Control Of Ground Subsidence
Philex Mine Opem Pit Area

Arthur Saldivar-Sali, MSc, DIC, FGS, PhD

Abstract

This paper characterizes the hydrotechnical parameters of the Philex Mine open pit area in Padcal, Benguet in order to determine the cause(s) of the accumulation of water ponds at the subsidence floor and evaluates appropriate engineering geotechnical solution(s). The serious concern for water ponds in this part of the mine in Padcal dates back to 1967 when a portion of the subsidence area collapsed into the underground workings directly below the open pit causing several deaths and damage to property. Since then Philex has dedicated a major portion of its budget to the continuous backfilling of the subsidence area to forestall the stagnation of water as ponds. Backfilling with borrow materials of weathered rocks and soils is meant to maintain a 4% gradient of the pit floor made obligatory by the DENR, in the belief that such a gradient prevents the development of water ponds. This paper shows that a 2 to 2.5% is the most optimal gradient as it is the optimal gradient that conveys surface water at minimized soil erosion. A lower gradient also means significantly lower volumes of borrow materials needed to backfill the open pit floor resulting in a more economical operating cost. Corollarily, the 4% gradient aggravates erosion of the pit floor increasing isiltation in areas downstream of the Philex mine camp and more expensive.