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Dams and reservoirs

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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 – PowerPoint PPT presentation

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Title: Dams and reservoirs


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

5
Reservoirs leakage
-
-
Water added
Leakage from reservoir
Water subtracted
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
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
  • Worlds 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
s1,WT
s3, WT
s1
s3
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
Embankment or Earth
Gravity
16
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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

gt10 2
0.07 0.002 lt0.0001 mm grain size
gravel sand silt clay
ground strengthening
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
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