Geological Considerations In Civil

Engineering

Engineering Geology

2

Considerations for:

• Selection of site for dams

• Selection of site for reservoirs

• Construction of tunnels

• Construction of mountain roads

3

Selection of site for dams

4

Need for dams

• For hydroelectric power generation

• For irrigation purposes

• To obtain water for domestic and

industrial purposes

• For fighting draughts and controlling

floods

• For navigational facilities

Additional benefits include

development of fisheries, tourism etc

5

Types of Dams

On the basis of design:

o Gravity dams

o Buttress dams

o Arch dams

o Earth dams

6

1. Gravity dam

• A solid concrete or masonry structures,

that withstands the water pressure, by

virtue of its weight

• All forces acting on the dam are

assumed to be directly transmitted to

the foundation rocks

• They are generally of triangular profile

and are among the safest

7

Concrete Gravity Dam

http://nptel.iitm.ac.in/courses/Webcourse-contents/IIT%20Kharagpur/Water%20Resource%20Engg/pdf/m4l04.pdf

8

Grand Coulee Dam on Columbia river

http://en.wikipedia.org/wiki/Grand_Coulee_Dam

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2. Buttress dam

• They have a continuous upstream face,

supported at regular intervals, by

buttress walls on the downstream side

• They are lighter than solid dams

• Likely to induce greater stresses at the

foundation, since most of the load passes

through the buttress walls and is not

spread uniformly over the foundation

10

Buttress dam

http://nptel.iitm.ac.in/courses/Webcourse-contents/IIT%20Kharagpur/Water%20Resource%20Engg/pdf/m4l04.pdf

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Buttress Dam on Lower Colorado Region

simscience.org

12

3. Arch dam

• Arch-shaped, convex at the upstream

side

• Major portion of the thrust forces acting

on the dam are transmitted by arch

action, onto the abutment rocks

• Structural efficiency is higher than that

of gravity dams, the presence of sound

abutments is a prime necessity

• Uses less amount of concrete

13

Idukki Dam

panoramio.com

14

4. Earth dam

• Non-rigid structures, built with naturally

available materials such as earth and rock

• Ideal, where the dam site is weak to

support concrete dams, or where

competent rocks are found at great depths

Homogenous, with toe drain Homogenous, with chimney drain

15

Hirakud Dam, across the Mahanadi, Orissa

panoramio.com

16

Forces acting on a dam

• Self weight

• Water pressure

• Uplift pressure

• Earthquake forces

• Other forces – due to silt, wave and ice

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Geological considerations

• Narrow river valley

• Occurrence of bedrock at shallow depth

• Competent rocks to offer stable

foundation

• Proper geologic structures

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1. Narrow river valley

• Narrow valley means smaller dam is

required, and hence, lower costs

Defective valleys include:

o Deceptive narrowing due to thick

superficial deposits

o Narrowing due to rock outcrops

o Presence of soluble material like

gypsum, renders the rocks unsuitable

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Some defective narrow river valleys

Chenna Kesavulu

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2. Occurrence of bedrock at shallow depth

• The presence of strong bedrock near the

surface, reduces the cost of the foundation

• The site should be explore using electrical

resistivity or seismic refraction methods, to

assess the nature of the bedrock

• The presence of buried river valleys, huge

boulders gives rise to problems, as they are

composed of lose material

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3. Competent rocks for stable foundation

• Igneous rocks are safer than sedimentary

ones

• Suitability of site depends on:

– The existing rock type

– The extent of weathering undergone

– The extent of fracturing

– The occurrence of geological structures

– The mode and number of rock types

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4. Proper geologic structures

• Undisturbed strata

• Disturbed strata

– Tilted beds

– Folded strata

– Faulted strata

– Jointed strata

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Undisturbed strata

Chenna Kesavulu

Gravity dam on horizontal beds

24Chenna Kesavulu

Dam on beds inclined in the upstream direction

Gentle inclination Steep inclination

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Dam on beds inclined in the downstream direction

Gentle inclination Steep inclination

Chenna Kesavulu

26Chenna Kesavulu

Dam over vertical beds

27Chenna Kesavulu

Dam over folded beds

28Chenna Kesavulu

Dam over faulted beds

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Selection of site for Reservoirs

30

Gibson Reservoir, Montana

http://en.wikipedia.org/wiki/Gibson_Reservoir

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Categorization of Reservoirs

• Storage and conservation reservoirs

• Flood control reservoirs

• Distribution reservoirs

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Storage Capacity of a Reservoir

Storage capacity is expressed in terms

of:

o Useful storage

o Dead storage

o Surcharge storage

33http://nptel.iitm.ac.in/courses/Webcourse-contents/IIT%20Kharagpur/Water%20Resource%20Engg/pdf/m4l05.pdf

