This thesis project was an initiative of the JKMRC and formed part of the project deliverables for the 2nd AMIRA Blasting and Reinforcement Technology Project. An improved ability to predict dilution in narrow-vein longhole stoping compared to conventional stoping facilitates more accurate economic comparisons between mechanised longhole stoping and other narrow-vein mining methods. Furthermore, understanding the underlying causes of dilution in narrow-vein mining is an essential element in dilution minimisation.
Sponsors of the 2nd AMIRA Blasting and Reinforcement Technology Project expressed concern about the applicability of empirical stability charts to narrow-vein stope design and dilution prediction. To address these concerns, a number of parameters identified as having a high potential to contribute to narrow-vein instability and not generally incorporated into stability charts were reviewed. Of these, several were considered rockmass dependent and therefore, to some extent implicitly taken into account by Q’. The possibility of developing site-specific stability charts to capture the potential sensitivity of narrow-vein stopes to Q’ was investigated. Monte Carlo simulations analysing trends in the variance of the Extended Mathews logit model indicated that a reliable stable-failure boundary requires at least 150 case histories, of which a minimum of 10 percent should be unstable stope surfaces. The time required to collate sufficient case studies for a site-specific chart is considered a significant limitation of this approach to improved narrow-veins stope design. Furthermore, dilution estimates are required for the feasibility stage of a greenfield project, prior to the availability of site data.
Only marginal site-specific effects were observed for the operating conditions captured within the Extended Mathews database (485 case studies). It has been concluded that the apparent site-specific effects referred to in previous literature are attributable to operating conditions inadequately represented in the database. Such operating conditions could induce erroneous stability predictions at any site, and therefore, are not truly site-specific. Following from these conclusions, a methodology for taking into account narrow-vein operating conditions has been proposed.
Stress relaxation is one of the narrow-vein operating conditions hypothesised to adversely affect narrow-vein stope stability prediction using existing stability chart methods. While many authors refer to the adverse effect of stress relaxation on excavation stability, some authors present compelling empirical evidence indicating that stress relaxation does not have a significant effect. Establishing clear definitions of stress relaxation was critical to understanding and quantifying stress relaxation effects. Three types of stress relaxation have been defined; partial relaxation, full relaxation and tangential relaxation. Once clear definitions were determined, it became clear that the theoretical arguments and empirical evidence presented by various authors to support their respective cases are not contradictory; rather the different conclusions can be attributed to different types of stress relaxation. In particular, when the minor principal stress is negative the intermediate principal stress has been identified as significantly affecting jointed rock mass behaviour. A new set of guidelines to account for the effect of stress relaxation within the stability chart approach has been proposed.
115 narrow-vein case studies from the Barkers mine in Western Australia showed a poor correlation to both the stability number (N) and hydraulic radius (HR). Given that both N and HR correlate well with stability in the vast majority of stability chart case studies, this suggests there is an overriding influence on stability at Barkers not accounted by stability charts. Blast pattern was found to have a statistically significant effect on overbreak. There was no evidence that tight backfill abutments (not continuous moving) behave differently from solid rock abutments in terms of determination of stable stope dimensions. These findings provided justification for further explicit consideration of narrow-vein operating conditions.
The effect of stress damage associated with the incremental extraction of long-hole rings resulting in a retreating high stressed zone at the stope brow was analysed. The study involved analysis of overbreak from 412 case studies from Barker mine. Numerical modelling of a 32 month extraction sequence for each case study demonstrated that stress damaged stope walls at this mine had an average 0.27 metres more overbreak than stope walls where stresses had not exceeded the site calibrated damage criterion.
Narrow-vein case studies from Barkers, Callinan and Trout Lake mine suggest that overbreak is not continuously related to N and HR well inside the stable zone. Based on this result it was concluded if a stope plots well inside the stable zone the cause of dilution is unlikely to be related to geotechnical instability. This interpretation implies that the causes of narrow-vein dilution can be separated into two independent causes:
A probabilistic benchmarking method has been used to estimate a benchmark stoping widths for three commonly used narrow-vein longhole blast patterns. The benchmark stability stoping width for each pattern defines realistic planned dilution limits. These limits provide the basis from which true unplanned dilution can be assessed. The probabilistic overbreak model has also been used to predict expected (average) stope widths for each of the patterns. Benchmark average stoping width, in conjunction with vein or ore width, can be used to estimate total dilution.
Benchmark stoping widths are primarily a function of the longhole stoping method (operating-condition specific). On this basis, the benchmark stoping widths determined for Barkers mine are applicable to narrow-vein longhole stoping with similar operating conditions. However, it was recognised that complex geology (e.g. cross-cutting structures) would require adjustments be made to both the benchmark stability stoping width and the benchmark average stoping width.
The narrow-vein dilution methodology (NVD method) proposed in this thesis is a tool for predicting narrow-vein dilution based on the benchmark average stoping widths. In addition to dilution prediction, the NVD method also includes recommendation and strategies for narrow-vein dilution minimisation generally, including; filling, cablebolting, stress relaxation, stress damage and blast overbreak. It is envisaged that improved dilution prediction will lead to more accurate comparisons of the expected cost of dilution in longhole stopes compared to other mining methods. In this sense, the title of this thesis could more accurately be described as ‘optimising dilution in narrow-vein mines’. This recognises that optimal mining method selection will, in some instances, accept a higher level of dilution as part of a higher overall NPV for an operation. Conversely, in other instances the higher mining unit costs may be acceptable to increase overall NPV.