Saprolite slopes: operational slope design at the Rosebel Gold Mine

SRK News | Issue 54: Rock Engineering and Slope Stability

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Saprolite slopes: operational slope design at the Rosebel Gold Mine

The Rosebel Gold Mine, located in Suriname, is comprised of a number of open pits which have been developed to varying depths. Due to their origin and tropical climate setting, the rocks throughout the site are deeply weathered with saprolite and transition (sap-rock) extending to depths greater than 70m. Existing interim saprolite and transition slopes have been excavated in several of the operating pits. Saprolites are complex materials with variable geomechanical behaviour that is not fully understood. The performance of these slopes is extremely variable, due to the impact of relict structures, groundwater, intense rainfall, and protolith.

A detailed geotechnical investigation program was undertaken during 2013 through 2014 to provide slope design configurations that could be implemented practically in this high-rainfall tropical environment, using the capabilities of the equipment on site. The slope design approach involved a detailed review of existing pit slope failures in various geotechnical settings, along with findings from the geotechnical drilling program, to aid in estimating strength parameters of the materials and analysing the factors controlling slope stability. A series of simple numerical models were generated to support the slope designs. One of the controlling factors was found to be the orientation of relict structures and foliation. Groundwater control was highlighted as another major controlling factor. Back-analysis indicated that the most critical period from a groundwater perspective was when the mine floor was located at the base of the saprolites. This is due to elevated pore pressure in the toe of the saprolite slopes. As mining progresses into the more permeable transition material, passive drainage of the transition layer acts as a natural drain beneath the saprolites. Identifying this process enabled mine plans to be modified so that the natural drainage could be used to depressurise the slopes. Adjusting mine plans to take advantage of these natural processes reduced the need for a more complex and costly dewatering system.

Interim slopes are mined at flatter angles until mining has progressed down into fresh rock, allowing under-draining of the saprolites to occur without the need for more complex depressurising techniques. A network of vibrating wire piezometers is in place to verify that such under-drainage is occurring. Final slopes can then be cut to the steeper design angles once the pore pressures are significantly reduced. Mining the pits in staggered benches enables good surface water management, reduced trafficability issues, and prevents saturation of saprolite slope toes.

Mark Raynor:
Bruce Murphy:

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