Displacement mechanisms of slow-moving landslides in response to changes in porewater pressure and dynamic stress

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

The integration of terrestrial laser scanning and numerical modelling in landslide investigations

Terrestrial laser scanning (TLS) can be used to either complement or replace traditional methods of characterizing both the geometry and structural geology of unstable slopes. TLS data collected from a failed bedrock slope threatening the main east–west highway in the Bhutan Himalaya are presented and interrogated for structural information. The structural data, along with TLS-derived slope geometry and cross-sectional profiles, are suitable for use within commercially available slope stability packages to derive solutions for the causes of instability, likely geometry of failure, and future activity under varied scenarios. The methods also allow the possibility of future model verification and calibration though TLS monitoring. The results of TLS-based numerical modelling utilizing a commercially available code are presented and the implications for slope surveying, numerical modelling, monitoring and management are discussed

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

Spatial distributions of earthquake-induced landslides and hillslope preconditioning in northwest South Island, New Zealand

Current models to explain regional-scale landslide events are not able to account for the possible effects of the legacy of previous earthquakes, which have triggered landslides in the past and are known to drive damage accumulation in brittle hillslope materials. This paper tests the hypothesis that spatial distributions of earthquake-induced landslides are determined by both the conditions at the time of the triggering earthquake (time-independent factors) and the legacy of past events (time-dependent factors). To explore this, we under\-take an analysis of failures triggered by the 1929 Buller and 1968 Inangahua earthquakes, in the northwest South Island of New Zealand. The spatial extents of landslides triggered by these events were in part coincident. Spatial distributions of earthquake-triggered landslides are determined by a combination of earthquake and local characteristics, which influence the dynamic response of hillslopes. To identify the influence of a legacy from past events, we first use logistic regression to control for the effects of time-independent variables. Through this analysis we find that seismic ground motion, hillslope gradient, lithology, and the effects of topographic amplification caused by ridge- and slope-scale topography exhibit a consistent influence on the spatial distribution of landslides in both earthquakes. We then assess whether variability unexplained by these variables may be attributed to the legacy of past events. Our results suggest that hillslopes in regions that experienced strong ground motions in 1929 were more likely to fail in 1968 than would be expected on the basis of time-independent factors alone. This effect is consistent with our hypothesis that unfailed hillslopes in the 1929 earthquake were weakened by damage accumulated during this earthquake and its associated aftershock sequence, which influenced the behaviour of the landscape in the 1968 earthquake. While our results are tentative, they suggest that the damage legacy of large earthquakes may persist in parts of the landscape for much longer than observed sub-decadal periods of post-seismic landslide activity and sediment evacuation. Consequently, a lack of knowledge of the damage state of hillslopes in a landscape potentially represents an important source of uncertainty when assessing landslide susceptibility. Constraining the damage history of hillslopes, through analysis of historical events, therefore provides a potential means of reducing this uncertainty

SURVEYING IN THE STUDIES OF THE STABILITY OF EARTHY CONSTRUCTIONS, FOCUS ON SELECTED HISTORICAL MOUNDS IN KRAKOW (POLAND)

Mounds, as anthropogenic constructions of a very delicate structure, are subdued to constant changes, which, due to the impact of external factors (prolonged precipitation, tremors) are subdued to deformations in the form of mass movements. These phenomena usually have the character of mild soil creep in time and sometimes, as a result of rapid loss of stability, they are seriously damaged by landslide. This phenomenon causes temporary exclusion of the object from use. In the framework of the protection of these objects, the maintenance was carried out within the preventive measures referring to the construction and surveying monitoring of the geometry changes in time, as a result of phenomena taking place in the ground medium under the influence of environmental factors causing strains. The process of the deformation of mounds is similar to the characteristic, according to the Terzagie's theory. The application of surveying technologies of high precision allows the monitoring of changes in their geometry in time. The properly defined study area and the proper selection of measurement technology in the aspect of the accuracy of the prediction of changes, can efficiently help in defining the scale of deformations in the decisive process referring to the way of efficient protection of barrows. The article presents the results of point monitoring carried out with surveying technologies within 11 measurement series carried out on the selected measurement base of the Wanda Mound. The use of measurement technologies of integrated and specialist software, allows complex assessment of the degree of deformation and the trends of these changes in time, as well as identifying anomaly zones in the framework of the landslide monitoring