Bangor’s sinkholes are a perplexing problem for the city’s public works crews.
Bangor public works routinely repair a handful of sinkholes each spring, but those sinkholes sometimes lead crews to discover previously unknown, abandoned underground infrastructure and can be a challenge to remedy when crews don’t know what caused a sinkhole.
Bangor has seen six urgent sinkholes so far this year, which residents usually report using an online public works database, according to Aaron Huotari, the city’s public works director.
“The popular thing is to grab a shovel and see how deep you can stick it in,” Huotari said. “A lot of people will call us and say ‘I have a shovel 3 feet into that hole.’”
While figuring out what caused a sinkhole and finding a way to fill it can pose a challenge for Bangor public works crew, sinkholes opening in roads isn’t a unique problem for some of Maine’s oldest cities. A common theme throughout those cities is that the sinkholes are sometimes caused by unknown or abandoned infrastructure failing.
In Portland, Maine’s largest city, public works teams have addressed about one sinkhole per month so far this year, which look like depressions in the pavement, Jessica Grondin, the city’s spokesperson, said. Lewiston, meanwhile, has seen four or five sinkholes this year, which is about average for the city, according to Jeffrey Beaule, Lewiston’s city engineer.
Like Bangor, both Portland and Lewiston public works officials said it’s not unusual to come across previously unknown underground utilities, such as pipes, tunnels, electrical lines and drains, while doing work.
While Bangor also saw six urgent sinkholes in 2021 and 2020, last year’s unstable freeze-thaw pattern and plentiful spring rain wreaked havoc on the city’s roads and ultimately caused 29 urgent sinkholes, Huotari said. Some of those, however, may have been potholes residents mistook for sinkholes.
Sinkholes are usually the result of broken infrastructure, poor workmanship or poor circumstances, according to Huotari.
Sinkholes often stem from underground construction that requires a hole to be dug. Once the work is done and the hole needs to be filled, the soil must be mechanically compacted into the hole every 6 inches, Huotari said. If contractors put too much soil in the hole before compressing it, it can’t be smushed properly.
Sometimes the ground can’t be pressed because layers of water pipes, sewage tunnels and electrical lines prevent compacting equipment from getting in the space and void pockets are left that aren’t visible from the surface.
“Over time, the earth will vibrate with passing traffic and eventually that void will collapse, opening up space over it, that will collapse, on and on until it reaches the pavement,” Huotari said. “Then it’s a matter of waiting for something heavy enough to break that pavement.”
Sometimes, cracked pavement or broken water or sewer pipes allow water to get into the soil and erode the ground until the pavement on the surface is unsupported.
Other times, abandoned, previously unknown infrastructure fails underground, giving public works crews a surprise when they arrive to fix the issue.
“You drive over these roads never knowing there’s a brick and granite structure down there that used to carry stormwater down to the Kenduskeag Stream,” Huotari said. “Bangor’s an old city, and the more work you’ve done underground, the more likely you are to have a sinkhole.”
To fix a sinkhole, crews will often fill the space with a thin cement called “flowable fill” to fill all the crevices that might not be visible from the surface. It’s important to fill all the void spaces, Huotari said, so the sinkhole doesn’t reappear over time, but there’s no guarantee that the hole won’t reopen.
Public works fought with a recurring sinkhole at the intersection of Birch and State streets for years until crews were able to find an abandoned sewer drain causing the problem.
This spring, Bangor saw significant sinkholes appear along Broadway, Main and Pine streets, but perhaps the deepest opened on Harlow Street where the road connects to Kenduskeag Avenue.
The chasm was about 35 feet deep, Huotari said, and was caused by a partial collapse of an abandoned brick and granite stormwater basin.
Most of the city’s sinkholes have appeared around downtown, Huotari said, as that’s the oldest part of Bangor and the area that has seen the most construction.
Sometimes crews will go digging for something an infrastructure map says is in one location, but they never find it, or it shows up somewhere else, Huotari said. Public works will update infrastructure maps based on what they find.
“We have infrastructure maps, but nobody believes that those are accurate because every time we go in the ground, we find things that we didn’t know about,” he said. “We don’t know what we don’t know until we get in there and figure it out.”
Last month, public works crews were looking for a pipe under Perry Road, but couldn’t find it. They reached a point when they couldn’t dig any deeper without closing the road for safety reasons, Huotari said.
They did, however, stumble upon a water pipe they believed would be elsewhere based on the infrastructure map.
“We don’t know if it was an abandoned or active line, but we felt lucky that we caught it before we broke it,” Huotari said.
While they can sound menacing, Bangor’s sinkholes don’t usually damage other property or infrastructure.
“This isn’t like Florida where huge caverns open in the ground and swallow houses,” Huotari said. “Our sinkholes might swallow a tire, but not a whole car.”
