questions & answers for engineering geologist

7/22/2019 questions & answers for engineering geologist

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MINISTRY OF SCIENCE AND TECHNOLOGY

DEPARTMENT OF

TECHNICAL ANDVOCATIONALEDUCATION

SAMPLE QUESTIONS & WORKED OUT EXAMPLES

FOR

Geol-03011

ENGINEERING GEOLOGY

B.Tech (First Year)

(FOR CIVIL ENGINEERING)


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YANGON TECHNOLOGICAL UNIVERSITY

DEPARTMENT OF ENGINEERING GEOOGY

ENGINEERING GEOLOGY FOR CIVIL ENGINEERING (EG-05011, 5th

Semester)

B.Tech. First Year (Civil)

Questions

Chapter 1. INTRODUCTION

*1. (a) Discuss the relationship between the Engineering Geologists and Civil Engineers.

(b) Describe the importance of Engineering Geology in Civil Engineering.

**2. Write short notes on any Four of the followings:-

(i) Engineering Geology (ii) Environmental Geology

(iii) Rock mechanics (iv) Geomechanics

(v) Mining Geology and Petroleum Geology

________________________________________________________________________ *

= must know

** = should know

*** = could know


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Chapter 3 ELEMENTS OF MINERALLOGY

*1. (a) How can you identify a mineral by the help of their physical and chemicalproperties?

(b) Add notes on the following physical characteristics that are useful for the

identification of rocks and minerals.

( i ) Colour (ii ) Streak (iii) Hardness (iv) Form

*2. Write short notes on the following rock forming minerals.

( i ) Quartz (ii) Feldspars (iii) Micas (iv) Calcite or Gypsum

________________________________________________________________________

* = must know

** = should know

*** = could know


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Chapter 4 PETROLOGY

* 1. Discuss thoroughly about the structures of Igneous Rocks. (Illustrate your answer

with neat diagrammatic sketches if you can.)

*2. Discuss what you know the processes of Sedimentation.

*3.With the help of neat diagrammatic sketches, describe briefly on Primary

Sedimentary Structures.

*4. Add notes on any Fiveof the followings:-

( i ) Granite (ii) Gabbro (iii) Syenite (iv) Sandstones

( v ) Limestones (vi) Shales (vii) Gneiss

(viii) Marble and Calc-silicate rocks

**5. Add notes on any Fiveof the followings:-

( i ) Diorite (ii) Serpentinite (iii) Schist (iv) Conglomerate and breccia

( v ) Slate and Phyllite (vi) Quartzite (vii) Evaporites (viii) Dolomite

_____________________________________________________________________

* = must know

** = should know

*** = could know


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Chapter 5 STRUCTURAL GEOLOGY

*1. With the help of neat diagrammatic sketches, describe briefly on Faults.

*2. With the help of neat diagrammatic sketches, describe briefly on Folds.

*3. From an Engineering Geological point of view, define Joints.

Discuss thoroughly about the Rock Joint Description in relation to Engineering

Geological investigation of rock materials.

**4. (a) Explain about the Mechanics of Faulting.

(b) Write short notes on the followings:-

(i) Unloading joints (ii) Cooling joints (iii) Dessicational joints

(iv) Joints due to the regional deformation

_____________________________________________________________________

* = must know

** = should know

*** = could know


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Chapter 6 EARTHQUAKES

*1. (a) Define the earthquakestechnically.

(b) Write short notes on any Threeof the followings:-

( i ) Causes of the Earthquake (ii) Seismic waves

(iii) Earthquake Magnitude and Intensity (iv) Effects of Earthquakes

( v ) Engineering Consideration

**2. Write a short account on Earthquake Belt and Seismic Zoing.

***3. Explain about the Earthquake prediction.

_____________________________________________________________________

* = must know

** = should know

*** = could know


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Chapter 7 FORMATION AND ENGINEERING USE OF SOIL

*1. (a) From an engineering geological view point classify the soil types.

(b) Add notes on any threeof the following engineering properties of soil.

( i ) Permeability (ii) Shearing strength (ii) Bearing capacity

(iv) Soil compressibility (v) Void Ratio and Porosity

** 2. Discuss briefly on physical properties of soil.

_____________________________________________________________________

* = must know

** = should know

*** = could know


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Chapter 8 GROUNDWATER

*1. Discuss what you about the Groundwater movements.

*2. With the help of neat diagrammatic sketches, write short notes on the followings:-

( i ) Vadose water (ii) Aquifer (iii) Unconfined aquifer

(iv) Confined aquifer (v) Artesian aquifer (vi) Fresh and salt groundwater

**3. (a) Discuss briefly on groundwater investigation.

(b) Explain about the water in rocks.

***4. Write a short account on Zonal Distribution of Groundwater.

_____________________________________________________________________

* = must know

** = should know

*** = could know


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Chapter 9 ENGINEERING GEOLOGY OF DAM SITES

*1. Discuss on any Fourof the following geological problems that are usually met with at

dam sites.

( i ) Dams on shale (ii) Dams on soluble rocks

(iii) Dams on strata dipping upstream (iv) Dams on strata dipping down stream

(v) Dams built across strike of rocks (vi) Dams on jointed and permeable

rocks

**2. (a) Write a short account on Forces acting on dams.

(b) Explain about the geological problem of Dams on Faults.

_____________________________________________________________________

* = must know

** = should know

*** = could know


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Chapter 10 ENGINEERING GEOLOGY OF TUNNELS

*1. Describe the following geological structures that influence the tunnel design, stability

and cost.

( i ) Tunnels in Horizontal Strata (ii) Tunnel axis parallel to the dipdirection

(iii) Tunnel axis driven parallel to the strike (iv) Tunnels in folded rocks

* *2. Discuss thoroughly about Geological Investigation of Engineering Problems

connected with Tunnelling.

***3. Describe what you know about the Soft Ground Tunnelling.

_____________________________________________________________________

* = must know

** = should know

*** = could know


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Chapter 11 ENGINEERING GEOLOGY OF ROADS

*1. Write short notes on the following geological investigation which are very important in

the design, stability and economical construction and maintenance of roads:-

( i ) Topography (ii) Lithological character (iii)Groundwater conditions

**2. Discuss briefly on Geological Structuresthat influence the construction of road.

***3. Add notes on any Three of the following engineering geological problems of the road

construction:-

(i) Roads in Hilly Regions (ii) Roads in Marshy Regions

(iii) Roads in Water Logged Areas (iv) Geological problem after Road Construction

_____________________________________________________________________

* = must know

** = should know

*** = could know


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Chapter 12 ENGINEERING GEOLOGY OF BRIDGES

*1. Write short notes on any Two of the followings geological characters that need to be

investigated:-

( i ) The depth to the bed rock (ii) The nature of the bed rock

(iii) The structural disposition of rocks

_____________________________________________________________________

* = must know

** = should know

*** = could know


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YANGON TECHNOLOGICAL UNIVERSITY

DEPARTMENT OF ENGINEERING GEOOGY

ENGINEERING GEOLOGY FOR CIVIL ENGINEERING (EG-05011, 5th

Semester)

B.Tech. First Year (Civil)

ANSWERS

Chapter 1 INTRODUCTION

*1. . (a) Discuss the relationship between the Engineering Geologists and Civil Engineers.

