Sinkholes in Bohol

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By Rox Peña

E-ssue

Thursday, October 24, 2013


AFTER the earthquake in Bohol and Cebu, residents are fearful not only of strong aftershocks but sinkholes. Like earthquakes, these holes appear on the ground without warning, sometimes eating up houses and buildings. This phenomenon happens when the soil underground collapses. It will be recalled that after the Cebu earthquake in February 6, 2012, sinkholes of varying sizes appeared in some barangays in the province.

This time in Bohol however, the sinkholes are more dangerous because some are not seen on the surface. They are called "concealed sinkholes." The top soil is intact but is hollow underneath. It’s a trap! According to news reports, the Mines and Geosciences Bureau (MGB) are trying to locate and map out these "concealed sinkholes" because they pose a "greater hazard" with the continuing aftershocks jolting Central Visayas.

The MGB said that a confirmed ruptured sinkhole was found in Lapu-Lapu City measuring 40 meters and spanned 20 meters and about one to two meters deep. It was also reported that yellow water came out of sinkholes which the MGB believed could be caused by natural deposits of phosphates underground.

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According to the United States Geological Service, sinkholes are common where the rock below the land surface is limestone, carbonate rock, salt beds, or rocks that can naturally be dissolved by ground water circulating through them. As the rock dissolves, spaces and caverns develop underground. Sinkholes appear when the ground above caves in. The MGB said that Bohol is 85 percent limestone. In fact, the chocolate hills are made up of limestone.

Human-induced sinkholes on the other hand can be caused by ground-water pumping and from construction and development practices. Groundwater extraction for water supply and for irrigation can produce new sinkholes. If pumping results in a lowering of ground-water levels and the upper layers of the ground cannot support the weight anymore, then the ground gives way suddenly to reveal a sinkhole.

Perhaps the most destructive sinkholes are those that occur in the middle of thickly populated cities like the 100 meter deep sinkhole that appeared in Guatemala City in 2007. According to wikipedia, the sinkhole was created by fluid from a sewer eroding uncemented volcanic ash, limestone, and other pyroclastic deposits underlying Guatemala City. Its collapse caused the deaths of five people, and the evacuation of over a thousand.

Three years after, another sinkhole appeared in downtown Guatemala in 2010 as the city was being ravaged by torrential rain during Tropical Storm Agatha. The hole is approximately 65 ft (20 m) across and 100 ft (30 m) deep. It claimed 15 lives, and swallowed a three-storey factory. The sinkhole occurred for a combination of reasons, including Tropical Storm Agatha, the Pacaya Volcano eruption, and leakage from sewer pipes.

I hope and pray that what happened in Guatemala City will not happen in Bohol.

Published in the Sun.Star Pampanga newspaper on October 25, 2013.

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