Mechanisms of rainfall-induced landsliding in Wellington fill slopes

<p>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.</p>

Mechanisms of rainfall-induced landsliding in Wellington fill slopes

<p>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.</p>

Toe drain size and slope stability of homogeneous embankment dam under rapid drawdown.

The slope stability of an embankment dam has been always a serious concern of any design team. Unfortunately, the information on the potential influence of a toe drain size on the slope stability of an embankment dam under rapid drawdown conditions is still scarce. This study investigated the potential effect of a toe drain size on the slope stability of a homogeneous embankment dam under rapid drawdown conditions. Three different sizes (5m, 10m, and 15m) of the toe drain were investigated under instantaneous (worst scenario) and 5 days (more realistic) drawdown rates with the help of numerical modeling in GeoStudio. From the results, it was observed that the pore-water pressures at the upstream face of the embankment decreased with the increase in the toe drain size, while the pore-water pressures at the downstream toe were increasing with the increase in the toe drain size. The factor of safety values were also observed to be affected by the changes in the toe drain size.

Effects of Hydropower Dam Operation on Riverbank Stability

The increasing number of extreme climate events has impacted the operation of reservoirs, resulting in drastic changes in flow releases from reservoirs. Consequently, downstream riverbanks have experienced more rapid and frequent changes of the river water surface elevation (WSE). These changes in the WSE affect pore water pressures in riverbanks, directly influencing slope stability. This study presents an analysis of seepage and slope stability for riverbanks under the influence of steady-state, drawdown, and peaking operations of the Roanoke Rapids Hydropower dam on the lower Roanoke River, North Carolina, USA. Although the riverbanks were found to be stable under all the discharge conditions considered, which indicates that normal operations of the reservoir have no adverse effects on riverbank stability, the factor of safety decreases as the WSE decreases. When the role of fluvial erosion is considered, riverbank stability is found to reduce. Drawdown and fluctuation also decrease the safety factor, though the rate of the decrease depends more on the hydraulic conductivity of the soils rather than the discharge pattern.

Long-term dynamic bearing capacity of shallow foundations on a contractive cohesive soil

The calculation of the long-term dynamic bearing capacity arises from the need to consider the generation of maximum pore-water pressure developed from a cyclic load. Under suitable conditions, a long-term equilibrium situation would be reached, when pore-water pressures stabilized. However, excess pore-water pressure generation can lead to cyclic softening. Consequently, it is necessary to define both the cohesion and the internal friction angle to calculate the dynamic bearing capacity of a foundation in the long term, being necessary to incorporate the influence of the self-weight of soil and therefore the width of the foundation. The present work is based on an analysis of the results of cyclic simple shear tests on soil samples from the port of El Prat in Barcelona. From these experimental data, a pore-water pressure generation formulation was obtained that was implemented in FLAC2D finite difference software. A methodology was developed for the calculation of the maximum cyclic load that a footing can resist before the occurrence of the cyclic softening. The type of soil studied is a contractive cohesive soil, which generates positive pore-water pressures. As a numerical result, design charts have been developed for long-term dynamic bearing capacity calculation and the charts were validated with the application of a real case study.