Massive drill surfaces after boring storm tunnel beneath South Boston

(George Rizer/Globe Staff)

Worker A.J. Gordon waved after the cutterhead broke through to complete the tunnel.

By John C. Drake, Globe Staff

A hulking 350-ton Japanese boring machine poked out through the earth in South Boston today after nearly a yearlong subterranean journey. The massive drill dug through rock to create a 19-foot wide sewage and storm water tunnel that runs for 2.1 miles beneath William J. Day Boulevard along the shore of Dorchester Bay.

The $215 million passage is designed to catch rainwater during storms and prevent the runoff from polluting the bay and Boston Harbor beyond. The goal is to eliminate the bacteria that can sully the water and force beach closures.

The giant bore emerged today behind the State Police barracks on the southern edge of Carson Beach. A crowd of about two dozen workers and gawkers oohed and ahed this morning as hulking slabs of concrete broke apart and crashed into a shaft, exposing the jagged edges of the cutting blade, which was seeing its first light in more than a year.

The tunnel it left in its path will store rainwater during a storm and send it to a new pumping station at Conley Terminal, which is near Castle Island. The pumping station will then push storm water to the Deer Island Sewage Treatment Plant.

Officials said it was ironic that the tunnel was completed today as Boston was emerging from a stretch of particularly rainy weather that has caused more beach closures than usual.

"Instead of the storm water and the combined flows, which includes human waste, going onto the beach, it will go into this tunnel," said Fred Laskey, executive director of the Massachusetts Water Resources Authority. "It's perfectly suited to deal with the negative impacts we've seen this year."

The project, which began last September, is expected to take four years to complete. It is part of the billion dollar effort to rejuvenate Boston Harbor.

Under normal conditions, rain and sewage are piped to Boston's water treatment plant. But during heavy storms, runoff can overwhelm the sewer system, forcing runoff and sewage into relief valves that empty into the harbor. High bacteria levels after storms have forced the closing of beaches in South Boston an average of 21 times a year, including this past weekend. The new tunnel and pumping station will hold overflows from all but the largest storms.

When the new system is in place, engineers will open gates when heavy rainfall approaches that will direct the combined sewer flow to the new North Dorchester Bay Combined Sewer Overflow Tunnel, where it will sit until the storm ends, before being pumped out. The 350-ton boring machine, custom-built for the project, included a train more than 300 feet long. It dug the tunnel 30 to 50 feet underground.

John C. Drake can be reached at jdrake@globe.com.

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