With the placement of mine infrastructure in undercut entries beneath cemented rockfill (CRF), a better understanding is needed of the long-term strength of CRF and also the long-term stability of CRF undercut spans. A previous case study conducted by researchers from the National Institute for Occupational Safety and Health (NIOSH) in cooperation with Placer Dome, Inc. at the Turquoise Ridge Mine in 2001 has provided a unique opportunity for investigating both of these issues. During this study, six parallel, adjacent drifts were mined and backfilled to construct a CRF sill that was partially undercut, successfully creating a stable 13.7-m (45-ft) wide by 30.5-m (100-ft) long span beneath the backfill. Instruments were installed at selected locations to monitor the geomechanical behavior of the host rock and backfill. This paper provides an overview of the original backfill span study, explains the response of the instruments during mining, and analyzes the long-term stability of the undercut span based on current instrument readings. Conclusions regarding the stability of the host rock and backfill are supported by the results of long-term compressive and tensile strength tests conducted with CRF samples from the test site and by an analytical analysis of the flexural stability of the CRF undercut span. By documenting the long-term strength properties and geomechanical behavior of CRF, this research will hopefully enhance the design of stable, long-term undercut spans beneath CRF.

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