The 4th International Symposium on Mine Safety Science and Engineering (ISMSSE2018)

246 / 2018-05-08 23:18:18

Preconditioning Hole Burden Evaluation for Deep Level Stopes

preconditioning,ground penetrating radar (GPR)

Mine Dynamics Disaster > Rock burst impact

Deep level gold mine panel stopes have an inherent risk of face bursts, due to the high face stress conditions encountered. As a face burst control measure, face perpendicular preconditioning of stope faces has been successfully implemented over the last 15 years throughout the industry.

The application of preconditioning is based on the SIMRAC issued ‘Precondition Implementation Guidelines’, which are based on the research work documented under projects GAP 030, GAP 336 and GAP 811. The guidelines recommended a hole burden spacing of 3.0m and a hole length of 3.0m as the initial values for the preconditioning standard.

The preconditioning guidelines further indicated that, the monitoring of the effectiveness of preconditioning should be conducted at implementation sites when conditions differ markedly from those investigated during the development of the technique. The initial investigations during GAP 030 and 336 projects were conducted at sites with a maximum depth of 2400m. Current mining panels have become deeper to depths of up to 3400m. It therefore became important to monitor the effectiveness of preconditioning at current mining depths, and to change some of the parameters in the standard to suit conditions were found necessary. Amendable face-perpendicular preconditioning parameters include: hole length and hole burden spacing.

During the initial preconditioning technique development project, one of the successful assessment tool used was GPR profiling. The tool can be applied to determine the effectiveness of preconditioning by determining the extent and intensity of fracturing ahead of the panel face. The project therefore used GPR profiling for the assessment of blasted preconditioned panels.

Following GPR profiling of 10 preconditioned sites, and analyses of the results obtained, the following conclusions were made:

 The depth of fracturing ahead of a preconditioned deep level panel is not determined by the precon hole length, but is influenced by face stress values.
 A minimum precon hole length of 2.4m has been shown to be adequate in achieving an adequate fracture depth for effective preconditioning, therefore the preconditioning guideline must be amended to indicate the minimum hole length as 2.4m for use by the industry, as opposed to the 3.0m indicated.
 The precon hole burden spacing of 3.0m as given by the guideline is confirmed as the effective spacing for achieving a continuous fracture profile ahead of the faces.
 Where efficient burden spacing is encountered, the depth of fracturing is increased, and inversely the areas where over spacing is encountered the depth of fracturing is reduced.
 The increase in fracture depth when hole burden spacing is reduced is insignificant at a range of 6cm increase for 0.5m burden reduction. Burden spacing reduction does not significantly increase fracture depth, however it can be used to ensure 100% probability of continuous fracture achievement.
 The precon guideline should indicate a burden spacing of minimum 2.5m and maximum 3.0m.