Thermal imaging has been used to monitor grout penetration behind precast concrete shaft rings for the first time.
The technique was pioneered by civils contractor Barhale on two 30m deep segmentally lined access shafts either side of the Thames, part of the £5M Chelsea to Battersea medium pressure gas pipeline project.
These shafts had to pass through made ground, alluvium and river terrace deposits before reaching the London Clay in which the 330m long sub-Thames tunnel that links the shafts was to be constructed. To achieve watertight shafts a cement grout was pumped around the concrete rings, a standard technique.
“Traditionally, grout penetration was monitored by pressure gauges attached to the concrete rings,” Barhale contracts manager Ovi Frunza explained. “This had several drawbacks.”
“On large shafts and tunnels the gauges could be 2m or more apart, making it difficult to be sure about grout penetration in the middle. Sometimes the pressure recorded can come from soft ground rather than grout. This could result in settlement.”
Cement-rich grout, rapidly rises in temperature as it is injected behind the rings. The chemical reaction between the cement compounds in the grout and the mixing water is exothermic, and the resultant heat can be picked up by a modern thermal camera even through the concrete lining.
“In practice there can be as much as 5°C difference between the areas where the grout has penetrated and where it hasn’t,” Frunza reported. “This makes it very easy for us to identify areas where secondary grouting or increased grouting pressure is needed.”
He added that standard off the shelf thermal cameras were ideal, and the technique also allowed the contractor to minimise the time staff had to spend in the shaft, one of Barhale’s priorities.
A different construction technique was used on each of the access shafts. The 7.5m diameter launch shaft, located within the Royal Chelsea Hospital Gardens, home of the Chelsea Flower Show, used caisson techniques.
On the north bank of the Thames the 6m diameter reception shaft located in Battersea Park was constructed by the underpinning method.
Barhale opted to increase the tunnel diameter from the initial 1.5m to 1.82m, to ensure easier and safer access for operatives. For the drive in the London Clay beneath the Thames an Iseki slurry tunnel boring machine was chosen.
To further reduce the need for operatives to work in high risk areas, the contractor developed an innovative roller system to transport the 460mm diameter gas pipe. The pipe was welded and coated within the launch shaft itself, as is the norm, but there was a key innovation, as Frunza explained.
“Normally there will be a winch in the reception shaft pulling the pipe through, with staff there to operate it. This puts them close to a wire cable under considerable tension, an obvious safety risk.
“Instead, we used an automatic winch controlled from the launch shaft itself.”
One of the design constraints was the proposed Thames Tideway tunnel that will pass beneath Barhale’s tunnel. On site the most sensitive issue was the absolute need to complete and cover the access shaft in time for the Chelsea Flower Show.
Client for the project is Cadent/National Grid; main contractor is tRIIO, a joint venture between Morrison Utility Services and Skanska Construction UK. Mott MacDonald was responsible for the design.
Currently, 29km of central London’s large diameter intermediate pressure gas mains are being replaced. The Chelsea to Battersea tunnel is a vital link in the new Fulham to Battersea main, which will ultimately support an estimated £15bn regeneration in the Battersea and Nine Elms area.
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