LAB MANUAL 9HydraulicConductivity

LAB MANUAL 9HydraulicConductivity - CIVE 310 Soil Mechanics...

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CIVE 310 Soil Mechanics Permeability Section Assigned Due 92
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PERMEABILITY Why do we need to know about permeability? “Reliable information on the permeability of sand strata may be required for either of two purposes. It may be necessary to estimate the quantity of water that will flow toward an excavation with specified dimensions, at a given position of the water table. Or it may be required to determine the depth to which the cutoff beneath a dam on a permeable foundation must be made to reduce the seepage losses from the reservoir to less than a specified amount.” (Terzaghi, Peck and Mesri, p. 61, 1996) Permeability data may also be needed for the design of the clay layer of a landfill liner, whose purpose is to significant levels of seepage into the ground water. In 1856, Darcy published a simple equation for the discharge velocity of water through saturated soils. This equation is based on the observed flow of water through clean sands: v=ki In this equation, k is the coefficient of permeability or hydraulic conductivity of the soil. Some typical values of permeability for soils are: Soil Type Typical Permeability – k-(cm/sec) >10 -1 Gravels and Coarse Sands 10 -1 to 10 -3 Fine Sands 10 -3 to 10 -5 Silty Sands Silts 10 -5 to 10 -7 Clays <10 -7 The smallest particles in a soil mix control the permeability. The fines content of a soil has a greater influence on permeability than does density. What ways do we have of measuring k? 1) Empirical Correlations a) Hazen’s equation – useful only for clean, uniform sands: k = 100 X (D 10 ) 2 b) Kozeny-Carman equation 2) Laboratory Tests a) Falling Head b) Constant Head – measures Q under a constant hydraulic gradient (i) c) Consolidation Test 93
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3) Field tests a) Borehole or piezometer test – raise the level of water by filling or lower it by bailing it out, then measure the time it takes the water to return to its original level. b) Pumping test – pump water from a discharge well while observing the level in observation wells. When a steady-state condition is reached, measure the drawdown of the water table in the discharge well. This is the most reliable/accurate method of determining the permeability of a soil, but it is also the most expensive. Possible problem with lab tests: a) Saturation – the soil mass is not fully saturated b) Stratification of the soil mass – anisotropy c) Soil fabric – the soil fabric in the lab sample is not the same as in the field d) Relative density – the relative density in the lab sample is not the same as in the field
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LAB MANUAL 9HydraulicConductivity - CIVE 310 Soil Mechanics...

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