Acoustic emission monitoring of coastal slopes in north-east England, United Kingdom

Acoustic emission (AE) monitoring of active waveguides (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 destabilising effects. The technique can also quantify decelerations in movement in response to stabilising effects (e.g. remediation or pore-water pressure dissipation). This paper details 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 on-going 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 ground water flowing through the active waveguide from relatively high permeability strata in response to rainfall events

Inclinometer casings retrofitted with acoustic real-time monitoring systems

The paper details the concept of retrofitting inclinometer casings with active waveguides in order to provide subsurface instrumentation that can monitor the stability of slopes continuously and in real-time. The operation of the active waveguide, the unitary battery operated Slope ALARMS acoustic emission sensor and warning communication system are described. A field trial previously reported by the authors demonstrates that acoustic emission rates generated by active waveguides are proportional to the velocity of slope displacements, and can therefore be used to detect changes in rates of movement (i.e. accelerations and decelerations) in response to destabilising (e.g. rainfall) and stabilising (e.g. remediation) effects. The paper presents the results of a field trial of the acoustic monitoring system retrofitted inside an inclinometer casing in a reactivated landslide at Hollin Hill, North Yorkshire, UK. The study demonstrates that this approach can provide continuous information on slope movements with high temporal resolution. Converting manually and periodically read inclinometer casings into continuously monitored active waveguides using Slope ALARMS sensors is a cost effective solution to provide real-time information that could be used in the protection of people and infrastructure

Field trial of an acoustic emission early warning system for slope instability

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 low cost instrumentation that can provide an early warning of slope instability to enable evacuation of vulnerable people and timely repair and maintenance of critical infrastructure. Current instrumentation systems are either too expensive for wide scale use or have technical limitations. An approach, Assessment of Landslides using Acoustic Real-time Monitoring Systems (ALARMS), has been developed and demonstrated through research. An approach developed using measurement of acoustic emission generated during the onset of slope failure to provide quantitative information on slope displacement is described. Sensor operation, deployment strategy, laboratory validation and field performance is considered. The paper presents the results of a field trial of acoustic sensors on an active landslide at Hollin Hill, North Yorkshire, and introduces additional ongoing tri-als in the UK and Italy. Real-time monitoring of acoustic emission generated by the deforming slope has been compared to traditional inclinometer slope displacement measurements. Analysis of the results of the field trial has established that there is a direct relationship between AE and displacement rate trends triggered by rainfall events. Slope deformation events have a characteristic ‘S’ shaped cumulative AE vs. time relationship indicating initial acceleration followed by deceleration of the slide body

An acoustic emission slope displacement rate sensor — case studies

Research over a period of 20 years has resulted in development of a battery operated unitary acoustic emission (AE) sensor which, when used with a standard active waveguide installation, can quantify soil slope displacement rates continuously and in near real‐time. The active waveguide is installed in a borehole through existing or anticipated shear zones, and comprises a steel tube with granular soil surround. The AE sensor is located at ground level and with the waveguide is encased in a cover. Deformation of the slope strains the granular backfill, which generates AE through rearrangement of the particles. The AE propagate as stress waves along the steel tube to the ground surface where they are detected and quantified by the sensor, which is used to provide alert text messages if pre‐determined thresholds are exceeded. The use of a reproducible waveguide allows standard interpretation of the generated AE to provide information on soil slope displacement rates, and the granular soil backfill generates measureable AE when the system is installed in slopes formed in ‘quiet’ fine grained soils. The approach monitors AE at high frequencies to exclude environmental background noise and hence ensure that false alarms are not generated. In rock slopes, the grouted waveguide is passive, with measured AE generated by rock deformation mechanisms. The sensors have been deployed on multiple sites in the UK and in Italy, Austria and Canada. At all sites performance of the AE sensors has been compared with traditional deformation monitoring instrumentation including ShapeAccelArray, inclinometer, extensometer and time‐domain reflectometry. Measurements from these field studies have demonstrated that generated AE are proportional to slope displacement rates. This paper outlines the AE measurement and the interpretation techniques developed, and presents field comparisons of measured AE trends and slope displacement rates obtained from extended trials at several sites. It is concluded that the AE technique can be used as a reliable early warning system for soil slope instability. Applications in rock slopes are promising but further work is required to link detected AE to rock deformation mechanisms and hence to derive thresholds as a basis for early warnings

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 sub-surface slope displacement monitoring system installed in a large coastal landslide complex at Flat Cliffs, North Yorkshire, northeast England. Cliff instability is in-dicated 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 rain-fall events

Development of a low cost acoustic emission early warning system for slope instability

Slope failures world-wide cause many thousands of deaths each year and damage built environment infrastructure. There is a clear need for low cost instrumentation that can provide an early warning of slope instability to enable evacuation of vulnerable people and timely repair and maintenance of critical infrastructure. Current instrumentation systems are either too expensive for wide scale use or have technical limitations. An approach, Assessment of Landslides using Acoustic Real-time Monitoring Systems (ALARMS), has been developed and demonstrated through research. An approach has been developed using measurement of acoustic emission generated during the onset of slope failure to provide quantitative information on slope displacement rates. Research is in progress to develop low cost acoustic sensors. A unitary acoustic emission slope displacement rate sensor has been designed and is being trialled in an active landslide. Continuous monitored acoustic emission rates show comparable trends to displacement rates measured using an inclinometer. Acoustic emission increase after rainfall events and this is considered to indicate increased displacement rates

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 2-D sections, which are insufficient to capture the full degree of spatial heterogeneity. This study is the first to employ 4-D (i.e., 3-D time lapse) resistivity imaging on an active landslide, providing long-term data (3 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 2 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

Assessment of ground-based monitoring techniques applied to landslide investigations

A landslide complex in the Whitby Mudstone Formation at Hollin Hill, North Yorkshire, UK is periodically re-activated in response to rainfall-induced pore-water pressure fluctuations. This paper compares long-term measurements (i.e., 2009 – 2014) obtained from a combination of monitoring techniques that have been employed together for the first time on an active landslide. The results highlight the relative performance of the different techniques, and can provide guidance for researchers and practitioners for selecting and installing appropriate monitoring techniques to assess unstable slopes. Particular attention is given to the spatial and temporal resolution offered by the different approaches that include: Real Time Kinematic-GPS (RTK-GPS) monitoring of a ground surface marker array, conventional inclinometers, Shape Acceleration Arrays (SAA), tilt meters, active waveguides with Acoustic Emission (AE) monitoring, and piezometers. High spatial resolution information has allowed locating areas of stability and instability across a large slope. This has enabled identification of areas where further monitoring efforts should be focused. High temporal resolution information allowed the capture of S’-shaped slope displacement-time behaviour (i.e. phases of slope acceleration, deceleration and stability) in response to elevations in pore-water pressures. This study shows that a well-balanced suite of monitoring techniques that provides high temporal and spatial resolution on both measurement and slope scale is necessary to fully understand failure and movement mechanisms of slopes. In the case of the Hollin Hill landslide it enabled detailed interpretation of the geomorphological processes governing landslide activity. It highlights the benefit of regularly surveying a network of GPS markers to determine areas for installation of movement monitoring techniques that offer higher resolution both temporally and spatially. The small sensitivity of tilt meter measurements to translational movements limited the ability to record characteristic ‘S’-shaped landslide movements at Hollin Hill, which were identified using SAA and AE measurements. This high sensitivity to landslide movements indicates the applicability of SAA and AE monitoring to be used in early warning systems, through detecting and quantifying accelerations of slope movement