The pounding starts at 7:00 AM every morning outside my house here in Pittsburgh, and it’s been like that for 8 months now. Every weekday, whether it’s a backhoe ripping through asphalt, a jackhammer shredding up the concrete, or a buzz saw dicing the sidewalk, there is a plethora of noise that I wake up to as they dig holes on my street. And why are they digging these holes? To fix a larger hole – a sinkhole.
Typical sinkholes, such as the one outside my window, share a common, natural source: water dissolving the rock formations that are just underneath the surface, creating a large cavity. For these sinkholes, the geology plays an important role, as certain rock formations dissolve more easily in water. But that geological process doesn’t create holes anywhere near as deep as the one in Guatemala City. In that sinkhole, there was another factor beyond geology at work: decaying sewer lines. In fact, the sinkholes in many urban areas, like Detroit and Pittsburgh, are likely the result of decaying sewer lines rather than the underlying geology.
The layering of most American city streets looks something like the figure above. When sewer lines begin to crack, either from age or root intrusion, the fluid that escapes starts to compact the underlying soil, creating a cavity. Without the support of the soil, the sewer line and all of the layers above it begin to dip, resulting in a man-made sinkhole..
This dip in the road can quickly transform into what looks like an open hellmouth (also known as a collapse sinkhole). But there are no supernatural forces at work: these collapse sinkholes are the result of decaying infrastructure. As the road and the soil dip, they apply enough pressure to rupture the fresh water line. This rupture results in a steady stream of rushing water that can eventually cause the collapse of the underlying sewer line, creating a man-made collapse sinkhole that is at least 4 feet deep and can often grow to be much deeper.
But 4 feet is a far cry from 100 feet, so how did the collapse sinkhole in Guatemala grow to a depth of 100 feet? Beneath Guatemala City, there are hundreds of feet of loosely consolidated volcanic ash. Water escaping from the sewer lines had most likely been leaking, eroding, and compacting this volcanic ash for months. The tipping point came when a tropical storm hit Guatemala in May of 2010 and a deluge of rain water escaped the sewer lines and the cavity expanded rapidly. Hundreds of feet of volcanic ash condensed, creating a massive sinkhole that killed 15 people.
Preventing these sinkholes is difficult because the only evidence of decaying sewer lines at ground level may be an unpleasant smell. Since sewer lines are dirty and compact spaces, human exploration of the lines is dangerous and inefficient. Innovations in robotic technology have led to the development of robots specifically designed to explore and assess sewer lines.
The city of Pittsburgh inspected all of its sewer lines over a five year period using robots from a local company. These robots were equipped with high resolution cameras along with sonar and laser sensors. These tools allowed the robots to create a 3D map of the sewer system and identify lines in need of repair. This effort led to the replacement of numerous pipes throughout the city and the closures of small streets, like mine, as well as main thoroughfares – to the chagrin of residents and business owners alike.
Fixing these sewer lines has its costs: snarling traffic jams, loss of business revenue, and hours of lost sleep for residents who live on these sinking streets. Despite these inconveniences, replacing our sewer lines now means a sinkhole won’t swallow our cars later. So while a fear of sinkholes doesn’t keep me up at night, I’m envious of my future self: a dude who can hit the snooze button at 6:50 AM without worrying about the revving jackhammers and whistling buzzsaws that lie in wait outside my window.
 Detroit Sinkhole, March 28, 2014; http://www.freep.com/article/20140328/NEWS01/303280131/detroit-sinkhole-intersection
Detroit Sinkhole, April 4, 2014; http://www.myfoxdetroit.com/story/25169063/watch-detroit-sinkhole-swallow-two-cars#ixzz2yD01T6w6
Detroit Sinkhole, April 27, 2014; http://www.wxyz.com/news/region/detroit/detroit-fire-department-fire-truck-gets-stuck-in-sinkhole-on-east-side
2010 Guatemala Sinkhole Entry in Wikipedia, http://en.wikipedia.org/wiki/2010_Guatemala_City_sinkhole
 Picture of Guatemala Sinkhole (May 30, 2010) from National Geographic; Article from June 3, 2010
 WPXI News, Clean-up begins on car-swallowing McKnight Road sinkhole, August 13, 2014, http://www.wpxi.com/news/news/breaking-massive-sinkhole-swallows-car-mcknight-ro/ngzz9/