A structural state model for interpreting residual strength transition behavior of land-sliding soils with different clay fractions

Abstract Residual friction angles of soils are commonly determined by laboratory tests, which are time-consuming and costly. It is more desirable to find practical means of estimating the residual strength of soils in the slip zones. Previous researchers attempted to correlate the soil residual strength with clay fraction or Atterberg limits using empirical equations. A large amount of laboratory data demonstrate that the magnitude of residual strength decreases with the increase in the clay content. However, the physical mechanisms of such correlation are not well interpreted. In addition, the decreasing trend has never been modeled with a unique empirical or semi-empirical equation because of the variety of influencing factors such as the clay mineralogy and the applied normal stress. In this study, a new approach is proposed to estimate the residual friction angles of land-sliding soils. The residual friction angle of soils is derived as a weighted average result of the friction angles of nonclay minerals and clay matrix. A structural state coefficient is used as the weight function, and the plasticity index is used to consider the difference in clay mineralogy. The percolation theory is used to physically explain and illustrate the structural state transitional behavior of soils with different clay fractions. The results demonstrate that the semi-empirical approach can be used for predicting residual friction angles of a wide range of soils that differ in geology, soil type, mineralogical properties, and shear strength. The effects of applied normal stress, pore fluid salinity, and particle size on the estimations are also discussed.