Hydrogeological Model Development and Horizontal Drainhole Effectiveness for Slope Stability

The presence of ground water, particularly pore pressures can have a significant detrimental effect upon slope stability. Successful open pit slopes, particularly in tropical environments, depend on a good understanding of the hydrogeological model and the development of appropriate depressurisation strategies. This paper presents the development of the hydrogeological model and depressurisation methods used at an open pit mine in Southeast Asia. The overall aim of the study was to reduce the number of horizontal drain holes installed while maintaining effective depressurisation objectives and achieving the projects economic and environmental goals.

A field observation program was conducted during wet season to support the study. The observations identified both lithology and structural controls on hydrogeological conditions within the open pit. This paper describes the resulting conceptual hydrogeological model, which is based on existing mine development data, recent groundwater monitoring and horizontal drain hole (HDH) installations. Preliminary model sectors were created to direct HDH design, including depth and spacing. These have then been tested against ongoing groundwater monitoring. This paper also presents the development of pore pressure trigger levels for areas where depressurization is insufficient and has implications for slope stability. Ongoing work is focused on the validation of the HDH design sectors.