Liquefaction sank Davao Sur building

Science Undersecretary Renato Solidum inspects the site of a commercial building that was damaged by the 6.9-magnitude quake on Dec. 15 in Padada, Davao del Sur. (Photo by ORLANDO DINOY / Inquirer Mindanao)

PADADA, Davao del Sur, Philippines — Science Undersecretary Renato Solidum stressed the need to follow the building code in putting up structures in areas like this town identified by experts to be prone to liquefaction.

Solidum made the call on Wednesday when he inspected the site of what used to be a three-story Southern Trade commercial building here whose first and second floors sank at the height of the 6.9-magnitude earthquake that jolted Davao del Sur and a large part of Mindanao on Dec. 15.

Experts from the Philippine Institute of Volcanology and Seismology (Phivolcs) said the area where the building used to stand had been found to be susceptible to liquefaction, as shown in the lateral spread, fissuring and sand boils on the ground surrounding it.

The website www.science daily.com describes liquefaction as the process by which saturated, unconsolidated soil or sand is converted into suspension during an earthquake. It has a devastating effect on buildings and structures.

Britannica.com also defines it as “ground failure or loss of strength that causes otherwise solid soil to behave temporarily as viscous liquid.”

No known fault

Solidum said Padada had no known fault passing through it but the soil structure of the area made it prone to liquefaction.

“If you’re going to build a house, make sure that its foundation is strong,” Solidum told reporters here. “Taller buildings needed to have a soil test.”

He added: “Make sure you have the proper foundation and follow the building code … which requires an analysis of the foundation.”

According to him, the first and second floors of the building did not collapse as was reported, but these had sunk or subsided.

“The foundation of the building went down, so it was subsidence … part of the building subsided; it was different when the frame of the building collapsed,” he said.

“Liquefaction happens in areas which are sandy, the water table is low, which means, the soil is soft just the way it is here in Padada. One manifestation of liquefaction is when the ground opens and water comes out,” he said.

“When they built the structure, they should have ensured that it had a stronger foundation,” Jeffrey Perez, supervising science and research specialist of Phivolcs, said in a separate interview.

‘Walk the fault’

Solidum also recommended to local governments not to allow the building of structures within the fault areas.

“I recommend that you don’t build your houses there, and one possible activity for the local government is to walk the fault,” he said.

“We will mark it so that people will know where the fault lies,” he added.

Solidum said the Tangbulan Fault, the 69-kilometer fault line that runs from Bansalan town in Davao del Sur to Sta. Maria in Davao Occidental, had caused the Dec. 15 shake.

But aside from this fault, the Makilala-Malungon Fault, a 65-km line that runs parallel 10 km to the west of Tangbulan Fault, could have also caused the shaking, said Perez.

The Makilala-Malungon fault is the longest fault within the Cotabato fault system that has the capacity to generate a 7.2-magnitude quake based on its length.