This study examines the hydro-mechanical influence of groundwater pore pressure on rock mass stability at the Detour Lake open pit mine in northeastern Ontario, Canada. Two groundwater scenarios are carried forward from companion analytical studies: conservative fractured bedrock conditions (K = 1×10⁻⁷ m/s) and fracture-enhanced shear zone conditions (K = 1×10⁻⁵ m/s). The hydraulic head difference of 100 metres established in the companion groundwater inflow study is used as the pore pressure input to an effective stress analysis of the pit wall. Slope design angles of 50° on the hanging wall and 48° on the footwall, and rock unit weight of 29.23 kN/m³, are derived from the 2018 NI 43-101 Technical Report. Under conservative bedrock conditions, pore pressure reduces effective normal stress on the hanging wall failure plane by 40.6%, giving a pore pressure ratio ru of 0.168 below the Hoek and Bray (1981) critical threshold of 0.3. Under fracture-enhanced shear zone conditions, effective normal stress drops by 81.2%, and ru reaches 0.336, exceeding that threshold. A single order of magnitude change in hydraulic conductivity; the difference between intact fractured rock and an active shear zone shifts the site from a manageable to a critical stability condition. The critical threshold is crossed at a water table height of approximately 89 metres above the failure plane, providing a practical dewatering target derivable entirely from public data. This paper is the third in a sequential analytical series on the Detour Lake site covering groundwater inflow, water balance, and hydro-mechanical stability assessment.
Isaiah Goodluck Ephraim (Sat,) studied this question.
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