Progressive shear-surface development in cohesive materials; implications for landslide behaviour

The aim of this study was to investigate mechanisms of progressive shear surface development using a series of specialised triaxial cell tests. Intact and remoulded samples of Gault Clay from the Ventnor Undercliff on the Isle of Wight in southern England were subjected to pore pressure reinflation (PPR) testing in a triaxial cell, in which failure is generated by increasing pore pressure under a constant total stress state. In addition, a novel very long term (> 500 days) creep test was undertaken, in which the sample eventually failed at a constant stress state below the failure envelope. The experiments showed that undisturbed samples of the Gault Clay failed in a brittle manner, generating a linear trend when plotted using the Saito technique. On the other hand, remoulded samples showed ductile behaviour, as indicated by a non-linear Saito trend. A number of otherwise identical PPR tests were conducted in which the rate of increase in pore water pressure was varied. These tests showed that strain rate generated at any point in the PPR tests depended on both the effective stress and the rate of change of effective stress. The latter is important because a change in stress generates a change in strain. Thus, whilst tests at different rates of change of effective stress are similar when plotted in q–p′ space and in strain–p′ space, they are markedly different in strain rate–p′ space. The long term creep test failed when the stress state had been constant for over 80 days. This mechanism was reminiscent of creep rupture, occurring below the failure envelope defined in the conventional experiments. We conclude that first time failure in the Gault Clay is a progressive mechanism dominated by the development of micro-cracking, which leads to strain localisation and the development of one or more shear surfaces at failure. Whilst this mechanism may usually occur in response to a change in stress, the study indicates that failure can develop progressively. In the remoulded Gault Clay shear strains cannot localise along a singular shear surface. The results provide new insight into the mechanisms of landslide movement operating within the Ventnor landslide complex and indicate that present movements are likely to be occurring on a pre-existing shear surface. The lab tests suggest that this material is unlikely to undergo catastrophic failure

Are microseismic ground displacements a significant geomorphic agent?

This paper considers the role that microseismic ground displacements may play in fracturing rock via cyclic loading and subcritical crack growth. Using a coastal rock cliff as a case study, we firstly undertake a literature review to define the spatial locations that may be prone to microseismic damage. It is suggested that microseismic weakening of rock can only occur in ‘damage accumulation zones’ of limited spatial extent. Stress concentrations resulting from cliff height, slope angle and surface morphology may nucleate and propagate a sufficiently dense population of microcracks that can then be exploited by microseismic cyclic loading. We subsequently examine a 32-day microseismic dataset obtained from a coastal cliff-top location at Staithes, UK. The dataset demonstrates that microseismic ground displacements display low peak amplitudes that are punctuated by periods of greater displacement during storm conditions. Microseismic displacements generally display limited preferential directivity, though we observe rarely occurring sustained ground motions with a cliff-normal component during storm events. High magnitude displacements and infrequently experienced ground motion directions may be more damaging than the more frequently occurring, reduced magnitude displacements characteristic of periods of relative quiescence. As high magnitude, low frequency events exceed and then increase the damage threshold, these extremes may also render intervening, reduced magnitude microseismic displacements ineffective in terms of damage accumulation as a result of crack tip blunting and the generation of residual compressive stresses that close microcracks. We contend that damage resulting from microseismic ground motion may be episodic, rather than being continuous and in (quasi-)proportional and cumulative response to environmental forcing. A conceptual model is proposed that describes when and where microseismic ground motions can operate effectively. We hypothesise that there are significant spatial and temporal limitations on effective microseismic damage accumulation, such that the net efficacy of microseismic ground motions in preparing rock for fracture, and hence in enhancing erosion, may be considerably lower than previously suggested in locations where high magnitude displacements punctuate ‘standard’ displacement conditions. Determining and measuring the exact effects of microseismic ground displacement on damage accumulation and as a trigger to macro-scale fracture in the field is not currently possible, though our model remains consistent with field observations and conceptual models of controls on rockfall activity

Modelling the effects of sediment compaction on salt marsh reconstructions of recent sea-level rise

