# unsat4 - Soil Suction Total Suction Total soil suction is...

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Soil Suction Total Suction Total soil suction is defined in terms of the free energy or the relative vapor pressure (relative humidity) of the soil moisture. ) ( ln 0 0 u u v v v v w RT ω = Ψ v u = partial vapor pressure of pore water vapor vo u = saturation vapor pressure of water vapor over flat surface of pure water For pure water at 20 ° C: Ψ 35022 =− 1 0 ln ( ) v v u u where Ψ is in kPa. The total suction consists of two components, matric suction ( u a - u w ) and osmotic suction ( π ). Ψ = (u a - u w ) + π Both components are due to differences in relative humidity of the soil vapor. UNSAT4 - 1

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Matric Suction A meniscus forms at the soil-air interface, due to the surface tension, resulting in reduced vapor pressure in the water. The vapor pressure decreases, becomes more negative, and the matric suction pressure increases as the radius of curvature of the meniscus decreases. The size of the soil pores decreases with a decrease in soil particle size which then affects the size of the radius of curvature and consequently the matric suction pressure. The vapor pressure decreases as the degree of saturation decreases. Osmotic Suction The presence of dissolved ions in water decreases the soil vapor pressures, relative humidity, which then increases the total soil suction. Osmotic suction can be a significant portion of the total soil suction. UNSAT4 - 2
The following figure can be used to illustrate osmotic suction. a) b) Excess Gas Pressure Pure H 2 O Soil Solution P a Semi-permeable Membrane P a P H a) Waters flow through the membrane into the solution due to the osmotic suction in the solution. b) Water flows through the membrane into the pure water due to the application of pressure on the solution. The pressure on the solution required in order to equalize the flow of water from the solution to the pure water is equal to the osmotic pressure of the solution. (From Tindall, J. A. and Kunkel, J. R., 1999, Unsaturated Zone Hydrology , New Jersey, Prentice Hall.) UNSAT4 - 3

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Capillary Model Soil matric suction is described in terms of capillary forces, i. e., capillary rise, acting in soil. Capillary rise is caused by surface tension and the attractive forces between the soil ions and the water molecules in the adsorbed water. The rise in a capillary tube is computed by setting the total upward forces due to surface tension equal to the downward force due to the weight of the water in a tube. R s T s α 2 r h c 2 π r T s cos α = π r 2 h c ρ w g The height of capillary rise, h c , is obtained by assuming α =0. c s w s s w h T R T gg == 22 ρρ r UNSAT4 - 4
The matric suction pressure can then be given as () a u w u w g c h s T s R s T r −= = ≅ ρ 22 The pores sizes in soil (comparable to r) are indeterminate so it is not possible to calculate pore water pressure directly.

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## This note was uploaded on 12/12/2011 for the course CIVIL ENGI 7004 taught by Professor Hasan during the Spring '11 term at Auckland.

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unsat4 - Soil Suction Total Suction Total soil suction is...

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