Assessing the stress field for the Jwaneng underground project

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 2030s; studies are currently underway 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 caving (SLC) and other caving methods; mining will extend to a depth of ~1,000 m below surface. The Jwaneng Mine is structurally complex with many large geological structures and a variable rock mass, with strengths ranging from weak kimberlites (25 MPa) to very competent dolomites (>250 MPa). Understanding the in situ stress regime is an important input for underground mine design and has a significant impact on designs analyses including numerical modelling. The magnitudes, ratios and directions of the principal stresses influence the choice of mining method, excavation stability, stand-off distances, extraction sequence etc. 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 has been undertaken on ~100 boreholes 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. The interpretation of the data is an iterative process with different approaches applied with the aim of developing reasonable stress field inputs and sensitivities for design analysis. The process followed and the outcomes will be discussed in this paper.