By Guy Page and Tim Page

A broken sewer pipe caused the gaping sinkhole that opened on Main Street in Burlington at about 11 pm Saturday night, city officials say. 

When the old, concrete pipe failed, its contents washed away roadbed materials, leaving a void that took city workers 12 hours to repair. No accidents or injuries were reported.

Observers spotted and photographed (above) the sinkhole in the eastbound lane near Memorial Auditorium and a ski shop, between South Union Street and South Winooski Avenue. They estimated the diameter at about six feet. Traffic in the eastbound lane was closed while repairs were made. No noticeable sewer smell was detected.

Concrete is not a common pipe construction material in Burlington, Robert Goulding, communications officer for the Burlington Department of Public Works, said. “My understanding is that there is not a lot, because we’re not coming across a lot,” he told the Burlington Daily News Tuesday morning. 

Sinkholes, too, are rare in Burlington, especially holes of this size: “Certainly, the magnitude of this one was not common,” Goulding said.

The sinkhole occurred during a multi-year, citywide assessment, replacement and relining of stormwater and sewer pipes. The Clean Water Resiliency Plan bond approved by voters in 2018 is funding assessment of the estimated 100 miles of pipes, replacing and relining as needed.

About 10 miles of streets are slated for replacement/relining – but Main Street is not among them. “With such a large system, there’s more to go,” Goulding said.

Sinkholes typically occur when heavy stormwater and snowmelt cause a pipe to break beneath the street and wash out the roadbed. “We’ve had a wild couple days of weather,” Goulding noted.

Although not common in recent years, Burlington does have a historic sinkhole problem.

The Oakledge Park Sinkhole: In the summer of 1927, Burlington residents were taken aback when a significant sinkhole appeared in Oakledge Park. This sinkhole, estimated to be around 30 feet in diameter, formed due to the collapse of an underground limestone cave system. Fortunately, no injuries were reported, but it served as a reminder of the geological complexities beneath the city’s surface.

The North Avenue Sinkhole: In 1954, a sudden sinkhole emerged on North Avenue, one of Burlington’s busiest streets. This sinkhole, measuring approximately 15 feet wide, was caused by the erosion of underground soil and rock layers due to a combination of heavy rainfall and aging infrastructure. The incident led to temporary road closures and subsequent repairs to reinforce the area’s underground infrastructure.

The Battery Street Sinkhole: In the spring of 1978, a notable sinkhole developed on Battery Street, near the waterfront. The sinkhole, estimated to be about 20 feet in diameter, formed due to the collapse of an underground stormwater drainage pipe. The incident disrupted traffic in the area and prompted the city to conduct inspections and repairs on its aging infrastructure.

The University Mall (South Burlington) Sinkhole: In 1993, a sinkhole appeared in the parking lot of the University Mall, causing a minor disruption for shoppers and employees. Measuring approximately 10 feet in diameter, the sinkhole was a result of the gradual dissolution of underlying limestone and the subsequent collapse of the surface layer. The mall management promptly took measures to repair and stabilize the affected area.

The Intervale Sinkhole: In 2008, a sinkhole emerged in the Intervale, a fertile agricultural area located along the Winooski River. This sinkhole, spanning about 25 feet in width, was attributed to the erosion of underlying glacial till and the collapse of an underground channel. Fortunately, no injuries were reported, but the incident highlighted the importance of monitoring and managing the region’s hydrology to protect both agricultural lands and nearby water bodies.

More recently, a burst pipe beneath Route 7 caused a sinkhole on March 15, 2019, between Flynn Ave. and Prospect Parkway. In June, 2020, the Burlington Free Press reported on a sinkhole discovered by Pizza delivery driver Tony Chea on Loomis St..

In addition to the pipe replacement, Burlington has been aggressively pursuing its pothole/aging street problem. “The City has been working on a fairly assertive plan to reinvest in the city’s roads,” Goulding said. Voters in 2016 passed a capital bond funding increased street rebuilding. City streets were being repaved at a rate of four miles a year (double the usual) until Covid and inflation slowed progress. The streets are now being repaved at about three miles per year, Goulding said.