Monitoring Strains and Temperatures in a Deep Excavation Base Slab Using Fibre-Optic Bragg Gratings

This paper describes the monitoring of strains and temperatures within a deep underground bunker base slab at the PROTOS Energy-from-Waste facility in Chester, UK, using fibre-optic Bragg Grating (FBG) sensors. A novel method for compensating the FBG measurements for temperature effects is outlined, which involves using a precisely machined aluminium block clamped around a single FBG on each fibre string to compensate all other FBGs. The compensation method is evaluated using data from secondary thermistors positioned adjacent to the fibres and the widely adopted industry method for temperature predictions during concrete curing. Interpretation of the monitored data allows for a quantitative assessment of the influence on construction activities on the observed axial loads and bending moments induced in the base slab.