Engineering Design of Backfill Systems in Undercut Mining

9th International Symposium on Mining with Backfill
Philip Dirige,
Abstract In order to maximize the recovery of ore in modern and highly productive mining methods, cemented backfill is normally placed in underground excavations to provide structural support. In order to save on costs, backfill of low cement content is used to fill the mined out excavations. This backfill mass is supported by a sillmat structure cast from cemented backfill of very high strength. Mining excavations progress under the sillmats, which must remain stable when exposed and subjected to mine induced stresses. The stability behavior of the sillmat elements must be carefully studied to provide very effective, safe and economic mining operations. Improper design of these support structures may result in catastrophic fill mass failure, with consequent losses of production, ore dilution, and in safety problems and substantial economic losses.

This paper presents a methodology, procedures and engineering standards for backfill design based on three engineering modeling approaches: analytical, numerical and centrifuge modeling. Analytical model designs were carried out using limiting equilibrium analysis. Numerical modeling was carried out using FLAC3D (Fast Lagrangian Analysis of Continua), a powerful three-dimensional elastic plastic-finite difference code. The centrifuge was an important physical modeling tool used not only to dynamically test backfill performance on a time-dependent basis but also to accommodate the three-dimensional aspects of the problem. This novel integrated modeling approach represents a powerful design tool in engineering practice.

Application of the design approach is demonstrated from an engineering design study conducted for an underground gold mine, aimed at minimizing backfill binder content and at producing cost efficient paste fill recipes for sillmat construction. The study aimed at establishing the effect of excavation geometry, excavation wall roughness and varying backfill heights on fill stability behaviour, when exposed or undercut during mining. Models were developed to conform with two stope conditions applied underground: stopes 3 m wide, 15 m long, and 30 m high, and stopes 5-7.5 m wide, 15 m long, and 40 m high. In all cases the stope walls were inclined at 75° and smooth, medium-rough and rough rock wall conditions were established for simulating typical excavation boundary modes encountered in the mine. Paste fill was prepared at 80% pulp density using unclassified tailings mixed with Type 10 Normal Portland cement (NPC) and Type C fly ash (FA). All sillmat recipes were prepared at 7% binder content; backfill recipes were prepared at 2.5% and 3% binder contents. The backfill was cured for 14 and 28 days before testing. The results of all three modeling techniques suggested that the 2.5% binder content paste fill supported by a 7% binder sillmat seems to be appropriate for the simulated mining conditions. Different modes of sill failure were exhibited by the physical and numerical modeling techniques. Centrifuge modeling indicated that approximately one-half to three-quarters of the fill column would plunge into the undercut (slip failure) when subjected to failure stresses. In contrast, numerical modeling indicated a rotational failure about the footwall contact. Application of the engineering design recommendations will result in annual cost savings to the mine in the order of hundreds of thousands of dollars.
Keywords: sillmat, centrifuge modeling, undercut mining
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