geology module: influence of different...

GEOLOGY

Paper: Hydrogeology and Engineering Geology

Module: Influence of Different Geological

Structures on Civil Engineering Constructions

Subject Geology

Paper No and Title Hydrogeology and Engineering Geology

Module No and Title Influence of Different Geological Structures on Civil

Engineering Constructions

Module Tag 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

Prof. P.P. Chakraborty


University of Delhi

Delhi

Paper Coordinator Content Writer Reviewer

Dr. Shashank Shekhar


University of Delhi

Delhi

Dr. M. Masroor Alam

Department of Civil Engineering

Aligarh Muslim University

Aligarh

Prof. Vinay Jhingran


University of Delhi

Delhi

GEOLOGY




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

7.1.1 Tectonic Joints

7.1.2 Non-Tectonic Joints

7.2 Geometric Classification

7.3 Joint Parameters and their influence on Rock Mass

Properties

7.3.1 Number of Joints per unit area/volume

7.3.2 Joint Spacings

7.3.3 Orientation or Attitude

7.3.4 Block Size and Shape

7.3.5 Length and Depth Persistence

7.3.6 Aperture or Openness

7.3.7 Asperities or Roughness

7.3.8 Joint Filling Material

7.3.9 Presence of Water

GEOLOGY




8. Shear Zone

9. Fault

9.1 Genetic and Geometric Classification

9.2 Identification and Effect of Faulting

9.2.1 Fault Plane Criteria

9.2.2 Topographic Criteria

9.2.3 Geological Criteria

9.2.4 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

GEOLOGY




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 geo-

architecture, 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

GEOLOGY




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 above-

mentioned 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

GEOLOGY




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. Large-

scale 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).

GEOLOGY




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.

GEOLOGY




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

GEOLOGY




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

(250C/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.

GEOLOGY




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: Joints, Faults

Ductile Deformation: Folds

Ductile - Brittle Deformation: 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

GEOLOGY




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 geo-

engineering 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 “non-

systematic” 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).

GEOLOGY




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.

7.1.1 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.

7.1.2 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).

GEOLOGY




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:

GEOLOGY




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 00 to 50

Gently Inclined Joints ► b-b Dip of the joints from 60 to 150

Moderately Inclined Joints ► c-c Dip of the joints from 260 to 450

Inclined Joints ► d-d Dip of the joints from 460 to 750

Steeply Inclined Joints ► e-e Dip of the joints from 660 to 850

Vertical Joints ► f-f Dip of the joints from 860 to 900

GEOLOGY




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

GEOLOGY




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).

GEOLOGY




Fig. 8 (a) Rose diagram showing three joint sets with direction and strength (each

circle represent 20%). The most prominent one is with direction N200-300/S2000–

2100 with strength 60%; (b) Block diagram showing different joint parameters used

for rock mass characterization.

7.3.1 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.

7.3.2 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.

7.3.3 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 geo-

engineering, the joint attitude decides the most favorable and unfavorable

joints with respect to slope, dam foundation and tunnel alignment.

GEOLOGY




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.

7.3.5 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.

7.3.6 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.

7.3.7 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,

GEOLOGY




inclined joints along which rocks may slide, roughness provides

resistance to such movements.

7.3.8 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.

7.3.9 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.

GEOLOGY




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.

GEOLOGY




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.

GEOLOGY




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.

GEOLOGY




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).

GEOLOGY




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.

9.3.1 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).

GEOLOGY




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.

9.3.2 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.

9.3.3 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.

GEOLOGY




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.

9.3.4 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,

GEOLOGY




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 geo-

engineering 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 900, to keep its effect minimum.

The strengthening along the fault zone by hacking and back filling by

shotcrete and fibercrete is a must.

GEOLOGY




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.

10.1 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.

GEOLOGY




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 1800 to 1200 Open fold 1500 to 1200

Closed fold 1200 to 900 Tight fold 900 to 600

Very tight fold 600 to 300 Extremely tight fold < 300

Fig. 17 Type of fold based on inter limb angle.

GEOLOGY




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.

GEOLOGY




Fig. 20 Map pattern of doubly plunging fold.

10.3 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.

GEOLOGY




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

GEOLOGY




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.

10.4 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.

GEOLOGY




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

GEOLOGY




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.

GEOLOGY




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.

11.1 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.

GEOLOGY




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.

12.1 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.

GEOLOGY




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).

12.3 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.

12.4 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).

12.5 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.

GEOLOGY




Fig. 28 (a) Different ways of formation of inliers and (b) outliers.

