TAGBILARAN CITY, Bohol - A team of geologists from the Region 7 office of the Mines and Geosciences Bureau (MGB) 7 of the Department of Environment and Natural Resources (DENR) recently discovered pockets of sinkholes in this city while conducting a geo-hazard assessment.
MGB7 data showed that pockets of sinkholes were found in seven barangays in this city, namely Poblacion 3, Bool, Mansasa, Cabawan, Tiptip, Dao and Cogon.
A sinkhole - also known as a sink, shake hole, swallow hole, swallet, doline or cenote - is a natural depression or hole in the surface topography caused by the karst processes or the chemical dissolution of carbonate rocks.
Eddie Llamedo, DENR7 information officer, said of the seven barangays, Dao and Bool face the highest risks because huge structures like malls, recreation centers, residential areas and dumpsite are situated in these areas.
"Huge structures adds more pressure to the ground that is why these areas were already given copies of the geo-hazard assessment, which will serve as an early warning for them," he said.
Meanwhile, MGB7 regional director Loreto Alburo urged all concerned local government units to conduct Engineering Geological and Geo-hazard Assessment (EGGA) as this will identify the possible geological hazards that occur within and in the adjacent areas of a development project.
He said LGUs should immediately take action about the situation and provide relocation sites to prevent bigger damages.
"Mapping should be given careful attention to the lithology, structural elements, and three-dimensional distribution of the earth materials exposed or inferred within the area," Alburo said.
He pointed out that by undertaking a full-blown EGGA, the proponent would be able to adequately and comprehensively address and mitigate the possible effects or impacts of such geologic hazards as sinkholes.
The MGB7 official bared that areas with vast limestone deposits are vulnerable to the natural occurrence of sinkholes like Cebu and Bohol.
Earlier, Al Emil Berador, MGB7 chief geologist, said sinkholes can appear in Cebu because a large portion of the island is composed of limestone. "Cebu is 60 to 70 percent composed of limestone," he said.
Berador said there is a big possibility that a sinkhole similar to the one that formed in Guatemala City will appear in Cebu, even in urban areas.
The Guatemala City sinkhole caught the attention of people all over the world because of its size, which is reportedly 66 feet across and 100 feet deep.
Berador said sinkholes are natural formations that occur after limestone deposits gradually corrode when in contact with weak carbonic acid, a combination of water and carbon dioxide.
Carbonic acid can gradually corrode hard limestone, which explains why there are limestone caves, he said.
Sinkholes are common where the rock below the land surface is limestone, carbonate rock, salt beds, or rocks that can naturally be dissolved by circulating ground water. As the rock dissolves, spaces and caverns develop underground.
These sinkholes can be dramatic because the surface land usually stays intact until there is no longer enough support. Then the surface area can suddenly collapse.
Berador said a sinkhole can only occur if a corroded limestone area is disturbed by natural tremors such as earthquakes or by human-induced activities.
He said the only way to detect if an area is susceptible to sinkhole formation is through core drilling or ground-penetrating radar.
Berador said there have been sinkhole formations in some areas in Central Visayas.
