Reactivated landslides are generally associated with residual shear strength. Residual shear strength mobilized in such landslides are important parameters in slope stability and landslide simulation analysis. The present study involves estimation of residual shear strength of landslide reactivated soil at Marappalam area of Nilgiris district, Tamil Nadu state, India where landslides being reactivated frequently due to rainfall infiltration. A series of torsional ring shear tests performed under different normal loads to study the shear behaviour and estimate the residual shear strength parameters of soil. Since the residual soil at Marappalam area formed due to weathering, an effort made to study the weathering effects along with clay fractions and clay minerals on residual shear strength of soil. The test conducted on soil samples collected from various depths which is having different clay fractions with varying plasticity characteristics and fines content. The mineralogical and micro fabric analysis shows the presence of kaolinite clay mineral formed due to weathering. As the depth increases, the gradation of soil increases that decreases the clay content and degree of weathering. Hence, it is observed that the residual shear strength of the soil decreases with increase in clay content in presence of kaolinite clay mineral. © Springer Nature Switzerland AG 2018.