Equipment supplied by Groundforce Shorco played a vital part in one the UK’s most important infrastructure projects.
Groundforce Shorco supplied their heavy-duty hydraulic braces and props to support 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 Project
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 either side of the river.
Challenges
The excavations each measured approximately 19m x 24m x 10m deep and were surrounded by tidal water on three sides. This, together with the extremely poor ground conditions, meant substantial structural support was required around their perimeters.
Solution
The pits were dug within robust cofferdams comprising a combination of interlocking steel sheet piles and tubular steel driven piles. The Groundforce equipment was 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.
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) were used to brace the frame.
Four MP 375s were installed as knee-braces in each excavation, one spanning each corner of each rectangular frame. A central MP 250 prop spanned each excavation at the mid-point along its length to provide added stability. Groundforce’s own load-monitoring system was fitted to one of the MP 250 props. These measure and record actual loads in real time and are monitored continually.
The system was calibrated with pre-set load limits and configured to trigger an automatic alert if the limit was approached or exceeded. To ensure maximum stiffness, the MP 250s were fitted with ‘super-tube’ extensions which at 1,220mm, are twice the diameter of the standard tubes.
Although fairly deep, the excavations were propped only at the top of the cofferdam. The tubular steel piles provided a lot of strength and were very deep – in excess of 30m in length. With plenty toe-in, it meant additional propping lower down the excavations was not required.
The equipment was installed in two phases - the west-bank cofferdam was braced first, followed a few days later by the east-bank cofferdam.
Client Comment
Kevin Percival, Sub-Agent for BAM Farrans JV, said: “The Groundforce solution has saved time and improved site safety on this project.”