Mechanisms of rainfall-induced landsliding in Wellington fill slopes

Recent landslides from Wellington fill slopes have occurred as potentially hazardous, mobile debris flow-slides with long runouts during heavy rainstorms. Globally, catastrophic landslides from fill slopes are well documented, and in many instances their rapid failure and long runout suggests that their shear zones may be subject to liquefaction. Various generations of fill slopes throughout Wellington, and urban New Zealand, have been constructed using different practices and at variable scales. Despite this, very few laboratory based studies to determine how different fill slopes may perform during rainstorms have been attempted, as conventional laboratory tests do not adequately simulate the failure conditions in the slope.  This study uses a novel, dynamic back-pressured shear box to conduct rapid shear and specialist pore pressure inflation tests in order to replicate rainfall induced failure conditions in fill slopes with different consolidation histories and particle size characteristics. During each test, excess pore-water pressures and deformation were monitored until failure in order to determine the failure mechanisms operating.  This study demonstrates that the failure mechanisms in fill slopes are strongly influenced by the consolidation history and particle size characteristics of the shear zone materials. In over-consolidated and fine grained (< 0.4 mm) fills where cohesion is present, brittle failure was observed. In these materials, failures occur more rapidly but require much higher pore-water pressures to initiate. Conversely, normally-consolidated fill slopes constructed from coarser material (0.4 - 2 mm) fail through ductile deformation processes, which typically initiate at much lower pore-water pressures but result in a less rapid slope failure. Although liquefaction was not observed, excess pore-water pressures can be generated during rapid shearing, indicating that liquefaction could occur after a landslide has initiated in conditions where excess pore-water pressures are unable to dissipate away from the shear zone. These results provide new insights into the types of failure that may be anticipated from different fill slopes, the hazards they may pose and potential mitigation measures that could be implemented.