Experimental Development of Distributed Optical Fibre Monitoring Methods for Underground Infrastructure

Doktorsavhandling, 2026

Monitoring during underground construction is essential for verifying design assumptions, assessing structural behaviour, and supporting safe and efficient excavation. However, conventional monitoring methods are often limited to discrete measurement points, which may be insufficient for identifying localised deformation and load-transfer mechanisms in jointed rock masses and tunnel support systems. This thesis investigates the use of distributed optical fibre sensors (DOFS) for high-resolution strain-based monitoring in hard rock tunnelling.

Two monitoring applications are studied experimentally. The first concerns load identification in fibre-reinforced shotcrete tunnel linings. Laboratory specimens were instrumented with DOFS and subjected to two load types: idealised loose rock block loads and distributed loads combined with bond loss between substrate and concrete. The results show that these load types can be distinguished from distributed strain and curvature responses before peak load is reached, demonstrating the potential of DOFS for early warning and targeted maintenance.

The second application concerns deformation monitoring in the surrounding rock mass. A DOFS-based shape sensing system was developed, evaluated in laboratory tests, and installed in a borehole adjacent to an active drill-and-blast tunnel excavation. A beam theory-based analysis method was developed to determine load direction, longitudinal deformation, and lateral deformation, including both bending and shear deformation. Laboratory results showed good agreement with reference measurements, while the field test demonstrated that the sensor can capture relative deformation in a hard rock mass.

The thesis shows that DOFS can provide spatially continuous information that improves the interpretation of local load and deformation mechanisms in underground infrastructure. This information can support safer and more efficient excavation by enabling earlier identification of unfavourable structural behaviour, more targeted inspections, and improved assessment of support performance.