(b) Describe the importance of Engineering Geology in Civil Engineering.

Answers.

(a) The engineering geologist presents geological data and interpretations for use by

the civil engineer. The civil engineers have to deal mostly with soil and rocks, timbers, steel,

and concrete. In a great majority of civil engineering, projects and the designs, involve the

soils and rocks almost directly.

Civil engineering is to construct the structure and facilities for transport, water supply,

hydropower, flood control, environmental protection, sewage and waste disposal, urban

development and more. In above fields, civil engineers construct and maintain waterways,

highways, railway, pipelines, dam and reservoirs and tunnels.

(b) The importance of engineering geology in civil engineering may briefly be

outlined as follows:

1. Engineering geology provides a systematic knowledge of construction material, its

occurrence, composition, durability, and other properties. Examples of such

construction materials are building-stones, road materials, clays, limestone, and

laterite.


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2. The knowledge of the geological work of natural agencies such as water, wind, ice

and earthquake helps in planning and carrying out major civil engineering works. For

examples, the knowledge of erosion, transportation, and deposition helps greatly I

solving the expensive problems of river control, coastal and harbour work and soil

conservation.

3. The knowledge about groundwater that occurs in the subsurface rocks and about its

quantity and depth of occurrence is required in connection with water supply

irrigation, excavation and may other civil engineering works.

4. The foundation problems of dams, bridges, and buildings are directly concerned with

the geology of the area where they are to be built. In these works, drilling is

commonly undertaken to explore the ground conditions. Geology helps greatly in

interpreting the drilling data.

5. In tunnelling, constructing roads, canals, and docks and in determining the stability of

cuts and slopes, the knowledge about the nature and structure of rocks is very

necessary.

6. Before starting a major engineering project at a place a detailed geological report,

which is accompanied by geological maps and sections, is prepared. Such a report

helps in planning and constructing the project.

7. The stability of the civil engineering structures is considerably increased if the

geological features like faults, joints, folding, and solution channels etc. in the rock

beds are properly located and suitably treated.

8. In the study of soil mechanics, it is necessary to know how the soil materials are

formed in nature.

9. The cost of engineering works will considerably be reduced if the geological survey

of the area concerned is done before hand.

For a major engineering project, precise geological survey is carried out and results

thus obtained are used in solving engineering problems at hand.


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Chapter 3 ELEMENTS OF MINERALLOGY

*1. (a) How can you identify a mineral by the help of their physical and chemical

properties?

(b) Add notes on the following physical characteristics that are useful for the

identification of rocks and minerals.

( i ) Colour (ii ) Streak (iii) Hardness (iv) Form

ANSWER

(a) : A mineral may be defined as an inorganic substance occurring in nature with a

characteristic chemical composition and usually possessing a definite crystalline

structure which is sometimes expressed in external geometrical form known as crystals.

It is clear that (a) many substances of organic origin such as coal, mineral oil,

guano, amber, organic bones and pearl are not minerals; (b) artificial substances such as

laboratory (man made) products; e.g. glass, brick, cement, etc. are not minerals since they are

not natural.

(b) (i) Colour Since the colour of a mineral is its most conspicuous property, it is also

one of the most distinguishing features of minerals. Colours depend upon the absorption ofsome and the reflection of others of the coloured rays or vibrations of which compose

ordinary white light.

Some minerals have a distinctive colour, e.g. the green colour of Malachite, the yellow

colour of Sulphur, the blue colour of Azurite and the lead colour of Galena. But, even in the

same species specimens are found having very different colours. The mineral quartz (SiO2) is

colourless or white but it is also found with pinkish, yellow, green, brown, and even black

and violet colours.

The different species are found having same colours, e.g., Orthoclase, Gypsum and

Quartz are different chemical composition but they can be found as pink colour.

(ii) Streak The streak of a mineral is the colour of the fine powder of a mineral. Such

powder is readily obtained by rubbing the mineral on a flat surface of unglazed porcelain

known as the streak plate. As a diagnostic feature, streak is better than the colour of a mineral


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being more stable, and therefore, more reliable. In many instance the colour of a mineral and

the colour of its streak are the same. Thus, both the colour and streak of graphite are lead. In

magnetite they are both black. In other case they differ materially.

The streak is very useful in distinguishing the various oxides of iron.

e.g. hematite red streak / limonite - brown streak / magnetite grey streak

(iii) Hardness: Hardness of a mineral is generally defined as its resistance to external

mechanical action such as scratching. The usual mineralogical practice is to define the

hardness of a mineral by scratching it with another, i.e. to establish its relative hardness.

Included in the Mohsscale are ten minerals arranged in order of increasing hardness as

follows.

1.Talc Mg (Si4O10) (OH)2 6. Orthoclase K (Al Si3O8)

2.Gypsum Ca SO4, 2 H2O 7. Quartz SiO2

3. Calcite Ca CO3 8. Topaz Al(SiO4)(F,OH)2

4. Fluorite Ca F2 9. Corundum Al2O3

5. Apatite Ca5(PO4) F 10. Diamond C

(iv) Form: Mineral assumes various indeterminable form that are not necessarily

dependent on crystal character. These forms are described by the following terms, which

have their customary meaning.

Accicular: in fine needle-like crystals as in stibnite.

Bladed: shaped like a knife-blade, commonly exhibited in wolframite.

Fibrous: consisting of fine thread-like strands, as exhibited by the variety of

gypsum.

Botryoidal: consisting of spheroidal aggregations, some what resembling a bunch of

grapes, as with Hematite, Chalcedony.

Foliate or sheet or Flake: consisting of thin and separable lamellae, with mica and the

micaceousminerals.

Granular: in grains, either coarse or fine, granular aggregates of minerals such as

in olivine.

Tabular: showing broad flat surface, as in feldspar.


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*2. Write short notes on the following rock forming minerals.

