Contractor Murphy GMC is progressing well on the £250M Gas to the West project for SGN Natural Gas in Northern Ireland after completing an auger bore to take the new gas pipeline below a local river.
Part of the new pipeline, which will bring mains gas supplied to the west the province, runs alongside the A4 Belfast Road and had to pass under the Colebrooke River and Murphy had to contend with unexpected ground conditions at the site.
Murphy used modular shoring equipment from Groundforce to create cofferdam to allow an auger boring rig to drive a micro-tunnel below the river to a similar 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 80t 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,” said 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.”
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,” explained Dunn. “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. Dunn said: “Overall this didn’t affect the schedule.”
The team is now working on another river crossing further downstream.