...productivity,” says Ronicle. The TBMs worked through a range materials including sand, clay and hard dolerite, a type of basalt.

There were more dolerite intrusions than expected, but they were fractured, eliminating the need for disc cutters, says Ronicle. The bigger machine advanced at about 20 m in clay and 12 m in sandstone per 12-hour shift, he adds.

Dolerite fractured by a TBM last year came to haunt the project this fall, when a section of busy Cromac Street sank. Remedial grouting had failed to fill a void created by the fracture and the ground above began settling, says a utility spokeswoman. Settlement reached the surface on Nov. 7, damaging the road, which now repaired. The tunnel was unaffected and recent investigations have revealed no more voids, she adds.

Ronicle cites contaminated land as a major tunneling impediment. Under the redeveloped site of a former coal-gas production plant, polluted water had migrated much further and deeper than expected. Without treating the ground, “We put the boys in chemical suits” to operate the TBM, he says. Suits provided good protection but cut productivity in the roughly 400-m-long contaminated stretch by 80%, he adds.

With more water than expected entering several large-diameter shafts, “We had some exciting moments,” says Ronicle. Four shafts needed to be temporally roofed with steelwork decking and filled with compressed air to allow excavation and lining.

Largely because of the ground conditions and work acceleration, Ronicle estimates the contract’s target cost to have risen some 30% from the original $150 million. “It’s one of the most challenging jobs I’ve ever done,” he says.

Morgan=Est will likely bid for the next phase of storm tunnel. But having intimate knowledge of Belfast’s tricky ground may put the firm at a pricing disadvantage, suspects Ronicle.

Valued at around $130 million, the new storm sewer will serve the city’s eastern part, says Gowdy. His team has begun short-listing design firms, aiming to have the project sufficiently drawn up to start procuring a turnkey contract early in 2011.