Quantification of slope displacement rates using acoustic emission monitoring

In soil slopes, developing shear surfaces generate acoustic emission (AE). The Authors have previously proposed the use of active waveguides for monitoring the stability of such slopes. Active waveguides comprise of a steel tube installed in a preformed borehole through a slope with coarse grained soil backfill placed in the annulus around the tube. Deformation of the host soil generates AE in the active waveguide. Field trials of this system reported previously have shown that AE rates are linked to slope deformation rates. This paper extends the study by detailing a method for quantifying slope movement rates using an active waveguide. A series of laboratory experiments are presented and used to define the relationship between AE event count rate and displacement rate. The method was shown to differentiate rates within an order of magnitude, which is consistent with standard landslide movement classification (i.e. 1 to 0.001 mm per minute), using a relationship derived between the gradient of the event count rate with time and deformation rate. In addition, it was possible to detect a change in displacement rate within two minutes of it occurring even at very slow rates (i.e. 0.0018mm/min). Knowledge of changes in displacement rate is important in situations where slope movements are suddenly triggered or displacements accelerate in response to a destabilising event. Field trials of a realtime AE monitoring system are currently in progress to compare performance against traditional instrumentation

Landslide hazard evaluation by means of several monitoring techniques, including an acoustic emission sensor

At Passo della Morte in the Italian Eastern Alps a geomorphological survey has identified potential instability of the valley side slope that could result in a debris/rock avalanche, which would threaten the Tagliamento River. A nationally important road passes through a tunnel 130 m long behind the potentially unstable slope. The stratum comprises a sequence of Limestone layers, dipping in the slope direction towards the river. Although currently there is no clear evidence of movement, the geological setting indicates a predisposition to instability that could involve a large landslide and extremely fast deformations can be foreseen. To appraise the physical characteristics of the rock mass and to provide an early warning of instability, monitoring instrumentation has been installed and monitored since late 2010. Extensometers, MEMS, TDR cables, a ver-tical inclinometer, a seismic station to monitor Limestone rock mass deformation generated micro-tremors and an acoustic emission (AE) monitoring system have been installed. The instruments are connected to real-time recording and transmitting units. The paper describes the geological setting and associated potential modes of instability. It details the design of the instrument installations and presents results obtained to date. In particular, the novel acoustic emission monitoring approach is described including sensor design, method of operation and comparison of the measured AE response with the deformation measurements and detected micro-tremor trends. Initial results indicate a strong response of the acoustic sensors to rainfall events. No sig-nificant rock mass deformations have been detected at depth within the slope to date, although a surface ex-tensometer has shown widening of a bedding tension crack. Upgrading of the instrumentation system is ongo-ing and it is planned to continue monitoring for the foreseeable future

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

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