This paper describes research to improve safety during transport of ore and waste in underground mines. Field tests are underway in mines in Idaho and Montana. Strains measured on structural support members in an ore pass provided information aboutotal forces acting on the structure as material was dumped into it. Results show that measured static loads were considerably less than actual total weight of the material dumped and that dynamic loads were subject to many factors, such as effects of blasting to remove hang-ups. Comparisons of measurements and computeresults using a particle flow code indicated that several difficulties remain before achieving realistic determinations and models of the dynamic effects of particle flow in ore passes and impact loads on the gates. Impact loads were overestimated in computer analyses as compared to loads measured in field tests. An alternative design approach based on softening the chute and control gate assembly is being proposed.
Hazards related to the operation of ore and waste rock passes have been identified as a significant safety problem in underground metal mines in the United States. Such hazards include structural failures, blocked gates, and water flow. Specifically, ore or waste rock hang-ups can collapse spontaneously or during freeing operations; the sudden release of hangups is the single most important cause of serious accidents. The dynamic loads induced by large falling blocks of ore or waste rock can weaken the chute and gate structure. Blocked gates can result in spillage of large volumes of material. Damage can also be caused from an air blast as material is released. Water flowing into an ore pass can result in catastrophic muck flows and inundation.
Existing design standards for ore passes are essentially rules-of-thumb based on simplified equilibrium analyses, model experiments, empirical observations and experience. This approach tends to assign high safety factors to the chute and gate structure so it can withstand excessive static and dynamic loads.
Ore pass design has structural and functional components, with one affecting the other and vice versa. The structural components are ore pass walls, liners, timber lagging, and chutes and gates, which control the flow of material. The functional component is concerned with the flow, or lack thereof (hang-ups), of ore and waste.
Important structural design factors are the static and dynamic loads that ore pass chutes and gates must withstand. Blight et al. (1994) conducted tests on model underground ore passes to determine factors associated with static gate pressure and dynamic loads. The effects of ore pass length, inclination, and the presence or absence of doglegs, which absorb impacts from the release of hang-ups, were determined.
The results indicated minimal change in static load when the material column exceeded a depth about 1 m above the gate, and that total static load and dynamic load factors decreased significantly when the inclination was less than 70øi They also found thathe presence of a dogleg had little effect on the static gate, and that peak impact loads could exceed four times the static load in vertical or near-vertical ore passes. Their conclusions were that static loads on the gate of an ore pass could be predicted accurately using equations developed by Janssen (1895) for vertical or inclined silos.
