Monitoring reused piles with distributed fibre optic sensing

Background

Building on underpinning research at the University of Cambridge, CSIC has been developing distributed fibre optic sensing (DFOS) for monitoring different types of infrastructure since 2011. Structural monitoring of infrastructure is crucial and DFOS is ideal for monitoring strain or temperature over distance or area. It is particularly useful for detecting phenomena such as cracks and material anomalies for embedded defects than cannot be observed with point sensors. 

The advantages of monitoring piles with DFOS is that, with a single instrumentation, one can measure the concrete's curing temperature, as well as strain and displacement during pile testing or during the operational phase of a working pile. Traditionally, strain gauges and extensometers have been used to monitor reinforced concrete piles. The information that they can provide, however, is limited as they only return measurements at discreet points rather than along the entire length of the pile. DFOS provides a complete profile along several sides and down the entire depth of the pile, thus delivering more information than strain gauges or extensometers, which provide measurements at only single points.

Bevis Marks: an early application of DFOS

An early demonstration of monitoring piles with DFOS was at the site of an eight-story, 1980s office block in Bevis Marks, London, where CSIC was brought in to monitor the effect of demolition on the building’s pile foundations and surrounding soil, and make a comprehensive assessment of their suitability for reuse.

The challenge
When a structure is demolished and another built on the same site, it is normally impossible to reuse the existing foundation piles because their condition is unknown. However, reuse of a building’s foundations following demolition offers huge commercial and environmental benefits, including savings in time, materials and carbon – provided that their performance is not compromised. During the construction of 6 Bevis Marks, it was proposed to reuse all existing foundation piles and the majority of the basement substructure. Project contractor Skanska asked CSIC to install a DFOS system in both existing piles and a borehole to observe the impact of the demolition process on the changes in piles' behaviour and ground response.

Monitoring to assess the integrity of existing piles
In this innovative technique, laser light is shone through the fibre optic cables to measure the strain along the entire length of the pile. The fibre optic cables were attached to a flexible pile that was embedded into the hollow core drilled through the entire length of the pile prior to demolition. The instrumented core was grouted to ensure full fixity between the instrumented pipe and the existing pile. The fibre optic instrumentation generated a continuous strain profile, along the entire length of the pile, at different stages of demolition. Fibre optic cables were also installed in an adjacent borehole to capture ground heave during demolition. Together, these outputs provided vital information about the performance of the existing pile and hence its suitability for reuse. The results at this site provided vital information about the strains that developed on an existing pile during demolition and eventually enabled all the existing piles to be reused, leading to huge reductions in terms of costs and materials.

Project impact
This monitoring produced key evidence proving that the existing piles were able to take the loads. The total cost saving on the project was GBP 6m, because it would have cost GBP 5m to remove the 66 old piles and associated walls/slabs plus GBP 1m to install replacement piles. Less material was used in construction, which equates to a saving of 1000 tonnes of CO2, which would otherwise have been released during the course of construction. In addition, three to six months of construction programme time were saved on the project. The project won the Ground Engineering Sustainability Award 2013 for Skanska.