At underground mines where cemented backfill is used for ground support, backfill strength properties are an important design consideration, particularly for underhand cut-and-fill mining operations where employees work directly beneath the placed fill. Following a backfill roof fall at a deep underground silver mine, standard tests were conducted to determine the strength and elastic properties of a typical paste backfill composed of cemented mill tailings. Unconfined compression tests and direct and indirect tensile tests were conducted with core samples obtained from paste fill slabs recovered from the roof fall. Test results indicated that the average tensile strengths determined by indirect methods (Brazilian and splitting tensile tests) were about twice the average tensile strength measured by direct tensile tests. To identify cold joints within the backfill, in situ direct tensile tests were also conducted on one of the larger backfill slabs using experimental test equipment. The results of these in situ tests were similar to the direct tensile tests and provided little evidence of additional cold joints within the slab. Elastic properties of the paste backfill were determined through compression tests with strain-gauged core samples. The results of this study are significant because they add to the sparse strength and elastic property data that are available for mine designs utilizing paste backfill.


Ground falls are typically the leading cause of fatalities in underground metal mines and a significant source of lost-time injuries [1]. As a result, a well-developed ground support plan needs to be consistently implemented to safely mine under these conditions. In deep underground metal mines, underhand cut-and-fill mining methods are used to mine narrow, steeply dipping veins of ore [2, 3]. In some of these mines, a paste backfill composed of cemented mill tailings is used to support the mined-out stopes or cuts. This cemented backfill forms a massive beam that provides a safe, stable back or roof for the miners who work beneath it on subsequently deeper cuts. The stability of this engineered beam is largely determined by two variables—its thickness and strength. Figure 1 shows computed factors of safety for twelve hypothetical backfill beams using four values of thickness and three values of strength [4]. As indicated by these curves, the thickness of the backfill beam is the primary immediate concern, but the backfill’s strength becomes more important as the thickness of the beam increases.

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