( i ) Quartz (ii) Feldspars (iii) Micas (iv) Calcite or Gypsum

ANSWER

(i) Quartz

Composition SiO2Hardness 7

Sp.Gr. 2.65

Lustre Vitreous

Colour Colourless, white, coloured by impurities

Streak White

Cleavage None

Fracture Conchoidal

Occurrence Acid igneous rocks, many metamorphic rocks, as a veinstone in

sedimentary rocks

(ii) Feldspars- Orthoclase and Plagioclase

Composition (Ortho) K, Al silicate

(Plagio) Na, Ca. Al silicate

Hardness (Ortho) 6

(Plagio) 6

Sp.Gr. (Ortho) 2.56

(Plagio) 2.7

Lustre (Ortho) Vitreous to perarly

(Plagio) Vitreous

Colour (Ortho) White to pink, also greenish grey

(Plagio) White, grey

Streak (both) White

Cleavage (both) Perfect


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Fracture (Ortho) Conchoidal to uneven

(Plagio) Uneven

Occurrence (Ortho) Essential constituent of acid igneous rocks, common in

metamorphic rocks,

(Plagio) Metamorphic and igneous rocks

(iii) Micas Biotite and Muscovite

Composition (Biotite) K, Mg, Fe, Al hydroxal silicate

(Musco) K, Al hydroxal silicate

Hardness (Both) 2.5

Sp.Gr. (Biotite) 3

(Musco) 2.85

Lustre (Biotite) Vitreous

(Musco) Vitreous to pearly

Colour (Biotite) Black, dark green, brown

(Musco) Colourless or pale brown, green

Streak (Both) White

Cleavage (Both) Perfect

Fracture -----

Occurrence (Biotite) Igneous rocks of all kind and many metamorphic rocks,

(Musco) Granite, pegmatite, schist and greisen

(iv) Calcite or Gypsum

Calcite

Composition Ca CO3

Hardness 3

Sp.Gr. 2.71

Lustre Vitreous

Colour Colourless or white, sometimes tinted by impurities

Streak White


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Cleavage Perfect

Fracture Conchoidal

Occurrence Common mineral in sediments, altered basic igneous rocks, etc.

stalactites, travertine, etc.

Gypsum

Composition CaSO4, 2H2O

Hardness 1.5 - 2

Sp.Gr. 2.3

Lustre Vitreous to pearly

Colour Colourless, white, tinted pink

Streak White

Cleavage Perfect

Fracture ------

Occurrence Evaporites, in clays and limestones, associated with sulphur


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Chapter 4 PETROLOGY

*2. Discuss what you know the processes of Sedimentation.

ANSWER

In the process of sedimentation, there are four main geologic works. They are (a) weathering,

(b) transportation, (c) deposition and (d) post depositional changes (diagenesis).

(a)Weathering: The rocks from the earth's surface are eroded and weathered all the time by

the natural forces such as the sun, the water, the rain and the ice etc., There are mainly three

weathering processes: (i) physical weathering, (ii) chemical weathering and (iii) biological

weathering.

(a-i) Physical weathering: This is brought about chiefly by temperature changes, e.g. the

expansion of water on freezing in pores or cracks of the rock; the differential expansion of the

rock or rock minerals when strongly heated by the sun. This latter process tends to cause thin

sheet of rock to split off (exfoliation or onion skin weathering).

(a-ii) Chemical weathering This is mainly brought about by the action of substances

dissolved in rainwater. They are usually acidic in character and leach rock quite actively.

Thus new minerals are formed. There are mainly five kinds of chemical weathering. They are

oxidation, carbonation, solutions, hydration and hydrolysis. Among them the hydrolysis

process is the most important role.

(a-iii) Biological weathering. This is mainly brought about by the action of living organisms

including the tree roots.

Rocks(earths

surface)

Erosionand

weatheringRock fragments Transportation

Sediments Post depositional changes(diagenisis)

Sedimentary Rocks


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(b) Transportation. The detritus and dissolved matter (caused by weathering) are carried by

the water, wind and ice. The chief transportation processes are rolling, leaping, floating,

colliding and dissolving. The rolling, leaping and floating processes are physical

transportation and others are chemical transportation.

(c)Deposition. It has three main processes: -

(i) Deposition of floating sediments become clastic sediments

(ii) Solidification of dissolved matter become chemical sediments

(iii) Deposition the remain of organism become organic sediments

All the sediments are deposited basically in the three classes of sedimentary environments or

depositional environments. They are marine, continental and transitional (mixed)

environments. Among them the most important sedimentation environments are continental

shelf environment(from marine 0 to 600), deltaic environment(from transitional), and

fluvial environment(from continental).

(d) Post depositional changes(diagenesis)

At the beginning, the grains of sediments from sedimentary rocks do not cement each

other. When more and more sediments deposit upon them, the connate water in the sediments

come out of from the sediments and they become harder and more compact. By deposition of

cementing materials among the grains of sediments which have no cementation, these

sediments may be harder and harder; e.g., if calcite deposits in sand grains, they become

sandstone.

**5. Add notes on any Fiveof the followings:-

( i ) Diorite (ii) Serpentinite (iii) Schist (iv) Conglomerate and

breccia

( v ) Slate and Phyllite (vi) Quartzite (vii) Evaporites (viii) Dolomite

( i ) Diorite


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Diorites: Diorites consist of plagioclase, within the oligoclase-andesine of which hornblende,

biotite and hypersthence are common. Sphene, apatite and magnetite may also be present.

Microdioriteand andesiteare medium-grained and fine-grained equivalent of diorite.

Diorite occurs as stocks in Yinmabin area of Thazi Township and at the top of

Pyetkhaywe Mountain Range of Myittha Township. Andesite is abundantly found at the

Andes MountainRange of South America. In Myanmar, it occurs in Popa, Jade Mineand

lower Chindwinarea.

(ii) Serpentinite

. Serpentinite: Serpentinites consist essentially of serpentine minerals and so they have green

color. In hand-specimen, sheet and fibrous form are found. Their surfaces are soapy. They

show mesh structure by mixing of white and black veins. Serpentinites are altered from

peridotite. This process is called serpentinization.

They are associated with jade in Jade Minearea. They are also occurred along Naga,

Chin,RakhineMountain Rangeas small intrusive. Ni, Crores are found in association with

ultra basicrocks in Northern Chin Hills.

(iii) Schist

These rocks are produced in medium grade metamorphic environments. The most characteristic

feature is their foliation, which is produced by the parallel growth of minerals such as mica chlorite

and hornblende. Roughly, these schist can be divided into three classes; low- grade schist (mica

schist, quartz schist, green schist);medium grade schist (amphibolites schist, and andalusite schist,

garnet schist);high grade schist(staurolite schist, sillimanite schist)

In Myanmar, they are found in Kyaukse Shan Taung Oo (mica schist, garnet schist); East of

Pyawbye, Pyinmana, Kanpalet(green schist and graphite schist); Thabaikkyin(hornblende schist);

Minwon Taung(Kyanite schist); Belin Taung(Silliminite schist)

(iv) Conglomerate and breccia

Conglomerate and Breccia In conglomerate, rounded pebbles contain; mostly three

or four kinds of rock pebbles can be observed. In breccia, angular pebbles contain, only

pebble of one kind of rock fragments, cemented from the running water of that mountain

and then become breccia at the bottom of that mountain.


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Type of conglomerate : ( a ) Orthoconglomerate ( b) Paraconglomerate

Mode of occurrence ( 1 ) Interformational conglomerate,

( 2 ) Intraformational conglomerate

( 3 ) Fanglomerate

(v) Slate and Phyllite

Slate & Phyllite: Located in zones of low-grade metamorphism. The rocks are altered from clay,

shale and tuffaceous rocks by regional metamorphism. They show distinct cleavage (e.g. slaty

cleavage). Most slate shows black colour but sometimes they show red and pale green- colour.

