Field performance and deployment challenges of a BOTDA-based distributed fibre optic inclinometer for microtunneling-induced ground deformation monitoring

Reliable monitoring of ground deformation during microtunneling is essential for safeguarding adjacent infrastructure and understanding soil–structure interaction under construction-induced loading. This study presents a performance evaluation of a Brillouin Optical Time Domain Analysis (BOTDA)-based distributed fibre optic strain sensing (DOFSS) inclinometer system applied to a microtunneling project beneath an active railway in Ipoh, Malaysia. Rather than introducing a new sensing methodology, the study focuses on assessing the practical performance, deployment behaviour, and limitations of a distributed fibre optic (FO) inclinometer under real construction conditions. The monitoring programme integrates laboratory calibration, field deployment, fibre integrity assessment, and numerical validation. Laboratory cantilever tests were conducted to verify strain transfer behaviour and calibrate strain-to-curvature and displacement reconstruction. Vertical and horizontal FO inclinometers were deployed to capture subsurface lateral deformation and near-surface settlement during pipe jacking operations. Field measurements identified clear deformation responses associated with the approach, passage, and departure of the tunneling machine. Fibre breakage events were analysed using Optical Time-Domain Reflectometry (OTDR), providing insight into installation-induced effects, grout stiffness, and casing rigidity on fibre survivability. Staged numerical simulations reproduced the observed deformation trends, supporting interpretation of field measurements. Overall, the results demonstrate that BOTDA-based distributed fibre optic inclinometers are effective for capturing spatially continuous trends of construction-induced ground deformation when installation strategy and mechanical coupling are appropriately controlled.