Weathered rock masses

SRK’s rock mechanics engineers have recently undertaken open-pit design studies for rock masses hosted within saprolites and highly weathered bedrock. One such example is Oromin Explorations Ltd. Sabodala gold project in Senegal. Since 2006, SRK has designed a series of field programs to satisfy geotechnical, structural and hydrogeological objectives for scoping, pre-feasibility, and feasibility studies.

Geologically, the project is characterised by steeply dipping, shear-hosted gold veins set within Precambrian greenstones. During early study stages, a vertical zonation of material properties was identified within the weathered profile. The degree of weathering was found to decrease systematically near surface from completely weathered soil through a transitional zone into a thin fracture-controlled weathered horizon above fresh rock. This zonation, separated into unique geotechnical 3D domains, has been determined to affect the behavior of the rock mass. Accurate characterisation of the weathered rock mass is vital not only for slope stability, but also for trafficability and metallurgical requirements.

The SRK structural understanding indicates that most brittle features occur along reactivated structures associated with earlier ductile shear zones. In these areas, the weathered profile (i.e. saprolite and saprock) often extends beyond 75m in depth; much deeper than typical background conditions distal to the shear zones.

In-situ sampling using clear plastic liners, as well as more traditional Shelby tubes, has provided the best means of obtaining relatively undisturbed samples of the weathered horizons.

Reverse circulation (RC) drilling has been used to complement geotechnical information gained through in-situ sampling, mapping and diamond drilling. The RC option has proven to be a rapid, cost-effective approach for characterising the weathered profile along proposed pit walls where existing core drilling intercepts are sparse. RC results have been used to calibrate adjacent diamond drillholes from the pre-feasibility geotechnical program. This correlation has improved the confidence of the various geotechnical domains within the weathered profile.

Ryan Campbell: rcampbell@srk.com


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