In phyllite, the grain sizes (composed of small sheet minerals) are larger than shale. Therefore,

phyllite is brighter than slates. The common colour of phyllite is gray but sometimes, greenish phyllite

can be found.

In Myanmar, they can be found in parts of Shan State (Chaungmagyi Formation, Saedawgyi,

Yeywa); in Taninthayi and Myeik(BaluKyun); Tatkone; Yinmarbin; Kanpetlet; Mt.Victoria.

(vi) Quartzite

These rocks are altered from siliceous sandstone show granular texture. Most of the quartzite

shows whitish in colour and sometimes they show gray and reddish colour. In Myanmar, they can be

found in Chaungmagyi Formation, Mogok belt; East of Thazi and Tatkone; and Shan Taung Oo.

(vii) Evaporites

(i) Gypsum CaSO4.2H2O; salinity 3.5 times than present

(ii)Anhydrite CaSO4 (iii) Halite NaCl ; salinity 10 times than present

-deposit in arid desert and lagoon; impermeable layer; so use as for oil storage

1. Thickness not more than

100' , show unconformity

e.g. Kalawconglomerate

2. Thin bed, do not show

unconformity, e. g.,

Paungyiconglomerate

3. Found in Uplift Mountain

at the side of it, >100' , e. g.,

Uruboulder conglomerate


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-Sipaw (N.S.S)-Pannyoevaporites; greater than 100; Khalainvillage (18 million tons; in

Pegugroup they can be found as thin bed.

(viii) Dolomite

CaCO3: MgCO3 = 1 :1; harder than limestone show granular texture; fossils are rare; react

slowly with HCL; grey to buff colour.

-dolomitization 2CaCO3 + Mg++

CaMg (CO3)2 + Ca++

limestone (from sea) (dolomite) (to sea)

-same electrical charge but different in ionic size; found in Napangyi near Ywa

Ngan; Pindayarange (Wunbye formation)


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Chapter 5 STRUCTURAL GEOLOGY

*3. From an Engineering Geological point of view, define Joints.

Discuss thoroughly about the Rock Joint Description in relation to Engineering

Geological investigation of rock materials.

ANSWER

In rock mechanics and engineering geology, all different types of discontinuity are

generally called joints.

Rock joint description

In engineering geological investigation of rock materials, the system of joints, faulting , etc should be

carefully described.

In addition to the type of jointing, other aspects to be described are:

(1)spatial orientation (dipping) (ii)extent

(iii)spacing (iv)opening

(v)filling (vi)roughness

These aspects will be treated on the following:-

(i)Spatial orientation: The orientation of a rock separation plane (joint) can be measured with acompass. Different kinds of compasses and different notation systems are used in geology .In

engineering geology, usually the direction of dipping and the dipping angle are measured and

noted. Preferably compasses are used which enable the reading of both values at the same time. The

direction of dipping varies from 0 to 360 from North over East, South and West to North. The dipping

angle varies from 0 for horizontal planes to 90 for vertical planes (for instance; 310/35,010/90,

005/40).

(ii) Extent: Many joints do not strike through completely. The percentage of the plane, which isdeveloped as a discontinuity is called the extent of a joint. In flat exposures only one direction of a

joint is seen. To determine the extent exactly also a second exposure, perpendicular to the first one

would be necessary. As (Fig.5. 21) shows, the extant different joint sets in an exposure can have

different extent percentages.


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A: horizontal= 100%; vertical= 100% A B

B: horizontal=100% vertical= 50%

C: horizontal= 60% vertical= 40%

D: horizontal= 40% vertical= 25% C D

Fig. 5.21 : Illustration of the Extent of rock joint planes (See fig. 5.21 in your lecture

notes)

( iii ) Spacing The spacing of joints refers to the average distance between the individual

joints from a joint set measured in the direction perpendicular to the joint. It is expressed in

cm or m. The reverse value is the degree of jointingwhich is the number of joints per meter

measured in direction perpendicular to the joints.

( iv ) Opening The opening of a joint is the distance between the rock faces at both sides of

the joint. The opening can vary considerably at different locations along the joint; in that case

it may be preferable to note maximum, minimum, as well as average opening for the

particular joints can be particular influence on the permeability of a rock mass.

( v ) Filling The presence of filling materials can have influence on the frictional

properties of a joint, clay for instance can reduce the frictional resistance considerably. The

secondary permeability of a rock mass depends completely on the type of filling material.

Clay fillings can be so impermeable that they are barriers for any water movement so that the

secondary permeability is lower than the primary permeability. When joints are filled with

coarse, crushed material or when joints are empty the secondary permeability is very high.

For these reason, it is important to mark the observed grain size (distribution) in the field

notes.

(vi) Roughness: The surface roughness of a joint has a great influence on the frictional

properties of the joints. Movement will not so easily take place along rough joints as along

flat joints as small scale (micro roughness over millimetres). Any roughness description

should thus also refer to the scale of roughness which is meant. For practical classification

procedures the following roughness description will be sufficient. (fig. 22). On large scale

(meters): planar, undulating, stepped; on the small scale (millimetres) slickensided, smooth,

rough.


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Rough

STEPPED Smooth

Slikensided

Rough

Smooth

UNDULATING

Slickensided

Rough

PLANAR Smooth

Slikensided

Fig. 5.22(See fig. 5.22 in your lecture notes)


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Chapter 6 EARTHQUAKES

*1. (a) Define the earthquakestechnically.

(b) Write short notes on any Threeof the followings:-

( i ) Causes of the Earthquake (ii) Seismic waves

(iii) Earthquake Magnitude and Intensity (iv) Effects of Earthquakes


ANSWER

(a) Define the earthquakestechnically.

Technically, earthquakes may be defined as:

Vibrations induced in the earths crust due to internal or external causes that virtually

shake up a part of the crust and all the structures and living and non- living things

existing on it.

( i ) Causes of the Earthquake

The causes of earthquakes may be either natural or artificial. The natural causes may be

further grouped into endogenous, which are due to the earth's inner energy (volcanic ortectonics phenomena) and exogenous, which are due to several external factors (meteorite

falls, collapse of cave roofs, sudden changes in the atmospheric pressure, the attraction of the

Moon or of the Sun). The artificial causes are generated by several human activities that

disturb the equilibrium of the crust (e.g. nuclear energy-blast, quarry explosions, impounding

of water into large reservoirs, intensive water pumping from the underground, etc).

Tectonics earthquake are the principal among natural earthquake. About 95% of the total

numbers of earthquakes recorded are of tectonic origin. They have the largest intensity, affect

extensive areas and are most destructive in effects. These earthquakes are triggered by a

sudden slip or collapse (of a portion in the earth's crust or in the immediately lower layers)along geologic fault. This occurs when the material of which the layer is composed has

reached the limit of strain accumulation. By slipping or collapsing the potential strain, energy

accumulated changes suddenly into kinetic energy (energy-release) and propagates vertically

through seismic waves to the earth's surfaces. This theory of tectonic earthquake is called the

elastic-rebound theory.


