Geological Considerations in Civil Engineering

Geological Considerations In Civil Engineering

Engineering Geology

Considerations for: • Selection of site for dams

• Selection of site for reservoirs • Construction of tunnels • Construction of mountain roads

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Selection of site for dams

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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 4

Types of Dams On the basis of design: o Gravity dams o Buttress dams o Arch dams o Earth dams

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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

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Concrete Gravity Dam

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

Grand Coulee Dam on Columbia river

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

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 9

Buttress dam

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

Buttress Dam on Lower Colorado Region

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simscience.org

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 12

Idukki Dam

13 panoramio.com

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 14

Hirakud Dam, across the Mahanadi, Orissa

15 panoramio.com

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 18

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 20

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 21

4. Proper geologic structures • Undisturbed strata • Disturbed strata – Tilted beds – Folded strata – Faulted strata – Jointed strata

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Undisturbed strata

Gravity dam on horizontal beds

Chenna Kesavulu

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Dam on beds inclined in the upstream direction

Gentle inclination

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Steep inclination

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Dam on beds inclined in the downstream direction

Gentle inclination

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Steep inclination

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Dam over vertical beds

Chenna Kesavulu

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Dam over folded beds

Chenna Kesavulu

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Dam over faulted beds

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Selection of site for Reservoirs

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Gibson Reservoir, Montana

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

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

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Reservoir Storage Capacity

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

Geological considerations • Influence of rock types

• Influence of geological structures • Influence of water table • Reservoir silting

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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 35

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

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Inclined beds and leakage at reservoir sites

Chenna Kesavulu

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Leakage at reservoir sites due to geological structure

Chenna Kesavulu

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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

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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 41

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

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Geological Profile

Parbin Singh

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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

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Tunnels in relation to joints, faults and shear zones

Joints parallel to tunnel axis

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Joints perpendicular to tunnel axis

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Tunnels parallel to the axis of fold

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Tunnels perpendicular to the axis of fold

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Tunnels on thick, inclined or horizontal beds

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Tunnels along inclined beds

Tunnel along strike of inclined bed

Chenna Kesavulu

Tunnel along dip of inclined bed

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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 58

Overbreak

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Cases where overbreak is less

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Overbreak (contd) Cases where overbreak is more

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Construction of roads

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A road in Vietnam, 1900m above msl

rickmann-uk.com

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Influence of Geological factors • Topography • Lithological characters – Consolidated hard rock – Unconsolidated material • Geological structures • Weathering • Groundwater conditions 63

Geological structures

Road cut parallel to dip

Road cut parallel to strike

Beds dip into the hill - safe Chenna Kesavulu

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Geological structures & weathering

Joint sets inclined towards free face

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Unequal weathering causing rock fall

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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

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Reference • 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

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