Importance of Numerical Analyses to Inform Laboratory Testing for Offshore Slope Stability Assessment

Abstract

An offshore pipeline will cross a series of steep rocky escarpments. Borehole samples show that there are thin layers of uncemented sand deposits at the head and middle of the largest escarpment. Bathymetry data also show evidence of past landslide activity, and the escarpments lie in a moderately seismic area. As a result, advanced, site-specific slope stability analyses were carried out to assess the failure potential of the slope in the event of project-specific design earthquakes. Analyses included deterministic and probabilistic static and pseudo-static slope stability analyses as well as dynamic time domain analyses. The initial results showed unacceptably low factors of safety against slope instability in the event of an earthquake based on empirically derived soil parameters. Consequently, additional advanced laboratory testing was performed to characterize better the dynamic properties and strength of the critical soil layer, and the pseudo-static and time domain analyses were repeated. Using the updated soil properties, the factor of safety and expected displacements during seismic shaking were reduced to satisfactory levels. The case history presented in this paper provides a clear example of the benefit of performing additional advanced laboratory testing to reduce uncertainty after numerical analyses have identified critical soil layers.