Reservoirs and Dams
description
Transcript of Reservoirs and Dams
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