Conference Paper

Saprolite Slope Design at the Rosebel Gold Mine

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Abstract

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.

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... However, the proportion of saprolite mining also reduced significantly around 2016 as several pits advanced into the transition and fresh rock. Interception of the more permeable transition and upper fresh rock has been shown to promote underdrainage and depressurisation of the overlying saprolite (Abrahams et al. 2015) and this is a likely cause of reduced seasonality in failure frequency. As mining continued into the fresh rock, the seasonality of failures is less evident than during mining of the saprolite and transition, as evidenced by the relatively linear cumulative failure curve from mid-2017 onwards. ...
... An alternative would be to undertake probabilistic design analyses. Figure 6 is a scatter plot of design cohesion versus friction angle from various sites in Australia, Africa, Madagascar and Suriname (Abrahams et al. 2015); the data is presented in this way to demonstrate the range of values for the datasets considered. Generally, the cohesion and friction angle form a cluster in the 10-35 kPa and 25-35° range respectively. ...
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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
... In addition, the conceptual scheme of a stratiform bedrock inherited from a single-phase weathering palaeoprofile since the Miocene (Dewandel et al. 2006;Koïta et al. 2013) allows questions of groundwater occurrence and flow anisotropy in shallow hard-rock aquifers to be tackled. The model stands for two superposed compartments, the saprolite or alterite layer (Abrahams et al. 2015) and the saprock or fissure layer (Fig. 1a), the latter enhanced by some vertical fault zones allowing for a deeper flow component (Dickson et al. 2015). Underneath the main valleys, other fault zones isolated from the saprock are assumed to be non-conductive. ...
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Randomness of fracture networks still makes channelized flow a challenge to track in hard-rock aquifers. While not underestimating geological and hydrological criteria that are also handled here through mapping exercises, this study raises an issue of water quality encountered in lifelong boreholes. Chemical classification checked against a recent conceptual model of bedrock aquifers gives birth to a new typology of groundwater in a complex granitic aquifer system located in the SW of Ivory Coast (West Africa). Major ion chemistry, borehole completion data, digital elevation model and satellite images are used to interpret the geochemical water facies as an expression of connexions between the saprolite and the saprock, or transient insulation. From major ions ratios, cumulate mineralization, carbonate equilibrium, stable isotopes, the maturation of ground waters and mixing between bedrock layers are described at seasonal and local scales. The results highlight some vertical feeding of the water table into the main saprock aquifer owing to shortcuts through the saprolite, along with the existence of dead-ends in the hydraulically active fracture network. Also, some influence of fault zones, either drain or barrier, is confirmed on the (Ca, Mg) bicarbonate water facies within the saprock.
... Remarkably, the VWPs shows a trend of strong downward gradient once mining advanced through the transition material into rock allowing the sap-trans overlying material to under drain easily between March and May 2016 in Figure 5. Also, elevated pore pressure response to rainy season can be seen clearly from January to August, 2017. It is however noted, that this strong downward gradient is not pronounced in all parts of the mine, which might be related to the significant role of rock fabrics and relict structures (Abrahams et al., 2015). Staggered benches were mined in the past to lower pore pressures locally in a narrower saprolite slopes while another part of the pit was operating in rock slopes at different elevation prior to opening up the adjacent saprolite slope segment. ...
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IAMGOLD Rosebel Gold Mines N.V. Rosebel Gold Mines is located in a tropical environment with high rainfall. The operation comprises several open pits with extensive slopes in saprolite and transition materials to a depth of more than 100m. The design of safe and economically viable slopes becomes challenging in the saprolite and transition materials due to the extremely variable thickness of the weathering profile, the high rainfall, and the complexity of relict structures. Interim slopes have performed variably and have been subjected to several types of failure mechanisms in saprolite and transition slopes. Documenting the slope height versus the slope angle relationship of the historical failures and stable areas of the interim slopes has provided valuable insight of slope performance. This paper presents an empirical approach to define slope design parameters and to provide an empirical guide to designing sustainable final walls.
... Various studies have shown that this simplification can result in reduced SRF estimates compared to full 3D analyses, as plane strain conditions increase overall kinematic freedom within 2D models (Cavounidis 1987;Chugh 2003;Albataineh 2006;Cala et al. 2006;Jiang et al. 2008). Despite these limitations, two-dimensional analysis is still widely used throughout geotechnical mine design as a simplification of three-dimensional behaviour (Hormazabal et al. 2013;Abrahams et al. 2015;Wen et al. 2015;Wolter et al. 2015;Argumedo et al. 2016;Tuckey et al. 2016). While geotechnical analysis in this study has applied 2D simplifications of the Ok Tedi pit, the same geostatistical methodology would also be applicable for full 3D analysis. ...
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