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(ii) Seismic waves

The elastic waves, which are generated at the focus, are called seismic waves. Three types of

seismic waves are produced in an earthquake. Two types of wave, which can travel through

the interior of the earth, are collectively termed as body waves and the other that travels alongthe outer part of the earth is called surface waves. Body waves are further divided into two

types, primary or P-wave and secondary wave or S-wave. (Fig. 6.1 A & B)

(i) P-wave: Push and pull wave, travels with back and forth vibration. Vibration

of P-wave is parallel to the direction and the fastest wave.

(ii) S-wave: on the other hand, vibrate at right angle to the direction of movement

like a rope does when it tied at one end and shaken form the other end. S-waves travel

slower than P-waves.

(iii) Surface waves or L-waves: These are also called long waves or surface waves

because their journey is confined mainly to the surface layers of the earth. These are observed

only after the arrival of the P and S- waves. In character, the surface waves are of two main

types:

(a) The Rayleigh Waves in which the displacement of the particle is of a complex nature,

partly being in the direction of propagation and partly at right angles to it.

(b) The Love Wavesin which the displacement of the particle is practically horizontal, that

is, in the direction of propagation. In term of their effects on solid material of crust of the

earth, the Rayleigh waves tend to distort the horizontal surface into a many zig-zag shape.

The love waves, however, tend to create shearing (breaking) ruptures.

(iii) Earthquake Magnitude and Intensity

The Magnitude Scale (M)

The scale has been devised by Charles F. Richterin 1935, an American seismologist,

and subsequently improved by Gutenberg and Richter. Today, refined Richterscale is used

worldwide to describe the earthquake intensity in magnitude.

As precise terms as understood today, the Richter Magnitudeis the logarithm to the

base ten of the maximum seismic wave amplitude recorded on a seismograph at a distance of

hundred kilometres from the epicentre of a particular earthquake.

Richter Scale consists of 10 grades, starting with zero (0) and ending at nine (9). The

highest magnitude ever recorded so far is 8.9. According to observations, the damage starts at

Richter 5. Generally, damage is directly proportionate to the earthquake intensity. However,


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other factors such as, the depth of the earthquake, focus, distance between epicentre and town

or city, density of population and nature of bed rock also determine the amount of damage.

The Intensity Scale -The Modified Mercalli Scale (MM)

Mercalli Scale was named after Italy seismologist, Mercalli, and it was modified in

1931 by the other two seismologists, Wood and Neumann. Mercalli Scale is based on thedamages, which develop in various types of structures.

Mercalli Scale is known as earthquake intensity scale. But as its expression is a

comparative value, it cannot represent the absolute value of earthquake intensity. This scale

comprises twelve grades, 1 being the lowest intensity hardly felt by people, and XII the

highest indicating the total damage of all structures. Roughly speaking there is no or

insignificant damage up to IV of Mercalli Scale; a little damage in Mercalli Scale V to VI;

considerable in VII to VIII; much damage in IX to X; catastrophic in XI to XII.(Table 6.1).

(iv) Effects of Earthquakes

Richter classified the effects of earthquakes into two main categories: primary effects and

secondary effects.

Primary effects

The tectonic earthquakes are often responsible for producing many important changes in the

geological structures of an area: creation of slopes or scarps, fissures, warping of strata

emergence, or subsidence of coastlines, changes in the courses of streams, origin of new

springs, and creation of sand dykein which saturated layers of sand may be forced up into

existing cracks and services. These primary effects cause damage directly.Secondary effects

If the damages are caused indirectly, they are called secondary effects. They are

observed on construction of all types. Many landslides are triggered mainly due to shaking

vibrations. Due to these vibration buildings, bridges, dams, poles and posts and fences, etc.,

may

be slightly or heavily damaged. Loose objects may be overturned or thrown away.

Telegraphic and electric cables, water and gas pipes may get broken.

Ground movement displaces stoves, breaks gas lines, and loosens electrical wires,

thereby starting fires. Because of breaking water mains, there often is no water available to

put out the fires.

Huge waves may be caused in the sea waters; these are called Tsunamis(seismic sea

waves) and may be as high as 20 m or more.


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Liquefaction(the sudden shaking and disturbance of water-saturated sediments and

regolith turning seemingly solid ground into a liquid mass of quicksand) occurs when

vibration causes sediment grains to lose contact with one another, allowing interstitial water

to bubble through. Many of the buildings were not structurally damaged, they simply keeled

over onto their sides.

Flooding is a secondary or tertiary effect of earthquakes usually resulting from ground

subsidence, the rupture of dams or tsunamis.

Loss of life and damage to property are two standard measures for describing the

effects of an earthquake.


Many engineers emphasize that it is not the earthquakes, which kill people, but the failure of

building that people construct. They maintain that the best approach is not to predict and

contact earthquake, but to erect sound building, bridges, tunnels and dams on relatively safe

sites.

Therefore the tasks of the engineers are:

1. To know the seismic history of the area

2. To assess the magnitude and probable loss or damage in the life period of the structures, in

quality and quantity, due to expected seismic shocks.

3. To introduce suitable factors of safety in the new construction and if possible to safe guard

the existing structures against the expected shocks.

4. Another factors that must be used in structure are to perform the aseismic design, toconstruct the quake resistant building (including foundation, body of the structures and roof)

and quite resistant bridge and dams.

Experience throughout the world has shown that much more damage is caused to stimulate by

earthquake shocks when they are founded on soil than they are directly in contact with firm

bedrocks.


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Chapter 7 FORMATION AND ENGINEERING USE OF SOIL

*1. (a) From an engineering geological view point classify the soil types.

(b) Add notes on any Threeof the following engineering properties of soil.

( i ) Permeability (ii) Shearing strength (ii) Bearing capacity

(iv) Soil compressibility (v) Void Ratio and Porosity

ANSWER

(a) From an engineering geological view point classify the soil types.

Three main soil types can be distinguished: residual soil, transported soil and pedogenic

material. Three possible combinations of residual soil and transported soil layers overbedrock are shown in figure (Fig. 7.1)

B = Bedrock

R = Residual soil

T = Transported soil, Pebble marker

P = Pedogenic material (may be present,

absent or weakly developed)

(See Fig. 7.1 in your lecture notes)

Fig. Possible combinations of residual and transported soil over bedrock

1.Residual soil: is formed insituby (decomposition), chemical weathering and disintegration

(physical weathering) of a parentrock. Residual soil generally changes downward into the

fresh parent material from which it was derived.

2. Transported soil: have been transported by a natural medium (ice, water, wind, gravity)

before they were deposited. The medium and speed of transport determine the grain size

distribution (texture).

