MODEL EXPERIMENTS ON FAILURE MECHANISM OF VOLCANIC SLOPES DURING SEISMIC LOADING AND RAINFALL

Abstract

This study aims to clarify the mechanical behavior until failure of slopes formed from pyroclastic fall deposits (Tarumae volcanic soil: Ta-d) in Atsuma Town, Hokkaido, Japan, under seismic loading and rainfall conditions. Experiments utilized highly fragile Ta-d PR layer samples for the model slopes. The experiment used a model soil container with a one-dimensional cyclic loading apparatus for seismic loading tests and a spray nozzle system to make it rain artificially. Instrumentation consisted of pore water pressure gauges, dielectric soil moisture sensors, and accelerometers. Particle breakage of soils was quantified by measuring the fine content increase rate (ΔFc) via sieve analysis of samples before and after testing. Shaking table tests revealed that strong seismic loading caused an increase in pore water pressure, resulting in slope failure, even at low saturation. Crushing of soil particles was confirmed under seismic loading, indicating that cyclic shearing in volcanic coarse-grained soils can induce particle breakage and lead to earthquake-induced slope instability. Rainfall tests showed that slopes failed due to infiltration, with pore water pressure and moisture changes varying by initial water content. Particle crushing of soils was also observed during rainfall, more so in wetter conditions. A unique relationship was found between initial and failure-time water content, consistent with other volcanic soils in Hokkaido. These findings offer fundamental data for predicting slope collapse and suggest practical stability assessment via field measurements.