Advanced techniques in measuring residual shear strength parameters of cohesive soils

The objective of this research was to find an effective method of measuring the residual shear strength of fine grained cohesive soils (specifically interseam clay from Loy Yang mine) using the direct shear apparatus. Due to multiple sources of error, which arise during large deformation testing of fine grained soils in the standard direct shear apparatus, results obtained from this test method are generally of low quality. Two simple modifications are introduced to prevent rotation of the loading platen, rotation of the upper shear box half and specimen loss. Test results and finite element method modelling show that the modifications minimise or remove these sources of error. When the tests are conducted with the modifications in place, the residual shear strength results obtained are generally improved. The test results require a simple correction for friction which occurs between the two shear box halves. Residual friction angles obtained from the modified apparatus (ranging from 14.6° to 16.6°) are compared to results from the ring shear apparatus (12.1° and 14.0) and are, as expected, slightly higher. As the transition between the interseam clay and lignite seams is not always distinct, but sometimes gradual, the effect of organic content (lignite content) on the geotechnical properties of a fine grained cohesive soil (interseam clay) is analysed. An increase in organic content results in an increase in shear strength, residual friction angle, liquid limit and plastic limit, but a decrease in plasticity index. It also decreases the shear deformation required to reach the residual shear strength, in overconsolidated, remoulded specimens.