The Mines and Geosciences Bureau (MGB) XII conducted Karst Subsidence Hazard Assessment and Mapping on June 21 to July 10, 2021 in the municipality of Alabel, Province of Sarangani.
MGB XII Supervising Geologist Ariel Austin A. Acosta presented the initial results of the Karst Subsidence Hazard Assessment and Mapping during the joint Municipal Disaster Risk Reduction Management Committee (MDRRMC) and Indigenous Peoples (IPs) meeting in Alabel, Sarangani on July 12, 2021.
Mr. Acosta reiterated, that based on the initial reports presented the Assessment Team validated sink depths that are mostly active and inactive landslides and tension cracks. MGB XII prompts the Local Government Unit of Alabel to propose at least three (3) relocation sites for the areas recommended for a relocation.
Based on the National Geohazard Mapping and Assessment Program of MGB, the Karst Subsidence Hazard Assessment aims to identify areas susceptible to subsidence due to sinkhole collapse and to produce karst subsidence susceptibility maps.
Karst is a type of landscape where the dissolving of the bedrock has created sinkholes, sinking streams, caves, springs, and other characteristic features while sinkholes are ground depressions or openings formed when the underlying soluble rocks are dissolved resulting in voids underground which can weaken the support for the overlying surface.
There are three types of sinkholes. First, the Solution Sinkhole occurs in areas where limestone is exposed at the surface or is covered by thin layers of soil or sand. Second, the Cover Collapse Sinkhole occurs where a solution cavity develops in the limestone to such a size that the overlying cover material can no longer support its own weight. Third, the Cover Subsidence Sinkhole occurs where the cover material is relatively incohesive and permeable, and individual grains of sand move downward in sequence to replace grains that have already moved downward to replace dissolved limestone.
Subsidence due to sinkhole collapse is a dangerous geohazard because of its unpredictability. In 2013, MGB added karst subsidence on the geohazard mapping aside from the landslide and flooding.
Currently, MGB XII assessed four municipalities that includes Maasin and Alabel in Sarangani, Lake Sebu in South Cotabato and Lebak in Sultan Kudarat. At least 100 sinkholes were identified. In 2022, three municipalities in Region XII will undergo Karst Subsidence Hazard Assessment and Mapping.