Influence of faulting on a mine shaft—a case study: part I—Background and instrumentation

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Abstract

The impact of geological structures on the stability of underground infrastructures is well recognised. Moreover, the influence of major geological structures such as faults, becomes critical when the stability of underground infrastructures which are essential for mine production, are threatened. The X41 shaft is the man and supply shaft at the Copper Mine, at Mount Isa, Australia. Here, there has been observed evidence of degradation manifested by the development of cracks in the shaft concrete lining since the early nineties. In addition, the shaft steel structure is being deformed and needs regular and meticulous maintenance. The shaft degradation has been attributed to the presence of two major geological structures, the W41 and W42 faults, which intersect the shaft in two distinct locations.

This paper presents a case study whereby the causes of shaft degradation were examined. The influence of faulting and mining sequence on the stability of the main mine shaft were investigated by means of field investigations and numerical modelling. This paper concentrates on the field investigation performed as part I of this project. It provides a review of work done prior to this study and exposes the past field monitoring practices and those that were implemented to characterise the observed displacement in the X41 shaft.

Introduction

In a mine environment, the size and shape of excavations, the presence of geological structures and the existing stress regime influence the stability of underground infrastructures. In mature mines, the influence of geological structural features and the modified stress regimes on the stability of underground excavations is even more dominant.

Copper Mine is one of five mines operated by MIM at Mount Isa in Australia. The X41 shaft is the man and supply shaft at the Copper Mine. It has exhibited evidence of degradation, manifested by cracking in the side walls. This has been attributed to the influence of faulting in the vicinity of the shaft. Since the X41 shaft gives a direct access to the Copper Mine, it is essential that it remains operational.

The primary goal of this investigation was to assess the degree of degradation of the X41 shaft and to quantify, if possible, whether it could potentially jeopardise the access to the Copper Mine. Such a turn of events would result in severe consequences for the mine. At a technical level it was of primary importance to identify and understand the mechanisms that induce damage to the shaft. Mine experience had suggested a series of possible causes. This work aimed to conduct a full-scale instrumentation study and to analyse and interpret the results.

Section snippets

Description of the mine site

Copper Mine is one of five mines operated by MIM at Mount Isa, located in Northwest Queensland, Australia, Fig. 1. It has been in continuous operation since 1942, save for a period from 1947 to 1952. As of December 1997, 255 million tonnes at 3.3% Cu has been extracted from the Copper Mine. Total orebody extraction is close to 90%.

At Mount Isa, all the known orebodies are found in the Urquhart Shale Formation. Robertson [1] reports that the Urquhart Shale Formation is truncated at depth by

Description of the case study

The X41 shaft provides access to the 1100 copper orebody. The X41 shaft is a circular downcast (air flows from surface down) shaft with a 6.1 m diameter and a length of approximately 1010 m. The single deck cage section dimensions are 5.6 m by 2.5 m. The cage runs at a winding speed of 7.6 m/s on four steel guides, Fig. 3 [3]. The X41 shaft concrete lining has an average thickness of 0.5 m. The access platforms are on the following working levels: 7, 9, 11, 13, 15, 17, 19B and 20.

The X41 shaft is

Shaft damage assessment

A preliminary investigation of the reported and observed shaft damage was conducted. The investigation includes a compilation of the shaft maintenance record data, a review of studies characterising the mining induced shaft displacement occurring along the faults intersecting the X41 shaft, diamond drilling and underground observations.

Field monitoring

The X41 shaft has experienced some degree of deterioration since the end of the eighties. The deterioration appears to be driven by fault displacement over discrete periods of time and is triggered, or is accentuated, by mining activities. To better understand the mechanisms that are damaging the shaft, many attempts were made to monitor the shaft induced displacement. This section presents an overview of what has been installed and measured over the years and highlights data invaluable to a

Monitoring results

All field monitoring indicate that displacement is occurring along both W41 and W42 faults. A broken glass slide and potentiometer (mounted on bar across one fault) readings taken, show evidence of displacement. In addition, shaft steel guide rails and concrete lining show signs of vertical tensional displacement around the 11 level and vertical compressional displacement near 17 level. Displacement taking place along the faults intersecting the X41 shaft can be related (over time, frequency

Assessment of methods and instruments used for this study

Several approaches have been used at the Copper Mine in order to assess the influence of the faults on the integrity of the shaft structure. Table 7, Table 8 show a subjective assessment, enjoying the benefit of hindsight, of the relative success of the different methods and instruments used to investigate the X41 shaft deterioration.

In order to evaluate the methods and instruments used for the X41 shaft study, a rating was assigned based on the cost, the installation ease, the data collection

Discussion and conclusions

Cracking of the shaft concrete lining was firstly attributed to mining induced displacement along the W41 fault and an associated splay. Diamond drilling results and underground observations suggested that what was perceived to be a splay is in fact a distinct fault, subsequently referred to as the W42 fault.

Field monitoring has been performed since 1993 in an attempt to characterise the observed displacement in the X41 shaft. The influence of two faults intersecting the shaft on this

Acknowledgements

This work was undertaken at a time when the authors were employed at Mount Isa Mines. The authors would like to acknowledge the contributions of mine personnel at the Copper Mine and Mining Research. The authors are grateful to the management of Mount Isa Mines for allowing the publication of this paper.

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