ABSTRACT:

Freeport-McMoRan Inc. (FMI) Chino Mines, located in Southeast New Mexico, undertook a new phase of expansion in the South Pit mining area from 2014 to 2018, including a major pushback and deepening, centered on the southern half of the pit. The pushback implemented is composed of typical porphyry copper geology and geologic structures which are characterized as barrier-conduit features with discrete zones of groundwater occurrence mostly associated with the major structures and rock fabric. The planned pushback floor was sunk to a depth of over 600 ft (183 m) below local groundwater levels. While groundwater inflow was not a major concern to the operation, the ambient hydrogeological conditions and depth of mining below groundwater combined to produce the presence of groundwater pressure in the rock mass. As a result, the new mine design involving buttress removal was sensitive to pore pressure and called for significant depressurization to reduce risk and support an acceptable toe to crest, overall slope factor of safety. In order to address the depressurization requirements and support the new design implementation, FMI Chino Mines implemented an aggressive program of horizontal drain drilling in the lower pit slope. This resulted in groundwater pressure reduction in the pit walls, moving the actual field condition toward the slope design criteria and acceptable factor of safety. The program was crucial to mine plan implementation, risk reduction, and, attaining the upside benefits from the new pushback design. This paper reviews the pit area hydrogeology, the drain program implementation and evaluation of its impact on pit slope pore pressures.

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