ABSTRACT:

The stability of cemented backfill systems is a prime concern and has been the subject of considerable research mainly due to (1) the high costs of stabilizing cemented fills, (2) the extensive economic losses associated with loss of production and ore dilution resulting from fill mass failure, and (3) safety problems. Conventional design approaches such as analytical or numerical models, backed by trial-and-error methods, are used to describe or predict the support performance of backfills during mining. Such approach offers inherent limitations and generally results in conservative designs; this is particularly true when designing backfill systems with paste fill, a relatively new form of backfilling. This paper presents an integrated design approach for paste backfill systems design based on three inter-related engineering approaches: centrifuge modeling, analytical modeling and numerical modeling. This design process, illustrated in a case study performed for establishing stable free vertical fill faces for an operating underground hard rock mine, can be effectively used to (1) describe or predict the support performance of backfills during mining, and (2) assess arching effects, failure modes and fracture mechanics involved in fill mass failure.

1. INTRODUCTION

The optimum recovery of ore in modern and highly productive mining methods requires the use of cemented backfills to provide structural support in underground excavations. Cemented backfills are utilized not only as ground support during adjacent mining but as sillmat elements to support excavations during overhead mining or sill pillar recovery. In order to save on costs, cemented backfill of high strength (sillmat), overlain by low or no cement content fill, is used to fill mined out stopes. Mining excavations progress under the sillmat or at the adjacent stope, either way, the backfill must remain stable when exposed and subjected to mine induced stresses. It is imperative that the stability behaviour of the backfill support systems be carefully studied to provide very effective, safe and economic mining operations. Improper design of mine backfills may result in fill mass failure and in extensive economic losses associated with loss of production and ore dilution, and in safety problems. The unique integrated design approach for paste backfill design would constitute the most practical means of rigorously and accurately engineering economic and effective fill systems. In the integrated methodology, centrifuge model studies are combined with analytical and numerical modeling analysis not only to describe or predict the support performance of backfills during mining, but also to understand the behaviour, potential failure modes, and fracture mechanics involved in fill mass failure. Although the approach is developed to (1) establish free-standing heights for different paste fill recipes and stope dimensions, (2) assess the stability of distinct sillmat designs, and (3) engineer economic paste fill recipes for diverse mining conditions and applications, the case study presented in this paper is an analysis of stable free vertical fill faces. In the case application, analytical modeling was carried out using limiting equilibrium analysis based on a method introduced by Mitchell et al [4] for the stability analysis of exposed vertical fill faces. Numerical modeling was carried out using FLAC3DM (Fast Lagrangian Analysis of Continua), a powerful three-dimensional elastic plastic-finite difference code developed by Itasca Consulting Group.

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