Alister Smith

Quantification of reactivated landslide behaviour using acoustic emission monitoring

Slope failures world-wide cause many thousands of deaths each year and damage built environment infrastructure, costing billions of pounds to repair, resulting in thousands of people being made homeless and the breakdown of basic services such as water supply and transport. There is a clear need for affordable instrumentation that can provide an early warning of slope instability to enable the evacuation of vulnerable people and timely repair and maintenance of critical infrastructure. An approach, Assessment of Landslides using Acoustic Real-time Monitoring Systems (ALARMS) is described in the paper, and results of a field trial of sensors on an active landslide at Hollin Hill, North Yorkshire, UK, are described. Continuous and real-time monitoring of acoustic emission generated by the deforming slope has been compared to traditional inclinometer slope displacement measurements. Analysis of the results has established that there is a direct relationship between acoustic emission rate and displacement rate trends triggered by rainfall events. The technique has provided insight into reactivated slope movement kinematics.

Reconstruction of landslide movements by inversion of 4‐D electrical resistivity tomography monitoring data

Reliable tomographic inversion of geoelectrical monitoring data from unstable slopes relies critically on knowing the electrode positions, which may move over time. We develop and present an innovative inverse method to recover movements in both surface directions from geoelectrical measurements made on a grid of monitoring electrodes. For the first time, we demonstrate this method using field data from an active landslide to recover sequences of movement over timescales of days to years. Comparison with GPS measurements demonstrated an accuracy of within 10 % of the electrode spacing, sufficient to correct the majority of artefacts that would occur in subsequent image reconstructions if incorrect positions are used. Over short timescales where the corresponding subsurface resistivity changes were smaller, the constraints could be relaxed and an order-of-magnitude better accuracy was achievable. This enabled the onset and acceleration of landslide activity to be detected with a temporal resolution of a few days.

Four-dimensional imaging of moisture dynamics during landslide reactivation

Landslides pose significant risks to communities and infrastructure, and mitigating these risks relies on understanding landslide causes and triggering processes. It has been shown that geophysical surveys can significantly contribute to the characterization of unstable slopes. However, hydrological processes can be temporally and spatially heterogeneous, requiring their related properties to be monitored over time. Geoelectrical monitoring can provide temporal and volumetric distributions of electrical resistivity, which are directly related to moisture content. To date, studies demonstrating this capability have been restricted to 2D sections, which are insufficient to capture the full degree of spatial heterogeneity. This study is the first to employ 4D (i.e., 3D time-lapse) resistivity imaging on an active landslide, providing long-term data (three years) highlighting the evolution of moisture content prior to landslide reactivation and showing its decline post reactivation. Crucially the time-lapse inversion methodology employed here incorporates movements of the electrodes on the unstable surface. Although seasonal characteristics dominate the shallow moisture dynamics during the first two years with surficial drying in summer and wetting in winter, in the months preceding reactivation, moisture content increased by more than 45 % throughout the slope. This is in agreement with independent data showing a significant rise in piezometric heads and shallow soil moisture contents as a result of prolonged and intense rainfall. Based on these results, remediation measures could be designed and early-warning systems implemented. Thus, resistivity monitoring that can allow for moving electrodes provides a new means for the effective mitigation of landslide risk.

Performance of an Acoustic Emission Monitoring System to Detect Subsurface Ground Movement at Flat Cliffs, North Yorkshire, UK

This paper describes the performance of an innovative acoustic emission subsurface slope displacement monitoring system installed in a large coastal landslide complex at Flat Cliffs, North Yorkshire, northeast England. Cliff instability is indicated by repeat deformation of an access road that serves a settlement of about 50 houses. As part of an extensive ground investigation, a sensor that can quantify acoustic emission was installed adjacent to a standard inclinometer, and continuous monitoring of acoustic emission has since taken place. The acoustic sensor has detected periods of slope deformation that are confirmed by manual surveys of the inclinometer. Performance is demonstrated using time series of acoustic measurements compared with ground deformations and triggering rainfall events.