Reservoir Storage

Capacity

34

• Influence of rock types

• Influence of geological structures

• Influence of water table

• Reservoir silting

Geological considerations

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1. Influence of rock types

• Igneous rocks such as granites are less

porous, hence will afford more stability

• Sedimentary rocks are often porous, but

are more abundant than igneous ones

• Metamorphic rocks like gneisses behave

like granites

• The nature of rocks are important, as

they determine the leakage of water

through the foundations

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2. Influence of geological structures

• Downstream dip of bedding planes,

contribute to loss of water, development

of uplift pressure

• Water can leak through a tilted

permeable bed extending to a lower

valley

• In certain cases, folding and faulting of

the strata can prevent leakage of water

37Chenna Kesavulu

Inclined beds and leakage at reservoir sites

38Chenna Kesavulu

Leakage at reservoir sites due to geological structure

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3. Influence of water table

Position of the water-table is the

single most factor influencing the

leakage of reservoir water. Rivers can be

of:

o Effluent nature

o Influent nature

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Influence of water table

Chenna Kesavulu

41

Reservoir silting

• Deposition of sediments by rivers, gradually

reduces the capacity of the reservoirs

• Silting can be controlled by:

– Growing vegetation on loose soil

– Covering weak zones with slabs

– Constructing retaining walls

– Diversion of sediment loaded waters

– Silt outlets

– Check dams and settling basins

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Construction of tunnels

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Purposes of Tunneling

• For facilitating rail and road traffic

• For public utilities

• For power generation

• For mining activities

• For diverting water during dam

construction

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Tunnel boring machine

http://en.wikipedia.org/wiki/File:Tunnel_Boring_Machine_(Yucca_Mt).jpg

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Objects of Geological investigations

• Selection of tunnel alignment

• Selection of excavation method

• Selection of tunnel design

• Assessment of cost and stability

• Assessment of environmental hazards

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Geological profile along the tunnel axis

Includes information regarding:

o Location and depth of exploratory bore

holes

o Types of rocks and their characteristics

o Structure of the rocks

o Hydrological conditions

o Ground temperature conditions

47Parbin Singh

Geological Profile

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Formwork installation for tunnel lining

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Geological considerations

• Types of rocks

• Geological structures

• Ground water conditions

• Overbreak

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1. Types of rocks

• Igneous rocks

– Competent, but difficult to work with

– Do not require lining

• Sedimentary rocks

– Less competent, compared to igneous

– Sandstones, shales etc are soft, easy to

work

– Requires lining

• Metamorphic rocks

– Gneisses are similar to granites

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2. Geological structures

• Effect of joints

• Effect of faults

• Effect of folds

• Effect of undisturbed or tilted strata

52Chenna Kesavulu

Tunnels in relation to joints, faults and shear zones

Joints parallel to tunnel axis Joints perpendicular to tunnel axis

53Chenna Kesavulu

Tunnels parallel to the axis of fold

54Chenna Kesavulu

Tunnels perpendicular to the axis of fold

55Chenna Kesavulu

Tunnels on thick, inclined or horizontal beds

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Tunnels along inclined beds

Tunnel along strike of inclined bed

Tunnel along dip of inclined bed

Chenna Kesavulu

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3. Ground water conditions

• Tunnel axis passing entirely through

impervious formations

• Tunnel axis mostly above the water table

• Tunnel axis below the water table

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4. Overbreak

• Tunneling through hard rocks requires the

removal of some rocks outside the

proposed perimeter

• This excess quantity of rock removed, is

called the ‘overbreak’

• Geological factors governing the amount of

overbreak are:

– The nature of the rocks

– Orientation of the joints

– Orientation of bedding planes

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Overbreak

Cases where overbreak is less

Chenna Kesavulu

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Overbreak (contd)

Cases where overbreak is more

Chenna Kesavulu

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Construction of roads

62rickmann-uk.com

A road in Vietnam, 1900m above msl

63

Influence of Geological factors

• Topography

• Lithological characters

– Consolidated hard rock

– Unconsolidated material

• Geological structures

• Weathering

• Groundwater conditions

64

Geological structures

Road cut parallel to dip

Road cut parallel to strike

Beds dip into the hill - safe

Chenna Kesavulu

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Joint sets inclined towards free face

Geological structures & weathering

Unequal weathering causing rock fall

Chenna Kesavulu

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Complicated regions for road construction

• Hilly areas - meandering

• Marshy regions - subsidence

• Waterlogged areas – capillary action

• Permafrost regions – blanket action

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Geological problems after road construction

• Frost action

– Replacing the porous soil

– Lowering the water table

• Erosion problems

– Provision of interception ditches

68

• Parbin Singh, Engineering and General

Geology, S K Kataria & Sons

• Chenna Kesavulu, N, Textbook of Engineering

Geology, MacMillan India

• Thompson, G R and J Turk, Introduction to

Physical Geology, Thomson Brooks/Cole

Reference