Analysis of failure pattern in cut slopes of bedded sandstone: a case study

Anisotropic sedimentary rocks are common around the world. Several factors, such as presence of primary and tectonic structures, type of weathering and rate of cohesion loss due to strength degradation, have significant effect on stability of slopes. In the present research, Geological strength index (GSI) has been used for quantitative comparison of rock mass quality and to investigate the rock mass damage. Other indicators of rock mass strength degradation, such as slake durability index tests, weathering index, were studied to understand the loss of strength with time leading to progressive failure in slopes. The failure pattern in bedded sandstones is controlled by bedding inclination, primary structures, vertical fractures. Folding affects bedding plane inclination and modify the failure mode along cut slopes. The cross-stratification significantly reduces rock quality and creates overhangs. The shape and size of overhangs are controlled by the presence of vertical cracks in addition to the type and scale of cross-stratification and failure generally occurs by block fall rather than block slide. The peculiar steeply plunging wedges associated with vertical fractures are very common in sub-horizontal bedded sandstone. Failure usually occurs by wedge fall rather than slide. Differential weathering affects the pinching strata against the massive beds and the characteristic wedges are common occurrence above this considerably fractured zone. Tectonic structures such as folds modify the style of failure in rock slopes as seen from kinematic analysis. The overall trend of lineaments is parallel to the strike of bedding plane (roughly E-–W) and the lineaments may be associated with reactivation of the major fault passing through the area. This study illustrates that the impact of tectonic damage further enhances the strength degradation caused by weathering, and climatic slaking. Overall rock mass quality lies in good to fair category, but preliminary stability analysis depicts slope in partially stable to unstable condition along SH-5; however, rock mass quality and cut slopes are relatively stable along SH-5A.