Engineers Australia

Suction-monitored direct shear apparatus: A simple device for unsaturated soil testing

Y.M Purwana, A. Chegenizadeh and H.Nikraz


Two very common devices used in unsaturated shear strength tests are suction-controlled direct shear apparatus and triaxial devices. During the tests, matric suction is applied to the specimen by controlling pore air and pore water pressure. Compared to using triaxial device, the suction-controlled direct shear apparatus is considered to be simpler to use due to the shorter drainage path of the specimen. A relatively new and a simpler technique for unsaturated shear strength is the suction-monitored direct shear test. The device is made by modifying the conventional direct shear apparatus with the attachment of low capacity tensiometer on its top cap; this is connected to an electronic readout monitor which assesses negative pore pressure during the test. The aim of the study is to find out the capability of the suction-monitored direct shear apparatus with regard to use in unsaturated soil. Conventional, as well as suction-monitored direct shear tests have been conducted on various compacted brown sand-kaolin clay mixtures of differing proportions. The first one was performed to obtain the effective shear strength parameters of the saturated specimen, with subsequent testing carried out on unsaturated specimens. The results indicates that in general, shear strength with respect to matric suction, exhibits a bilinear envelope with an initial value of φb higher than the effective internal
friction φʹ. This phenomenon was due to the effect of dilation on the strength development of sand. Matric suction of the specimen was generated indirectly by adjusting the specimen’s water content to the desired value, and for this purpose the Soil-Water Characteristic Curve (SWCC) can be a very useful tool for predicting the required water content. Despite the suction capacity of the tensiometer being relatively low, the suction-monitored direct shear apparatus was effective for soil with a low high entry value, such as sand or sand with relatively small portion of fine grained material.