Groundforce provides special shoring solution
27 July 2023
Groundforce Shorco has provided specialist equipment to support a second air treatment unit (ATU) excavation on the Thames Tideway Tunnel, London’s ‘super sewer’ project.
A number of such units are to be installed along the route of the 15-mile tunnel to treat exhausted air from the sewer.
Last year Groundforce Shorco, which is part of the Vp group, provided Costain, Vinci Construction Grands Projects and 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. Now, Groundforce Shorco is supporting a similar installation at the Greenwich Pumping Station site.
This latest excavation is especially challenging as it is in a very confined location, bounded on one side by the pumping station, which is a listed building, and on an adjacent side by an acoustic enclosure used by the joint venture to receive precast concrete tunnel sections for the Greenwich connection tunnel.
“Although the ATU is only about 8m by 8m in plan, a sheet piled structure of 13m by 15m was designed to enable adjacent ventilation pipework and ducting to be installed simultaneously,” said Civils 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 2 listed Thames Water pumping station.”
Access to the acoustic enclosure also had to be maintained throughout the works. The solution was to create a chamfer to one corner of the 14m by 13m by 5m deep excavation, allowing 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,” he added.
Groundforce Shorco Area Manager, Arpad Nagy, said: “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, meaning a total of 12 props were deployed.
The MP150s are attached to brackets welded to the waling beams. These act as shear-stoppers to prevent the props sliding along the beams.
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 2 listed Thames Water pumping station in order to facilitate the installation of gallows brackets required to support the temporary frames,” explains Arpad.
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, as loadings had to be recalculated and the knee braces repositioned.
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 1m below ground level.
“Groundforce have served us well on the ATU package, engaging at the design stage, attending site to deliver toolbox talks on the EMV [piling hammer], and generally with how the works were progressing,” said Mr Bellamy.
