Instrumentation is vital to many modern geotechnics projects, but work being led by Cambridge University looks set to create a step change in capabilities.
Visit one of London’s current landmark ground engineering projects and start talking about monitoring and it probably won’t be long before Cambridge University is mentioned. Chances are you’ve stumbled across one of the 30 demonstration projects currently being undertaken by its Centre for Smart Infrastructure & Construction (CSIC).
“The aerospace and automotive industries routinely use sensors, but we don’t routinely incorporate them into construction activities or the finished product” says Robert Mair, professor of geotechnical engineering at the University of Cambridge. “By not doing this we are unable to say how a structure is performing during the operational phase, but the data could help create true performance-based design on future projects.”
The five-year CSIC project has just reached the half-way stage and project leader, Professor Mair, believes it will drive changes in instrumentation and monitoring. The project was initiated by a £10M grant from the Technology Strategy Board (TSB) and the Engineering & Physical Sciences Research Council (EPSRC), in 2011. It now has 30 industry partners who have contributed a further £7M of funding.
While this particular project is still relatively new, instrumentation and the need to improve techniques have been a recurring theme throughout Mair’s career. He studied civil engineering at Cambridge University and was introduced to the world of soil mechanics by lectures given by Peter Wroth. “I found his lectures inspirational and they really caught my attention”, he says.
On graduation, Mair wanted to get out into industry and he felt that consulting work suited him best, so he joined Scott Wilson Kirkpatrick (now URS) in 1971. His first two years were spent in the UK on a mix of geotechnical and structural projects; he then moved to work in Hong Kong for three years.
On his return to the UK, Professor Mair was seconded from Scott Wilson to work on a TRRL-funded research contract at Cambridge University, which looked at tunnelling through soft ground.
It was there that I met Andrew Schofield, who was the supervisor for the project. He had a passion for centrifuge testing and had built a 10m diameter centrifuge at Cambridge, so we applied the technique to our tunnel research. The stress levels that would exist within tunnels can be replicated and allowed us to create collapse scenarios to understand the stress conditions and the influence of tunnel stability more clearly. The tests being undertaken now are far more sophisticated than they were then, but they were still accurate in the 1970s, and we could instrument the models to get the quality of data needed for numerical modelling. In the field, less instrumentation is used and it is very rare for failure to occur so the centrifuge offers huge learning opportunities.
This was probably Mair’s first step into the world of instrumentation.
After three years on the project, and with a PhD under his belt, Mair returned to Scott Wilson in 1979 where he got involved in a wide range of geotechnical projects before leaving in 1983 to set up GCG with another Scott Wilson engineer, David Hight.
According to Mair, GCG had a new agenda: to put the very latest research developments into practice.
Mair and Hight already had good links with Cambridge and Imperial College, and made connections with Oxford University, Barcelona, and MIT in Boston, US. “We built a group of experts who we could call on”, he explains.
One of the projects that stands out in Mair’s mind from that period was the Jubilee Line Extension (JLE), where GCG acted as a specialist geotechnical adviser to London Underground. It was another scheme that highlighted the need for instrumentation.
“We had worked with London Underground to develop the technique of compensation grouting – a phrase coined by David – to enable an escalator tunnel to be built below the Victory Arch and the Waterloo and City Line at Waterloo Station for the Eurostar redevelopment work in the early 1990s”, he says. “Both the arch and tube line were sensitive structures, and, by grouting as the tunnelling progressed, we were able to keep settlements to less than 15mm on the tube line and it was very successful.”
Through GCG’s input, compensation grouting was used widely on JLE with the most critical part of the work used to stabilise Big Ben as tunnelling work passed underneath.
“I feel very proud when I look round Westminster station now with the diaphragm walls and struts exposed as part of the design”, says Mair.
It was not a straightforward job though, with the Heathrow Express tunnel collapse happening just before work started on JLE. “The Health & Safety Executive (HSE) was concerned about the sprayed concrete lining (SCL) work that was planned for JLE and the potential for a similar collapse to occur”, Mair adds. “We helped the HSE with its investigation and in the end SCL work went ahead on the project.”
Instrumentation used on JLE was critical to not just the compensation grouting but to reassurance that the SCL work was safe.
Mair says that he was very happy in his work for GCG, and he says, when approached in 1998 to head up the civil engineering department back at Cambridge University, his immediate reaction was that he had no wish to be an academic.
"The then vice chancellor, Alec Broers, had been a path finder in the development of the silicon chip and had a great career with IBM before taking up his post at Cambridge”, he explains. “He had strong views that modern engineering departments must be linked to industry and that I shouldn’t give up on consultancy if I took the post, in fact he encouraged it.”
During his time at Cambridge the issue of instrumentation has continued to feature in Mair’s work, and 10 years ago he and Kenichi Soga formed links with MIT to jointly work on research and teaching in the field of advanced sensors for civil and geotechnical projects.
We started to work closely on developing wireless sensors. There had been a revolution in sensor technology in other industries and the collaboration aimed to develop the technology for the applications we were involved in. At that stage the industry was still using wired sensors which limited the number of sensors that could be installed and also created the potential for damage during construction work.
Mair and Soga pitched the idea of a research project on the subject to the TSB and EPSRC, which resulted in the CSIC initiative.
The 30 demonstration projects, several of which are on Crossrail contracts, are using fibre optics to monitor the distribution of strain throughout the structures they are fitted within.
“We are able to monitor the front and back faces of a diaphragm wall to see the deflection, and within circular shafts to get a clear picture of the hoop stresses”, he says. “We can see how the stress levels change during the excavation – the data we are getting is fascinating.”
Mair points to the removal of planned strut levels at the Tottenham Court Road station box on Crossrail as an example of the potential benefits. “Interpretation of the instrumentation and monitoring allowed this, helping to save time during prop installation, and ease the excavation phase”, he says.
On the Bevis Marks redevelopment scheme in the City of London, use of the technology allowed Cementation Skanska to convince its client that it was safe to reuse existing piles within the new construction and helped it to win the sustainability category at the GE Awards this year.
“The piles were drilled through and fibre optics installed over the full length so we could see how the pile was performing before, during and after demolition”, says Mair.
Mair believes that the technique has potential in a huge number of geotechnical applications, including pile testing and segmental concrete tunnel linings.
The CSIC project is now at the half-way stage, but there’s no doubt it is creating a buzz within the sector and the benefits are already becoming clear.