In most underground mines material transfer relies on ore and waste pass systems. A major challenge is ensuring that the ore pass systems remain operational. The unexpected failure of an ore pass can result in major problems to a mine including closure. Ore pass longevity is influenced by ground conditions; ore pass configuration, operational considerations and ground support. Based on several case studies a methodology has been developed to estimate ore pass longevity.
1. INTRODUCTION
The profitability of a mining operation is strongly influenced by the reliability of its material handling system. In underground mines, ore and waste pass systems provide a low cost method for the gravity transport of material through large vertical distances. The stability of underground excavations is often quantified as a function of stand-up time, i.e. the time an excavation can remain stable without support. A more appropriate measure of the longevity of an ore pass is the amount of material that can be safely transferred without necessitating re-habilitation or major intervention. In estimating the costs for repairing, or in extreme conditions replacing an ore pass, it is necessary to consider both material and labour but also costs associated with delays in production. Such delays can have significant monetary consequences. It is an accepted fact that ore pass longevity depends largely on such design factors as rock mass quality, ratio of field stress to rock mass strength and support. However, operational parameters are equally or perhaps more important to the economical viability of an ore pass. Once the ore pass is actually commissioned, design parameters should dictate the way the ore pass is operated. Unfortunately, it would appear that very few ore pass systems are operated as per their design specifications. Consequently, quite often mine specific ore pass operation practices have quite an adverse impact on the longevity of ore pass systems. There are no reliable design tools to predict the longevity of an ore pass. This paper discusses the limitations of existing tools, and introduces an empirical ore pass longevity estimation procedure.
2. CONSIDERATIONS IN ORE PASS DESIGN
2.1. Ground Conditions
The influence of ground conditions on the stability of raise bored shafts has been investigated by McCracken and Stacey [1]. Using the Q system [2] as an indicator of rock mass quality, they introduced a series of modifications to account for wall stability rather than the roof, excavation orientation relative to structural features and rock degradation. The relationship between rock mass and ore pass operational failure has also been investigated in 10 underground mines [3]. This field study defined a "failed" ore pass section if it had expanded to twice its initial volume as recorded in the original layout. In this study ore pass expansion was quantified using cavity monitoring surveys, volume reconciliation from actual tonnage capacity and/or comments from mine operators. The performance of ore passes is summarized in Table 1, making a distinction between supported and non supported ore pass sections.
Table 1. Performance of ore pass sections in 10 underground mines. (available in full paper)
