Abstract

In practice, forces calculated in geotechnical structures where the structures interact with soil such as piled foundations and tunnels are not fully understood due to complexity of deriving actual loading conditions and boundary value problems. Measuring complete strain regime across the whole length of these structures is one of the ways to check how the structures are actually performing under various loading conditions and hence validate assumptions made in design. An innovative technique based on optical fibre sensing that allows such distributed strain measurement has recently been introduced in structural health monitoring known as Brillouin Optical Time-Domain Reflectometry (BOTDR). The distributed optical fibre sensing allows measurement of strain along the full length (up to 10km) of a suitably installed optical fibre. In the context of monitoring strain in piled foundations, retaining walls and tunnels, capturing the continuous strain profile is often invaluable to pinpoint localised problem areas such as joint rotations, cracks, deformations and other non-uniformly distributed soil-structure interaction loads.

As BOTDR sensing is practically new in the field of monitoring underground infrastructures, various key issues are examined before the technology can be said to be a fully developed tool. Some of the issues covered include best practice of installation / attachment techniques, type optical cables, robustness, accuracy of measurements and data processing / interpretations.

The research focuses on developing BOTDR technology for application to piled foundations and tunnels by deploying the sensors on real construction sites. Two case studies of tunnel-tunnel interactions in London and Singapore are presented followed by two examples of pile instrumentations of high capacity load bearing pile and laterally loaded secant-piled wall both in London. Several important findings from laboratory works are included to validate the instrumentation techniques conducted in the field sites.