HYDROLOGICALLY-DRIVEN LANDSLIDE INITIATION AT AN INTENSELY INSTRUMENTED FIELD SITE: A SIMULATION-BASED PERSPECTIVE

Slope failures continue to be a global problem, causing loss of life and costly property damages. While conceptual understanding of hydrologically-driven landslides and debris flows has advanced considerably, major challenges remain for simulating failure initiation. In particular, hysteretic soil-hydraulic properties, bedrock fracture flow, representation of unsaturated soil mechanics, and initial hydrologic conditions remain enigmatic. We present results from a simulation-based study at a site in the Oregon Coast Range, where a detailed observational record provides the parameterization and evaluation data for the 3D unsaturated/saturated hydrologic-response model. Simulation of three controlled sprinkler experiments and nearly seven years of natural storms serve as the foundation for modeling of the hydrologic response to an intense storm that triggered slope failure within the instrumented field site in November of 1996. Estimates of Factor of Safety using an Infinite Slope approach in the region where the slope failure occurred suggest that the failure was triggered by positive pore-water pressures from bedrock fracture flow driven by the steep slope, echoing the conclusions drawn from piezometer data. The stability estimates also show that the wetting soil-water retention curve should be used rather than the more commonly used drying soil-water retention curve if hysteresis cannot be considered. Comparison of multi-year instrumental records and simulation results indicates that the seasonal saturated-zone dynamics can play an important role in setting initial conditions conducive to landslide initiation. Further work is needed to incorporate 3D unsaturated soil mechanics to better understand failure initiation in steep terrain.