Pore-water pressure response during undrained isotropic load changes in layered soils

Abstract

Low pore-water pressure responses observed during undrained isotropic loading of thinly interbedded varved clay specimens were related to internal pore-water pressure equalization and internal shearing between soft clay seams and stiff silt layers of the varved clay. Both processes were analyzed in two separate models: a finite element analysis of the layered soil specimen with different elastic properties for each layer showed that shear stresses can develop along the layer interfaces during undrained isotropic loading. However, because the shear stresses are small and restricted to a narrow zone close to the surface of the cylindrical specimen, it appeared that the effect of shearing on the overall pore-water responses is negligible. The analysis of the pore-water pressures during undrained, isotropic loading demonstrated that hydraulic gradients between the two layers will develop. As a result, pore water will drain from the clay into the silt, leading to consolidation of the clay and swelling of the silt seams. The stabilized pore-water pressures should be the same as the pore-water pressures measured for the overall specimen, if the effect of internal shearing is negligible. Comparison of the computed with the measured overall pore-water pressure responses during testing for Skempton's pore-pressure coefficient B indicated reasonable agreement.Key words: Skempton's pore-pressure coefficient B, pore-water pressure response, varved clays, internal shearing, internal pore-water pressure equalization.