Landslides are the most common type of geohazard in mountainous areas. They have tremendous socio-economic impact, especially in developing countries. The reactivation of ancient/old landslides is a hot topic in engineering geology and geotechnical engineering. It is well known that the residual shear strength of slip zones determine the stability of ancient landslides. However, the measurement and interpretation of residual shear strength, especially for composite soils, is still a pending scientific and technical issue. This leads to uncertainties in estimating the strength parameters...