1. 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

2. 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)

3. Reservoirs: purpose
Water storage
Flood prevention
Power

4. 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
Evaporation
Transpiration
Runoff

5. Reservoirs: leakage - - 1. Dam bypass 2. Water table effects
Water added -
Water subtracted -
Leakage from reservoir
Net amount of water available for storage
Rainfall in river basin
Infiltration
Evaporation
Transpiration
Runoff
1. Dam bypass
2. Water table effects

6. Reservoirs: leakage Leakage via subsurface bypass due to siphon effect
Dol-y-gaer dam
reservoir level
Carboniferous strata:
fracture and dissolution flow routes
Devonian strata
Devonian strata
Devonian strata
Subsurface water flow

7. Sautet dam and reservoir
Reservoirs: leakage
Leakage buried channels beneath drift
Bypass of reservoir in drift
R Drac
Ancient river/valley
Sautet dam and reservoir
Modern river/valley
50 km

8. Reservoirs: water table leakage-1
land surface
before
water table
water table divide
Bedrock with a water table and finite permeability
river
Leakage to next valley
new water table
reservoir
after

9. Reservoirs: water table leakage-2
Water table in aquifer
High permeability
layer
Land surface
before
Bedrock with low permeability: aquiclude
river
Modified water table in aquifer
High permeability
layer
reservoir
after
Leakage to next valley

10. Reservoirs: sedimentation
World’s largest dam; 180m tall, 2km wide
84% sediment in rainy season (june-sept)
drawdown and sediment sluicing during this period

11. Reservoirs: raised water table
Before
Water table
river
After - 1
Raised water table
reservoir
After - 2
Failure and slumping due to weakened rock mass
reservoir

12. Viaont dam disaster, Italy

13. Reservoirs: raised water table
ss = c + m . (sn - p)
p = pore fluid pressure
sn – p = effective stress
Unstable
Stable
Shear stress ss
Raising water table
s3, WT
s1,WT s3 s1
Normal stress sn

14. 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

15. Types of dam
Arch
Buttress
Gravity
Embankment
or Earth

17. Emosson Dam, Switzerland

18. The Vaiont dam today

19. 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

20. Dam failure: earthquake

21. Dam failure: asteroids

22. Dam failure: bombs

23. Dam sites: geology
Poor geological characterisation of dam foundation responsible for 40% of dam failures
Need proper site investigation

24. Dams: ground improvement
Poor geological conditions can be improved in 2 ways
improving load bearing properties
controlling seepage
> < mm grain size
gravel sand silt clay
Rolling, bolting and pre-loading
gravity drainage
well-points with drainage
ground strengthening
electro-osmosis
vibro flotation
explosives
grouts
chemical treatments
thermal treatment

25. 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

26. Reservoir 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