NUMERICAL STUDY ON THE DEFORMATION MECHANISM OF A SHALLOW LANDSLIDE IN NORWOOD TUFF, NORTHERN UTAH

Over the past decade, land development activities on hillsides in northern Utah have resulted in a significant increase in landslide activity throughout the region. The majority of recent landslides are shallow and occurred on cut gentle slopes especially during spring and early summer due to snowmelt induced elevated groundwater tables. The geologic material documented at numerous landslide sites is a soft gray-green completely decomposed Norwood Tuff. The present study employs numerical simulation in order to better understand the mechanism of a shallow landslide in completely decomposed Norwood Tuff. Detailed slope surface geometry obtained from laser-scan surveys together with strength and stress-strain parameters derived from laboratory triaxial tests on undisturbed samples of completely decomposed Norwood Tuff collected from the landslide site were used in a finite-element analysis addressing the relationship between the ground surface deformation patterns and yielding behavior of the slide mass. The numerical results indicate a gradual retreat of the yield zone with progressive landslide deformation, which eventually becomes concentrated within the accumulation zone of the landslide, compared to a well-developed yield zone within the entire slide mass at the onset of landslide movement. Limit equilibrium stability analyses along potential sliding surfaces of extent limited within the yield zone of the displaced slide mass produce lower safety factors than an analysis based on the original sliding surface comprising the entire slide mass.