The authors of this paper wanted to test something different: the conductivity of concrete. Because when that dips it's a strong indicator of possible tunnel lining voids.
The paper, titled Raspberry Pi-Based IoT System for Grouting Void Detection in Tunnel Construction, opens by explaining that railways tunnels are built with primary and secondary tunnel linings.
They assessed techniques like manual inspection and ground penetrating radar, finding limitations in both methods, especially the high cost and lack of continuous monitoring with radar.
It's possible the integrity of a tunnel could be compromised if secondary linings contain voids, leading to structural damage or collapse.
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