Measurement of the Shear Strength of an Expansive Soil by Combining a Filter Paper Method and Direct Shear Tests

Abstract

Abstract The measurement of the shear strength of unsaturated soils in terms of two independent stress state variables is usually difficult, expensive, and time-consuming. This paper presents a proposal to combine a filter paper method and a conventional direct shear test to obtain this measurement. The feasibility of this approach is illustrated through tests on an expansive soil. First, the filter paper method is used to establish the soil-water characteristic curve of the soil, and a series of conventional direct shear tests is subsequently conducted to measure the shear strength of the soil. The matric suction of the soil at failure is estimated from the soil-water characteristic curve based on the water content of the soil tested. The test results show that the failure envelopes of the expansive soil are nonlinear on the shear strength versus the matric suction plane for different net normal stresses. The unsaturated shear strength parameter ϕb is equal to the effective friction angle ϕ′ when the soil is close to being saturated, and ϕb decreases as the soil becomes drier. The combined method proposed in this paper may be a practical technique for analyzing the shear strength of unsaturated soils in terms of two independent stress state variables because it can be conducted in most geotechnical laboratories.