In open pit mining, a rock mass model can provide important input for the optimization of production in relation to stability challenges and a safe workplace. In this sense, rock mass classification systems are useful for defining rock mass domains that identify areas of poorer ground conditions that may require more detailed investigation or modifications to mine designs. These methods are widespread among geotechnical engineers as one practical way to assess quality of the rock mass. In open pit mine bench faces with no clear discontinuities, one single joint may dominate the pit slope plane failure mechanism. Analysis of low rock mass quality zones combined with the jointing orientation may provide an improvement in the assessment of instabilities. The aim of this paper is to develop a rock mass model of an open pit mine under active operation, based on extensive field mapping. Through this process, surface maps based on different rock mass classifications (RMR 89 , GSI, and Q-system) systems will be established for the mine. The mine was classified and assessed using RMR 89 . Finally, a three-dimensional rock mass model was developed based on the values obtained from surface and underground mapping. It was hoped that such a model allowed the identification of areas of the mine that are likely to have poorer ground conditions and may require more attention during the design and operational phases of the mine life. INTRODUCTION The planning of an open pit is a matter of determining the most profitable pit limit and the most economical mining sequence for a given orebody. In the exploitation phase, the excavation sequence implies the development of artificial slopes the stability of which is a crucial factor both for safety and economics of mining. Therefore, the assessment of rock slope stability is an essential requirement for the open pit mining industry, not exclusively during the engineering studies, but also throughout the operational life of a project. For this matter, rock mass classification systems are extensively used in order to quantitatively address the quality of the exposed rock mass in open pit slopes. A variety of rock mass classification systems have been proposed in the last 50 years, many of which were originally intended for tunnel design and subsequently adapted or modified for use in slopes. Four systems have gained broad acceptance in the mining industry: the RMR89 (Bieniawski, 1989), the Q-system (Barton, 1974), the MRMR (also known as Laubscher's RMR: Laubscher, 1990), and the GSI (Hoek, 1994). Flores and Karzulovic (2002) stated that the most widely used methods for rock mass classification in underground mines are Laubscher's RMR89, or MRMR, (53%), followed by Barton's Q (26%), and Bieniawski's RMR89 (15%). On the other hand, the most widely used method for rock mass classification in open pit mines is Hoek's GSI (39%), followed by RMR89 (26%) and MRMR (22%). This is shown in Figure 1.

The study sought to establish the applicability of rock mass classification as a primary input to wall control blasting at a hard-rock mine in Botswana. Conventional rules of thumb are used to develop blast designs based on parametric ratios with insufficient consideration of the rock mass factors that influence the achievability of final wall designs. Control of the western highwall of the mine's pit had proved to be challenging in that the designed catchment berms and wall competence were perpetually unachievable from the pit crest to the current mining levels. This exposed the mining operation to safety hazards such as local wallrock failure from damaged crests, frozen toes, and rolling rockfalls from higher mining levels. There was also an effect of increased standoff distances from the western highwall, which reduce the available manoeuvring area on the pit floor and subsequently the factor of extraction that is safely achievable. The study investigated the application of rock mass classification and the blastability index (BI) as a means to improve wall control. This was achieved by establishing zones according to rock type forming the western highwall rock mass, in which the distinguishing rock mass classification factors were categorized and used to establish the suitable wall control designs through a design input tool (DIT). The DIT consolidated rock mass classification methodologies such as the Geological Strength Index (GSI) and the Rock Mass Rating (RMR) and related them to the BI and discontinuities of the rock mass to produce a tool that can be used to develop objective wall control designs. The designs developed by the tool inherently take into account the rock mass characteristic factors at the centre of the rock mass classification methods above and significantly reduce the dependence on rules of thumb. It was found that this approach yields designs with explosive energy measures that are consistent with the rock-breaking effort and the behaviour of discontinuities, all the while remaining biased towards preservation of perimeter wall rock. The work presented in this paper is part of an MSc research study at the University of the Witwatersrand.