13. Summary

All the rocks, on or near the earth surface are deformed to some level or degree

suggesting presence of stresses naturally and their application on rocks and rock

masses. A civil engineer has to work on these deformed rocks with different kinds of

heterogeneities. A very common result of deformation is generation of discontinuity

surfaces, pulverization, disruption, dislocation, repetition and omission of rocks. The

chaos and unpredictability, which is incorporated due to deformation need to be

identified, classified by engineering geologist, so that their effect is predicted,

rectified and minimized by civil engineers while planning and designing any mega

construction project. The important deformation structures are contacts, joints, shear

zones, faults, folds and unconformity. Some less found structures are outliers,

inliers, nappe and its variants. Exposed outcrops, open pits, mines, road, rail, river

GEOLOGY




cuttings and excavations for civil engineering works are important locales for

observing different deformation structures.

Joints as discrete fractures, being ubiquitous have profound control on rock mass

properties hence are of utmost importance from the geo-engineering point of view.

There numbers, orientation, continuity, spacing, aperture, condition etc. exert the

most dominant control on the performance of rock mass to geo-engineering loading

and unloading.

Next to contacts and joints, shear zone is the most prevalent deformation structure. It

represents partly brittle to partly ductile deformation in form of tabular planar to curvi-

planar zone of highly strained rock within a largely non-deformed rock block.

Depending upon its length, thickness and depth persistence different problems may be

encountered. The most common way of treating it is its excavation and back filling.

Faults are not that common but in tectonically active areas their presence cannot be

overlooked, as they may be active at these places and are locales of earthquakes.

Fault with known history of activity should be avoided on or near the major geo-

engineering projects. The fault planes commonly have pulverized rocks sometimes

with groundwater stored in them hence locales of rock failure and other problems.

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, tunnels and other underground structures its antiquity and

activity should be established before final selection of the site. In case of multiple

faults and fault zones, the hacking of pulverized and degraded rock material must be

taken up and should be backfilled by reinforced concrete, shotcrete or fibercrete as

the case may be for the strengthening.

Folding of rocks at different scales can bring in lot of issues in geo-engineering.

Starting from parasitic folds to large scale folds have some or other effect on

behavior of rock mass and its disposition. The relationship of fold axis with respect

to the length and the width of civil engineering structure and its alignment is an

GEOLOGY




important factor, which will decide the overall approach for design, and execution of

construction works in rocks, especially in anticlinal and synclinal zones.

Unconformity, not so common structure does bring rocks of two different geological

sequences of rocks juxtaposed to each other. The rocks below are usually much

older, more deformed, more lithified and metamorphosed, than the rocks above the

unconformity surface. The weathered and erosional remnants in form of palaeosols

and residual conglomerates are problematic horizons; need to be identified, as there

are instances of dam failures over them. There are some other structures, which are

directly or indirectly related to deformation, such as diapirs, nappe, klippe, outlier

and inlier. Many of these though rare, are found along most tectonically active

zones, usually convergent plate boundaries. The major issue is different rocks at

different locales with in the project site.

The folds and faults can also give an idea of major stress directions currently under

play. 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. 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. 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, while the

least principal axis will be horizontal and perpendicular to both the axes.

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. The presence of these stresses is reflected when folded rocks undergo brittle

deformation and develop joints.

GEOLOGY




Frequently Asked Questions-

Q1. What are the important deformation structures, enumerate their

importance in civil engineering?

All the rocks, on or near the earth surface are deformed due to past and present

prevailing stresses. A civil engineer has to work on these deformed rocks with

different kinds of heterogeneities. A very common result of deformation is

generation of discontinuity surfaces, pulverization, disruption, dislocation, repetition

and omission of rocks. The chaos and unpredictability, which is incorporated due to

deformation need to be identified, classified by engineering geologist, so that their

effect is predicted, rectified and minimized by civil engineers while planning and

designing any mega construction project. The important deformation structures are

contacts, joints, shear zones, faults, folds and unconformity. Some less found

structures are outliers, inliers, nappe and its variants.

Joints as discrete fractures, being ubiquitous have profound control on rock mass

properties hence are of utmost importance from the geo-engineering point of view.

There numbers, orientation, continuity, spacing, aperture, condition etc. exert the

most dominant control on the performance of rock mass to geo-engineering loading

and unloading.

Shear zones are other prevalent deformation feature found in rock masses present in

form of tabular planar to curviplanar zone of highly strained rock within a largely

non deformed rock block. Depending upon its length, thickness and depth

persistence different problems may be encountered. The most common way of

treating it is its excavation and back filling.