3. Pedogenic materials are secondary materials concentrated in certain layers after

solution and transport by soil moisture. The type of pedogenic material depends on the

combination of various climatic factors and the original soil composition. The ferricrete,

calcrete and silicrete refer to soils, which are strongly cemented or replaced by iron oxide,

calcium carbonate and silica respectively.


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b. ( i ) Permeability

It refers to the capacity of a rock to transmit fluids (water or petroleum) through it. It

is often expressed as Intrinsic Permeability, the unit which is the capacity of a rock of 1

cm length and 1 cm2cross sectional area to allow a flow of 1 cm

3/ sec. at a difference of

1 atmosphere.

b.(ii) Shearing strength

It is defined as the resistance of soils to shearing forces and is regarded as one of the

most important engineering properties of soils. It is the net result of at least three qualitative

characters of the soil, such as (a) the frictional resistance existing between the solid

components of the soil; (b) the degree of cohesion and adhesion between the soil particles; (c)

the textural arrangement of the solid particles such as degree of interlocking etc.

The shear strength of soils is determined in laboratory by direct sheer tests,

unconfined compression tests and triaxial compression tests. A number of methods are also

available for determination of shear strength of soils in the field itself. Among them, the

Vane-shear tester, the penetrometers and the split-spoon sampler are used commonly.

b. (iii) Bearing capacity

Bearing Capacity may be defined as the capacity of a soil to withstand building loads

without undergoing excessivesettlement or shear failure. Hence, this forms most important

field property that needs firm evaluation before any construction programme is proposed over

a soil. In practice, ultimate bearing capacityis determined by loading the soils to be tested

through contact (bearing) plates and observations of settlement. From this allowable bearing

capacity is determined for design purpose. Conventionally, for ordinary types of building

construction in a planned residential colony local building codes are prepared and followed in

a general way with respect to bearing capacity. For major construction, such as multistoreyed

buildings and industrial buildings, however, elaborate tests are carried out to arrive at safe

values of allowable bearing capacity. A number of factors have then to be taken into

consideration such as soil compressibility, water table, depth of the soil cover, shape of the

foundations to be given on the soil and so on. The subject has been dealt by different workers

in different ways. For a comprehensive understanding of the subject, reader must refer to

some standard text on Geotechnical Engineering.

b. (iv) Soil compressibility

Many natural soils undergo considerable deformation when loaded from above. This

deformation commonly takes the shape of a decrease in volume in vertical direction which

may be due to (i) expulsion of air and/or water from within the voids; (ii) collapse of solid

particles by closure of voids; (iii) deformation of solid particles. The net result due to this

compression is called consolidation of soils, which takes place at some rate with time, i.e. it is

a time related process.


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Granular, cohesionless soils consolidate at a fast rate compared to fine structured

cohesive soils. However, the total consolidation may be much less in the first type of soils,

where it may be completely achieved within a short span compared to in cohesive soils where

this process may keep going on for many years.

Consolidation may lead to settlement of the structures built over the soil and if this settlement

happens to be beyond the allowable limits, collapse or deformation of built-up structures may

follow. As such, the soil engineer is always required to investigate thoroughly the

compressibility related characteristics of the soil by practical methods.

b. (v) Void Ratio and Porosity

The Void ratio is defined as the ratio between the volume of voids and volume of solid

particles in a given soil mass. Numerically, void ratio, e, is given by the relationship:

e = Vv / Vs where Vv= volume of voids; Vs= volume of solid

The porosity, n, of the soil mass is, however, ratio between the total volume of

voids and the volume of soil sample:

[ n = Vv/V x 100] where V= total volume of the sample]

It is expressed in percentage terms.


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Chapter 8 GROUNDWATER

*2. With the help of neat diagrammatic sketches, write short notes on the followings:-

( i ) Vadose water (ii) Aquifer (iii) Unconfined aquifer

(iv) Confined aquifer (v) Artesian aquifer (vi) Fresh and salt groundwater

( i ) Vadose water

Vadose waterThis type of water occurs from surface downwards up to a variable depth and

is in a state of downward movement under the influence of gravity. It movements is

commonly described as INFILTRATION. The thicknessof soil and rock through which the

Vadose water infiltration is called zone of aeration. Obviously, in the zone of aeration the

soil and rocks remain unsaturated with water.

The Vadose Zone: This zone can be distinguished into three different types of

environments; soil water, intermediate Vadose water and capillary water.

Z.A= zone of aeration

S.Z= zone of soil water

C.F= capillary fringe

P.S= zone of saturation


Fig. : Zones of groundwater

(ii) Aquifer

An aquifer is a rock mass, layer or formation, which is saturated with groundwater and which

by virtue of its properties, is capable of yielding the stored water at economical costs when

tapped.

Gravels, limestone, and sandstones generally form good aquifer when occurring in

suitable geological conditions and geographic situations.


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(iii) Unconfined aquifer

It is also called water table aquifer, and is the most common type encountered in the field. In

this type, the upper surface of water or the water-table is under atmospheric pressure which

may be acting through the interstices in the overlying rocks (Fig.8.2) Water occurring in this

type of aquifer is called Free Groundwater. When tapped through a test well, the free water

will rise to a level equivalent to the water table of the area. (See fig. 8.2 from your lecture

notes)

(iv) Confined aquifer

It is a rock formation saturated with water and capable of yielding water when tapped but unlike

unconfined aquifer, has an overlying confining layer (an impermeable rock mass) that separates it

from the influence of atmospheric pressure. Naturally, water held in this type of aquifer is not under

atmospheric pressure but under great pressure due to the confining medium. The upper surface of

water in a confined aquifer is called piezometric surface (Fig 8.4). For establishing a piezometric

surface, level of water in a number of test wells has to be made. (See Fig. 8.4 from your lecturenotes)

(v) Artesian aquifer

It is, in fact, confined aquifer of such a geometry and developed in suitable geological situations so

that the piezometric surface is above the ground level at many places when projected in elevation.

When water is tapped from such a confined aquifer, it rushes up to and even above the surface and

may rise to the heights theoretically equivalent to the projected piezometric surface. Such wells are

called Artesian Wells, or flowing wells and the type of groundwater obtained from them, which oftenneeds no pumping, as Artesian Wells. (Fig. 8.4 )

AQ= Aquiclude

AF= Aquifer

PS= Piezometric surface

(See fig. 8.4 in your lecture notes)

(8.4) Artesian aquifer

(vi) Fresh and salt groundwater

The neighbourhood of ocean or sea, salt water encroaches on fresh water, and contaminates it and vice

versa. Fresh water may over ride and displaces salt water. (Fig. 8.5)

(See Fig. 8.5 from your lecture notes)

Fig. (8.5) Fresh groundwater floating on salty water.


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Chapter 9 ENGINEERING GEOLOGY OF DAM SITES

*1. Discuss on any Fourof the following geological problems that are usually met with at

dam sites.

