EFFECT OF WET AND DRY CYCLES ON SHEAR STRENGTH OF CLAYS UNDER LOW OVERBURDEN STRESS

Abstract

The seasonal variations in soil water content are one of the major important environmental factors that causes the reduction of shear strength of expansive clays. Highway embankments which are constructed on expansive soils all over the world pose a severe maintenance problem due to shallow slope failures. Proper understanding of strength loss mechanism of the expansive clays will give some useful guidelines to design embankment with adequate factor of safety for long term drained conditions. Effect of wet and dry cycles on the high plastic clay strength has been studied by several researchers but the effect of wet dry cycle at low overburden stress and self-healing phenomenon was not clear, which might be very important consideration in the geotechnical design. This study focuses on the shear strength parameters of the soil samples collected from ten different locations in the vicinity of Islamabad region. Shear strength parameters were determined after 2nd ,4th, 6th wet and dry cycles and 7 days strength recovery period after 6th wet and dry cycle by Direct Shear Test (DST). X-Ray Diffraction (XRD) analysis of the soil samples was conducted. Other parameters like void ratio, density and moisture contents were also determined for the same number of cycles. Several anomalies in the trends of shear strength parameters were observed. It is concluded that cohesion is a non-reliable parameter and it is safe to use zero cohesion for long term design of slopes. Angle of internal friction is reduced both for low plastic and high plastic. This reduction is less in low plastic soils. The self-healing in low plastic clays is much more than in high plastic clays.