Faults normally dislocate rocks and in tectonically active areas their presence with

known history of activity should be taken into consideration and properly analyzed

to minimize its effect in case of major geo-engineering projects. The fault planes

commonly have pulverized rocks sometimes with groundwater stored in them hence

locales of rock failure and other problems. The location of faults are also a common

site of landslides, hence should be taken into consideration for any road or rail

GEOLOGY




project and hill area development. In case of dam and reservoir site, tunnels and

other underground structures its antiquity and activity should be established before

final selection of the site. In case of small multiple faults and fault zones pulverized

and degraded rock material must be excavated out backfilled by reinforced concrete,

shotcrete or fibercrete as the case may be for the strengthening.

Folding of rocks at different scales can bring in lot of issues in geo-engineering. The

relationship of fold axis with respect to the length and the width of civil engineering

structure and its alignment is an important factor, which will decide the overall

approach for design, and execution of construction works in rocks, especially in

anticlinal and synclinal zones.

Unconformity, not so common structure does bring rocks of two different geological

sequences of rocks juxtaposed to each other. The rocks below are usually much

older, more deformed, more lithified and metamorphosed, than the rocks above the

unconformity surface. The weathered and erosional remnants in form of palaeosols

and residual conglomerates are problematic horizons; need to be identified, as there

are instances of dam failures over them. There are some other structures, which are

directly or indirectly related to deformation, such as diapirs, nappe, klippe, outlier

and inlier. Many of these though rare, are found along most tectonically active

zones, usually convergent plate boundaries. The major issue is different rocks at

different locales with in the project site.

The folds and faults can also give an idea of major stress directions currently under

play depending upon faults being normal, reverse or of strike slip nature. Folds

though represent compressive stresses across the axial plane but tensile stresses

along the axis and shearing stresses oblique to it can be unraveled.

Q2. Enumerate different kinds of joint related parameters and there overall

effect on rock mass?

The rock mass has innumerable discrete fracture system termed as joints forms the

most common and formidable discontinuity surfaces dividing a rock mass into rock

blocks of different shapes and sizes. Most joints are planar but curvilinear surface

are not uncommon. The joints of regional dimensions (1-10 km) are called as

GEOLOGY




“master joints”. 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 “non-systematic” joints. 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. The

ultimate cause of large scale jointing in rocks are due to compressive, shear and

tensile stresses produced in different tectonic regimes. Residual stresses may also

cause development of sheet joints parallel to the ground surface.

Geometric classification is a descriptive one, simple and is easy to apply specially

from geo-engineering point of view as Strike Joints, Dip Joints, Diagonal Joints and

Bedding or Foliation Joints, when related to bedding or foliation of layered rocks. In

non-layered rocks such as igneous and non-foliated metamorphic rocks, dips of

joints are used for the classification.

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. Almost all the engineering

classifications of rock mass 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 parameter controlling geotechnical

behavior of rock mass. These joints need to be plotted on the plan of civil

engineering structure on suitable scales. The important parameters and their effects

are as follows:

i. 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.

ii. Joint Spacings: It is related to the distance between joints from a single set,

may vary from less than a centimeter to more than 5m. The number of joints

GEOLOGY




per unit volume as well as joint spacing will decide the size of the blocks in

rock mass.

iii. 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 geo-engineering

the joint attitude decides the most favorable and unfavorable joints with

respect to slope, alignment of dam foundation and tunnel opening.

iv. Block Size and Shape: Depending upon the spacing between the joints and

their orientation results into separation of rock mass 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.

v. Length and Depth Persistence: The length of joints, which may be

discontinuous or continuous, may vary from < 1 m to > 20km, on the surface

as well as in sub surface. Normally with increasing depth, the joints get

reduced in numbers.

vi. Aperture or Openness: Depending upon the origin, weathering and exposure

of rocks the joints may have varying degree of openness. Some joints are

seen as hair cracks, few may have opening less than 1 mm to 5mm and can

go up to 1 meter. The openness of joints is more in sedimentary rocks

especially limestone due to easy weathering and erosion caused by

moving/flowing water. The openness of joints is generally more on the

surface and diminishes with depth. The open joints may have different levels

of infilling.

vii. Asperities or Roughness: It is related to the irregularity 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 joint

roughness offers the resistance to slippage and is a very important factor for

the overall strength of rock mass.

GEOLOGY




viii. Joint Filling Material: The open joints are amenable to filling by sediments

brought in by wind and water. The sediments may vary from pure gravel,

granule, sand, silt and clays or their mixture. Sometimes minerals such as

clays, calcite, epidote and quartz are precipitated out of flowing water and

other low to high temperature 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. The presence of clay or gouge more than 5

mm continuously along a fracture opening will make the surface roughness

redundant.

ix. Presence of water: The joints are the avenues for natural ground water to

reside. 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 solution/weathering effect as well as by creating fracture water

pressure.

