Significance of substrate soil moisture content for rockfall hazard assessment

摘要

Rockfall modelling is an important tool for hazard analysis insteep terrain. Calibrating terrain parameters ensures that the model resultsmore accurately represent the site-specific hazard. Parameterizing rockfallmodels is challenging because rockfall runout is highly sensitive to initialconditions, rock shape, size and material properties, terrain morphology, andterrain material properties. This contribution examines the mechanics ofterrain impact scarring due to rockfall on the Port Hills of Christchurch,New Zealand. We use field-scale testing and laboratory direct shear testingto quantify how the changing moisture content of the loessial soils caninfluence its strength from soft to hard, and vice versa. We calibrate the three-dimensional rockfall model RAMMS by back-analysingseveral well-documented rockfall events that occurred at a site with dryloessial soil conditions. We then test the calibrated “dry” model at asite where the loessial soil conditions were assessed to be wet. Thecalibrated dry model over-predicts the runout distance when wet loessialsoil conditions are assumed. We hypothesize that this is because both theshear strength and stiffness of wet loess are reduced relative to the dryloess, resulting in a higher damping effect on boulder dynamics. For bothrealistic and conservative rockfall modelling, the maximum credible hazardis usually assumed; for rockfall on loess slopes, the maximum crediblehazard occurs during dry soil conditions.