Debris flow-slide initiation mechanisms in fill slopes, Wellington, New Zealand

Although catastrophic debris flow-slides from anthropogenic fill slopes are common, their failure mechanisms during both earthquakes and extreme rainfall events remains under-studied. We have used a suite of tests using a dynamic back-pressured shear box on fill materials with varying grain size characteristics and stress histories to explore their potential failure mechanisms in response to seismic loading and elevated pore water pressures. Our experiments demonstrate that whilst looser coarse-grained fills display a ductile style of deformation in response to elevated pore water pressures, denser fine-grained fills display a brittle style of deformation and require higher levels of pore water pressure to initiate failure. Dynamic loading of these fills did not generate significant excess pore water pressures or liquefaction but instead resulted in densification and seismic compression. This process of densification made these fills more prone to brittle failure in response to subsequent elevation of pore water pressures. Our results show that grain size characteristics and stress history (density) significantly impact fill slope failure mechanisms and indicate that, although in some instances, fill slopes may be strengthened by earthquake shaking, seismic compression often results in significant deformation, resulting in tension crack formation, severing of services and the development of new pore fluid pathways. This may allow high pore water pressure to develop in the slope in future rainstorms, which would increase their vulnerability to rapid debris flow-slides. The results provide new insights into the styles of failure that may be anticipated from different fill slopes and the hazards they may pose. These findings may help to inform future long-term management practices for engineered fill slopes in dynamic environments.