Early warning systems for slope instability need to alert users of accelerating slope deformation behaviour to enable safety-critical decisions to be made. This study shows that acoustic emission (AE) monitoring of active waveguides (i.e. a steel tube with a granular backfill surround installed through a slope) can both detect shear surface development and quantify increasing rates of movement during slope failure, thereby providing an early detection of slope instability. A large-scale physical model was designed and built to simulate slope failures on elements of soil, through which full-scale active waveguides were installed. A shear surface develops in each test and the sliding mass accelerates during failure, reaching velocities greater than 300 mm/h and shear deformations of 50 mm. Continuous measurements were obtained to examine the behaviour of active waveguides subjected to first-time slope failure dynamics (i.e. development of new shear surfaces and accelerating deformation behaviour). Comparisons with continuous subsurface deformation measurements show that AE detection began during shear surface formation, and AE rates increased proportionally with displacement rates as failure occurred. Empirical AE rate-slope velocity relationships are presented for three granular backfill types, which demonstrate that generic AE rate-slope velocity relationships can be obtained for groups of backfill types; these relationships allow displacement rates to be quantified from measured AE rates to provide early detection of slope instability.KEYWORDS: deformation; failure; field instrumentation; landslides; monitoring; slopes INTRODUCTION Shear surfaces can develop in slopes formed of strain-softening materials (e.g. overconsolidated clay) after very small deformations (millimetres) (Skempton, 1985;Bromhead, 2004). Shear zones develop when the shear stress exceeds the peak shear strength locally within the slope, causing reductions in strength to occur. These shear zones propagate through the slope, developing a continuous shear surface, leading to slope failure (Skempton, 1964;Skempton & Petley, 1967;Chandler, 1984;Leroueil, 2001). These first-time failures can have high post-failure velocities and experience large displacements, leading to potentially catastrophic consequences. During this failure process, the rate of movement increases by orders of magnitude; from the gradual development of a shear surface producing low velocities, to the high velocities that are reached after the shear surface forms, shear strength reduces and failure occurs. Early warning of this process (i.e. shear surface development and accelerating deformation behaviour) is critical to enable evacuation of vulnerable people and timely repair and maintenance of critical infrastructure.The Selborne cutting stability experiment (Cooper et al., 1998) provides a detailed example of a first-time failure. The progressive failure process began at the toe of the slope shortly after the cutting was formed, and the shear surface retrogressed up-slope le...

It has become common practice to conduct numerical analyses to assess the stability and integrity of side slope landfill lining systems, however, information that can be used to validate such models is extremely limited. This paper contains data from a series of large scale laboratory tests containing geosynthetic elements of a multilayered lining system exposed to downdrag forces from a compressible synthetic waste material (rubber crumb). These data are compared to the results from numerical analysis of the same problem. The numerical results are from initial best estimate analyses, with interface and synthetic waste properties derived from a laboratory testing programme and geosynthetic material properties from manufacturers. The observed trends of tensile stresses in the geosynthetics and relative displacements at interfaces in the laboratory testing are reproduced by the numerical models to an acceptable degree of accuracy that would be appropriate, using site specific input data, for use in commercial design. KeywordsLandfill design, numerical analysis, geomembrane tension, multiple layer lining systems, interface displacements.

Seasonal wetting and drying stress cycles can lead to long-term deterioration of high-plasticity clay slopes through the accumulation of outward and downward deformations leading to plastic strain accumulation, progressive failure and first-time failures due to seasonal ratcheting. Using recent advances in hydro-mechanical coupling for the numerical modelling of unsaturated soil behaviour and development of nonlocal strain-softening regulatory models to reduce mesh dependency of localisation problems, the mechanism of seasonal ratcheting has been replicated within a numerical model. Hydrogeological and mechanical behaviours of the numerical model have been compared and validated against physical measurements of seasonal ratcheting from centrifuge experimentation. Following validation, the mechanism of seasonal ratcheting was explored in a parametric study investigating the role of stiffness and long-term behaviour of repeated stress cycling extrapolated to failure. Material stiffness has a controlling influence on the rate of strength deterioration for these slopes; the stiffer the material, the smaller the seasonal movement and therefore the more gradual the accumulation of irrecoverable strains and material softening. The validation presented provides confidence that the numerical modelling approach developed can capture near-surface behaviour of high-plasticity overconsolidated clay slopes subject to cyclic wetting and drying. The approach provides a tool to further investigate the effects of weather driven stress cycles and the implication of climate change on high-plasticity clay infrastructure slopes.

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