R. Trueman

Two- and three-dimensional elasto-plastic analysis for coal pillar design and its application to highwall mining

Abstract An elasto-plastic model suitable for the analysis of coal pillars has been developed and implemented in both two- and three-dimensional finite element codes. Yield was modelled with a modified Hoek-Brown criterion and strain softening was introduced so that in the post failure region, the yield criterion reduced continuously to a weaker modified Hoek-Brown criterion. For a global average coal, yield parameters suitable for in situ conditions were determined so that a three-dimensional analysis of coal pillars fitted established empirical pillar design formulae. The model was used to determine preliminary design guidelines for a proposed highwall mining trial in central Queensland, Australia.

Extending the Mathews stability graph for open–stope design

Abstract The Mathews method of predicting open-stope stability was first proposed in 1980. The initial stability graph was based on a limited number of case studies, primarily from deep, North American, steeply dipping open stopes in strong rocks of medium to good quality. Since then new data have been added by various practitioners to modify, update and validate the method and support its use as a preliminary open-stope design tool. The original Mathews method has been extended with use of a significantly increased database of mining case histories. The format of the Mathews stability graph has been changed to reflect the broader range of stope geometries and rock mass conditions now captured within the database. The extended database now contains in excess of 400 case histories. Logistic regression has been performed on this larger database to delineate and optimize placement of the stability zones statistically. Isoprobability contours have been generated for all stability outcomes. The advantage of the logistic regression lies in its ability to minimize the uncertainties reflected in the method through the use of maximum likelihood estimates. The risks associated with use of the Mathews method can now be quantified and the true statistical significance of the stability zones understood.

Numerical modelling as an aid to the determination of the stress distribution in the goaf due to longwall coal mining

Abstract To date, it has not been possible to take accurate in-situ measurements of the stress rise in the goaf as a result of longwall mining, due to the difficulty in maintaining contact between in-situ equipment and data loggers. For this reason, research into stress distributions within the goaf, loaded by overlying strata, has been addressed through numerical modelling, with a strain stiffening constitutive law for the goaf. The modelling results have been compared with existing methods and significant differences noted.

Assessing longwall support-roof interaction from shield leg pressure data

Abstract A concept has been developed where characteristic load cycles of longwall shields can describe most of the interaction between a longwall support and the roof. A characteristic load cycle is the change in support pressure with time from setting the support against the roof to the next release and movement of the support. The concept has been validated through the back-analysis of more than 500 000 individual load cycles in five longwall panels at four mines and seven geotechnical domains. The validation process depended upon the development of new software capable of both handling the large quantity of data emanating from a modern longwall and accurately delineating load cycles. Existing software was found not to be capable of delineating load cycles to a sufficient accuracy. Load-cycle analysis can now be used quantitatively to assess the adequacy of support capacity and the appropriateness of set pressure for the conditions under which a longwall is being operated. When linked to a description of geotechnical conditions, this has allowed the development of a database for support selection for greenfield sites. For existing sites, the load-cycle characteristic concept allows for a diagnosis of strata-support problem areas, enabling changes to be made to set pressure and mining strategies to manage better, or avoid, strata control problems. With further development of the software, there is the prospect of developing a system that is able to respond to changes in strata-support interaction in real time.

Quantifying stresses and support requirements in the undercut and production level drifts of block and panel caving mines

A numerical modelling strategy has been developed in order to quantify the magnitude of induced stresses at the boundaries of production level and undercut level drifts for various in situ stress environments and undercut scenarios. The results of the stress modelling were in line with qualitative experiential guidelines and a limited number of induced stress measurements documented from caving sites. A number of stress charts were developed which quantify the maximum boundary stresses in drift roofs for varying in situ stress regimes, depths and undercut scenarios. This enabled many of the experiential guidelines to be quantified and bounded. A limited number of case histories of support and support performance in cave mine drifts were compared to support recommendations using the NGI classification system, The stress charts were used to estimate the Stress Reduction Factor for this system. The back-analyses suggested that the NGI classification system might be able to give preliminary estimates of support requirements in caving mines with modifications relating to rock bolt length and the support of production level intersections

Factors influencing overbreak in the Barkers orebody, Kundana Gold mine: narrow vein case study

Abstract A detailed analysis of stope performance data from the narrow vein Kundana Gold mine in Western Australia has been made using the extended Mathews stability graph method and comparative statistics. For these Kundana stopes, a poor correlation was found between stope stability and both the Mathews stability number N and hydraulic radius HR. Given that both N and HR correlate well with stability in the vast majority of Mathews method case histories, this suggested that there is an overriding influence on stability at Barkers not accounted for in the Mathews method. Drill and blast issues were isolated as the most likely cause of this poor correlation. Blast pattern was found to have a statistically significant effect on overbreak. In terms of the drill and blast patterns used at the mine, the inline pattern performed significantly better than both the ‘staggered’ and ‘dice-5’ patterns for the vein geometries at the time. Undercut footwalls were found to behave in a similar manner to non-undercut hangingwalls. There was no evidence that backfill abutments behave differently from solid rock abutments in terms of determination of stable stope dimensions. Drillhole accuracy was indirectly examined by considering the effect of stope heights within the limits of 13–20 m. Within these limits, stope height did not affect the magnitude of overbreak.

Development of benchmark stoping widths for longhole narrow-vein stoping

Abstract In narrow vein mining it is often not possible to limit stope width to the vein width when utilising blasting for rock breakage. A probabilistic benchmarking method is used to estimate benchmark stability stoping widths and benchmark average stoping width for three commonly used narrow vein longhole blast patterns. Average stoping widths for inline, staggered and dice-5 blast patterns have been estimated at 1˙3, 1˙5 and 1˙7 m respectively. Average stoping width can be used to assess planned and unplanned dilution. Additionally, a concept termed the benchmark stability stoping width has been defined and quantified for the three blasting patterns. Stability stoping widths for inline, staggered and dice-5 have been estimated at 1˙6, 2˙0 and 2˙1 m respectively. Narrow vein stopes within these limits after blasting can be regarded as stable.

Quantifying effect of concurrent draw on extraction zones in block caving mines using large scale 3D physical model

Abstract There remains a debate within the literature and among practitioners of caving methods as to the effect on draw zone geometry for the concurrent drawing of multiple drawpoints. Concurrent draw refers to an extraction schedule where a limited amount of material is drawn from each drawpoint before moving to the next drawpoint to draw the same amount. One hypothesis concludes that the flow geometries of a single drawpoint increase while another assumes no change from that of isolated draw. The largest 3D physical model constructed using gravel as the model media has been used to further investigate interactive draw of extraction zones as part of an International Caving Study (ICS) and Mass Mining Technology Project supported by major international companies with interest in caving methods. All extraction zones were measured in 3D. To date a maximum of 10 drawpoints have been modelled. Model results so far indicate no growth in the horizontal width of extraction zones using concurrent draw. Experiments conducted with multiple drawpoints that were spaced less than the width of isolated extraction zones showed that the combined horizontal area of draw appears to reduce with the increasing overlap of isolated extraction zones. The horizontal widths of extraction zones continued to increase within the height of the draw tested.