The company's heavy-duty hydraulic braces and props are supporting two large excavations on opposite banks of the River Yare in Great Yarmouth, where a new lift bridge – the Great Yarmouth Third River Crossing – is being built by the BAM Farrans Joint Venture (JV).
The £121 million project will provide a link from the A47 at Harfrey's roundabout on the west bank of the river to the port and the enterprise zone via South Denes Road on the eastern bank.
The new structure will be a twin leaf bascule bridge, also referred to as a counterbalanced lift bridge, to allow access for commercial vessels. The two halves of the bridge will swing up to give clearance to ships. The best-known bascule bridge in the UK is Tower Bridge in London.
The bridge mechanism will be housed within two deep bascule pits, one on either side of the river, which are currently under construction.
The excavations each measure approximately 19m x 24m x 10m deep and are surrounded by tidal water on three sides. This, together with the extremely poor ground conditions, means that substantial structural support is required around the perimeter of both excavations.
The pits have been dug within robust cofferdams comprising a combination of interlocking steel sheet piles and tubular steel driven piles. The Groundforce equipment is installed around the top of the cofferdams to support the high lateral loads.
As these loads exceed 400kN/m, props from the heavier end of the Groundforce range were required.
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A high-capacity waling beam comprising a Super Mega Brace frame was installed around the top of each excavation and high-capacity MP 375 and MP 250 props (375kN and 250kN respectively) are used to brace the frame.
Four MP 375s are installed as knee-braces in each excavation, one spanning each corner of each rectangular frame. A central MP 250 prop spans each excavation at the mid-point along its length to provide added stability. Groundforce’s own load-monitoring system is fitted to one of the MP 250 props. These measure and record actual loads in real time and are monitored continually.
The system is calibrated with pre-set load limits and configured to trigger an automatic alert if the limit is approached or exceeded. So far, the loadings have remained well within the design parameters.
To ensure maximum stiffness, the MP 250s are fitted with ‘super-tube’ extensions which, at 1,220mm, are twice the diameter of the standard tubes.
Although fairly deep, the excavations are propped only at the top of the cofferdam. Thomas Hughes, Major Project Manager (South) for Groundforce, explained: "The tubular steel piles provide a lot of strength and they’re very deep – in excess of 30m in length – so there’s plenty of toe-in." This means additional propping lower down the excavations is not required.
Construction work started in January 2021 and the first Groundforce components arrived on site in August as excavation within the two cofferdams got underway.
The equipment was installed in two phases: first the west-bank cofferdam was braced, followed a few days later by the east-bank cofferdam.
Kevin Percival, Sub-Agent for BAM Farrans JV, says that the Groundforce solution has saved time and improved site safety on this project.
"This is a design-build contract and our designers produced drawings that assumed the use of fabricated steelwork to support the cofferdams.
"I didn't specify the Groundforce equipment, but I've worked with it before and it’s much quicker than fabricated steel. With steelwork there would have been a lot of welding to do on-site. This is much quicker and doesn’t require welders to work inside the cofferdam," he explained.
Another factor influencing the decision to specify a modular system like Groundforce's was site access. "Access is difficult because most of the excavation is in the river. It’s much easier to install modular components in these conditions," added Kevin.
Progress has been smooth and free of snags so far, he added: "We've had a lot of good support from Groundforce. They've been really attentive and very, very thorough."
Both excavations were completed and the base slab for both bascule pits were cast just before Christmas 2021. BAM Farrans JV is now casting the side walls of the pits.
Concrete for the first side-wall (in the western cofferdam) was poured in early January. "We're aiming to cast a wall per week," explained Kevin. "As there are six walls in total, they should all be done by March. Then the supports can come out," he added.
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