On August 10, 2005 a landslide of approximately 133,000 m3 occurred along the Lahore-Islamabad Motorway (M-2) near the village of Simbal, in the Salt Range area of Pakistan. A program of research was undertaken to evaluate the likely impacts of percent saturation and bulk density on mobilized shear strength along the basal rupture surface of the Simbal Landslide. A series of direct shear tests [1] were performed on remolded samples at different densities. The percent saturation was then varied on these samples to evaluate its impact on mobilized shear strength. The results of these tests suggest that soil cohesion and friction tended to decrease with increasing percent saturation. The tests also showed that the shear strength parameters tended to increase with increasing dry density; however, all of the samples exhibited a noticeable loss of shear strength with increasing degree of saturation, independent of soil density.

Limit equilibrium slope stability analyses were performed along the most probable failure planes, based on shear strength parameters corresponding to degrees of saturation, varying between 20% to 100% over a wide range of in-situ densities. The results of these analyses suggests that the factor of safety drops significantly, (from FS = 1.6 down to 0.41) as the degree of saturation approaches unity.

These results suggest that the causative factor in triggering the Simbal Landslide was the partial saturation of the zone that developed the basal rupture plane. As rain infiltrated the slope, the bulk unit weight of the soil increased along well the pore water pressure, while the shear strength along the developing plane of rupture decreased sufficiently to concentrate shear strain when the material became more than 60% saturation (FS < 1.0).