Samantha Espley Inco, Mines Research Engineer
Chris Langille Inco, Ground Control Engineer
Dr. Dougal McCreath Laurentian University, Professor
In the past year, trials of two innovative applications of liner support involving the replacement of bolts in the ground support system were undertaken at INCO mines. The first trial involved an evaluation of the effect of dynamic loading on mesh-reinforced boltless shotcrete in a sub-level cave mining area on 2400 level at Stobie Mine. The second trial involved an evaluation of the MIROC Mineguard spray-on liner as a replacement for bolts and screen support, or as a replacement for screen support alone, in a topsill heading of a bulk, VRM (vertical retreat mining) stope on 3240 level at Lower Coleman Mine.
In the Stobie Mine trial, an uppers retreat heading was supported using a two-pass system. First, shotcrete was applied 2-inches thick, followed by pinning of mesh screen to the back and walls. A second pass of another 2-inches of shotcrete was then performed. For recovery of the ore zone, a vertical slot was created using a Borepak blind boring machine, then the uppers rings were blasted, one or two rings at a time, starting at the slot and retreating out of the heading. During blasting of the production rings, blast monitoring was used to record the ppv's (peak particle velocities). For a series of 5-ring blasts, the maximum ppv's were measured to be 6 in/s (153 mm/s), with a maximum charge weight per delay of 800 lb (357 kg). Six 5-point convergence measurement stations were set up and after each blast the total rockmass convergence was measured using a tape extensometer. Although a maximum displacement of 4 in (100 mm) was measured, the shotcrete still managed to provide some support function. A support damage assessment was also done to try and relate blast vibration to effectiveness of the mesh reinforced shotcrete liner. This trial demonstrated the effect that blasting has on a support system, and it also demonstrated the necessity of having the mesh reinforcement properly placed within the shotcrete.
The Lower Coleman Mine trial involved the development of a engineered design to create a dead-weight loading system to test the standard rockbolts and screen support system versus a polyurethane liner support system, with and without the installation of rockbolts. Calculations indicate that a dead-weight load, or slab of rock having dimensions 12 ft by 20 ft by 4-ft thick, could be supported by rockbolts (using a standard 2.5 ft by 5 ft staggered bolting pattern) and screen. Using this information, a development and blasting scenario was designed and implemented for the creation of the dead-weight loading system, as described. Initially, a topsill heading was developed 20 ft high and 20 ft wide. The topsill heading was then advanced an additional 100 ft using a reduced back height of 15 ft. A 5 ft hanging breast, or brow area, was created along the length of the topsill for the testing of each support system. For each test, 12 to 13 ft of advance was required such that the brow was drilled off with closely spaced horizontal perimeter holes, each being 12 ft in length and with a 4 ft burden. Additional horizontal perimeter holes were drilled along the shoulders of the brow area to within 1 ft of the back. Further down the drift from the brow, closely spaced up-holes were drilled, each 5 ft in length, to intersect the toe-locations of the horizontal perimeter holes. Each alternate perimeter hole was loaded with ETI's Trimrite-2C perimeter blasting product. After detonation of the blast, a hanging slab of rock (20 ft wide, by 12 ft long, by 4 ft thick) was created.
The design was tested in situ using the standard rockbolts and screen as the support system. With the failure of the initial test, the loading and blasting strategy was re-designed and the test was repeated. The design modifications were successful such that the rockbolts and screen were able to effectively support a loose slab of rock which was 4 ft thick. The trial was then expanded to test the support capacity of the polyurethane liner as a stand-alone support system, without the addition of rockbolts. All of the tests from the trial were fully documented with still and video photography and instrumented with gmm's (ground movement monitors). Blast monitoring and calculations were conducted for the trial with an estimated range of ppv's of 12 in/s to 37 in/s (305 mm/s to 940 mm/s). An analysis of the trial results proved inconclusive whether the polyurethane Mineguard liner can effectively replace the full rockbolts and screen support. However, the trial indicated that the Mineguard liner can be considered acceptable as a replacement for screen support within specific rock types. Further testing of the liner, with the additional installation of rockbolts to provide the necessary radial support to the rockmass, is currently under way. A support system utilizing the Mineguard liner and rockbolts allows for increased efficiency of the underground operations through an improvement of the rate of development.