Piezo based transducers are considered smart sensors in contemporary structural health monitoring (SHM) technologies due to their dual functionality of both actuation and sensing. This function can be used to extract structural impedance in the form of electro-mechanical impedance (EMI) from any host structure over which they are bonded. Further, these sensors are capable of acquiring acoustic emissions (AE) as well. In the present study, an attempt has been executed to use thin piezo based transducers for acquiring EMI signatures and AE from a sand-stone specimen under compression. The load has been applied in stages and after every stage load has been reduced to zero. The EMI signatures for damage state have been recorded sequentially and have been compared with the baseline signature. The transducers used are thin Lead Zirconate Titanate (PZT) patches of dimensions 10×10×0.2 mm. The acoustic emissions generated are recorded in low frequency range of 20 Hz-22 kHz, which covers 90% of emitted events in geological materials. In a comprehensive manner, the results are promising and indicate capability of a single PZT patch in condition monitoring of rocks using both EMI technique and AE method.
The use of smart sensor-networks for monitoring the structures under construction is the new emerging trend in the field of construction technology. The underground structures are now becoming complex in designs just like any super-structure rather than simple caverns and tunnels. The increase in design complexity raises the bar for their safe construction. Underground structures are designed mostly safe against natural calamities like earthquakes and landslides however, most of the accidents occur during the construction period. In 2017, a tunnel of Kaleshwaram large irrigation project in Telangana collapsed (Fig.la) while construction killing seven workers at the spot. It was suspected that absence of the rock bolts to hold the destabilized rock led to this accident. In another incident in 2016, a tunnel roof collapse confined three workers alive for nine days in Bilaspur, Himachal Pradesh while construction. National Disaster Response Force (NDRF) could survive only two of them alive (Fig.1b).
A smart monitoring system should include both global and local monitoring techniques. It should be easy and quick to install to reduce the overall construction time. It should provide early warnings against upcoming issues so that preventive measures can be taken in time. Use of wave propagation for damage detection is one of the fastest technique in comparison to the other available techniques. However, rocks and concrete being non-homogeneous are difficult to be monitored rather than metallic structures which are homogeneous (Park et al., 2006).