unsat6 - Measurement of Unsaturated Permeability Water...

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Measurement of Unsaturated Permeability Water Coefficient of Permeability Direct Permeability Methods 1) Steady State Methods Principle: Under steady state conditions, i.e., (u a - u w ) = constant, the flow rate is independent of time. Test Description: a) a saturated high air entry plate at the top for applying a constant head; b) a saturated high air entry plate at the bottom for measuring the discharge; c) two tensiometers at the side for measuring the pore water pressures; d) an outlet at the side for varying the soil air pressure. UNSAT6 - 1
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Methodology: a) apply a constant water head at the top of the soil specimen and a constant air pressure to the soil; b) measure the outflow (Q) from the bottom of the soil and the tensiometer readings over time (t); c) steady state conditions occur when the inflow and outflow rates are equal; d) repeat for different applied air or water pressures. Calculations: w Q At x d hh = 12 k ( ) aw ave a uu u −= + 2 Results: ) ( ) ( ) (( ) w k k k k u u k k w w w w w ave w a w w = = = θ UNSAT6 - 2
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Limitations: a) difficult to measure volume of flow, Q b) osmotic flow due to concentration gradients c) air diffusion into water; d) tensiometers limited to suction pressures less than 1 bar e) long period for steady state conditions and flow measurement; f) assumes pore water pressure/moisture distributions a r e u n i f o r m ; g) separation between shrinking soils and the high air entry interfaces. 2) Laboratory Instantaneous Profile Methods Principle: The volume of flow and pore water pressures can be used in an unsaturated soil to determine permeability under changing (transient) conditions. UNSAT6 - 3
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Test Description: a) an unsaturated soil sample with a constant flow rate is induced on the left end of a test specimen with the right end is exposed to the atmosphere; b) a series of tensiometers or psychrometers placed along the sides to measure the variation of pore water pressure. Methodology: a) inject water at the left end at a constant rate so that the hydraulic gradient across the specimen is approximately uniform (It will be necessary to adjust the flow rate to obtain a uniform gradient.); b) measure the pore water pressures at various times to get the soil matric pressure (equal to the absolute value of pore water pressure); c) at the end of the test (nonnegative pore pressure), divide the soil and run water content tests to get the soil moisture characteristic relationship.
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unsat6 - Measurement of Unsaturated Permeability Water...

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