Optical Fibre Sensing for Monitoring Shallow Buried Utilities Under Dynamic Loading

The construction of urban light railway systems often requires costly diversion of buried utilities to mitigate potential damage from surface loads. However, understanding the behaviour of shallowly buried utility ducts due to dynamic loading is limited. This study investigated the structural behaviour of shallow buried utility pipes under cyclic loading (simulating tram loads). A 90 mm diameter high-density polyethylene (HDPE) pipe was buried at a depth of 300mm to the crown in a 10 m × 5.8 m x 5 m deep test pit at the National Buried Infrastructure Facility (NBIF), University of Birmingham, UK. To simulate a realistic surface track system, a reinforced concrete slab was constructed above the pipe to represent a tramway track bed. The pipe was offset by 600 mm from the edge of the track slab which was loaded using a 250kN dynamic actuator with dynamic loads ranging from 80–200 kN at frequencies of 1–8 Hz to simulate tram axle loads. Distributed optical fibre sensors (DOFS) were attached along the crown of the pipe and at three parallel circumferential positions. A Luna ODiSI 6100 interrogator was used to obtain dynamic, high-resolution strain measurements.

Results showed that pipe strains remained within the elastic range under simulated tram axle loads, with a maximum measured strain of 36.7 με. However, they exceeded predictions from the Spangler method in BS 9295, which assumes homogeneous, elastic soil and underestimates soil pressures and bending moments at shallow depths, especially under dynamic loading. The findings reveal limitations in conventional design codes and highlight the capability of DOFS to capture detailed strain distributions with high spatial and temporal resolution. This study underscores the value of large-scale controlled testing and integrating optical fibre sensing into geotechnical monitoring frameworks, supporting future research on ageing infrastructure and pipe materials.