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Illustrations of flow nets 3D6 Environmental Engineering II Dr Gopal Madabhushi

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Trench supported by sheet piles Impermeable clay Uniform sand 5m 6m

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Trench supported by sheet piles Impermeable clay Uniform sand 5m 6m

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Trench supported by sheet piles Impermeable clay 5m 6m h=6m Nh=10 Nf=2.5+2.5 Uniform sand

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Excavation supported by a sheet pile Shale Uniform sand Water pumped away Steel sheet

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Excavation supported by a sheet pile Shale Uniform sand Water pumped away Steel sheet

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Reduced sheet penetration; possible liquefaction v = 0 Shale Uniform sand Steel sheet

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Reduced sheet penetration; possible liquefaction v = 0 Uniform sand Reservoir Tail water Shale

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Concrete dam or weir Reservoir Tail water Shale Uniform sand

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Concrete dam with cut-off; reduces uplift pressure Reservoir Shale Uniform sand

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Concrete dam with cut-off; reduces uplift pressure Reservoir Shale Uniform sand

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Pumped well in confined aquifer Observation well pumped well Elevation Aquifer heads H D aquifer Radial flow Impermeable stratum Plan

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Pumped well in confined aquifer Observation well pumped well Elevation Aquifer heads H D aquifer Radial flow Impermeable stratum Plan

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Clay dam, no air entry Shale clay reservoir atmospheric line drain

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Clay dam, no air entry Shale clay atmospheric line drain reservoir

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Clay dam, no air entry Observation well Shale clay atmospheric line drain reservoir

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Clay dam, no air entry, reduced drain; seepage out of downstream face Shale clay atmospheric line Not possible reservoir

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Clay dam, with air entry Shale clay reservoir drain

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Clay dam, with air entry Shale clay reservoir drain

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Clay dam, no capillary, reduced drain; seepage out of downstream face Shale clay reservoir

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Clay dam, no capillary, reduced drain; seepage out of downstream face Shale clay reservoir

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Flow of water in earth dams The drain in a rolled clay dam will be made of gravel, which has an effectively infinite hydraulic conductivity compared to that of the clay, so far a finite quantity of flow in the drain and a finite area of drain the hydraulic gradient is effectively zero, i.e. the drain is an equipotential

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The phreatic surface connects points at which the pressure head is zero. Above the phreatic surface the soil is in suction, so we can see how much capillarity is needed for the material to be saturated. If there is insufficient capillarity, we might discard the solution and try again. Alternatively: assume there is zero capillarity, the top water boundary is now atmospheric so along it and the flow net has to be adjusted within an unknown top boundary as the phreatic surface is a flow line if there is no capillarity. Flow of water in earth dams

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If then in the flow net, so once we have the phreatic surface we can put on the starting points of the equipotentials on the phreatic surface directly Flow of water in earth dams

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Unsteady flow effects Consolidation of matrix Change in pressure head within the soil due to changes in the boundary water levels may cause soil to deform, especially in compressible clays. The soil may undergo consolidation, a process in which the voids ratio changes over time at a rate determined by the pressure variation and the hydraulic conductivity, which may in turn depend on the voids ratio.

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Liquefaction (tensile failure) The total stress normal to a plane in the soil can be separated into two components, the pore pressure p and the effective inter- granular stress ’: By convention in soils compressive stresses are +ve. Tensile failure occurs when the effective stress is less than the fracture strength ’ fracture, and by definition for soil ’ fracture =0. When the effective stress falls to zero the soil particles are no longer in contact with each other and the soil acts like a heavy liquid. This phenomenon is called liquefaction, and is responsible to quick sands. Breakdown of rigid matrix

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Uniform soil of unit weight Upward flow of water Large upward hydraulic gradients:

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Uniform soil of unit weight Upward flow of water Plug of Base area A Water table and datum standpipe Gap opening as plug rises Critical head Pressure h crit Critical potential Head z

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At the base of the rising plug, if there is no side friction: So if v =0 then v = p and :, i crit =0.8~1.0

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where i crit is the critical hydraulic gradient for the quick sand Condition. As 18~20 kN/m 3 for many soils (especially sands and silts) and w 10 kN/m 3 :

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Frictional (shear failure) Sliding failure of a gravity concrete dam due to insufficient friction along the base:

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Uniform sand Reservoir Tail water W H1H1 H2H2 W´W´

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Limiting condition on shear force T is: where tan ’ max is the co-efficient of friction, so considering the base of the dam we are looking for: where W‘ = W-U is the effective weight of the dam, U is the total uplift due to the pore pressure distribution p along the base of the dam, and F = H 1 - H 2 is the shear force along the Dam base

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