Effects of confinement and initiation delay on fragmentation and waste rock compaction : Results from small scale tests

Abstract: Sub-level caving (SLC) is classified as a mass mining method and there is an increasing interest in its application worldwide and in a very wide range of geotechnical environments. Design of sub-level blasting rounds and optimization has become more important now when the sizes of the blasting rings have become larger. Sufficient fragmentation is one of the key factors and in confined blasting, as in sub-level-caving, this influences the mobilization of the blasted ring. The caved rock or debris at the interface first acts as a wave trap i.e. the debris at the SLC interface reduces the fragmentation and the swelling of the blasted ring and it dissipates the explosive energy. These phenomena may immobilize the blasted ring, causing ore losses.Small-scale blasting has been carried out to investigate and clarify this phenomenon. To minimize geometrical and geological effects, tests were made on Ø140 mm cylinders of magnetic mortar, which fractures like magnetite but is a less variable material. The cylinders were placed inside a Ø300 mm steel cylinder and confined by packed aggregate. The explosive source was decoupled PETN cord with different strengths in a center hole, which gave a specific charge between 0.2 and 2.6 kg/m3. The magnetic mortar and the nonmagnetic aggregate allowed for post-blast magnetic separation. The setup provides very repeatable results. The fragmentation of the blasted mortar and the aggregate plus the swelling of the confined mortar cylinders have been measured. As a reference, free mortar cylinders without radial confinement were used. More than 160 tests have been made with the cylindrical set-up.The results show that the confinement results in fragmentation being coarser when compared to that from free cylinders, and that the properties of the debris have a strong influence on the fragmentation and the swelling of the blasted materiel/compaction of the confining material. For the latter, a freezing-slicing method has been developed. The cylinders could thus be sliced perpendicular to the charged hole and then photographed to measure the radial expansion at different heights. The acoustic impedance between blasting material and confining debris has been applied to give a material description in a simple and physical form. This, in combination with the specific charge, has been shown to influence the fragmentation and compaction to a great extent. For the compaction, the porosity of the confining debris is also an influencing factor. Two prediction equations have been presented with high coefficients of determination, both for fragmentation and compaction. The tests have also been shown to be a good input for numerical modelling of blast compaction and reliable input for future numerical modelling of blast fragmentation. Further, a series of detailed small-scale tests have been made to investigate the use of short delays to promote better fragmentation caused by shock wave interactions. The block design had a size of 650/660×205×300 mm (L× W× H) and two rows with five Ø 10 mm blastholes in each row. The spacing and burden were 110 mm and 70 mm respectively, giving an S/B ratio of 1.6. The results showed no distinct differences or high improvements of the fragmentation when the delays were in the time range of interactions compared to no shock wave interactions. The decrease of x50 was around 20 % at a delay time ~1.1 ms/m burden compared with longer delays like 2 ms/m. A statistical analysis of the results has been made to evaluate the minimum at short delays and it is not significant.