Reduction in Factor of Safety for Various Landslide Repair Works with Earthquake Induced Ground Shaking

While designing landslide repair works, static factor of safety for the landslide geometry right after the initiation of slide is considered as one and stabilization measures are designed to increase the planned safety factor by 50%. However, very few slides are checked for the seismic ground motion. After 2004 Central Niigata Earthquake, which triggered several thousands of reactivated landslides, geo-practitioners have realized the importance of rechecking the static safety factor for different seismic coefficients that can occur during different seasons. We collected topographic and subsoil exploration information of a reactivated landslide area and performed numerous slope stability analyses with the Method of Slice, Bishop's Simplified Method, Janbu's Corrected Method, Spencer's Method, and the Army Corp of Engineer's Method for static condition and designed prevention measures for combination of, a) ground water level reduction with subsurface drainage, b) toe loading with a retaining wall, and c) application of piles to increase the safety factor to 1.5. Then, modifying the same equations for seismic slope stability analysis using Newmark's Pseudo-static Analysis Method, the seismic factor of safety for the designed repair works was checked for different pseudo-static seismic coefficients. The result shows that the combination of prevention measures that provides the best stability condition for static situation does not necessarily provide the optimum stability for dynamic loading.