Reservoirs_and_dams their components.ppt

Description of reservoirs and Dams , their components

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

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

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

8. 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

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

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

12. Viaont dam disaster, Italy

13. Normal stresss
n
Shear stress
s
s
s
1,WTs
3,WT
Unstable
Stable
s
1s
3
s
s= c +m. (s
n-p)
p = pore fluid pressure
s
n–p = effective stress
Raising water table
Reservoirs: raised water table

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

24. 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
grouts
chemical treatments
thermal treatmentground strengthening
Dams: ground improvement

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
–Freezingwith injected liquid N2 to consolidate loose ground during excavation. Heatingby
burning petroleum under pressure in subsurface –causes thermal metamorphism -hardens
ground and cuts porosity

26. Injected
grout curtain
Pre-stressed
anchors
Drain
Apron drains (to
individual aquifers)
Excavation
to rock
Regolith
Reservoir
Hard face to dam
Aquifer layers
Aquiclude layers
Core and rear of dam