Comparison of site-specific and empirical correlations for drained residual shear strength

Analyses of landslides, slopes and embankments often require reliable drained residual shear strength estimates for cohesive deposits involving pre-existing shear surfaces. Laboratory tests to measure residual shear strength can be costly and time consuming. Therefore, empirical correlations based on more readily available index properties such as liquid limit and clay-size fraction appear as an attractive alternative in practice. This paper presents residual strength and index properties for 25 natural samples, predominantly claystone and siltstone, collected from a single geologic setting over an approximately 0·25 km² site. The data show substantial variability in residual strength against any single index parameter considered in a site-specific correlation. Comparisons of these data, along with three additional data sets, generally indicated notable variability among data sets and relatively poor fit to published empirical correlations. Empirical correlations for residual strength may be useful in approximating the expected range of residual strength. However, given the significant variability in the data, both within and between data sets, empirical correlations should not entirely replace carefully performed site-specific laboratory tests measuring residual strength, when reliable values are required.