added an update
Mining in residual soils is a characteristic of some open pit mines, particularly those mines in the tropical and sub-tropical regions. With residual soils’ prevalence on the earth’s surface almost as common as that of sedimentary rocks (Wesley 2013), mining in such soils requires special understanding of the behaviour and characteristics of the residual soil to determine slope designs that are both safe and economic. Due to the presence of relict structures, and the relatively low strength of the residual soils and weathered rock, design slope angles in these materials should be developed by blending the results of the kinematic assessments of geologic structures with rock mass stability analyses and traditional soil mechanics (Newcomen & Burton 2000). It is thus imperative that geotechnical designs should be site/location-specific and based on soil’s field performance, back-analyses and risk zoning. Understanding the variability of these materials is important for developing robust designs. This paper outlines the different aspects that are to be considered when conducting slope designs in residual soils, and in particular, saprolites, and summarises shear strength data from various mine sites that highlights the uncertainty associated with these parameters. Keywords: saprolite, residual soils, slope design, uncertainty, variability, reliability
The Rosebel Gold Mine, located in Suriname, is comprised of eight 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 70 m. Existing interim saprolite and transition slopes have been excavated in several of the operating pits. 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 practically implemented in this high-rainfall tropical environment using the capabilities of the equipment on site. This paper documents the methodology used to generate the saprolite and transition slope designs and the implementation requirements. 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 the estimation of strength parameters of the materials and analysis of controlling factors on slope stability. A series of simple numerical models were then generated to support the slope designs. One of the controlling factors was found to be orientation of relict structures and foliation. Groundwater control was highlighted as another 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. Identification of this process enabled mine plans to be modified such that the natural drainage could be used to depressurize the slopes. Adjusting mine plans to take advantage of these natural processes reduced the need for a more complex and costly dewatering system.