Question by Waddell (for Board and Crouch)

Rockbursts occur in many different ways in mines in the Coeur d'Alene area, those in the Star mine being the most difficult and least dramatic of all of them. With 2-dimensional analysis, in which it is not possible to take into account changes in vein width among other things, can detailed investigation of the difference between one mine and another be made? Would you also comment on related studies you have been carrying out?

Reply by Board

The Star mine differs from seine others in the area in that the vein is nearly vertical in all places, and quite regular except for one place at which it widens out to 40 ft; that is in one of the stopes in the west end of the mine. The analysis we have done on the analog computer is really preliminary in that we considered the vein to be of uniform thickness. It would be possible to incorporate the thickness of the vein into our existing procedure, approximating some of the constants used to compute the surface enclosures. We have not done this. We have a good general solution for an attempt to cure the problem of the sill pillar bursting. Vein thickness is a specific problem that has to be dealt with in the mine. In the past there has been a problem of bursting at the point there the vein widened at the Star mine. It was primarily a problem of the mining method being used, and is not such a problem now with the new mining method. The main problem at present is mining in really hard bands of silicified quartzite. With the aid of a microseismic system and a mini computer we are now correlating data from these areas, and then using destressing techniques around the area to try to cure the problem.

Question by Maury (for Board and Crouch)

With what precision can you locate rockbursts? For instance, do you find them in the pillar, very close to the top or bottom of the pillar, or in the vicinity of the pillar? Since installing the microseismic recording system we have been able to locate the position of rockbursts to within 20-40 ft. (as determined by test blasts). Rockbursts observed so far seem to be coming from the south wall at the elevation of the pillar and not actually from the vein. The vein material is very broken, tending to squeeze rather than burst, whereas the south wall, being really hard silicified quartzite, takes most of the stress and hence is the site of most of the rockbursts. These usually occur right in the wall rock in the direct vicinity of the workings. About 92% of the observed seismic events appear to have originated within the main line workings in the south wall of the vein.

Question by Leahy (for Board and Crouch)

You mentioned destressing practised in the Coeur d'Alene district of Northern Idaho.

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