A study of micro seismicity and attenuation in unstable soil slopes

• Main Author: Wilson, S. A.
• Published: Swansea University 1994
• Subjects: 624.15
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.636670

An acoustic emission monitoring system consisting of a pair of accelerometers mounted on steel waveguides and a digital recording system, was deployed at three sites of slope instability. These sites are thought to represent reactivated glacial flows and are located along the Tawe valley in South Wales. Difficulties in detecting prompted a laboratory investigation into the magnitude of emission from soil samples taken from these localities and into a field investigation of the attenuation that such acoustic waves would experience. Acoustic emission amplitudes of up to ≃ 0.05g, within a bandwidth of 200-1500Hz, were recorded from soils sheared in the laboratory. The measurement of soil quality factors and the modelling of the waveform propagation of laboratory data have shown how rapidly the magnitude of such signals decays with the distance of propagation. It is calculated that within several metres they attenuate to a level below the detection threshold of the monitoring system. The use of this technique in monitoring soil slopes is thus greatly hindered by the small effective volume that can be monitoried per sensor array. Based on this information further field monitoring was conducted during the winter of 1990/1991. Analysis of this data together with rainfall data show how the rate of acoustic emission is greatly influenced by heavy precipitation and changes in the hydrological characteristics of the soil. Comparison with survey data demonstrates that during this period of slope instability, identified by the acoustic emission data, slope movement noticeably accelerated within the monitored area.