Monitoring of Piles and Diaphragm Walls with Distributed Fibre Optic Sensors

Distributed fibre optic sensing systems can measure strain and temperature at closely spaced intervals along a fibre optic cable. Distributed fibre optic sensor (DFOS) cables can be embedded in concrete piles and diaphragm walls during construction, in order to measure the temperature profile of the concrete during the curing process and the strain profile of the pile or wall under loading. The temperature profile is useful to detect and identify localised anomalies in the concrete, such as potential voids, soil inclusions, necking or over-breaks. The strain profile can be used to derive the load-bearing capacity of a test pile, or to assess the structural performance of an operational pile or wall throughout its lifetime. The use of DFOS for pile and diaphragm wall monitoring is described in this paper and illustrated with an example of a 44.5 m-deep instrumented test pile which was constructed in London. The temperature profiles obtained from the DFOS instrumentation during the concrete curing showed that the pile had a localised (unintended) defect. The strain profile measured during the subsequent load testing provided important insights on the soil-structure interaction and the load-bearing performance of the pile.