The video below provides an introduction to Slope ALARMS including method of operation and examples of its use.
Slope failures world-wide annually cause many thousands of deaths and damage infrastructure, costing billions of pounds to repair and resulting in thousands of people being made homeless. The 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 is self-evident. Current systems are either too expensive for wide scale use or have technical limitations. An approach based on acoustic emission (AE) real-time monitoring has been developed. Funded research has established relationships between AE generated by a deforming slope and displacement rates.
Research and development projects carried out in collaboration with the British Geological Survey have developed a unitary acoustic slope monitoring sensor, Slope ALARMS, and demonstrated field performance of this sensor. The sensor detects and quantifies acoustic emission when the slope starts to move. The sensor is located on an active waveguide. This comprises a steel tube surrounded by granular soil backfill and is installed in a borehole that penetrates an existing or potential shear surface beneath a slope. Deformation of the slope strains the waveguide (i.e. granular backfill) generating AE. Relationships between AE and displacement rates are derived through laboratory calibration. If trigger values are exceeded, an alert message is sent to nominated persons to enable relevant action to be taken. Watch the video to find out more.
Field trials of the sensors are currently underway where performance is being compared to that of traditional instrumentation (i.e. manual and in-place inclinometers). Trials have been implemented on a natural slope containing a landslide, a slip in a railway cutting, coastal landslides that threaten houses, slope failures impacting on rural roads, a large rock slide in the eastern Italian Alps that threatens a road tunnel and two sites in Canada where major road and rail links are in danger from slope failures. More installations are planned over the next 18 months. These field trials are being supported by a number of organisations.
Description of measurement system
Materials undergoing deformation generate acoustic emission. Studies of acoustic emission have sought to use capture and measurement of the signal to determine the extent of material deformation. In soil, acoustic emission is generated from inter-particle friction and in rock by fracture propagation and displacement along discontinuities. The presence of AE is an indication of microscopic defect growth or localised inter-particle displacements within the soil or rock body. Within a soil slope the shear stresses induced by destabilising forces cause re-arrangement of particles along developing shear surfaces resulting in slope deformation. Use of acoustic emission to assess the stability of soil slopes has been investigated by researchers for many years. However until the work of Dixon and his co-workers, funded by EPSRC, it has not been possible to reliably measure AE generated by a deforming soil slope and to interpret the measured AE to quantify slope displacement rates.
Slope ALARMS is a real-time AE slope displacement rate sensor. It is comprised of the following elements: transducer, pre-amplifier, filters, an integrated signal processing, data storage and communication device, power supply and a secure, acoustically insulated chamber. The integrated signal processing, data storage and communication device performs the following functions: measurement and quantification of detected AE as Ring Down Count (RDC are a simple measure of AE energy) rates; conversion of RDC rates into displacement rates; comparison of these with trigger values and generation of alert communication. The sensor is located on an active waveguide. Deformation of the slope results in straining of the active wave guide system (i.e. deforms the granular backfill surround) leading to generation of AE. In real-time, acoustic emission RDC rates generated by the active waveguide are recorded at pre-defined time intervals and using a relationship between acoustic RDC rates and displacement rates, derived through a laboratory calibration process, quantitative information on slope displacement rates can be obtained. RDC rates are recorded and are compared to pre-determined trigger/action values based on both magnitude and changes in rate. If the trigger values are exceeded, an alert message that includes measured AE rates is sent using a wireless communication system to a nominated person(s) to enable relevant action to be taken. Watch the video to find out more.
The Slope ALARMS sensor is highly sensitive and can detect displacement rates as low as 0.01 mm per minute. It is designed to detect changes in soil slope displacement rate for orders of magnitude (for example, 0.01, 0.01, 0.1 and 1 mm per minute), which is consistent with established slope displacement rate classification and monitoring procedures, and it can detect changes in soil slope displacement rate within a few minutes of the change occurring. The sensor design can be configured for any length of the waveguide (i.e. including backfill type, sensor specification and cover system). Measurements can be made at continuing large magnitude displacements (>500mm), as the interaction between granular backfill and waveguide will continue to generate AE proportional to displacement rates at the shear surface. The sensor can be re-used on other waveguides of the same design without the need for re-calibration or modification as the design is independent of the soil materials that form the slope being monitored and the slope geometry (i.e. slope height, length and slope angle). Monitoring is of subsurface slope deformations for the depth of the waveguide using one Slope ALARMS sensor located at the surface. Alerts obtained from continuous measurements are real-time therefore providing the possibility of a timely response to reduce consequences of slope failures and hence reduced risk.
The Slope ALARMS system was re-engineered in 2012 by the British Geological Survey to produce improved performance. This includes a lower power requirement that means sensors can operate in remote locations for periods in excess of 12 months without the need to replace batteries or provide battery charging facilities. Performance of these new sensors has been proven in multiple field trials. Conformance testing has been completed on the sensors leading to the sensors achieving CE marking.