2D or 3D Pore pressure modelling of mine pit slopes is a key input to pit slope stability evaluations and is often informed by limited piezometric data. Hydraulic data is usually sourced from onsite hydraulic testing, although often modified through numerical model calibration. Modelling typically ignores the hydraulic effects of open drill holes on the hydraulic characteristics of the rock mass. Each of these holes, if not cemented following completion, represents a conduit for groundwater movement, dependant in part on the tendency of the hole to collapse following completion. These drill holes may significantly alter the hydraulic properties of the pit slope, particularly in the case of sub-horizontal geological units with pronounced vertical layered anisotropy.
This paper evaluates the influence of open drill holes on the overall hydraulic properties of the rock mass and on the pore pressures present in a series of “typical” pit slopes. A simple, limited extent 3D MODFLOW model was developed to evaluate pore pressure distributions with, and without, the effects of open vertical drill holes. The evaluation found that the presence of open drill holes can have a significant effect on the vertical hydraulic conductivity characteristics of the rock mass, particularly in rock masses with layered vertical anisotropy. Greatest effects will typically occur in interim pit walls, where density of drill holes is usually highest. Exclusion of these effects can lead to overly-conservative estimations of pore pressure. The evaluation also highlights the opportunity for utilizing open pit slope drill holes as passive vertical drains. Where these holes cave, maintenance of the holes through installation of perforated casing may improve pit slope drainage. Where the holes extend into a deep aquifer, pumping of the aquifer will further drain the slope via the passive vertical drains.