Open stope design

Common back analysis techniques, such as empirical stability graph methods, are limited in their ability to identify and quantify the relative contributions of the various factors that influence excavation performance. A comprehensive back analysis of existing stoping operations was deemed necessary to critically assess key contributing factors to existing and future performance of open stoping at Kanowna Belle, such as drill and blast, development undercutting, induced stress rock mass damage, rock mass quality, and large scale geological features. The back analysis strategy firstly adopted targeting earlier, shallower, primarysecondary stoping blocks to potentially minimise the contribution of the effects of stress induced rock mass damage on stoping performance. In this regard, Block A stopes were chosen for this critical review. Performance of Block A stopes was assessed through analysis of post extraction geometric data, utilising the results of CMS surveys. The studies undertaken highlight the impact of various stress-path dependent damage mechanisms, in situ rock mass quality and the influence of stope-scale geological structure on open stope performance.

This paper was prepared for presentation at Alaska Rocks 2005, The 40th U.S. Symposium on Rock Mechanics (USRMS): Rock Mechanics for Energy, Mineral and Infrastructure Development in the Northern Regions, held in Anchorage, Alaska, June 25-29, 2005.

A methodology and a number of software tools have been developed to assess the operational and geotechnical factors affecting the performance of open stoping operations. The paper describes the tools and techniques for collecting and analysing common factors affecting performance, such as drill and blast, development undercutting, stress induced damage, rock mass quality, and large scale geological features. Example data has been collected and analysed with some results presented.

Stope performance is reviewed with respect to the overall stope design process. Global and detailed design issues are identified along the way, and the stope design note is described in detail. Stope performance is quantified based on the depth of failure measurements, which are calculated using block models of Cavity Monitoring System wireframes and tested against the stope design boundaries. Finally a stop performance assessment summary data sheet is also provided.

A review of the most widely used global extraction sequences in sublevel is undertaken. The review includes techniques used to extract massive as well as single or multiple steeply dipping tabular orebodies. The paper also deals with thick flat lying orebodies suitable for open stoping. Techniques used to minimize the effect of stress re-distributions on a global scale are discussed for all the extraction sequences analyzed.

Back analysis of open stope performance is essential in the dilution control process, as an improved understanding of mechanisms allows one to check the validity of any assumptions and refine geotechnical parameters used in the design process. This paper describe the results of a back analysis of over-break geometries, using non-linear elasto-plastic finite element modelling undertaken at Barrick Gold Australia’s Kanowna Belle Gold Mine. The entire sequence of bench and longhole open stopes at the Kanowna Belle mine was modelled. The back analysis study involved analysing hangingwall over-break data from longhole open stopes from a number of key mining blocks. For the numerical modelling exercise, results grids were placed such that various components of velocity and plastic strain could be ascertained into the hangingwall rock mass, with modelling steps calculated at the individual stope’s extraction, prior to backfilling. Stochastic analysis of the CMS over-break data, together with the results from the modelling was used to establish global relationships between velocity and plastic strain and marked increases in overbreak.The back analysis results were then used to develop over-break criteria which can be used as a planning tool to estimate probability and volumes of over-break for any future stope design and/or sequencing option.

Open stope performance is generally assessed by the ability to achieve maximum extraction with minimal dilution. Hence, the success of the open stoping method relies on the stability of large (mainly un-reinforced) stope walls and crowns as well as the stability of any exposed fill masses (Villaescusa, 2004). The performance of an open stope can therefore be judged on the actual outcome versus the planned outcome, in terms of the final volume, tonnage and grade of material extracted, and the timeliness of extraction, compared to the planned design and schedule. Performance can be described in a number of ways, from subjective qualitative terms to quantitative numbers, based on a number of parameters and/or physical quantities. A number of quantitative measures of stope performance, such as ELOS (Clark and Pakalnis, 1997), have been used in the past, however some of these measures fail to adequately capture certain geometrical aspects of over-break or under-break. Back analysis of open stope performance is essential in the dilution control process, as an improved understanding of mechanisms allows one to check the validity of any assumptions and refine geotechnical parameters used in the design process. A number of new shape descriptors are introduced and, in conjunction with existing performance parameters, an improved method for quantification of over-break and under-break will be presented. To illustrate the methodology, data from two cases study will be presented.

The main aim of underground mine planning and design is to develop a mine design that will accommodate company business objectives and, when implemented, operates within specified performance criteria such that financial and operational risks are minimised. The inherent uncertainty associated with rock engineering implies that there is always a risk that designs may not perform as intended, along with attendant economic and safety implications. The objective of the design engineer is to ensure that the level of design reliability is commensurate with the stage of project development and acceptable business risks. This paper presents an optimised rock engineering approach to open stope span design in underground hard rock mines. The principal philosophy behind the approach is to ensure that appropriate methodologies are utilised in data collection, data analysis, rock mass model formulation and stope design at relevant project stages in order to maximise design reliability.