Current Projects

Minor Cave-in Halts Work on CSO Tunnel

James Finlaw - Herald New Staff Reporter: 8/18/2003

Drilling operations at the city's combined sewer overflow tunnel were temporarily halted earlier this week when a minor cave-in occurred inside the tunnel.

Project engineer Robert M. Otoski of Camp, Dresser & McKee Inc., said the cave-in took place last weekend when no workers were inside the tunnel. He said the amount of rock that actually fell from the roof of the tunnel could have been brought out "in a couple of wheelbarrows," and that the debris did not block off the remainder of the 20-foot-diameter tunnel.

"It happens. If somebody had gotten hurt it certainly would have been a big deal, but from a tunnel boring standpoint, it's not a big deal," he said. "The tunnel is not in danger of collapse," said Otoski. Otoski said the incident cost J.F. Shea Inc., the California-based company drilling the 3-mile sewer tunnel, a "couple of days" of drilling. However, he said the interruption did not take the $55.9 million project off schedule. Otoski said the work continues to remain ahead of schedule, with completion expected at the close of 2004.

Otoski said the cave-in occurred along a segment of the tunnel where the granite being bored through was fractured. He said work crews remedied the problem by inserting 10-foot rock bolts into the area where the fractures were located, and placing wire mesh over the location. "If you have fractured rock and you're concerned about a piece of it falling apart, you put a 10-foot bolt into the rock and back it up with concrete or grout material," Otoski explained.

The tunnel, which is designed to resolve the city's existing sewage control problem, is currently 11,850 feet long. It will be 16,194 feet long when completed. Once work is finished, the tunnel will stretch from the city's wastewater treatment plant on Bay Street to Lowell Street.

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