New lifting bridge reaches construction milestone

Aerial view of bascule pit excavation for Great Yarmouth Third River Crossing (Photo: Groundforce Shorco)

Joint venture company BAM Farrans has reached a key stage in the construction of the new £121 million Great Yarmouth Third River Crossing in Norfolk, UK.

Connecting the A47 on the west bank of the River Yare with the port and business area on the east bank, the structure - known as a twin leaf bascule or counterbalanced lift bridge - will comprise two halves that will lift upwards to allows ships to pass under it.

BAM Farrans has now completed the installation of structural supports to the two large excavated pits - one on either side of the River Yare - that will house the bridge’s lifting mechanism.

The company used heavy-duty hydraulic braces and props from Groundforce Shorco - a specialist in trenching and shoring equipment - to support the excavations, which measure 19m x 24m x 10m deep and are surrounded by tidal water on three sides.

According to the company, the bascule pits have been dug within cofferdams comprising a combination of interlocking steel sheet piles and tubular steel driven piles.

Thomas Hughes, Major Project Manager (South) for Groundforce, said the use of tubular steel piles - which “provide a lot of strength” and are in excess of 30m in length - mean that additional propping lower down the excavations was not necessary.

Aerial view of bascule pit excavation for Great Yarmouth Third River Crossing (Photo: Groundforce Shorco)

Instead the company’s equipment was used around the top of the cofferdams to support the high lateral loads, which exceed 400kN/m.

BAM Farrans’ collaboration with Groundforce saw the installation of a high-capacity waling beam - comprising a Super Mega Brace frame - around the top of each excavation and high-capacity MP 375 and MP 250 props (375kN and 250kN respectively) to brace the frame.

Groundforce said, “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.” 

Kevin Percival, Sub-Agent for BAM Farrans JV, said, “This is a design-build contract and our designers produced drawings that assumed the use of fabricated steelwork to support the cofferdams.

Aerial view of bascule pit excavation for Great Yarmouth Third River Crossing (Photo: Groundforce Shorco)

“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.”

Both excavation and shoring works to the bascule pits, including the pouring of concrete base slabs, have now been completed and BAM Farrans 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,” said Percival. “As there are six walls in total, they should all be done by March. Then the supports can come out.”

The Great Yarmouth Third River Crossing is due for completion in March 2023.

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