Sinkholes in Philadelphia: What Causes Them and Why Repairs Take So Long

Recent sinkholes in West Oak Lane, the Schuylkill River Trail and Point Breeze

In early January, a very large sinkhole opened up at a crossroads in West Oak Lane, a neighbourhood in North Philadelphia, after a burst water main.

Only a fortnight earlier, the city had reopened a stretch of the Schuylkill River Trail in Center City. That section had been closed for two months because of a sinkhole.

Last summer, residents on a block in Point Breeze in South Philly also ended up waiting two months for repairs after a sinkhole appeared.

Laura Toran, a hydrogeologist and professor emeritus of environmental geology at Temple University, was asked by The Conversation US to explain what causes sinkholes, whether Philly is especially susceptible, and why repairs can be drawn out.

What are sinkholes and how do they happen?

A sinkhole is a cavity that suddenly opens at the ground surface because something has changed below ground.

Broadly, sinkholes form for two reasons. One type is linked to carbonate rock. Carbonate rock can develop underground voids and caves because it dissolves when groundwater is even mildly acidic. If the “roof” spanning one of these cavities gives way, the ground collapses and a sinkhole forms.

The other type is connected to buried water-supply lines or sewer pipes. If a pipe leaks, the soil or sediment alongside it can erode and be washed out. That creates an underground gap; when the surface collapse becomes large enough, it is recognised as a sinkhole.

What do we know about West Oak Lane and the Schuylkill River Trail sinkholes?

West Oak Lane has had two recent water main bursts. Material carried along by the flowing water made it difficult to reach the point of failure.

Repairing a major leak involves multiple steps and can be complicated. Crews have to stop the leak, remove the debris, obtain the replacement components, fix the pipe itself, and then rebuild the road surface. Situations like this also highlight why teams need time to check nearby sections of pipe for age-related problems and, if needed, repair them while the excavation is already open-so the work should not be rushed.

The sinkhole on the Schuylkill River Trail late last year-which took two months to put right-was likewise caused by a leaking pipe. The water department had to take part in the repair work alongside the parks and recreation department. It is also worth noting that the city’s budget for pipe repairs is limited. As one of the oldest cities in the country, Philadelphia has extensive infrastructure that requires ongoing attention.

Why carbonate rock sinkholes are harder to fix

Even so, dealing with a leaking pipe is preferable to addressing a carbonate rock sinkhole. With cavities in carbonate rock, you often cannot tell how large the void is, and a common approach is to fill it with concrete. Sometimes, though, the cavity turns out to be far bigger than the amount of concrete available.

Is Philly prone to sinkholes?

The wider Philadelphia region experiences both kinds of sinkholes. Within the city itself, carbonate rock is not present, but it does occur just outside Philadelphia-such as around King of Prussia-where carbonate-related sinkholes can happen.

In Philly, sinkholes generally occur where pipes leak and the surrounding ground is washed away. Because the region has suitable conditions for sinkholes and the city also has an extensive, ageing water network, sinkholes are fairly common.

That said, there are places where sinkholes are even more frequent. Florida, for example, sits entirely on carbonate rock, and sinkholes occur there quite regularly.

Can residents tell when a sinkhole is forming in Philadelphia?

There is a statewide map showing where carbonate rock is located, but not every area of carbonate rock develops sinkholes. Exactly where and when a sinkhole might form in carbonate rock cannot be reliably predicted.

In Philadelphia, sinkholes are also hard to anticipate because the triggering process is underground and out of view. We cannot know in advance when a pipe will start leaking. In some cases, the ground surface sags before a larger hole opens. In other cases, the leak is noticed before any sinkhole forms. However, not every leak or patch of sagging ground will turn into a sinkhole, and there may be no warning signs at all.

Even so, reporting leaks and any sagging ground is important so the issue can be checked before it worsens. Leaks can be reported to the Philadelphia Water Department by calling their emergency hotline on 215-685-6300.

If the city could replace all its ageing infrastructure, there would be fewer sinkholes. But doing so would be expensive and highly disruptive, so it is not really practical. For now, the city has to repair sinkholes as they appear.

Laura Toran, Professor of Environmental Geology, Temple University

This article is republished from The Conversation under a Creative Commons licence. Read the original article.