Dynamic displacement analysis of a shallow landslide in Norwood Tuff

Northern Utah has a long history of landslides occurring on natural and man-made slopes. Earthquake activity in this region may exacerbate the incidence of landslides, thus increasing the potential of landslide related damage to residential structures and transportation corridors. In this context, the present study addresses the earthquake response in both drained and undrained conditions of a typical, shallow landslide in northern Utah occurring in completely decomposed Norwood Tuff. The slide mass geometry was obtained using a 2-D seismic refraction profile and previously collected geotechnical borehole data. The Newmark sliding block analysis was employed with a translational failure mechanism to determine the permanent dynamic slope displacements under various input accelerograms. The yield coefficient for both drained and undrained conditions was obtained from pseudo-static limit-equilibrium slope stability analyses. Based on the computational results, a methodology to evaluate the peak ground acceleration threshold that would distinguish between insignificant ground movement and potentially damaging slope displacements during an earthquake was developed. A normalized Arias intensity was subsequently used to compare the dynamic displacements in dry and partially saturated conditions. Dry slopes comprised of completely decomposed Norwood Tuff that experience earthquake accelerations greater than 0.55g are considered unsafe against damaging displacements. Slopes that are partially saturated or contain a perched water table are considered unsafe when earthquake accelerations exceed 0.47 g. Dynamic displacements in completely decomposed Norwood Tuff increase exponentially for partially saturated slopes relative to dry slopes as the normalized Arias intensity increases.