Reservoirs and dams

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Dams and reservoirs:

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:

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:

Reservoirs: purpose Water storage Flood prevention Power

Reservoirs: site selection:

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

Reservoirs: leakage:

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

Reservoirs: leakage:

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

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

Reservoirs: water table leakage-1 :

land surface water 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

Reservoirs: water table leakage-2:

Land surface 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

Reservoirs: sedimentation:

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

Reservoirs: raised water table:

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

Slide 12:

Viaont dam disaster, Italy

Reservoirs: raised water table:

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

Dams: types:

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:

Types of dam Arch Gravity Buttress Embankment or Earth

Emosson Dam, Switzerland:

Emosson Dam, Switzerland

The Vaiont dam today:

The Vaiont dam today

Dams: forces applied:

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: earthquake

Dam failure: asteroids:

Dam failure: asteroids

Dam failure: bombs:

Dam failure: bombs

Dam sites: geology:

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

Dams: ground improvement:

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 treatment ground strengthening Dams: ground improvement

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

Slide 26:

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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