An unexplained sinkhole has formed near the Dublin Town Library, a problem town officials say will likely need to be addressed before repairs can be made to an adjacent stone wall.
Chris Raymond of the town’s Capital Improvement Programs Committee discussed the issue with the Select Board at meetings on Dec. 30 and Jan. 5.
Though the town is focused on repairing the library’s stone wall with the help of the Milford-based Fieldstone Land Consultants, Raymond recommended they shift their attention to fixing the sinkhole that’s damaging the stone wall. He proposed a two-phase approach: Phase I would allocate $20,000 to $30,000 to excavate, investigate and fix the sinkhole, and Phase II would focus on repairing the stone wall.
“We should address the sinkhole before we do any work on the wall. Without figuring out what is really going on with the drainage, it would be imprudent to do any other work,” Raymond said. He said that Fieldstone’s plan doesn’t address the drainage problem.
The Dublin town administrator said the sinkhole appeared about two years ago, and the town will now have to raise funds to fix it.
Sinkholes can form in a variety of ways, depending largely on geology and location. Fred Rogers, a professor of environmental science and geology at Franklin Pierce University, said sinkholes may be either natural or human-caused.
According to Rogers, natural sinkholes often occur in areas where the bedrock is made of limestone, sand or other brittle or soft minerals.
“Brittle bedrock is water-soluble, so when percolating groundwater flows over something like limestone, the water becomes acidic, thereby dissolving the ground around it. Eventually, in places like Kentucky or Florida where the bedrock is soft and brittle, caverns begin to form,” he said. Over time, these caverns can collapse if enough weight is put on them.
Rogers said that, as the Granite State, New Hampshire’s bedrock is not brittle, making sinkholes rare in the state.
“For a state like New Hampshire, if a sinkhole forms, it is more than likely going to be artificial. By that I mean it will come about as a result of human development.”
“In the case for the Dublin Library, it sounds like it might be due to old piping,” Rogers said. “Pipe and sewer systems, particularly old ones in rural areas, serve as conduits for sinkholes. A pipe may rust and corrode, leaving a man-made cavern in the ground that eventually becomes a sinkhole as water from the piping system flows into the open gap.”
The Dublin Select Board proposed drafting a $50,000 warrant article to eliminate the sinkhole.