ABSTRACT:

A polyurethane product called Mineguard was sprayed on the roof and walls of two tunnels to determine the effectiveness of thin sprayed-on membranes for tunnel support. A continuous membrane that is firmly adhered to the rock creates effective support and is resistant to damage from blasts and scoop abrasion. Careful rock scaling, cleaning, washing, and drying are essential for good adhesion. Stress fractured or slabby ground can result in a "cantilever" loading mechanism in a membrane liner which can progressively tear the membrane and result in a fall of ground. A robotic arm is required to spray a continuous membrane up to the face of a freshly blasted round and to minimize the cantilever loading mechanism.

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INTRODUCTION

INCO Ltd. has been investigating the use of rapidly deployable, sprayed-on membranes for ground support. Initial work focused on evaluation of the physical and chemical properties (including toxicity and flammability) of various products. A polyurethane product called Mineguard was one of several products selected for further laboratory and field tests. Two field trials were designed to assess the in situ performance of Mineguard. Mineguard is made from two reactive chemicals, one being a resin and the other a catalyst. The two chemicals are pumped at a 1:1 ratio through a heated hose and as they mix at the tip of the spray gun a chemical reaction begins immediately. The material sets up within about 20 s creating a tough flexible coating that adheres to the rock surface (Archibald et al. 1992; 1997, Espley et al. 1996). The field trials were conducted at depth of 1295 m within a mine operating in the Sudbury basin. The area selected for the evaluation was a narrow-vein orebody that was being mined using a drift and fill method. Drifts at these depths in the mine experience stress-induced fracturing. The fractures occur subparallel to the roof. The primary function of the sprayed-on membrane is to maintain interlock between pieces of rock and to support the stress-induced slabs that are created as well as any potential small unstable wedges created by intersecting joints at the roof.

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FIELD TRIAL # 1

The first trial involved the application of Mineguard as primary support for a drift being driven along the strike of a steeply dipping 0.2 to O.5-m wide vein of sulphide ore. The drift was excavated as part of a drift-and-fill mining method for the narrow vein and it was the second of many drifts that were planned in order to mine the vein in a bottom up mining sequence. Figure 1 shows the location of the test drift on the 4250 foot level of the mine (1295 m deep). The first cut (drift) excavated was immediately below the left wall of the test drift. The test section of the drift comprised eight rounds (individual blasts) with spans that ranged between 1.5 and 3.2 m and heights that ranged between 2.1 to 2.7 m. The first six rounds were supported with Mineguard and the final two were supported with welded-wire mesh and bolts. The drift was prepared for the Mineguard application after each blast by mucking out the broken rock, scaling any loose rock, and washing the roof and walls. The adhesion of polyurethane to rock can be adversely affected by wet rock. Therefore, each cleaned and washed round was allowed to dry under good ventilation conditions before the Mineguard was sprayed onto the rock. Figure 2 shows the appearance of the test drift after application of Mineguard to the first three rounds. The Mineguard was sprayed on the exposed roof and walls after each round is blasted, mucked,' and washed The face of the drift was not sprayed. A thickness of 4 mm was specified for the roof while only 2 mm of Mineguard was required for the drift walls. Generally the actual Mineguard thickness exceeded these specifications. Mineguard is white in colour, which helps to improve the lighting situation underground.

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