Influence of clay mineralogy in residual shear strength of soil

Thirty four samples were prepared with different combination of bentonite, kaolin and standard sand. Those samples were tested in Imperial College type ring shear machine. Residual Φ had shown good relationship with consistency limit, especially liquid limit and plasticity index. There was distinct upper and lower bound in the relationship curve, for corresponding samples dominated by bentonite and kaolin respectively. Likewise, residual Φ of the soil samples from landside area and other 40 samples collected from debris flow area, slope failure area and volcanic eruption area had also shown good consistency with the data for sand-bentonite-kaolin mixture. Particle crushing along the shear zone, confirmed by particle size and mineralogical analysis of post shear samples, caused the increase in shearing zone thickness along with the displacement. Liquid limit of the sample from shearing zone was about 20% higher than the initial sample for the samples from main scarp of landslides and other disaster area. Measured fully softened/critical Φ did not vary considerably with residual Φ. However, fully softened shear intercept (C) was almost double of the residual c. Residual c was decreased with the increase in displacement. Residual shear strength from ring shear test and corresponding clay mineralogy was used to make contour of residual Φ, based on the proportion of smectite and kaolinite, plotted as two sides of a triangle, the third side of which represents the proportion of non clay minerals. There was less than 7% variation in the estimated and measured residual Φ for the natural soil samples. Seven different equations for the estimation of residual Φ with smectite proportion have been proposed, corresponding to 7 zones in the triangular relationship chart