The Impact of Blasting on Sublevel Caving Material Flow Behaviour and Recovery

nSublevel caving (SLC) is a mass mining method based upon the utilisation of gravity flow of blasted ore and caved waste rock. The method functions on the principle that ore is fragmented by blasting, while the overlying host rock fractures and caves under the action of mine induced stresses and gravity. The caved waste from the overlying rock mass fills the void created by ore extraction. A major disadvantage of the SLC mining method is the relatively high dilution of the ore by caved waste. A major factor influencing this dilution is the flow behaviour of the ore and waste material. For this reason, flow behaviour has been studied and quantified through theoretical, small and full scale experimental programs for almost 50 years. These programs have attempted to identify parameters which have a significant influence on flow behaviour, and therefore ore recovery and dilution results. Parameters directly influencing flow performance have been found to include the geometry of the extraction layout and drives, sublevel height, blast ring design, material characteristics of the blasted and waste material, and draw control methodology. Due to the complex interaction of these parameters with one another, a total understanding of the impact of SLC mining on flow behaviour is not fully understood. Drill and blast issues have been identified in the literature to have a substantial impact upon SLC material flow. These issues relate to both drill and blast design parameters and design implementation. Increases in SLC geometry size have meant the development of longer and larger diameter blast holes, and improvements in explosive and blasting methods. This has in turn lead to large mine cost savings due to decreased development costs. Such significant changes in the drill and blast design would be expected to have an impact on blasted and caved material properties, and therefore material flow behaviour. Dominantly, the literature has made general conclusions concerning the interaction of drill and blast parameters and flow behaviour, with respect to the knowledge that blasting has a direct impact on ore and caved flow material properties. A limited number of studies have related drill and blast parameters to indirect measures of material flow behaviour, in particular ore recovery and dilution. To date, no detailed analysis of the impact of drill and blast parameters on material flow behaviour in full scale SLC operations has been documented in the literature. The implementation of full scale SLC marker trials has been noted in the literature to be crucial for the ongoing success of the mining method. Such trials provide detailed information concerning the development and shape of the extraction zone, identify possible sources of waste ingress into the ring, and ascertain the degree of flow behaviour variability. The marker trials undertaken at the Ridgeway SLC gold mine provide a unique opportunity to assess these factors. These trials are considered to be the most comprehensive to date, with 69 individual ring trials completed from July 2002 to April 2005. The Ridgeway marker dataset was used in this thesis to assess and quantify factors influencing material flow behaviour and extraction zone recovery. It can generally be concluded from the Ridgeway marker trials that the shape of the extraction zones were irregular in nature (not described by an ellipsoid shape), with primary recovery consisting of an area of lcontinuous flowr near the blast ring plane and lfingersr of recovery further from the blast ring plane. The backbreak extraction zone is relatively common, with highest recoveries occurring in close to the previously fired blast burden. Secondary, tertiary, and quaternary recoveries occur as relatively small discrete zones within the blasted material. An analysis was undertaken to identify factors that influence extraction zone recovery for the Ridgeway marker trial dataset. Factors analysed included parameters related to drawpoint location, drill and blast design, geology, drawpoint geometry, and draw control. To identify factors influencing extraction zone recovery, a neural network technique was adopted. The analysis indicated that a number of blasting parameters are directly or inversely correlated to extraction zone recovery for the marker trials. Blasting parameters appear to dominate correlations with recovery when compared to geological and drawpoint related parameters. Although the neural network methodology provides a rigorous means to identify possible correlations between parameters, further data analysis was required to understand the nature and characteristics of these relationships. Correlations identified by the neural network analysis were analysed in two dimensions, and did not consider multivariable relationships. Traditional statistical methods were employed to investigate and characterise these correlations. For categorical blasting parameters, non-parametric tests were used to determine if significant statistical differences existed between categorical groups. For continuous blasting parameters, the assumption of a linear correlation was made to quantify the strength and significance of such a relationship existing. Based upon the statistical analysis a number of possible theories were proposed with regard to the impact of blast parameters on extraction zone recovery. Although the correlations analysed do not necessarily prove causality, the correlations can be linterpretedr in causal terms to propose a number of blast related theories with respect to recovery. To develop these theories, both correlations between blasting parameters and extraction zone recovery as well as blast related inter-parameter correlations were considered. The most likely theory based upon this analysis is that a fundamental change in drill and blast design at Ridgeway with the removal of blast holes resulted in lower extraction zone recoveries (in particular primary recovery close to the blast ring plane). The reduction in the number of blast holes in turn impacted on total secondary to quaternary extraction zone recovery either directly (due to factors such as poor fragmentation and limited swell) or indirectly through reduced primary recovery (leading to subsequent lower total secondary to quaternary recoveries). Recovery in the marker ring planes is directly related to proximity of the blast ring plane. The results and conclusions presented in this thesis provide an improved understanding of full scale SLC flow behaviour and recovery. In particular, parameters significantly influencing extraction zone recovery were identified. The analysis highlights and quantifies the importance of drill and blast design and implementation on improved extraction zone recovery.