This paper quantifies the potential influence of sediment compaction on the magnitude of nineteenth and twentieth century sea-level rise, as reconstructed from salt marsh sediments. We firstly develop a database of the physical and compression properties of low energy intertidal and salt marsh sediments. Key compression parameters are controlled by organic content (loss on ignition), though compressibility is modulated by local-scale processes, notably the potential for desiccation of sediments. Using this database and standard geotechnical theory, we use a numerical modelling approach to generate and subsequently ‘decompact’ a range of idealised intertidal stratigraphies. We find that compression can significantly contribute to reconstructed accelerations in recent sea level, notably in transgressive stratigraphies. The magnitude of this effect can be sufficient to add between 0.1 and 0.4 mm yr−1 of local sea-level rise, depending on the thickness of the stratigraphic column. In contrast, records from shallow (<0.5 m) uniform-lithology stratigraphies, or shallow near-surface salt marsh deposits in regressive successions, experience negligible compaction. Spatial variations in compression could be interpreted as ‘sea-level fingerprints’ that might, in turn, be wrongly attributed to oceanic or cryospheric processes. However, consideration of existing sea-level records suggests that this is not the case and that compaction cannot be invoked as the sole cause of recent accelerations in sea level inferred from salt marsh sediments

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

Controls on post-seismic landslide behavior in brittle rocks

Earthquakes trigger widespread landsliding in tectonically active landscapes. The effects of strong ground shaking on hillslope stability persist into the post-seismic stage; rates of landsliding remain elevated in the years following an earthquake. The mechanisms that control the spatial pattern and rate of ongoing landsliding are poorly constrained, hindering our ability to reliably forecast how landscapes and landslide hazard evolve. To address this, we undertook a detailed geotechnical investigation in which we subjected representative rock samples to dynamic loading, simulating the effects of earthquake ground shaking on hillslopes of different configuration. Our results indicate that post-seismic hillslope strength is not an intrinsic rock property; rather, it responds to the amplitude of imposed dynamic loads and the degree of pre-existing shear surface formation within the rock. This path-dependent behavior results from differences in the character of fractures generated by dynamic loads of different amplitude, and the ways in which apertures are mobilized or degraded in subsequent (post-seismic) shearing. Sensitivity to dynamic loading amplitude is greater in shallow landslides in which shear surfaces are yet to fully form; such hillslopes can be strengthened or weakened by earthquake events, depending on their characteristics. In contrast, deeper landslides on steeper hillslopes in which shear surfaces have largely developed are less likely to display differences in behavior in response to dynamic loading because strain accumulation along pre-existing fractures is dominant. Our results demonstrate the need to consider path-dependent hillslope stability in numerical models used to forecast how landscapes respond to earthquakes and how post-seismic hazard evolves

On the impact of urban landslides

Although considerable attention has been paid to the issue of urban landslides in the literature since Alexander's 1989 paper on the topic, few attempts have been made to quantify their importance. In this paper, such an attempt is made, based upon global landslide fatality data from 2005. First, a new definition for the term ‘urban landslide’ is proposed. This is then applied in the analysis of the 2005 landslide fatality data. It is shown that fatal urban landslides occur primarily in the tropical regions, whereas rural landslides dominate the mid- and high-latitude regions. The causes for this are explored using a detailed dataset for landslide fatalities in Nepal during the period 1968–2006. It is proposed that the key difference between the tropical and the extra-tropical regions is that in the case of the former areas, landslides are mostly triggered as a result of disturbance of the thick weathering layers, whereas in the latter case they tend to be shallow, high-energy events triggered by seasonal rainfall. Finally, an analysis is presented of the distribution by latitude of papers presented in the ‘Urban Landslides’ session at the International Association for Engineering Geology and the Environment (IAEG) Congress in 2006. It is shown that the distribution of papers mirrors the distribution of all landslide types, and of the global population, and not the distribution of urban landslides. This suggests that urban landslides in the tropical regions, specifically, are not receiving adequate attention from the research community