Groundforce Shorco has provided specialist equipment to support a second air treatment init (ATU) excavation on the Thames Tideway tunnel.
The project requires a number of ATUs to be installed along the route of the 25km tunnel to treat exhausted air from the super-sewer.
Last year, Groundforce provided the Costain Vinci Bachy Soletanche Joint Venture on the eastern section with sheet piles and a range of medium and heavy-duty bracing frames for the ATU excavation in the Deptford Church Street site.
Groundforce Shorco is now supporting another Tideway ATU excavation, this time at the Greenwich Pumping Station site.
This excavation is especially challenging, Groundforce says, because it is in a confined location, bounded on one side by Greenwich Pumping Station, a listed building, and on an adjacent side by an acoustic enclosure used by the JV to receive precast concrete tunnel sections for the Greenwich connection tunnel.
"Although the ATU is only about 8m x 8m in plan, a sheet piled structure of 13m x 15m was designed to enable adjacent ventilation pipework and ducting to be installed simultaneously," said Costain senior engineer Andrew Bellamy.
"Stringent limits on the deflection of the sheet were set due to the proximity of the excavation to the acoustic enclosure foundations, which includes a gantry crane, working continuously to supply the secondary lining works. The excavation was sheet piled on three sides, the fourth being the Grade II listed Thames Water pumping station."
Access to the acoustic enclosure needed to be maintained throughout the works, adding further restrictions to the already confined working area. The solution was to create a chamfer to one corner of the 14m x 13m x 5m deep excavation. This chamfer allowed articulated lorries to continue delivering materials.
"Due to the orientation of the site access gate it was physically impossible to move the vehicle route away from the ATU and without the chamfered corner vehicle access would have been completely impossible for HGVs," Andrew Bellamy added.
Groundforce Shorco area manager Arpad Nagy explained: "There are two excavators on site, including one 45-tonner, and three lorry deliveries per shift bringing tunnel segments for the tunnel boring machine, which meant that our design had to accommodate an increased surcharge of 15kN/m2."
The excavation is lined with 6m-long GU13N steel sheet piles and braced with three levels of Groundforce's hydraulic Mega Brace frames. Four MP150 modular hydraulic props were installed as knee-braces (spanning the corners of the excavation) at all three levels within the excavation – a total of 12 props.
The MP150s are attached to brackets welded to the waling beams. "These act as shear-stoppers to prevent the props sliding along the beams," said Arpad Nagy.
One side of the excavation runs along the top of the pumping station's 10m-deep foundations.
"The JV had to drill into the foundation of the Grade II listed Thames Water pumping station to facilitate the installation of gallows brackets required to support the temporary frames," he said.
The support structure is designed to limit deflections in the sheet piles to within 10mm.
Designing the support system was complicated by the need to chamfer the corner of the excavation. "Once you lose that corner you need to recalculate the loadings and reposition the knee braces," he added.
With the bracing installed, the 800mm-thick base slab was cast. When this had reached full strength, the two bottom levels of bracing could be removed to allow formwork to be assembled for the concrete wall lining.
The top level of bracing is due to be removed when the lining walls have reached one metre below ground level.
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