ABSTRACT:

Backfill, particularly hydraulically placed, uncemented, deslimed tailings, is being used increasingly for support in the deep level stopes of the South African gold mines. Non-linear stress analyses are described, the results of which show good agreement with observed behaviour. The results of multi-step mining and backfilling analyses are compared with those of single step analyses, and it is concluded that the latter are satisfactory, particularly for comparative situations. Finally, mining at a depth of 5000m, using soft backfill for support, is analysed, and it is concluded that such support will provide adequate support at great depth.

1 INTRODUCTION

The depth of operation of gold mines in South Africa is continually increasing and, at present, the deepest mining production is taking place at some 3400m below surface. Mining at such depths is a very difficult operation - travelling distances are great, transport facilities are limited and transportation of men and materials is time consuming. In addition, temperatures and stresses are very high. Backfill was first used for support in the South African gold mines eighty years ago. Its use was subsequently discontinued, however, and it is only in recent years that it has been reintroduced. Some of the delay in this reintroduction is no doubt due to the results of elastic mining simulation stress analyses, which indicated that significant benefit could only be gained from the use of backfill for support when spans were such that significant closure occurred in the stopes (Ryder and Wagner 1978). However, results of trial filling programmes showed that even soft, hydraulically- placed deslimed tailings fill provided good local support, and appears to be effective under seismic loading. In addition, it allows easier control of ventilation, an important aspect in the very hot mining environment. The long-recognized benefit of improved regional support is also present (Piper and Ryder 1988, Ryder and Wagner 1978). In this paper reported cases illustrating the effective behaviour of hydraulically placed backfill will be summarised briefly. A successful method of analysis for the backfill-rock mass interaction will then be dealt with, and finally the application of this method to the prediction of conditions resulting from mining at a depth of 5000m will be described.

2 REPORTED EFFECTIVENESS OF BACKFILL

Substantiation, by measurement, of the effectiveness of backfill is limited at this stage. There have been several reports, based on observations, of the performance. Gay et al (1988) present two cases in which rockburst effects were recorded. In the one case, gullies in filled panels remained in good condition. Damage was minimal where the fill was close to the face. Fill was being placed at the time of the burst, and the fill paddock was slightly damaged, but no liquefaction of the fill occurred. In the second case conditions remained good in totally filled panels and there was only limited closure. Gulley conditions remained good. In partially filled panels substantial closure occurred.

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