BENGUET, Philippines – The Cordillera Mines and Geosciences Bureau (MGB) under the Department of Environment and Natural Resources (DENR) has sought the assistance of experts from the University of the Philippines (UP) for a geophysical investigation of the 20-meter-wide sinkhole in Virac, Itogon that already swallowed at least 7 houses as of November 2.
An initial report of the agency said that the subsidence occurred on October 22, two days after the heavy downpour brought about by Typhoon Lando which hit Northern Luzon for almost a week.
MGB Geologist Sydney Lalwet said what caused the void has yet to be identified.
The MGB report said the houses and the road are noticeably constructed along a steep slope adjacent to a small channel where water drains during the wet season.
The Virac hole exposed what appeared to be chlorite clay material, according to the initial examination by MGB geologists.
In a presentation of findings to the Benguet Provincial Board on Monday, November 2, MGB Geologist Jerali Rodrigo said their site investigation showed a rupture in one of the diversion tunnels of an old mine in Itogon, Benguet, which spans 80 meters beneath residential houses.
The diversion tunnels were built in the 1960s to steer the Ambalanga River away from Benguet Corporation's old mine tailings dam facility.
"It is not yet clear if [the tunnel breach] was the effect or the cause of the subsidence," Rodrigo said, thus further investigation of the tunnel with experts from UP is recommended.
The breach was detected when residents found debris like stonewalling material and wood in the area. However, Rodrigo clarified they have not found traces of any parts of a house from the 7 structures that fell into the hole.
The MGB also failed to determine how deep this funnel-shaped void extends. Rodrigo explained that a "void" – the preferred term of MGB to describe the sinkhole – is an open space underground which may occur due to an opening in the rock ground.
Meanwhile, Rodrigo noted that active ground subsidence as well as tension cracks continue to form in the abandoned houses.
Photos courtesy of Cordillera Office of Civil Defense thru Ompong Tan
Hundreds of families evacuated
In a dialogue between the MGB and Benguet Corporation, it was mentioned that the mining firm operating there had once excluded the area for settlement because it served as a natural gully, based on the mine's archival records.
The mining firm presented in their report the existence of 3 underground workings within the vicinity of the affected subsidence area.
This includes the "Vegas Tunnel," a main haulage underground opening where mine tours are conducted. The tunnel is 57 meters below the ground.
Below the Vegas Tunnel are twin old diversion tunnels constructed in 1966 to 1967, based on the report of the mining firm.
An inspection of water discharge from the tunnel by the Benguet Corporation team in the morning of October 23 showed no signs of debris from the subsidence, and the water was reportedly clear.
After the 1990 earthquake, Benguet Corporation declared the area a danger zone where no construction should take place, according to the firm's geologist, Paul Damasco.
According to the company, the land was sold by a man named Carantes to the present landowners.
Although there were no reported casualties, the village is now akin to a ghost town as residents have fled their houses after sighting cracks in the ground.
The police evacuated 166 families composed of 314 individuals who temporarily occupied rooms of the Virac Elementary School and Balatoc Elementary School.
At present, the evacuees have been transferred to bunk houses of the Benguet Corporation while the company is still looking for permanent relocation sites.
Itogon-based Benguet Corporation – the oldest mining company in the country – initially took in 29 of the evacuated families because their relatives work for the firm.
The MGB has added sinkholes and ground subsidence to the geohazards it will identify all over the country.
After looking into 18 municipalities in Bohol and one in Negros Occidental in 2014, they have expanded the assessment to 15 other municipalities in the country this year.
A mysterious underwater sinkhole also appeared in Purok Tinago in Barangay South Dadiangas, General Santos City. It is feared that if the hole keeps growing bigger, it could affect more than 1,000 families living in the area. – Rappler.com