Field monitoring has provided many important insights into the real behaviour of geotechnical transport infrastructure such as embankments, tunnels and retained or battered cuts, resolving uncertainties for research, design, construction control or economic purposes. Where such monitoring is carried out, it is usually over a relatively short period of time for example during construction or in connection with a specific maintenance or remediation requirement. Professor Robert Mair's March 2006 Rankine lecture demonstrated the value of longer term field measurements, which may indicate unexpected and unforeseen continuing changes in the behaviour and condition of the infrastructure and the state of the surrounding ground. As the owners and custodians of our transport infrastructure seek to extend its economical life through sometimes extensive in-service maintenance and refurbishment, an understanding of the factors governing its long-term behaviour and state will become increasingly important.In recent years, the Geomechanics Research Group at the University of Southampton has installed loggable instrumentation in connection with a number of research projects to investigate the performance during and for a short period after construction of geotechnical structures such as slopes and retaining walls for transport infrastructure. In some cases, this instrumentation is still in place and working, offering a unique opportunity to continue monitoring to gain an insight into the long-term performance of the structures as equilibrium conditions are gradually reached, perhaps in response to new or unforeseen boundary conditions such as changing climate patterns and groundwater conditions or further construction nearby. The proposed research offers the opportunity to answer some questions concerning the long-term performance of geotechnical transportation infrastructure whose answers have remained elusive for decades. These are the potential for the re-establishment of in situ lateral stresses on retaining structures in overconsolidated deposits; the interpretation of strain gauge readings in underground concrete structures as the concrete ages; the impact of cyclic seasonal variations on the stability of unreinforced and remediated cutting and embankment slopes; and the interactions between buried structures and the groundwater regime. All of these will have major benefits in terms of the design of new infrastructure and predicting the service life and impacts of climate change on existing structures.

Design limits are frequently based on strain developing in the structure. Although strain measurement is well established, current practice has until recently been restricted to measurement of point-wise strains by means of vibrating wire (VWSG) or metal foil strain gauges and more recently by fibre optics utilising Fibre Bragg Grating (FBG) technology. Where structures interact with soil, (e.g. underground infrastructure such as foundations, tunnels or pipelines) or indeed in the case of a soil structure (road or dam embankments), the state of the structure is not fully understood unless the complete in situ strain regime is known. In the context of monitoring strain in piled foundations, tunnels, pipelines, slopes or embankments, capturing the continuous strain profile is often invaluable to pinpoint localised problem areas such as joint rotations, deformations and non-uniformly distributed soil-structure interaction loads. In this project, we propose to use a unique fibre optics technology called the 'Brillouin optical time-domain reflectometer (BOTDR)'. The novel aspect of this new technology lies in the fact that tens of kilometres of fibre can be sensed at once for continuous distributed strain measurement, providing relatively cheap but highly effective monitoring systems. The system utilizes standard low cost fibre optics (potentially 0.1/m) and the strain resolution can go down to 2 micro strains. We will demonstrate the importance of distributed strain measurements to monitor the performance of building foundations at field sites in the UK and US. Using the distributed strain data, a design tool that optimises the performance of foundations that require rehabilitation, repair and reuse will be developed with industrial collaborators. The project has supports from UK Industrial partners as well US collaborators (National Science Foundation and Northwestern University).