The above mentioned parameters play major role in controlling the rock mass

behavior hence most of the engineering classes of rock mass incorporate all of these

in one way or another.

Q3. What do you understand by shear zone? What are the problems created by

them and suggest remedies for different types of shear zones with

diagrams?

Shear zones are most prevalent deformation structure, next only to joints. They

represent partly brittle to partly ductile deformation in form of tabular planar to a

curviplanar zone of highly strained rock within a largely non-deformed rock, across

them. The size may vary from microscopic ones to normal outcrop size to as large as

tens of kilometers in length and accordingly may have large depth persistence as

compared to its thickness. 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.

GEOLOGY




The effectiveness of deformation can be gauged from the fact that the rocks get

pulverized to form cataclastic rocks as breccia, mylonite and as a gouge. The rocks

loose their original character and have altogether different character along the shear

zone. The shear zones have different geometries as horizontal, inclined, vertical,

parallel, diverging, converging, anatomizing, conjugate, folded or faulted shear zones.

The overall effect is weakening of rock mass due to the presence of a number of shear

zones in unit area or volume with respect to the dimensions of the civil structure.

It is highly unpredictable and most problematic feature in rock masses in the field of

geo-engineering especially very fine powdered gouge, whose thickness and continuity

has a profound effect on the behavior of rock mass, as they are the locales of least

fraction along the rock wall surfaces. To counter this problem, if shear zones are close

to the surface then it can be excavated and backfilled with concrete or any other sealing

material. In case shear zones are going very deep, then grouting is the best option.

Q4. What will be the effect of folding on geo-engineering projects?

Folding of rocks can bring in lot of issues in the arena of 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. The

most important issue is of repetition of rocks. If some weak rocks are present in the

stratigraphic sequence of that area then that rock will keep coming after some

distance depending upon the wavelength of the folds and dimensions of structure

involved. Also, 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 comparatively less fractured. For example if tunnel is

constructed parallel to fold axis then it will be easy to drive a tunnel along the

anticlinal, rather along synclinal zone, but chances of providing support may be

more in first case as compared to second case. However, 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. In case tunnel is being

made perpendicular to the fold axis, the problems encountered in anticlinal and

GEOLOGY




synclinal zone will keep coming after some distance depending upon the wavelength

of the folding and length of the structure.

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 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.

Q5. What will be the effect of faulting on rock strata and topography?

Faults are not very frequent in rock masses, but their absence cannot be taken for

granted. If a fault is present then its antiquity should be established and faults with a

known history of activity should be avoided on or near the major geo-engineering

projects. Along the fault plane, it is common to have pulverized rocks sometimes

with a 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 the 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 a 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 with concrete to avoid seepage

through fault planes. Similarly, in the case of tunnels, the alignment chosen should

be such that if any fault comes in its way it should be negotiated at 900, to keep its

effect minimum. The strengthening along the fault zone by hacking and back filling

by shotcrete and fibercrete is a must.

GEOLOGY




Multiple Choice Questions-

1. Which of the following structure is not a deformation structure

(a) Joint

(b) Contact

(c) Rock Cleavage

(d) Klippe

2. Which of the following parameter is not related to joints

(a) Asperties

(b) Aperture

(c) Persistence

(d) Hade

3. Which of the following structure is present as sets

(a) Joints

(b) Shear Zones

(c) Nappe

(d) Fenester

4. Which of the following structure results in repetition of rocks

(a) Fault

(b) Thrust

(c) Fold

(d) Shear Zone

5. Residual Conglomerate is found along

(a) Reverse Fault

(b) Translational Fault

(c) Plunging Fold

(d) Unconformity

GEOLOGY




Suggested Readings:

1. Subinoy Gangopadhyay (2013), Engineering Geology, Oxford University

Press, New Delhi.

2. Krynine, Dmitri P and Judd, William R (2005), Principles of Engineering

Geology and Geotechnics, CBS Publishers, New Delhi.

3. Tony Waltham (2002), Foundation of Engineering Geology, 3rd Edition,

CRC Press, London.

4. Bell, F G (1983), Fundamentals of Engineering Geology, Butterworths,

London.

5. Marland P. Billings (2001), Structural Geology, Prentice Hall India, New

Delhi.

6. Alam Masroor M. (2013), Fundamentals of Engineering Geology and Geo-

Engineering, Axioe Books, India.

geology module: influence of different...

Documents