Early detection of first-time slope failures using acoustic emission measurements: large-scale physical modelling

A Smith,N Dixon,G J Fowmes

doi:10.1680/jgeot.15.p.200

Abstract

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.

Highlights

Shear surfaces can develop in slopes formed of strain-softening materials after very small deformations (Skempton, 1985; Bromhead, 2004)
Empirical acoustic emission (AE) rate–slope velocity relationships were derived for three different active waveguide backfill types, which allow slope displacement rates to be quantified from measured AE rates when using these instruments and monitoring in the 20–30 kHz range
(a) Evidence has been obtained showing that AE monitoring of active waveguides can both detect shear surface development and quantify increasing rates of movement during slope failure, thereby providing an early detection of slope instability

Summary

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. 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

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