Dunn, MJ & Kebatsetse, LQ 2024, 'Development of the geotechnical model for the Jwaneng underground project', in Daniel Johansson & Håkan Schunnesson (eds), MassMin 2024: Proceedings of the International Conference & Exhibition on Mass Mining, Luleå University of Technology, Luleå, pp. 213-233.
(https://papers.acg.uwa.edu.au/p/2435_B-03/)
Abstract: The Debswana Diamond Company (DDC) Jwaneng Mine in Botswana, is a large open pit diamond mine extracting three kimberlite pipes. Open pit operations will cease in the early 2030’s; studies are being undertaken to transition to an underground operation by the time open pit mining ceases. It is anticipated that underground mining will be undertaken using a combination of sublevel retreat (SLR), sublevel caving (SLC) and block caving (BC) methods; mining will extend to a depth of ~1000 m below surface.  

The Jwaneng open pit has been in full production since 1982 and has a mature high-quality geotechnical model that has been progressively developed and improved over many years. Debswana makes use of a Geotechnical Review Board to guide the development of the geotechnical model. The geotechnical model comprises of several components: lithology model, major structures model, rock mass model, fabric model, hydrogeology model and stress field model. The geotechnical model is based on a significant amount of data (>226 km of logged core and >13,000 laboratory tests) and various components of the geotechnical model are regularly updated; generally, this is planned to coincide with the study stage, the intent being to meet the confidence level required for each stage. 

Understanding the in-situ stress regime is an important input for underground mine design and has a significant impact on design analyses including numerical modelling. Various evaluations have been undertaken and this includes stress measurements using the Sigra In-situ Stress Test (IST) and the Deformation Rate Analysis (DRA) methods. In addition, borehole ovality analysis using televiewer data was undertaken to assess the orientation of the maximum horizontal stress. In addition to these measurements, the literature has been reviewed and independent reviews of all work have been undertaken. The outcomes from the various methods do not always agree and the interpretation of the stress field in a complex geological environment is not straightforward and has associated uncertainties. A back analysis of these data was undertaken using a large scale 3DEC model to assess the far field stresses.  

This paper focuses on the rock mass and stress field model components of geotechnical model for the underground studies. The process used in developing these components will be discussed and the results summarised.