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

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.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...

An acoustic emission landslide early warning system for communities in low-income and middle-income countries

“…Extensive research including multiple field trials (e.g. Berg et al 2018;Dixon et al 2015a;Dixon et al 2015b;Smith and Dixon 2015;Smith et al 2014;Smith et al 2017a) has demonstrated that AE rates are indicative of slope displacement rates, and hence, this instrumentation is defined as a slope displacement rate sensor. Activity status of the slope (e.g.…”

“…Accelerating landslide events have been simulated in two ways: (i) a large scale simulated first-time landslide failure experiment using a waveguide with external fill to compare performance of CSS directly with the published performance of the Slope ALARMS system (Smith et al 2017a) and (ii) series of experiments conducted using displacement-controlled bending tests on active waveguides with internal fill to understand repeatability and establish relationships between AE and displacement rates for different tube diameters and infill types. In all tests, the waveguides were subjected to accelerating deformations to replicate known behaviour in first-time landslides, which accelerate as Fig.…”

“…3 Particle size distributions of the soils used in the physical model experiments Fig. 4 Annotated photograph of the large-scale slope failure physical model used to accelerate the top box relative to the bottom and to compare displacement behaviour with AE measured using both the Slope ALARMS and CSS HPP sensors (after Smith et al 2017a) series of tests in (ii), and subsequently, a re-designed for mass manufacture factory production prototype sensor, denoted CSS MP, was used for a second series of tests in (ii). These two CSS sensors have comparable component architectures and operating systems, but evolution of the electronics and components means that the magnitudes of AE produced by the two devices are not directly comparable, although AE trends are analogous.…”

“…AE monitoring is not as well developed in geotechnical monitoring applications as in other industries due to low energy levels of generated AE and the high attenuation of signals as they propagate through the ground, both of which make it challenging to detect and quantify AE. However, recent research and advances in instrumentation have resulted in successful applications using AE to monitor soil slope stability (Berg et al 2018;Dixon et al 2015aDixon et al , 2015bSmith and Dixon 2015;Smith et al 2014Smith et al , 2017a. This body of work has proven that monitoring AE can be used to provide an early warning of slope instability by detecting both the development of shear surfaces and accelerating deformation behaviour (Chichibu et al 1989;Fujiwara et al 1999;Koerner et al 1981;Michlmayr et al 2017;Nakajima et al 1991;Smith et al 2017a).…”

Monitoring buried pipe deformation using acoustic emission: quantification of attenuation

“…Generated AE rates are proportional to applied displacement rates, where the coefficient of proportionality Smith et al Monitoring Buried Pipe Deformation Using AE: Quantification of Attenuation 17/08/2016 3 is dependent on the depth to the shear surface and the distance AE propagates along the waveguide to the ground surface where it is measured. Quantifying this magnitude of attenuation for each installation is essential to deriving slope displacement rates from measured AE rates, which can be used to warn users of accelerating slope deformation behaviour to enable evacuation of vulnerable people and timely repair and maintenance of critical infrastructure (Koerner et al, 1981;Nakajima et al, 1991;Smith et al, 2014a;Smith et al, 2014b;Dixon et al, 2015a;Dixon et al, 2015b;Smith, 2015;Smith & Dixon, 2015;Smith et al, 2016).…”