Distributed Optical Fibre Strain Sensing of Geotechnical Structures

Hisham Mohamad, Cambridge University
Geotechnical Engineering Group


This research describes the work conducted on developing the use of Brillouin Optical Time-Domain Reflectometry (BOTDR), a distributed strain sensing technology, for monitoring the performance of geotechnical structures. BOTDR is capable of measuring strain at every point along a standard optical cable, meaning a suitably installed fibre can potentially replace many closely spaced point sensors and therefore can be regarded as a cost-effective sensing tool. In the context of monitoring large-scale underground structures such as tunnels, piles, and retaining walls where the structures interact with soil, the true state of structure is not easily predicted unless the complete strain profile is known. By measuring strain from a single optical cable wrapped around or embedded in a structure, the performance of the structure can be quantified.

Because BOTDR technology is relatively new in geotechnical instrumentations, there are many implementation issues and challenges that require careful consideration and hence are examined in this research. These include methods of installation, cable protection, data collection, data processing and interpretation. This thesis summarises the findings made from works conducted in the laboratory and in several unprecedented field trials.

In the experimental work, the performances of optical cables embedded in concrete beams are studied via uniaxial loading tests. It was found that all optical cables generally behave similarly even though they are subjected to different installation methods, i.e. either endpoints attachment or spot-glued and either pre-strained or without pre-strained. Only one optical cable did not perform well when it was under pre-strained. Separate tests were conducted to study thermal behaviour of optical cables using thermal bath and during concrete curing. The thermal tests indicate BOTDR has the potential of measuring thermal strain and temperature of a concrete structure simultaneously.

The first tunnel instrumentation was implemented in an old masonry tunnel when a new tunnel was constructed obliquely underneath the old tunnel. Strain measurements showed the brick lining deformed in a rather flexible manner which correspond to the positions of the new tunnel face. In another tunnel instrumentation consisting of precast-bolted lining, strain distributions showed the circular tunnel deformed into an uneven oval shape as the new tunnel was excavated closely side by side, i.e. springline nearer to the new tunnel recorded higher compressive strain than the opposite springline.

Fibre optics were implemented in a load bearing pile undergoing construction loading and ground heaving. The strain distribution obtained was not only useful for analysing the load transfer mechanism between the pile and the ground, but also able to pinpoint local strain events. The lateral behaviour of a pile was investigated on a secant-piled wall of a multi-prop system. By manipulating BOTDR data, information such as deflection, inclination, bending moments as well as axial movement can be established. All BOTDR measurements were validated with other independent instrumentation.