Underhand cut-and-fill mining has allowed for the safe extraction of ore in many mines operating in weak rock or highly stressed, rockburst-prone ground conditions. However, the design of safe backfill undercuts is typically based on historical experience at mine operations and on the strength requirements derived from analytical beam equations. In situ measurements in backfill are not commonplace, largely due to challenges associated with instrumenting harsh mining environments. In deep, narrow-vein mines, large deformations and induced stresses fracture the cemented fill, often damaging the instruments and preventing long-term measurements. Hecla Mining Company and the Spokane Mining Research Division of the National Institute for Occupational Safety and Health (NIOSH) have worked collaboratively for several years to better quantify the geomechanics of cemented paste backfill (CPB), thereby improving safety in underhand stopes. A significant focus of this work has been an extensive in situ backfill instrumentation program to monitor long-term stope closure and induced backfill stress. Rugged and durable custom-designed closure meters were developed, allowing measurements to be taken for up to five successive undercuts and measuring closures of more than 50 cm and horizontal fill pressures up to 5.5 MPa. These large stope closures require the stress–strain response of the fill to be considered in design, rather than to rely solely on traditional methods of backfill span design based on intact fill strength. Furthermore, long-term instrument response shows a change in behavior after 13–14% strain, indicating a transition from shear yielding of the intact, cemented material to compaction of the porosity between sand grains, typical of uncemented sand fills. This strain-hardening behavior is important for mine design purposes, particularly for the use of numerical models to simulate regional rock support and stress redistribution. These quantitative measurements help justify long-standing assumptions regarding the role of backfill in ground support and will be useful for other mines operating under similar conditions.

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Initial results are reported on determining the applicability of the toxicity characteristic leach procedure (TCLP) in predicting the potential for heavy metal contamination associated with mining wastes. TCLP tests, variations of the TCLP tests, and baseline tests were run on tailings samples from two mills to determine the sensitivity of laboratory protocol to slight procedural errors and to determine if any inherent factors produced a fatal flaw in using the test to evaluate mine tailings. Results from tailings A showed that metal ion concentration increased with higher liquid-to-solid ratios, with an increased volume of HOAc, and with longer mixing times. Results from tailings B showed the same general trends, but varied significantly in sensitivity to variations. The average yield of the standard TCLP tests as a percentage of the total digestion assay is 20% for tailings A and 10% for tailings B. This is a significant difference between the two types of tailings. Evaluation of these initial results indicates that the sensitivity of the three parameters to laboratory errors is probably acceptable if laboratory procedures are followed with normal attention to detail. These results are only the initial phase of a systematic evaluation of the TCLP method. Future research should emphasize how well the TCLP test actually simulates mine waste contamination phenomena. In particular, studies should focus on the applicability of the extraction fluid, chemical and mineralogical effects in and around the disposal area, time, pH effects, and oxidation effects.

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