Abstract
Abstract. Although slow-moving landslides represent a substantial hazard, their detailed mechanisms are still comparatively poorly understood. We have conducted a suite of innovative laboratory experiments using novel equipment to simulate a range of porewater pressure and dynamic stress scenarios on samples collected from a slow-moving landslide complex in New Zealand. We have sought to understand how changes in porewater pressure and ground acceleration during earthquakes influence the movement patterns of slow-moving landslides. Our experiments show that during periods of elevated porewater pressure, displacement rates are influenced by two components: first an absolute stress state component (normal effective stress state) and second a transient stress state component (the rate of change of normal effective stress). During dynamic shear cycles, displacement rates are controlled by the extent to which the forces operating at the shear surface exceed the stress state at the yield acceleration point. The results indicate that during strong earthquake accelerations, strain will increase rapidly with relatively minor increases in the out-of-balance forces. Similar behaviour is seen for the generation of movement through increased porewater pressures. Our results show how the mechanisms of shear zone deformation control the movement patterns of large slow-moving translational landslides, and how they may be mobilised by strong earthquakes and significant rain events.
Highlights
Landslides are a significant natural hazard, responsible for up to 14 000 fatalities per annum globally (Petley, 2012; Froude and Petley, 2018)
We compare the displacement patterns we observe in the laboratory to high-resolution monitoring records collected from the landslide – along with numerical modelling of (1) static stability, caused by changes in porewater pressure measured above the slide surface, and (2) dynamic stability, potential ground displacements caused by earthquakes – in order to get insights into the processes controlling the complex movement patterns observed in this landslide complex
Strains when accelerated by strong earthquakes. These results show that dynamic changes in shear stress, which exceed the monotonic failure envelope of the shear surface material, result in permanent landslide displacement and movement rates several orders of magnitude greater than would be anticipated by similar magnitudes of normal effective stress reduction during periods of elevated porewater pressure
Summary
Landslides are a significant natural hazard, responsible for up to 14 000 fatalities per annum globally (Petley, 2012; Froude and Petley, 2018). In this study we present a suite of laboratory experiments that simulate a range of porewater-pressure and dynamicstress scenarios on samples of smectite-rich clay taken from the slide surface of the Utiku landslide, a very large slowmoving slip Such smectite-rich clays control many landslides in this area of New Zealand (Thompson, 1982; Massey, 2010). We compare the displacement patterns we observe in the laboratory to high-resolution monitoring records collected from the landslide – along with numerical modelling of (1) static stability, caused by changes in porewater pressure measured above the slide surface, and (2) dynamic stability, potential ground displacements caused by earthquakes – in order to get insights into the processes controlling the complex movement patterns observed in this landslide complex. The landslide has generally moved slowly (varying between 16 mm yr−1 and 1.6 m yr−1; Stout, 1977) but it has repeatedly damaged the North Island Main Trunk railway (NIMT) and State Highway 1 (SH1), both of which cross the landslide (Fig. 1a and b)
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