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Dams and reservoirs Reservoirs Site selection Leakage from reservoirs

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Presentation on theme: "Dams and reservoirs Reservoirs Site selection Leakage from reservoirs"— Presentation transcript:

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

16

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

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