During a September afternoon on a bustling street in Sandy, Utah, the earth opened up. Asphalt cracked and crumbled away as the road gave way to a massive sinkhole—ten feet deep and large enough to fit two cars into. Water gushed out of the sinkhole as the street became a river. Traffic ground to a halt in all directions as the city assessed the safety of the road, only to resume in one lane of traffic as the damage was repaired. While no injuries were reported, flooding caused damage to the property of several residents and the water had to be turned off in the area for several hours.
So what had happened? This is a prime example of an urban sinkhole. While some sinkholes are caused by rain, others, like the one in Sandy, are caused by water main breaks. Sinkholes can occur in heavily populated urban areas or on relatively unpopulated land, but the urban ones are especially dangerous because there is a greater likelihood of property damage or injury to people nearby. Land, like water, is susceptible to acting as a fluid (at a much slower flow rate than the flow of water). It is pushed out of the oceans by the movement of tectonic plates, and can similarly be driven out of the way when exposed to other water sources—like a broken water main. As stable as land seems, it is always at risk of disappearing right out from beneath our feet.
Man-Made Sinkholes Threaten Decaying Urban Infrastructure
Sinkholes are often caused by crumbling urban infrastructure, and in many cases can exacerbate the damage. In Guatemala City, a 100-foot deep sinkhole caused by a combination of its decaying sewer lines and the local geology devastated the area and swallowed several buildings. Detroit, famous for its deteriorating infrastructure, has experienced three sinkholes, one of which was 16 feet deep. Sinkholes like this, in Detroit and nearby Pittsburgh, are likely caused by damaged sewer lines or water mains.
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Though cities may differ wildly on the surface, underneath the earth they can be very similar. For example, the typical urban street sits atop the same pattern of layers:
- A layer of asphalt or concrete, making up the road itself
- A bricklayer to serve as a foundation for the road
- About 4 feet of soil
- Fresh water lines (about 15 inches in diameter)
- Another layer of soil (3 to 6 feet)
- The sewer lines (24 to 36 inches in diameter)
If either the fresh water or the sewer lines begin to crack, the leaked water can create a cavity by compacting the soil underneath. With no soil to support them, the pipes and all the layers above them begin to dip, creating a sinkhole. When the water main is broken, either in the initial breach or, in the case of a sewer line rupture, when the underlying layers collapse, it produces a steady stream of water that can cause even more damage to the sewer line (sometimes collapsing it entirely) and the surrounding area. This is one example of how already-dilapidated infrastructure can be made worse by letting the problem linger.
Repairing vs. Preventing Urban Sinkholes
In the case of sinkholes, an ounce of prevention is worth a pound of cure. Maintaining sewer lines and water mains is of the utmost importance. Either regular pipeline condition assessment or continuous water main monitoring are helpful for cities looking to avoid the damage of a sinkhole.
The city of Pittsburgh ended up using robots to inspect its sewer lines over a five-year period. Equipped with sonar, laser sensors, and high resolution cameras, they were able to create a three-dimensional map of the sewer system and identify lines in need of repair. This led to the city replacing and repairing many sewer lines that, if they had been left alone, could have led to more sinkholes in the future.
Though repairing damaged lines as a preventative measure can be costly, the cost of repairing a sinkhole is often worse. Not even touching on the possible loss of life, the property damage alone is often astronomical. By inspecting water and sewer lines regularly, and addressing damaged infrastructure before it becomes a problem, these dangerous events can be prevented.