7 Reservoirs and Dams

8/12/2019 7 Reservoirs and Dams

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Dams and reservoirs

Dams

Types

Forces on a dam

Geology and dam sites Rock types and dams

Dams on soils

Ground improvement

Reservoirs

Site selection

Leakage from

reservoirs

Sedimentation

Stability: effect ofraised WT


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Dams and reservoirs - literature

Bell F.G., Engineering geology andgeotechnics

Ch 6 (Reservoirs)

Ch 7 (Dams)

Blyth F.G.H. and de Freitas M.H., Ageology for engineers

Ch 14 (Reservoirs and dams)


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Reservoirs: purpose

Water storage Flood prevention

Power


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Reservoirs: site selection

Hydrological considerations

Fundamental controls topography

climate geology

Water

added

Net amount of water

available for storage

Water

subtracted+

Rainfall in river

basin

Infiltration

EvaporationTranspiration

Runoff


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Reservoirs: leakage

- -WateraddedWater

subtracted

Net amount of water

available for storageLeakage from

reservoir

Rainfall in

river basin

Infiltration

EvaporationTranspiration

Runoff

1. Dam bypass

2. Water table effects


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Reservoirs: leakage

Leakage via subsurface bypass due to siphon effect

Devonian strata

Devonians

trata

Devonian strata

Dol-y-gaer dam

Carboniferous strata:

Subsurface water flow

reservoir level

fracture and dissolution

flow routes


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Reservoirs: leakage

Leakage buried channels beneath drift

50 km

Modern river/valley

Ancient river/valley

RDrac

Sautet

dam and

reservoir

Bypass of reservoir in drift


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Reservoirs: water table leakage-1

landsurface

watertable

river

beforewater table divide

Bedrock with a water

table and finite

permeability

reservoir

Leakage to next valleynew

water

table

after


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Reservoirs: water table leakage-2

Landsu

rface

river

High

permeability

layer

Water table in aquifer

before

Bedrock with low

permeability: aquiclude

reservoir

after

High

permeability

layer

Modified water table in aquifer

Leakage to next valley


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Reservoirs: sedimentation

Worlds largest dam;

180m tall, 2km wide 84% sediment in rainy

season (june-sept)

drawdown andsediment sluicing

during this period


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Reservoirs: raised water table

Before

Water table

river

After - 1

reservoir Raised water table

After - 2

reservoirFailure and

slumping

due to

weakened

rock mass


12/26

Viaont dam disaster, Italy


13/26

Reservoirs: raised water table

s = c + . (n - p)

p = pore fluid pressuren p = effective stress

1,WT3, WT

UnstableStable

13

Shearstresss

Raising water table

Normal stress n


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Dams: types Gravity dam: rigid monolithic structure

Trapezoidal cross section

Minimal differential movement tolerated

Dispersed moderate stress on valley floor and

walls

Arch dam: high strength concrete wall

Convex faces upstream

Thin walled structure

Relatively flexible

Huge stresses imposed on valley walls and floor

Earth dams: bank or earth or rock withimpermeable core

Core of clay or concrete, extended below ground

Sand or gravel drains built to cut fluid pressure

Low stress applied to valley floor and walls


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Types of dam

Arch Buttress

Embankment

or Earth

Gravity


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Emosson Dam, Switzerland


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The Vaiont dam today


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Dams: forces applied

Vertical static forces Lateral force applied by water body

Dynamic forces

wave action

overflow of water (controlled by spillway

channels) earthquakes and tremors

ice/freezing


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Dam failure: earthquake


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Dam failure: asteroids


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Dam failure: bombs


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Dam sites: geology

Poor geological characterisation of dam foundation

responsible for 40% of dam failures

Need proper site investigation

D d i t


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Dams: ground improvement

Poor geological conditions can be improved in 2 ways improving load bearing properties

controlling seepage

gravel sand silt clay

>10 2 0.07 0.002


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Dams: ground improvement

Rock bolts

Rolling and preloading compresses ground in prep for structure

improves post dam compaction

Gravity drainage and well points: sand and gravel channels and shallow wells (for pumping) Electro-osmosis: insert conductionrods into fine grained clay-rich bedrock and have an electric field - de-waters ground via theflow of electric current

Vibroflotation mechanical vibrating plate with load compresses low density gravels and sands

Explosives useful in water-saturated gravel and scree increases bulk density

Grouts material injected into the ground

Chemical treatments

react solutions injected into ground. React with material to alter properties. NaCl solutioninjected into smectite-rich mud, shale etc. to alter expansivity of smectite stabilizes groundpre-construction

Thermal treatment Freezing with injected liquid N2 to consolidate loose ground during excavation. Heatingby

burning petroleum under pressure in subsurface causes thermal metamorphism - hardensground and cuts porosity


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Injected

grout curtain

Pre-stressed

anchors

Drain

Apron drains (to

individual aquifers)

Excavationto rock

Regolith

Reservoir

Rip

rap

tok

ill

wave

energy

Hard face to dam

Aquifer layers

Aquiclude layers

Core and rear of dam

Documents

7 Reservoirs and Dams