Monitoring buried pipe deformation using acoustic emission : quantification of attenuation

Deformation of soil bodies and soil-structure systems generates acoustic emission (AE), which are high-frequency stress waves. Listening to this AE by coupling sensors to structural elements can provide information on asset condition and early warning of accelerating deformation behaviour. There is a need for experimentation to model the propagation of AE in buried pipe systems to enhance understanding of real behaviour. Analytical solutions are often based on many assumptions (e.g. homogeneity, isotropy, boundary conditions and material properties) and cannot exactly represent the behaviour of the in situ system. This paper details a series of experiments conducted on buried pipes to investigate AE attenuation in pipes due to couplings and soil surround. The attenuation coefficients reported provide guidance to engineers for designing sensor spacing along buried pipes for monitoring ground deformations, and active waveguide installation depths for slope deformation monitoring. Attenuation coefficients have been quantified for both air–pipe–air and air–pipe–soil trilayer systems for the frequency range of 20–30 kHz.

Current and future role of instrumentation and monitoring in the performance of transport infrastructure slopes

Instrumentation is often used to monitor the performance of engineered infrastructure slopes. This paper looks at the current role of instrumentation and monitoring, including the reasons for monitoring infrastructure slopes, the instrumentation typically installed and parameters measured. The paper then investigates recent developments in technology and considers how these may change the way that monitoring is used in the future, and tries to summarize the barriers and challenges to greater use of instrumentation in slope engineering. The challenges relate to economics of instrumentation within a wider risk management system, a better understanding of the way in which slopes perform and/or lose performance, and the complexities of managing and making decisions from greater quantities of data.

Photographic feature: Acoustic emission monitoring of coastal slopes in NE England, UK

Acoustic emission (AE) monitoring of an active waveguide (a steel tube with a granular backfill surround) installed through a slope can provide real-time warning of slope instability by quantifying increasing rates of movement (i.e. accelerations) in response to slope destabilizing effects. The technique can also quantify decelerations in movement in response to stabilizing effects (e.g. remediation or porewater pressure dissipation). This paper describes the AE monitoring approach and presents results from a field trial that compares AE measurements with continuous subsurface deformation measurements. The results demonstrate that AE monitoring provides continuous information on slope displacement rates with high temporal resolution. Case studies are presented where the AE technique is being used to monitor coastal slopes at Filey and Scarborough in North Yorkshire, UK, to inform continuing risk assessments for these slopes. The results demonstrate that the AE approach can successfully be used to monitor slopes with relatively deep shear surfaces (>14 m); however, they also show that potentially contaminating AE can be generated by groundwater flowing through the active waveguide from relatively high-permeability strata in response to rainfall events.

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

Early warning systems for slope instability are needed to alert users of accelerating slope deformation behaviour, enable evacuation of vulnerable people, and conduct timely repair and maintenance of critical infrastructure. Communities exposed to landslide risk in low- and middle-income countries seldom currently instrument and monitor slopes to provide a warning of instability because existing techniques are complex and prohibitively expensive. Research and field trials have demonstrated conclusively that acoustic emission (AE) monitoring can be an effective approach to detect accelerating slope movements and to subsequently communicate warnings to users. The objective of this study was to develop and assess a simple, robust, low-cost AE monitoring system to warn of incipient landslides, which can be widely deployed and operated by communities globally to help protect vulnerable people. This paper describes a novel AE measurement sensor that has been designed and developed with the cost constrained to a few hundred dollars (US). Results are presented from physical model experiments that demonstrate performance of the AE system in measuring accelerating deformation behaviour, with quantifiable relationships between AE and displacement rates. Exceedance of a pre-determined trigger level of AE can be used to communicate an alarm to users in order to alert them of a slope failure. Use of this EWS approach by communities worldwide would reduce the number of fatalities caused by landslides.

Listening to Infrastructure, EPSRC Fellowship

This science report is published under the Creative Commons Attribution 4.0 International Licence (CC BY). Full details of this licence are available at: http://creativecommons.org/licenses/by/4.0/