I-93 Sinkhole Caused by Concord Storm Drain

A sinkhole that formed on I-93 northbound in Concord on Aug. 19, 2015, is presumed to have been caused by a city storm water drain that runs from behind Market Basket on Storrs Street to the Merrimack River, according to a New Hampshire Department of Transportation official.

William Boynton, the public information officer for NHDOT, stated, “There’s definitely a link to a storm drain underneath the highway … that’s where it starts.”

A thunderstorm last night that lasted for about 20 minutes caused some flash flooding around the Granite State, including in Concord. As reported on Concord NH Patch earlier today, Concord General Services workers were seen behind the store working on a storm drain right across from the sinkhole. Patch readers noted online that the section of I-93 that is near the sinkhole was completely flooded during the storm and vehicles were hydroplaning while trying to navigate the water.

Boynton said the NHDOT has requested that Audley Construction out of Bow – the company contracted for the Exit 12 I-93 bridge project less than a mile from the sinkhole site – to come to the scene to perform the emergency repair work.

“It’s a bit out of our scope,” he noted when speaking about the repair work required.

Audley will fill the drain with concrete, will load backfill over it and tar it over, for a temporary fix. He expects the repair work to be done through early evening. During that time, Boynton noted, the Interstate will remain closed.

The sinkhole measures about 20 feet in depth and is cone shaped at the bottom, according to Boynton, and is 12 feet long and 14 feet wide.

Commuters, he noted, were calling in reports of a large dip that continued to collapse and eventually became the sinkhole. The department closed the highway at just after noontime.

Officials quickly discovered that the right side low speed lane had also been compromised by the sinkhole which is why officials have kept the highway completely closed. For a short time, the southbound left passing lane was closed by officials inspected the area but determined it to be safe.

“We are very fortunate that no one was injured and no cars were damaged,” Boynton said.

Officials from Concord General Services were not available for comment at post time since they were at the scene inspecting the drainpipe. The age of the pipe is unknown but probably dates back to long before the railroad yard was turned into the Capital Plaza Shopping Center on Storrs Street in the late 1950s and early 1960s.

About 12 or 13 years ago, city officials proposed a concept to bury that section of the highway to create a walkway connecting the shopping plaza to the river as part of the Reconstructing I-93 widening project but the idea was quickly scrapped due to costs and comparison to Boston’s Big Dig fiasco.

Commuters have been reporting traffic backups on both sides of the highway for most of the day. Boynton said he heard that at one point I-93 North traffic was backed up to the Hooksett Tollbooth. NHDOT put alerts up along the Interstate as far aback as Londonderry in an effort to alert commuters about the problems, he stated. 

Caption: Pictures of a very old Concord city storm drain that runs from Market Basket to the Merrimack River and is presumed to be the cause of a sinkhole on I-93. Credit: NHDOT