unsat10 - Unsaturated Soil Shear Strength Parameters...

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Unformatted text preview: Unsaturated Soil Shear Strength Parameters Measurements • Control of Stress States The stresses σ , u a and u w can be measured or controlled so that the stress state variables ( σ- u a ), ( σ 1- σ 3 ) and (u a- u w ) will be known. The axis translation method is used to obtain matric suction pressures greater than atmospheric. Base Cap Cap Soil Regulated / Measured Air Pressure Regulated / Measured Water Pressure High Entry Air Flow Material Coarse Porous Material Cell Pressure / Soil Volume Change Measurement Use extended Mohr-Coulomb failure envelope to determine shear strength parameters c', φ ' and φ b . Use shear strength testing to determine parameters for other shear strength theories. UNSAT10 - 1 • Triaxial Shear Testing Consolidated Drained Test Both the air and water are permitted to drain and the pressures are controlled. The matric suction pressure can therefore be held constant throughout the test. The strain rate must be sufficiently slow to allow for flow of water from the soil through the high air entry material. Constant Water Content Test Only the air is permitted to drain so the gravimetric water content remains constant (undrained) and the water pressure must be measured. The strain rate can be faster since air permeability is much higher than water permeability. The matric suction does not remain constant but changes. The variation of matric suction can be explained in terms of volumetric strain. See Fig. 9.32 below for an explanation. UNSAT10 - 2 When the loads are applied (low strains) the volume decreases (negative volumetric strain) and the pore water pressure increases (decrease in matric suction). At higher strain levels the volumetric strain becomes less negative or becomes positive. Matric suction typically increases as the sample dilates....
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unsat10 - Unsaturated Soil Shear Strength Parameters...

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