Evaluation of intensive preconditioning in block and panel caving – Part I, quantifying the effect on intact rock

Abstract

A combination of preconditioning by hydraulic fracturing and confined blasting referred to as intensive preconditioning has been implemented for the first time at a major panel caving operation. The reasoning behind this application was the potential to alter rock mass behaviour in a manner that could improve caving mechanics and overall extraction performance. A comprehensive sampling and testing campaign was conducted before and after the application of large scale intensive preconditioning. Intact rock data was analysed with the combined use of uniaxial compressive strength tests and acoustic emission monitoring. The aim was to define damage thresholds of intact rock and study specific boundaries associated with different degrees of preconditioning. To evaluate the intact rock failure process, four stages or damage were studied, they included crack initiation stress, crack damage stress, uniaxial peak strength and cumulative count of Acoustic Emission (AE) events. Results from this analysis indicated that between in situ and post hydraulic fracturing preconditioning, the initiation of damage occurs almost at the same stress levels, which as expected, demonstrated that hydraulic fracturing does not affect intact rock strength. However, after preconditioning by confined blasting, results indicated that the on-set of excessive damage prior to failure was occurring at lower axial stresses, this confirmed the presence of micro-fractures in the samples which may be attributed to confined blasting induced stresses.

Keywords:

• Intensive preconditioning
• hydraulic fracturing
• preconditioning by blasting
• block and panel caving
• acoustic emission
• rock damage thresholds
• blast induced damage