If today were April 1, readers might assume this story is a joke. But no. The now-infamous Water Street sewer project is in trouble, again.
In the area closest to the latest construction zone, one corner of the Plymouth Waterfront Visitor Center on Water Street appears to have suddenly dropped nearly 2 inches, and a sinkhole opened in the Memorial Hall parking lot behind the visitor center.
“We are still evaluating what is going on,” Plymouth Department of Public Works Director William Coyle said Wednesday.
The new sinkhole is the latest in a four-year-long series of problems plaguing the sewer line project almost since the beginning, prompting infighting and finger-pointing as the project hits snag after snag.
This time, the contractor, Northern Construction, mobilized on Water Street in September to replace the last 70 feet of sewer pipe – out of the quarter mile of new pipe already installed – because it had settled more than expected into unstable soil.
That last 70 feet runs in front of the visitor center near the rotary at Water Street and Town Wharf.
Indications there might be a new problem first surfaced Tuesday, Sept. 29, when a survey crew doing regularly scheduled measurements in the construction zone reported the southeast corner of the visitor center had dropped nearly 2 inches from the previous reading.
Then, in the afternoon of Friday, Oct. 2, part of the asphalt in the parking lot behind the center cracked and fell in, revealing a 3-foot-deep sinkhole.
“Out of an abundance of caution, our engineers and Northern agreed we should shut the wells down on Friday to give us time to understand what happened,” Coyle said.
Coyle is referring to nine “dewatering” wells installed in the construction zone to draw down the groundwater to allow for stable digging. As of last week, the wells had dropped the groundwater table to about 16 feet below the surface, Coyle said, with another 7 feet of dewatering still needed before deep excavation can begin.
The wells pump the groundwater into large green filter tanks on Town Wharf that remove the sediment, then discharge the clear water into the harbor.
Coyle said he didn’t want to speculate on whether pulling so much water out of the ground so quickly could have caused the soil in the area to shift, leading to the drop in the building and the sinkhole. He said he is waiting for an engineering analysis and more data.
“We are bringing in ground-penetrating radar on Friday to see what else may be going on,” Coyle said.
The radar team will scan the whole area to a depth of about 8 feet, looking for any voids in the ground or unstable structures that are problematic, he said. The scan is expected to be completed Friday, Oct. 9, with a report to follow within a week.
Until then, since the area appears to be stable for now, the town has allowed the dewatering wells to be turned back on to limit delays, Coyle said.
The ongoing saga stems from the need to replace an aging quarter-mile section of 30-inch sewer pipe installed in 1969 that runs under the edge of the harbor adjacent to Water Street.
Town officials were concerned a break in the old line would be an environmental disaster for the waterfront.
Michael Cohen can be reached at michael@plymouthindependent.org.