Abstract:

Cemented rockfill (CRF) is commonly used in conjunction with underhand cut-and-fill mining methods to provide ground support in weak rock conditions, particularly in the underground gold mines in Nevada. Because miners work directly beneath the backfill at these operations, a thorough understanding of the material properties of the in-place CRF is needed in order to design safe undercut spans beneath the fill. To assess the quality and strength of the backfill, unconfined compression tests are typically conducted with standard-sized samples of CRF—15×30-cm (6×12-in) cylinders. However, depending on the preparation and testing of the CRF sample, the resultant unconfined compressive strength (UCS) may not be representative of the in-stope fill. To address this issue, the National Institute for Occupational Safety and Health (NIOSH) has been conducting research in cooperation with several underground mines in United States to develop a better means of relating the material properties of in-stope CRF to test results obtained from standard-sized samples.

This paper presents the results of tests conducted with CRF samples from Nevad Gold Mines’ Goldstrike Operations near Carlin, NV and compares these findings with similar tests conducted with CRF samples from other mines. Strength and elastic properties are reported for tests conducted with CRF samples ranging in size from 15×30 cm (6×12 in) to 46×91 cm (18×36 in). The effects of sample size, density, maximum aggregate screen size, and curing age are discussed along with the consequences of unintended changes in mix design during sampling. Developing better methods of relating the strength of CRF samples to the properties of the in-stope material should lead to more clearly defined target strengths, more appropriate factors of safety, and thus, safer backfill mine designs.

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Josef Bourgeois