Dams and reservoirsReservoirs• 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 subtract

ed+

Rainfall in river basin

InfiltrationEvaporation

Transpiration

Runoff

Reservoirs: leakage

Water added

Leakage from

reservoir

Water subtracte

d-

Rainfall in river basin

InfiltrationEvaporation

Transpiration

Net amount of water available

for storageRunoff

-

1. Dam bypass2. Water table effects

Leakage via subsurface bypass due to siphon effect

Devonian strata

Devo

nian

str

ata

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/valleyAncient river/valley

R Drac

Sautet dam and reservoi

r

Bypass of reservoir in drift

Reservoirs: leakage

land surfacewater tableriver

reservoir

before

after

water table divide

Leakage to next valley

Bedrock with a water table and finite permeability

new wate

r tabl

e

Reservoirs: water table leakage-1

Land surface

river

beforeBedrock with low permeability: aquiclude

High permeabili

tylayer

Water table in aquifer

reservoirafter

High permeabili

tylayer

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 - 1reservoir Raised water table

After - 2reservoir Failure

and slumping due to weakened rock mass

Reservoirs: raised water table

Viaont dam disaster, Italy

Normal stress n

Shear

stress

s

1,WT3, WT

UnstableStable

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-loadinggravity drainage

well-points with drainageelectro-osmosis

vibro flotationexplosives

groutschemical treatments

thermal treatment

ground strengthening

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

kill wave

energy

Hard face to dam

Aquifer layers

Aquiclude layers

Core and rear of dam