By Sherri Hall
Imagine walking along the grounds of your community when you notice what appears to be a small rabbit hole. You look ahead and see a few more similar holes. Filling them with dirt doesn’t seem to help. In fact, they just keep coming back and getting larger and larger. That’s when you realize, you might just have a sinkhole issue on your hands. But how can you know for sure? And, what should you do about it?
According to Professional Engineer Dan Fischer, of FWH Associates in New Jersey, “sinkholes can cause a variety of damage to property as well as the health and safety of the community’s residents.”
If a sinkhole gets close enough to a building, it can start undermining its foundation or if a sinkhole forms under a road it has the potential to cause a serious accident, he added.
According to Fischer, sinkholes can form under both paved and unpaved areas; however, they may not always be noticeable at first. On lawn areas, they may look like a depression or be similar to a rabbit hole. With regard to paved areas, you may not see a hole right away. Instead, you might notice the road starting to crack and lower, he said.
“Of course, in the case of lawn areas, someone may trip and fall due to the holes. However, with regard to roads, sidewalks and other paved areas, sinkholes can undermine their strength and integrity, making them incapable of carrying the loads they were meant to,” Fischer said.
Sinkholes within a roadway can cause serious problems for vehicular traffic. A sinkhole that forms under a roadway can be temporarily masked by the above bituminous pavement layers. When enough of the underlying subgrade deteriorates, the right pressure at the right time from a passing vehicle can cause an abrupt failure, Fischer explained.
How do these sinkholes form in the first place? According to Fischer, there are two types of sinkholes, “natural” sinkholes (which aren’t as common in New Jersey) and “mechanical” sinkholes. “Natural sinkholes usually occur in karst topography; an area containing limestone, gypsum or salt beds, which dissolve easily in water underground,” he said. “This leaves a void underground that eventually starts showing up on the surface.”
On the other hand, mechanical sinkholes are caused by issues with man-made infrastructure such as breaks in water mains or storm sewer systems, separation of pipe joints, improper pipe connections to a stormwater structure or improper soil compaction under roads, Fischer explained.
“With the exception of improper soil compaction under roads, what happens in these instances is the water from a damaged system starts eroding the adjacent soil. This erosion is more prevalent in areas containing sandy soil,” he noted.
Naturally occurring sinkholes aren’t typical in New Jersey, said Fischer. They are more common in other areas of the Northeast, such as Pennsylvania, where they are caused by underground caves or abandoned mines.
Once sinkholes form, how should they be addressed? When it comes to the naturally occurring sinkholes, Fischer recommends excavating down to where the soil is more compact and not sinking any further. Then, the area should be backfilled with a soil that has a high clay content. He noted that clay does not as easily allow water seepage. The soil should be compacted in one (1) foot layers, as it is being replaced, until the normal surface elevation is obtained.
“If it is a mechanical sinkhole, the area still has to be excavated down to the cause of the problem. Most often, it’s occurring adjacent to a broken pipe or water main, the location of which can be determined by referring to utility maps of the area prior to excavation, said Fischer.
Once a problem, such as a broken pipe or separated joint is found, it must be fixed. Then, the area needs to be backfilled, compacting the soil in one (1) foot layers.
In the case of paved areas, the process is a little different than that with unpaved land, Fischer noted. He explained that’s because the pavement has to be properly remediated as well.
In his own experience, Fischer has dealt with a variety of sinkhole formations in community associations. One involved a stormwater pipe that was poorly connected to an inlet structure. This allowed water that flowed through the system to erode the soil all around the structure, he explained. As a result, the back of the inlet structure developed a very large sinkhole. After excavating down to the stormwater pipe, water that remained within the system had to be pumped out prior to remediation of the connection. Once the connection was fixed, the soil was backfilled and compacted. The issue was remediated and there has been no formation of a sinkhole in that area since, he said. If fixed properly, sinkholes caused by mechanical issues are not likely to reoccur,” said Fischer. “If you stop the water from eroding the surrounding soil, that should solve the problem.”
Another recent sinkhole Fischer worked on was caused by something entirely different – buried organic material. The community started noticing sinkholes on a lawn area adjacent to a woods line within their community. It began with five small sinkholes that grew significantly in a short period of time. During the excavation process it was discovered that, years ago, the developer buried tree stumps, limbs and other organic material 5-8 feet beneath the ground of the sinking area. “Over years of varying groundwater elevation the organic material lost its cohesion with the surrounding soil which began to create a void, explained Fischer. The void accelerated with respect to time. The more time that went by, the quicker the sinkhole grew.
The organic material was removed and separated from the reusable soil. The area was backfilled with whatever soil was reusable, as well as additional fill, and compacted. Fischer noted that if the organic material hadn’t been removed, the problem would have just continued to worsen.
Although insurance policies do not typically cover damage resulting from sinkholes, such coverage may be available depending on the carrier. Associations can specifically request information from their insurance provider regarding this type of coverage.
If a property manager or board member starts seeing signs of a potential sinkhole within their community, it’s important to contact an engineering professional to perform a site visit and evaluate the situation, said Fischer.