( i ) Dams on shale (ii) Dams on soluble rocks

(iii) Dams on strata dipping upstream (iv) Dams on strata dipping down stream

(v) Dams built across strike of rocks (vi) Dams on jointed and permeable

rocks

ANSWER

( i ) Dams on shale

Shales are of two types: (i) cementation shale and (ii) compaction shales. The cementation

shales are stronger and do not disintegrate when subjected to wetting and drying. The

compaction shales on the other hand are soft and they slake when subjected to alternatewetting and drying. Their bearing strength is low and they become plastic when wetted. The

compaction shales have a tendency to flow away from the loaded area and therefore the

structure settles. Swelling and caving may result during the excavation work, which may

cause trouble. If dams have to be built on compaction shales, heavier structures like gravity

dams should be avoided. After excavating the weathered rock either concrete should be

placed immediately without delay or its surface should be coated with asphalt to avoid

swelling and caving.

(ii) Dams on soluble rocks

The soluble rocks include limestone and dolomites and marbles. These rocks are generally

sufficiently strong to support the weight of the dam, they may contain under ground solution

channels and caverns. If such solution channels are present at a dam site, the leakage through

them may be on such a large scale that the reservoir may not hold water for long. The

treatment of such openings is very expensive therefore; they should be carefully looked for in

the soluble rocks before constructing a dam.

(iii) Dams on strata dipping upstream

The dams located on rocks dipping upstream represent ideal foundation conditions. They are

the most capable of supporting the weight of dams and the pressure of the reservoir because

the resultant of these two forces acts nearly at right angles to the bedding planes of rocks.

(Fig. 9.2). Further the upstream dip of rocks does not allow the water in the reservoir to

percolate below the dam. As a result the leakage of water and the development of uplift

pressure will be minimum.

(See Fig. 9.2 in your lecture notes )

Fig.(9.1) Showing forces acting Fig. (9.2). Dam on rocks dipping Fig.(9.3) Dam on strata

on dams upstream dipping downstream


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(iv) Dams on strata dipping down stream

The dams constructed on rocks dipping down stream (Fig. 9.3 ) may not be safe due to the

following reasons.

(a) The percolation of water may lubricate the junctions of rock bed, which mayfacilitate sliding of dam.

(b) The water percolating through the strata dissolves the cementing materials ofrocks and enlarges the openings by mechanical erosion. This undermines the

strength of the rocks and increases the seepage of water.

(c) The water, which enters into the openings of rocks below the dam, causes thedevelopment of uplift pressure, which tends to decrease the stability of the

structure.

(d) In figure(9.3) R is the resultant of the weight of the dam and pressure of thereservoir water. In this case, this resultant acts nearly parallel to the bedding

planes and endangers the stability of the dam.

(See Fig. 9.3 from your lecture notes)

(v) Dams built across strike of rocks

The best foundation condition is when only one uniform rock is present along the length of a

dam. If a dam is aligned across the strike of strata, its foundation will be on different rocktypes of varying properties. In such a case there are changes of unequal settlement of the

dam. Further, as the bedding planes of the strata lie across the axis of the dam, there is a

possibility of serious leakage of water not only through the porous beds but through bedding

planes also (fig.9.4).


Fig. (9.4) Dam aligned across the strike of rocks.

(vi) Dams on jointed and permeable rocks

Where highly fissured, jointed and permeable rocks exist below the dam, they will not only

leakage of water, but also build uplift pressure at the base of the dam. The uplift pressure acts

opposite to the weight of the structure and it may cause sliding such rocks may be

consolidated by grouting.


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Chapter 10 ENGINEERING GEOLOGY OF TUNNELS

*1. Describe the following geological structures that influence the tunnel design, stability

and cost.

( i ) Tunnels in Horizontal Strata (ii) Tunnel axis parallel to the dip

direction

(iii) Tunnel axis driven parallel to the strike (iv) Tunnels in folded rocks

ANSWER

( i ) Tunnels in Horizontal Strata

Horizontal strata : Such a situation is rare in occurrence for long tunnels. When

encountered for small tunnels or for short lengths of long tunnels, horizontally layered rocks

might be considered quite favourable. In massive rocks, that is, when individual layers arevery thick, and the tunnel diameter not very large, the situation is especially favourable

because the layers would overbridge flat excavations by acting as natural beams (Fig. 10.3).

However, when the layers are thin or fractured, they cannot be depended upon as beams; in

such case, either the roof has to be modified to an arch type or has to be protected by giving

a lining (Fig. 10.4).


Fig. 10.3 Safe situation Fig. 10.4 Unsafe at the top

(ii) Tunnel axis parallel to the dip direction

When the tunnel axis is parallel to the dip direction (which means it is at right angles to the

strike direction), the layers offer uniformly distributed load on the excavation (fig. 10.5). The

arch action where the rocks at the roof act as natural arch transferring the load on to sidescomes into maximum condition. Even relatively weaker rocks might act as self- supporting in

such cases. It is a favourable condition from this aspect. However, it also implies that the axis

of tunnel has to pass through a number of rocks of the inclined sequence while going through

parallel to dip.



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(iii) Tunnel axis driven parallel to the strike

When the tunnel is driven parallel to strike of the beds, the pressure distributed to the

exposed layers is asymmetrical along the periphery of the tunnel opening : one half would

be bedding planes opening into the tunnel and hence offer potential planes and conditionsfor sliding into opening. The bridge action, though present in part, is weakened due to

discontinuities at the bedding planes running along the arch (Fig. 10.6).

(See fig. 10.6 from your lecture notes)

Fig. 10.5 Tunnelling parallel to the dip of Fig.10.6 Tunnelling

parallel to

layers (against the dip direction) to the strike

(iv) Tunnels in folded rocks

Folded rocks show bends and curvatures and store a lot of stain energy in the rock.Their

influence on design and construction of tunnel is important at the their position of angles.

Considerable variation and uncertainly folded rocks

Folded Rocks with peculiar rock pressure

(See Fig. 10. 9 and 10.10 in your lecture notes)

high pressure low pressure high pressure low pressure high pressure low pressure

Fig. 10.9 Anticline (low pressure in Fig.10.10 Syncline (high pressure in

middle region) middle region)


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Chapter 11 ENGINEERING GEOLOGY OF ROADS

*1. Write short notes on the following geological investigation which are very important in

the design, stability and economical construction and maintenance of roads:-

( i ) Topography (ii) Lithological character (iii)Groundwater conditions

ANSWER

( i ) Topography

Topography or the landform of a region is single most important factor that controls the

selection of the alignment of a road project. Topographic maps would reveal the existence of

various land features like valleys and inflowing streams, the hills and their undulations, the

plateaus and the plains with their entire configuration from place to place. Obviously,

knowledge of all such features is not only important but very essential for a right alignment.

Moreover such knowledge would also be necessary to decide where cuttings would be

required and in which areas it would be fillingwhere necessary or where the slopes could be

left at their natural inclination and where these would have to be flattened protected by

retaining walls and so on.

(ii) Lithological character

Ground may be divided into two types: consolidated, massive hard rock type and soft,

unconsolidated type.

