Monitoring of rocks using smart sensors

This paper examines the feasibility of employing the new generation fibre optic and piezoelectric sensor systems for comprehensive monitoring of rocks, covering load history monitoring/retrieval as well as damage assessment. A detailed experimental study was conducted on the Bukit Timah granite rock samples obtained from a cavern site in Singapore. The rock specimens were subjected to cyclic loading and their conditions were continuously monitored using such smart sensors. Optical fibres with multiplexed fibre Bragg gratings (FBGs), surface bonded on the rock specimens, were used as strain and temperature sensors. Their performance was compared with the conventional electric strain gauges (ESGs). Piezo-electric ceramic patches made up of lead zirconate titanate (PZT) were employed as piezo-impedance transducers for detecting the onset of damages. These typically employed high frequencies of the order of kHz to extract impedance information of the specimens. Typically, the piezo-impedance transducers detected cracks even before they reached macroscopic dimensions. The results of this experimental study demonstrate the superior performance and cost-effectiveness of the new generation smart technologies as compared to the conventional sensor systems for monitoring rock-structures such as caverns and tunnels.