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.