Shear strength behaviors of unsaturated expansive soils with dual-porosity structure

A comprehensive set of experimental tests over a wide suction range and mechanism analysis are carried out to study the shear strength of Neixiang expansive soil with dual-porosity structure and bimodal soil-water retention curve (SWRC). Under the low and medium suction range, all the deviator stress versus strain curves show the strain-hardening behaviors with the shear contraction. Under the high suction range, the deviator stress versus strain curves show the post-peak strain softening stress-strain responses and the initial contraction of 1%~3% followed by the shear dilation. The specimens become stiffer with the increase of suction, and show a ductile failure mode with barrel shape or bulging at center under the low to medium suction ranges and strain localization with sliding surface under the high suction ranges. Also, the brittleness increases the amplitude of the post-peak softening, which is consistent with the increase of the difference between the magnitude of peak and the residual shear strength. Based on the bimodal SWRC considering capillarity and adsorption, the shear strength is analyzed and investigated in the capillary suction stress space for the soils with dual-porosity structure and bimodal SWRC. It is concluded that the strength shows the bi-linear characteristics if the capillary saturation has been used to upscale the suction from pore-scale stress to macroscopic one. The theoretical studies show that for the soils with double-porosity structure, the capillary suction stress under the low suction range can better characterize the macroscopic effective stress transferred between skeletons. The peak shear strength under the high suction range is provided by capillarity and closely related to the intra-aggregate cementation.