While working in an Northern Ontario underground nickel mine in the 90’s, a problem arose that required the invention and design of a methodology using a shear line that could secure the integrity of a hanging wall (Norite) at the contact of an orebody where impurities were located. These impurities would affect the milling operation when mixed in with ore.
The idea was to develop a shear line (a controlled crack line – shear hole to shear hole) between the hanging wall contact and the rest of a blast to prevent the mixing of ore with the impurities. The shear line would stop cracking from the blast reaching and damaging the hanging wall. In theory this should prevent the impurities from mixing with the ore. Electronic detonators were off in the distant future, so this author still had to cope with NONEL delays.
If the creation of a viable shear line was achievable, it could be extended to those cases when a hanging wall needed to be stabilized in the mining of narrow vein gold mines with the view of limiting dilution resulting from blasting close to the contact zone of host rock and gold veins. In this case shear blastholes would be drilled parallel to the ore contact at a set distance away to avoid damage to the contact. Presplitting or splitting investigations in the past found that high gas producing explosives would produce a better fracture and might reduce or eliminate fine hairline cracks that may appear on borehole walls. In addition, there were claims the splitting action required the collision of stress wave reflections from adjacent shear holes from a shock viewpoint which may be in error.
This example/application of shear splitting in front of a hanging wall seemed to indicate that the shock wave magnitude from a detonating decoupled explosive might have had the strength to pull out scabs from the inside walls on opposite sides of shear boreholes. This scabbing action would create a notch that could easily concentrate gas expansion for the purpose of creating a well-defined split line. Figure 1 illustrates the idea of a scabbing mechanism produced by shock wave reflection from shear borehole walls.
In this case, it was desirous to use a high-shock cord based explosive (Primaflex) in order to create a notch from tensile scabs of adjacent borehole walls for gases to concentrate for the purpose of creating a well- defined split line.
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