Relating equal smectite content and basal spacing to the residual friction angle of soils

The residual shear strength of soils is increasingly becoming an active research area, as landslides occur almost daily all over the world. Since Skempton published his landmark paper in 1964, several researchers proposed correlations between soil index properties and residual friction angle of soils for preliminary estimates. However, recent data gathered from Japanese colluvium soils indicate that previously proposed charts correlating soil index properties with residual friction angle yield very poor estimates. In this study, first previously published Japanese data are reevaluated to establish a theoretical foundation for such correlations. Then, it is shown that equivalent smectite content (ESC) largely controls the residual friction angle of soils. In the second stage of the paper using the data of several researchers, it is shown that there is piecewise linear correlation between the equivalent basal spacing (EBS) and the residual friction angle. This study lays down a foundation for theoretical correlation between EBS and residual friction angle of soils.