Modular shoring equipment from Groundforce has helped the Murphy GMC joint venture install a new gas main under the Colebrooke River at Maguiresbridge, Co Fermanagh, Northern Ireland.
Murphy GMC is installing the new gas pipeline for SGN Natural Gas as part of the £250m Gas to the West energy infrastructure project to bring mains gas supplies to eight towns in the western part of the province.
A section of the new gas main runs parallel to the A4 Belfast road and, in two locations where this passes over the meandering Colebrooke River, the buried pipeline must go underneath. At Maguiresbridge, this was done using an auger-boring rig operating from a pit dug into the ground near the riverbank. The machine drives a micro-tunnel under the river to a reception pit on the other bank.
The 13m long, 5m wide excavation was lined with a cofferdam comprising Larssen AR605 sheet piles. The final dig depth was approximately 6m.
To support the excavation, Groundforce designed a structure comprising three hydraulic frames. The middle and bottom frames both consisted of Groundforce’s Mega Brace units, each propped at the mid-point with one 80 tonne capacity HSK80 hydraulic prop.
These two frames were designed to be removed as soon as the concrete base slab had been cast and reached full strength.
The top frame, however, had to remain in place throughout in order to support the opening at the top of the excavation.
“For the top frame we used our Super Mega Brace,” says Groundforce technical sales executive Richard Dunn. “We couldn’t have a prop across the top of the excavation because the client wouldn’t then have been able to lift the auger boring machine into the pit. That’s why we used our strongest bracing system,” he adds.
After Groundforce had designed the support structure and assembled the necessary components, it was discovered that the ground conditions were not as had been expected:
“The test borehole hit a low spot in the limestone bedrock and so our calculations were out: the piles were meant to be driven to 8m but there was refusal at 6.5m,” explains Richard. “There wasn’t enough depth for the sheet piles to toe-in”.
Groundforce’s design engineers were able to modify the design, using only the equipment already on-site, to provide a workable alternative at short notice, says Richard: “Overall this didn’t affect the schedule”.
The first river crossing was completed during the early summer; Murphy GMC and Groundforce are now completing the second river crossing about four miles further downstream.