The paper discusses the reasons for the introduction of new mining system. Details of investigations to determine in situ rock mass strength are presented. Back analysis of existing mine tunnels based on visual tunnel observations, diamond drill core analysis and numerical simulations and the RFC-system for estimating tunnel conditions in deep gold mines yielded similar rock strength values. Hoek-Brown method of rock mass strength determination resulted in unrealistically low uniaxial compressive strength and relatively low global compressive strength values. Behavior of 7 m wide stope pillars during a full mining cycle comprising of development of 5 m high top and bottom headings, removal of 11 m high sill pillar between these headings, backfilling of stope and mining of adjacent stope has been monitored using multiple rod extensometers. Results of a full stoping cycle are presented. The beneficial effects of early backfilling of the high stopes have been demonstrated.

1 Introduction

At the Breitenau mine a string of magnesite lenses of irregular shape having strike and dip extensions of up to 200 m and a height of up to 100 m are mined using post pillar mining method. Pillar and room dimensions are about 7.5 m. The height of individual lifts is 3.5 m. After completing a lift it is backfilled using gravity fill. Up to 35 lifts have been mined in some of the larger magnesite lenses. With increasing mining depth rock pressure problems have been encountered. For this reason it was decided to change the stoping method in the deeper areas of the mine from post pillar mining to a cut and fill method where 70 m to 80 m long 6 m to 7 m wide and 21 m high stopes are excavated and subsequently back filled using paste fill. The stopes are separated by 7 m wide pillars. Between mining panels of four to six stopes wide barrier pillars are left in place to ensure regional stability.

In contrast to the post pillar mining method where the pillar workings extend over the full areal extent of the mineral body with the new method only one stope is excavated in a mining panel at any moment in time. After backfilling a stope the adjacent stope is excavated. Mining sequence in a stope is such that at the top and bottom of the stope 5 m to 6 m high headings with a width corresponding to the stope width are developed. The sill pillar between the two headings is then mined by bench blasting. The advantages of new stoping method are that the stope does not have to be entered by mining personnel and that only one stope is mined in a panel at any moment in time. This paper describes the rock mechanics investigations that have been carried out in connection with the introduction of the new stoping method.

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