The Unsaturated Shear Strength of Dual Porosity Soil

One of the most frequently occurring geo-environmental problems around the world is rainfall-induced slope failures. A good understanding of unsaturated soil mechanics (i.e. soil-water characteristic curve and unsaturated shear strength) is required for the analyses of rainfall-induced slope failure mechanisms. Rainfall-induced slope failures are commonly observed in residual soils. Some of the residual soils show bimodal characteristics in their grain-size distribution, called gap-graded soils. The results of the soil-water characteristic curve (SWCC) tests demonstrated that gap-graded soils may exhibit either unimodal or bimodal SWCC depending on the dry density and the water content of the soil. This paper focuses on the unsaturated shear strength characteristics of soils with bimodal SWCC, called dual porosity soils. The results from the unsaturated shear strength tests indicate that the characteristics of Ib angle of dual porosity soil are different from those of soils with unimodal SWCC at matric suctions beyond the air-entry value. For dual porosity soils, the desaturation process occurs first within the macro pores of soil while the micro pores of soil are still saturated at suctions between first and second air-entry values. Air starts to enter the micro pores of the soil at suctions beyond the second air-entry value.