A sinkhole, 65 feet across and 100 feet deep, swallowed up a small factory and some telephone poles in Guatemala City last weekend. Police have been stationed around the hole to prevent bystanders from falling in, and those who live nearby are staying elsewhere for the time being. How do you fill a massive sinkhole in the middle of a major urban area?
With cement or rocks. Sinkholes develop when water flows through pores in bedrock and gradually enlarges them. When these subterranean cavities get big enough, the ground above collapses and fills them in. Guatemala City's last major crater, which opened in 2007, dropped three houses 330 feet below the city's streets. The government spent $2.7 million redirecting sewer pipes around the area and filling the hole with cement. This week's sinkhole will probably be filled the same way.
It's not clear whether cement is the best option, however. A 6,500-cubic-foot wad of concrete may serve to concentrate water runoff in other areas, leading to more sinkholes. Many engineers prefer the graded-filter technique, in which the hole is filled with a layer of boulders, then a layer of smaller rocks, and, finally, a layer of gravel. This fills the hole, more or less, while permitting water to drain through the area.
No matter what is used to fill the crater, more sinkholes are on the way. Guatemala City's location and leaky sewage system make it particularly prone to these events. The capital lies downhill from seven major volcanoes, two of which are active members of the Pacific Ring of Fire. For hundreds of thousands of years, the gargantuan Amatitlan Caldera dumped volcanic ash on the ground where the city now sits. As a result, the local bedrock consists mostly of loose volcanic pumice. (Many cities sit atop volcanic deposits, but the ash from Amatitlan has not had the time or the appropriate pressure and temperature conditions to compress into a solid, reliable foundation.)
Large sinkholes sometimes open up in North America, too. While the process is similar, the geology is different. The most sinkhole-prone parts of the U.S. consist of limestone and dolomite bedrock rather than volcanic ash. The easily-eroded materials are responsible for natural wonders like New Mexico's Carlsbad Caverns, but they can also lead to a terrific collapse. Florida and Kentucky are the most vulnerable states. In 1994, a 15-story-deep sinkhole opened near Mulberry, Fla.
Cover-collapse sinkholes—the dramatic and instantaneous events that swallow up unsuspecting victims—are relatively rare. Most sinkholes happen slowly over time, and bottom out at a foot or two. As the bedrock erodes, the ground begins to subside at an imperceptible rate. When the soil finally lowers to the level of the groundwater, there is a small, visible drop.
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