A complex shoring solution supplied by Groundforce Shorco for another Tideway excavation. 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. The project requires a number of ATUs to be installed along the route of
Vinci Construction Grands Projects and Bachy Soletanche JV, the Costain, demanded mining support to install an air treatment unit (ATU) at Tideway’s Deptford Church Street site in Lewisham. The ATU will control and treat air exhausted from London’s 25km long “super sewer” tunnel. The Costain has installed the ATU alongside
BDC 319 : Aug 2024
A complex shoring solution supplied by Groundforce Shorco for another Tideway excavation. 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. 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 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 another Tideway ATU excavation, this time at their Greenwich Pumping Station site. This ATU excavation is especially challenging as it is in a very 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,” says 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 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,” says Mr Bellamy. “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,” explains Groundforce Shorco Area Manager Arpad Nagy. 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 (ie 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,” explains Arpad. 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, says Arpad: “Once you lose that corner you need to recalculate the loadings and reposition the knee braces.” 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. Despite the complexity of the job, design and installation of the support system has gone very smoothly. “Groundforce have served us well on the ATU package, engaging at the design stage, attending site to deliver toolbox talks on the EMV, and generally with how the works were progressing,” says Mr Bellamy. Building, Design & Construction Magazine | The Choice of Industry Professionals
Vinci Construction Grands Projects and Bachy Soletanche JV, the Costain, demanded mining support to install an air treatment unit (ATU) at Tideway’s Deptford Church Street site in Lewisham. The ATU will control and treat air exhausted from London’s 25km long “super sewer” tunnel. The Costain has installed the ATU alongside a 60m deep, 16m diameter shaft that is said to move effluent from street level to the main sewer tunnel downward. In turn, Groundforce Shorco supplied a sheet piling and structural support solution for the ATU excavation. It is located in a confined area adjacent to the main shaft and includes three separate sections. The section engineer, Freddie Gorman, pointed out: “When the sewer is operating, air will build up inside and needs to be vented,” “We can’t just let it come straight out; it needs to be filtered. When the shaft is completed, it will be capped and any air trapped inside will be fed through the ATU, where it’s treated. It then goes into a ventilation chamber before being vented out into the atmosphere.” For supporting the excavation, Groundforce supplied KD6 sheets and Larssen sheet piles with its Mechshore, Maxi and Mega Bracing frames. Three levels of the frame with the larger capacity Mega Brace frames located at the bottom to support the higher loads were installed. Arpad Nagy, Groundforce area manager, illustrated that the temporary works design was complicated since the three sections of the excavation, each a distinct size and depth, were built back-to-back. Nagy expressed: “The frames work with and against each other and are interdependent. That means we couldn’t make any changes to one frame without affecting all of the others.” In addition, the steel sheets lining the excavation act as the outer skin of the formwork necessitates to pour the reinforced concrete walls of the ATU chamber. He continued: “The original plan was to use formwork to construct the ATU walls, but by pouring the concrete against the sheet piles the project cost has been reduced.” The excavation’s complexity, merged with varying ground levels and the proximity of the main shaft, meant the temporary works design process was tough. Nagy claimed: “The whole evolution of the temporary works design took around seven months and went through eight different versions.” The first design meeting was in April 2021, and the first delivery of equipment to the Tideway site was not until December 2021. Another obstacle for the site team was the ground conditions on the site, which were extremely hard and stony. This made it challenging to install the steel sheet piles just with the excavator-mounted vibrating hammer. Therefore, a pre-augering method had to be acquired. Also, several sheet piles still remain to be installed. However, the proximity of the current shaft resulted in additional problems. Ross Hope, a senior engineer, explained: “We were tasked with designing a system to allow the removal of the sheets which were blocking the connection, as well as providing enough clearance to allow the concrete walls to be fully cast. We came up with using a trapezoidal frame in order to transfer loads to the existing capping beam, followed by a secondary temporary frame propping off the top of the capping beam in order to allow the removal of the frames below.” The task has now advanced onto a second phase which was the excavation to obtain a long culvert that channels the filtered air from the ATU to the ventilation chamber. Nagy clarified: “This excavation will be just as complicated, but we’ve learned a few things in phase one, so I’m sure we can provide a suitable solution.” The ATU is located near the historic St Paul’s Church in Deptford and is an element of the £4.3bn Thames Tideway Tunnel project. It is said to create a combined sewer, running mainly beneath the tidal section of the River Thames across London, to capture, store and convey 95% of the raw sewage and rainwater which presently overflows into the Thames. Building, Design & Construction Magazine | The Choice of Industry Professionals
