Dams and reservoirs

Reservoirs• Site selection• Leakage from

reservoirs• Sedimentation• Stability: effect of

raised WT

Dams• Types• Forces on a dam• Geology and dam sites• Rock types and dams• Dams on soils• Ground improvement

Dams and reservoirs - literature

• Bell F.G., Engineering geology and geotechnics – Ch 6 (Reservoirs)– Ch 7 (Dams)

• Blyth F.G.H. and de Freitas M.H., A geology for engineers– Ch 14 (Reservoirs and dams)

Reservoirs: purpose

• Water storage

• Flood prevention

• Power

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

InfiltrationEvaporationTranspiration

Runoff

Reservoirs: leakage

Water added

Leakage from reservoir

Water subtracted-

Rainfall in river basin

InfiltrationEvaporationTranspiration

Net amount of water available for storage

Runoff

-

1. Dam bypass2. Water table effects

Leakage via subsurface bypass due to siphon effect

Devonian strata

Dev

onia

n st

rata

Devonian strata

Dol-y-gaer dam

Carboniferous strata:

Subsurface water flow

reservoir level

fracture and dissolution flow routes

Reservoirs: leakage

Leakage buried channels beneath drift

50 km

Modern river/valley

Ancient river/valley

R Drac

Sautet dam and reservoir

Bypass of reservoir in drift

Reservoirs: leakage

land surfacewater table

river

reservoir

before

after

water table divide

Leakage to next valley

Bedrock with a water table and finite permeability

new water table

Reservoirs: water table leakage-1

Land surface

river

before

Bedrock with low permeability: aquiclude

High permeability

layer

Water table in aquifer

reservoirafter

High permeability

layer

Modified water table in aquifer

Leakage to next valley

Reservoirs: water table leakage-2

Reservoirs: sedimentation

• World’s largest dam; 180m tall, 2km wide– 84% sediment in rainy

season (june-sept)

– drawdown and sediment sluicing during this period

Before

Water tableriver

After - 1

reservoirRaised water table

After - 2

reservoirFailure and slumping due to weakened rock mass

Reservoirs: raised water table

Viaont dam disaster, Italy

Normal stress n

Sh

ear

stre

ss

s

1,WT3, WT

Unstable

Stable

13

s = c + . (n - p)p = pore fluid pressuren – p = effective stress

Raising water table

Reservoirs: raised water table

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 with impermeable 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

Types of dam

Arch

Gravity

Buttress

Embankment or Earth

Emosson Dam, Switzerland

The Vaiont dam today

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

Dam failure: earthquake

Dam failure: asteroids

Dam failure: bombs

Poor geological characterisation of dam foundation responsible for 40% of dam failures

Need proper site investigation

Dam sites: geology

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 <0.0001 mm grain size

Rolling, bolting and pre-loading

gravity drainage

well-points with drainage

electro-osmosis

vibro flotation

explosives

groutschemical treatments

thermal treatmentgrou

nd s

tren

gthe

ning

Dams: ground improvement

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 conduction

rods into fine grained clay-rich bedrock and have an electric field - de-waters ground via the flow 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 solution

injected into smectite-rich mud, shale etc. to alter expansivity of smectite – stabilizes ground pre-construction

• Thermal treatment– Freezing with injected liquid N2 to consolidate loose ground during excavation. Heating by

burning petroleum under pressure in subsurface – causes thermal metamorphism - hardens ground and cuts porosity

Injected grout curtain

Pre-stressed anchors

Drain

Apron drains (to individual aquifers)

Excavation to rock

Regolith

Reservoir

Rip

rap

to k

ill

wav

e en

ergy

Hard face to dam

Aquifer layers

Aquiclude layers

Core and rear of dam