The massive group of rocks include all varieties of igneous, sedimentary and

metamorphic rocks which can stand even with vertical slopes. For making roads through

them, however, these rocks require extensive blasting operations. They cannot be simply cut

out or dug out. Once cut, especially if they are free from joints and fractures and

unfavourably inclined bedding planes, these rocks stand erect for years without much

maintenance.

The unconsolidated group presents the engineers many complicated problems.

Thorough soil investigations regarding their mode of origin, texture, structures, porosity,

permeability, degree of compaction, consolidation, characteristics or compressibility etc. all

are required to be known within broad limits to design safe and stable roads over them.

Residual solid are generally homogeneous and properties evaluated from selective bore hole

samples might prove sufficient. In transported type of soils, however, variation in properties

both laterally and vertically might be more complicated nature. Presence of clay seams or

layers at critical places should be investigated as some types of these rocks often swell on

coming in contact with moisture, and create adverse situations for road stability and safety.


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(iii)Groundwater conditions

It is always necessary to investigate thoroughly the position of water table of the

area. Not only that, water bearing qualities should also be known along the

proposed route. It is quite likely that water bearing zones (aquifer) might be

intersecting the base or slopes of an alignment. Specific care and design would berequired for these natural water conduits. These are always to be taken as weak

and hazardous zones in the road.

Ground water conditions are very important as they influence on the bearing

capacity of the rocks and soil. Hence when the ground is rich with moisture it

would not bear the design loads. Sometimes free flow of ground-water through the

soil is quite dangerous for the stability of the road surface.

**2. Discuss briefly on Geological Structuresthat influence the construction of road.

ANSWER

The structural features of rocks include dip and strike, joints, fault planes and shear

zones.

1 Dip and Strike: There may be three possibilities for making a cut in the

inclined beds: it can be made parallel, at right angles or inclined to the dip

directions.

(i) Cut is parallel to the dip direction: In such a case (Fig.11.1 A), thelayers offer a uniform behaviour on either side of the cut and as such therisk of failure is minimal on this account.

(ii) Cut is made parallel to the strike, that is, at right angles to the dipdirection. In such a case, strata plunge across the cut, offering different

inclinations of the layers on either side of the cut. On the dipping in side of

the cut, there is always likelihood of slips, especially when the planes are

inclined steeply and get lubricated very often due to rainwater, or

groundwater movement (Fig. 11.1 B). In some cases where the layers dip

into the hill rather than in the road, the cut is considered quite stable

(Fig.11.2)


Fig.11.1 A: Road cuts parallel to the dip; B. parallel to strike Fig 11.2 Road cutparallel to strike

of inclined layers (beds dip into the hill)


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(iii) Cutting inclined to Dip and Strike: In such cases also, the strata will dipacross the cutting and the slope of cutting will be unequal on both sides.

Hence such a condition would give rise to similar difficulties as

encountered in cuts parallel to strike.

2. Joints:These influence the stability of the cuts in the same way as the bedding

planes. When present in great abundance, joints reduce even the hardest rock as a

mass of loosely held up blocks on the side of a cut which could tumble down on

slight vibrations. Further, even if the joints are few, but are continuous and inclined

towards the free side of the cut, these offer potential surfacesfor slips during the

presence of moisture.

3. Faults:Faulting generally leads to the crushing of the rock along the fault planes

and shear zones. Such a condition is, of course, very unfavourable for a cut when it

happens to form upper or lower slope or even base of the cut. It should not be left

untreated in any case.These are the worst type of planes of potential failure.


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Chapter 12 ENGINEERING GEOLOGY OF BRIDGES

*1. Write short notes on any Twoof the followings geological characters that need to be

investigated:-

( i ) The depth to the bed rock (ii) The nature of the bed rock


ANSWER

( i ) The depth to the bed rock

In alluvial channels the thickness of loose sands and gravels may be so great that it is not

economical to reach the bed rocks for placing the piers. In such case pile foundation is used.

The piles are generally driven through the material to the bed rock. Friction piles are used

where the bed rock is not available up to a great depth.

In most cases, the river bed below the water is covered by varying thickness of

unconsolidated natural deposits of sand, gravels and boulders. Such loose materials are not

safe as foundations for bridge piers for at least two reasons:

Firstly, piers placed directly on them would be unstable;

Secondly, the cover material is liable to be removed due to scouring by river water.

As such, the pier must be placed on a stable foundation, preferably of rock, under a

suitable thickness of cover material so that it is sage from scour by river water.

The height of pier from under the span to the foundation depends on the depthof the

bed rockbelow the river water.

Such sound bed rocks depend on the local geology which has to be investigated and

understood. To achieve this, drill holes are made all along the centre line of the proposed

bridge, even on the right or left of it, till they reach the sound rock sequence or up to a

reasonable depth. Utmost care is needed not to mistake isolated big boulders buried

underneath the river bed as the bed rock. Boulders are rocks but they are not bed rocks and

cannot be trusted as foundations for bridge piers.


Fig. 12.1 Depth of bed rock


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(ii) The nature of the bed rock

The very first rock encountered below the bed cover material may not be suitable as a

foundation. It should be kept in mind that three types of loads are to be borne by a bridge pierfoundation:

- the compressive, vertical loads due to the weight of the bridge span and that of pier

material;

-the horizontal loads due to the thrust of the water flowing above as transmitted directly and

through the pier;

-the dynamic, complex load, often inclined and shearing in character, due to heavy traffic

on the bridge.

Consequently, the bedrock selected as foundation for the pier must be strong enough

to bear the sum total of all these loads, not temporarily, but throughout the proposed life ofthe bridge.

The nature of the bed rock is commonly determined through study of petrological

characters and engineering properties, especially the strength values, using the core samples

obtained during drilling of test bore holes. In fact complete and very useful geological

profiles could be prepared all along the centre line of the proposed bridge from the study of

such core logs.

Most igneous and massive type of sedimentary and metamorphic rocks are quite

strong, stable and durable as foundations for bridge piers and abutments. The group of weak

rocks, which might behave badly in the presence of water includes such types as cavernous

limestones, chalk, friable sandstones especially with clayey cements, shales, clays, slates,schists and the layers of peat and compressible organic material.


Fig. 12.2 Nature of rocks below piers



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The horizontal attitude and uniformly massive structure with depth are desirable characters in

the foundation rocks as these offer inherent resistance against failure. However, even inclined

rocks in a confined situation under the bridge piers are considered quite safe if these possess

normal strength values.

Fracturing and jointing is, hover, undesirable at the foundation levels as these might

cause settlement beyond the allowable limits.

When the bridge sites are located in the zones of seismic activity, the foundations are

required to be designed for additional seismic loads as specified in the codes of respective

areas.

If a fault runs across the bridge alignment, this will be a source of many troubles. The

highly crushed and weathered rocks which exist in the fault zones make the foundation

treatment extremely expensive. It is therefore advised that the possibility of avoiding the fault

by shifting the bridge alignment upstream or downstream may seriously be considered.

questions & answers for engineering geologist

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