Successful mine plan execution at operations with high rainfall and deep and irregular saprolite profiles requires strong geotechnical, hydrogeological and hydrology design integration, coupled with a realistic assessment of mine production capabilities and clearly defined environmental objectives.
Vancouver’s rock mechanics and Cardiff’s hydrogeology teams have provided long-term open pit operational support to the Rosebel mine operated by Rosebel Gold Mines (RGM) in Suriname. Since 2017, this has extended to multidisciplinary support for open pit slope stability and depressurisation, waste storage, surface water and sediment dams at RGM’s new Saramacca Operation approximately 25 km from the primary mine area. Saramacca’s open pit comprises saprolites, both homogenous and relic structured, up to approximately 120 m in expected slope height with shallow pre-mining groundwater levels.
Fundamental to the Saramacca materials characterisation was the development of a rigorous 3D weathering model that considered the deposit lithology, structural and Implementation of mine strategy driven by slope depressurization mineralisation controls. SRK generated the 3D weathering and structural models using implicit techniques and all exploration and geotechnical drilling data with a view to calibrate models as the open pit advances. The weathering model comprised four domains: saprolite; structured saprolite (saprolites with relic structures); transition (sap-rock); and fresh rock units that were coded into block models utilised by the RGM mine planners.
Multi-purpose geotechnical, structural geological and hydrogeological field investigations were performed. This involved a network of vibrating wire piezometers, and airlift and pumping testing to evaluate the feasibility of the proposed depressurisation strategies within the weak and strong rock units, including anisotropic controls on groundwater flow and predicted influence on stability.
An integrated 3D surface and groundwater model (MODFLOW-USG + SWAc) was developed to evaluate the dewatering and depressurisation requirements considering the seasonality at site. An optimized pumping well construction schedule was developed to depressurise the saprolite sequence to achieve the required slope design acceptance criteria, as defined by the supporting stability analyses. The 3D model exports, and more simplistic 2D sensitivity approaches were used to evaluate the expected stability conditions for the critical mine phases.
As the mine plan was developed, SRK supported mine planners to advance the pit into areas of shallower transitional rock to promote passive under-drainage of the overlying saprolites. Where depressurisation was reliant on pumping wells, in-pit and ex-pit pumping well locations are being defined. This ongoing integrated effort and the establishment of clearly defined rules for vertical advance rate and depressurisation requirements allows for early risk identification and adoption of mitigation strategies to support execution of the short-and long-term mine plans.
SRK continues to support RGM with new data collection and monitoring to evaluate the performance of the slopes and calibrate the strength, structural and groundwater parameters to improve the reliability of the geotechnical and hydrogeological designs.
