It has long been recognized that the slip zones are the weakest features of ancient/old landslide masses and that the mechanical properties, especially the residual strength, of these zones play a critical role in the stability and reactivation of these former landslide masses. In the TGP area, approximately 2,490 ancient landslides of various scales have been identified during the earlier field investigations. The reactivation potential of these landslides poses a great threat to navigation in the reservoir and to operation of the TGP dam.