younger rocks below the thrust plane are seen then it is called as window or fenester (Fig. 27b) Outlier and Inlier: A limited exposure of younger rocks completely surrounded by older rocks form due to, either erosion of a hill or syncline or a fault (Fig. 28a) is called as outlier. Similarly a limited exposure of older rocks completely surrounded by younger rocks, form due to either erosion of anticline or valley or a fault (Fig. 28b) is called as inlier. Not of much significance to geo-engineering but there, presence can give indication of presence of folding or differential erosion.