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Effect of delayed backfill on open stope mining methods Caceres Doerner, Cristian Andres

Abstract

Open stoping is comprised of large rectangular voids separated by intervening pillars so as to minimize the size of the exposed surface and thereby reducing the potential for wall slough and in turn external dilution. These pillars provide support to the exposed wall; however, they result in ore loss and increased costs such as having to establish slots for blasting. Longhole mining methods such as Avoca or longhole retreat as practiced at the Musselwhite mine of Placer Dome employs 100% extraction with the use of fill walls to provide support to the adjacent stope. Transverse open stoping also practiced at Musselwhite employs cemented rock fill adjacent to a mined stope. The question is how to account for the overall stope wall stability when the adjacent support is backfill. The backfill does not provide the same support as that of a rock pillar, however, due to the increased use of fill abutments one has to develop a methodology that accounts for this reduced overall support element as it does reduce the overall stope surface exposed. It has been shown in this thesis that the backfill wall does not provide the same overall stability to an individual stope as would avail itself if the stope had rock abutments. This is the focus of this study in order to establish design criteria to enable one to employ existing methods for stope design such as the Stability Graph by augmenting input parameters that have been calibrated through field measurements, analytical assessments, numerical modeling and laboratory testing to evaluate the effect of mining adjacent to a backfilled stope. Sill pillars are employed at the Musselwhite mine to allow for multiple mining horizons with unconsolidated backfill placed immediately above the intervening sill. These sills can be comprised of unmined ore when the economics are such as to negate their mining or alternatively they are replaced by a constructed sill mat to allow for mining underneath by non-entry methods and thereby containing the overlying backfill. Numerical studies were conducted to investigate modeled results of mining under a cemented rockfill sill mat and to develop criteria for sill mat design. The results obtained from this analysis can be extrapolated to other operations that utilize backfill as part of the mining sequence. Design curves were developed for the stability of sill mats for various stope configurations and cemented rockfill strength properties This, coupled with defining the effect of mining adjacent to backfill, forms the focus of this thesis.

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