8.7 Simulation of seismic effect
Slope movement is secondary effect of earthquake or blasting. For rock slopes, earthquake induced slope movement is divided into falls and slides. In general there are two basic approaches to incorporate the seismic effect on slope stability i.e. Inertia slope stability analysis and weakening slope stability analysis.
It is preferred if material retains its shear strength during earthquake. Weakening slope stability analysis is preferred if material experiences significant shear strength reduction during earthquake. During liquefaction, there are two cases of weakening slope stability analyses. Flow slide develops when the static driving forces exceed shear strength of soil along failure surface. In lateral spreading, static driving forces do not exceed shear strength of soil along slip surface. Instead, driving forces only exceed resisting forces during those portions of earthquake that impart net inertial forces in the downward direction. This results in progressive and incremental lateral movement.
Soils which dilate during seismic shaking, or do not exhibit reduction in shear strength with strain, clay with low sensitivity, soils located above water table and landslides having distinct rupture surface are examples, where material retain shear strength during earthquake. Inertia slope analysis is preferred for these cases. Foliated or friable rock, which undergo fracture during earthquake, sensitive clays, overloaded soft and organic soils as well as loose soils located below water table and under liquefaction induced excess pore water pressure are examples where material experiences sufficient shear strength reduction during earthquake. Weakening slope stability analyses is preferred for them.