Sinkholes: The Philippine experience

By Caroline J. Howard, ANC

Posted at Jun 08 2010 12:28 AM | Updated as of Jun 11 2010 12:26 AM

Expert downplays fears of sinkholes ever happening in Metro Manila

MANILA, Philippines - Geologists are still dumb-founded as to how a gigantic sinkhole appeared in an intersection in Guatemala more than a week ago.

Some geologists say human activity, not nature, was to blame for the Guatemala sinkhole.

A scientific investigation into the formation of the 70 X 102 foot crater launched on Thursday revealed it may have been the result of excessive ground water, coming on the heels of Tropical Storm Agatha.

Sinkholes are known to form when the ground collapses into a void beneath the surface, usually in places where water dissolves away rocks like limestone and gypsum.

As such, geologists say, sinkholes are not exclusive to Guatemala. In fact, they are found in many countries, including the everglades in the U.S. state of Florida where limestones are extensive, such that the state requires sinkhole insurance for homes.

However, Sam Bonis of the Dartmouth College in New Hampshire, points out there is no limestone beneath the section of Guatemala City where the new sinkhole appeared. He believes a burst sewer pipe, or storm drain, may have hollowed out the underground cavity, mostly made up of pumice fill from past volcanic eruptions, resulting in the chasm.

But when it comes to sinkholes, even the Philippines is no exception.

Limestone and sinkholes

Speaking on Dateline Philippines on Sunday, Dr. Carlo Arcilla of the National Institute of Geological Sciences in U.P. Diliman says, limestone is a natural setting for this phenomenon owing to the rock's porous composition.

"Limestone can be dissolved by acid rain over time," Arcilla says. "But it's strong so you can actually build a house even with a one- to two-meter deep foundation. But when the cracks develop and you put a load, it could suddenly collapse."

Arcilla explains the nature of limestone: "It is a rock formed mostly under the ocean. It precipitates in the ocean, or sometimes, the shells of sea animals collect at the bottom of the ocean and it formes a big pile over millions of years. In geologic time, all of that limestone is brought up to the surface by tectonic forces. This material, called calcium carbonic acid, is soluble. So after millions of years of dropping, there are cracks, and yet it still holds. Sometimes, it can even engulf rivers."

Such cracks forming, or bodies of water suddenly disappearing, are among the warning signs of sinkholes.

Arcilla says this is the case of the Palawan underground river, when the river suddenly disappears after encountering hollow areas in the ground and goes underneath.

Sinkhole-prone areas in RP

"It's basically how caves form, but a sinkhole is basically a cave that bursts to the surface. So the caves and holes were there in Guatemala before it collapsed," Arcilla says.

He notes the phenomenon has happened in small scale in areas with limestone including Bohol, Cebu--60% of which is made up of limestone--and parts of Baguio.

Bohol's Chocolate Hills were also formed the same way, as the mounds surrounding those that made up the Chocolate Hills collapsed.

Arcilla notes the case of Pangasinan's Hundred Islands is even more spectacular.

"Between them are limestone that all collapsed over eons. And in an island beside it, the Anda Island in Pangasinan, there are sinkholes developing," he says, adding he had deemed it useless to build roads on top if it after the provincial governor asked him to investigate the area.

Probing sinkholes

Geologists like Arcilla have ways of probing potential sinkholes. He cites ground-penetrating radars, and the gravity method, which he likens to doing an x-ray on the ground.

However, Arcilla laments, despite environmental policies that require an engineering, geological and geohazard assessment, some people are loathe to hire geologists and would rather hire feng shui experts when building.

As for repairing a sinkhole, some geologists (including Dan Doctor of the US Geological Survey) say, it requires an engineering solution: filling it with rock, sediment, or some type of cement of grout.

Manila on solid ground

While there are some limestones in Rizal and Tanay, Arcilla says, most of them don't have buildings. He, however, downplays fears of sinkholes ever happening in Metro Manila, which sits on solid adobe rock.

"Most of Metro Manila is volcanic in origin, the result from a violent eruption of the Laguna de Bay volcano," Arcilla says, adding it happened some 30,000 to 40,000 years ago.

Sinkholes are the least to worry about in Metro Manila. Arcilla says more worrisome are active earthquake faults, including the Marikina Valley Fault, which extends from Montalban to Tagaytay.