Was King of Prussia Built On a Sinkhole?

Our editorial director gets to the bottom of this long-standing Main Line rumor.



Hobart Rowland

If, like me, you were a weekend fixture at the King of Prussia Mall in the 1970s and ’80s, you should get a kick out of the throwback images we included in “Retail Royalty”, contributing editor Tara Behan’s well-researched piece on the past, present and future the mall, one of the biggest shopping hubs on the planet. 

And you may have been privy to a rumor that the mall was sinking, slowly, an inch or two a year. At least, that’s the story I heard when I was teenager, prompting a nagging question that’s remained with me through the years: Why on earth would anyone build something as monumental as KOP on mushy ground? Was I misremembering? Was it a hoax? 

So, with some help from Behan, I set out to solve this mystery. “Upper Merion Township hasn’t had any major sinkhole situation in a couple of years—maybe a few small ones, but none that were going to swallow a car,” Todd Lachenmayer, Upper Merion’s highway superintendent, told Behan. “King of Prussia Mall is on private property. If they have had any issues during their construction, we don’t know about it,” he says. “Nothing has been reported to our office.”

In February of 2011, a 75-foot-wide sinkhole did swallow up a section of Route 202 near Mall Boulevard. Think of the number of cars a hole that size could accommodate. And there was this 1992 report from the Philadelphia Inquirer:

“Upper Merion Township residents and business owners have become well-rehearsed in telling tales of sinkholes. The ground routinely opens, gobbling up lawns, roads, telephone poles and, in one case, graves in a local cemetery. In the last four years, 78 sinkholes have opened up, township officials said.”

Sinkholes are so common in Upper Merion that township officials have a long-standing ordinance that outlines procedures for dealing with them. In fact, the entire Chester Valley, from Montgomery through Chester counties, is at risk due to its limestone bedrock. Water erodes the sedimentary material, creating holes. According to a 2001 story in the Times Herald, limestone erosion wreaked havoc on the 1999 work zone for the Route 202 improvement project. At the time, PennDOT estimated it had spent $17 million “pumping the crevices full of a cement-like grout.”

“Some [holes] were as deep as 150 feet,” said the project manager. 

More recently, a small sinkhole appeared on a portion of Mall Boulevard in 2014. “[It] was on township property, and we were able to fix it in a day. It was nothing major,” said Lachenmayer. “I don’t think there’s a story here.” 

That is, until the next sinkhole. 

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