Murray Grabinsky

Estimating the as-placed properties of mine backfill is fundamental to optimizing the safety and productivity of underground mines. For cemented paste backfill (CPB), an important consideration is the extent to which self-weight consolidation during deposition may reduce the void ratio and enhance the binder’s effectiveness. Field monitoring and sampling campaigns can help investigate this phenomenon but they are expensive and logistically difficult. Therefore, mesoscale column experiments are performed in the controlled laboratory environment to better understand the coupling between self-weight consolidation and cement hydration. In this paper, columns are backfilled with uncemented paste tailings and with CPB and the pore water pressure, electrical conductivity, and volume changes are monitored during and after backfilling. The pore water pressure profiles at the end of backfilling are compared with an available analytical solution and restrictions on the solution’s validity are identified. Void ratios of samples taken from columns are compared with the ones obtained from field samples and the similarity indicates the filling conditions simulated in the laboratory are representative of field conditions. The changes in void ratio after curing were small compared to the initial void ratio of fresh CPB. The tests’ results help explain why self-weight consolidation during backfilling is not significant at the studied mine, which may be the case for many other mining operations as well.

Tensile strength is a crucial parameter involved in the design and analysis of cemented paste backfill (CPB). The ability of CPB to withstand tensile forces is essential for the stability of the backfilled stopes, particularly in areas with high stress or deformation. The tensile strength is a critical design parameter used in sill mats to perform underhand cut-and-fill operations. This study presents a novel technique that utilizes rectangular dog-bone specimens and compression to tensile load converters to perform the direct determination of tensile strength. This study indicates that the prevailing assumption regarding the ratio of unconfined compressive strength (UCS) to tensile strength (i.e., 10:1 or 12:1) underestimates the strength. The results suggest a ratio closer to 3:1 or 4:1. The findings indicate that the ratio varies with the curing interval. Specifically, the tensile-to-compressive strength ratios were higher in early-age specimens, as tensile strength values do not increase at the same rate as those of compressive strength. This disparity has notable implications, as underestimating tensile strength via traditional UCS-to-tensile strength ratios could potentially inflate binder consumption. Our study underscores the importance of using direct tensile strength measurements to optimize mining operations.

The Unconfined Compressive Strength (UCS) test is the most widely used method in the underground mining industry to assess strength development during binder hydration. However, testing errors are common for weak materials (UCS less than 100 kPa, i.e., cemented paste backfill at early curing stage), and reliable test methods must be established for designs relying on early age strengths, particularly for continuous pours. Therefore, the standard geotechnical laboratory vane shear test is investigated as an alternative. A newly designed “cast-in-place” mold is used to avoid specimen damage arising from vane insertion. Parametric tests with vane rotation speeds of 30 to 120 degrees per minute show strength insensitivity to these shear rates. Vane shear strengths correlate linearly with UCS over the strength ranges used; however, the trend does not follow the usual assumption for clays in the undrained state, because the material does not shear at a constant volume as clays do. The influence of suppressed dilation is investigated using direct shear tests, but these results do not fully explain the discrepancy, suggesting other mechanisms also have an important influence. Therefore, vane shear tests can provide a useful complement to UCS tests not only at the laboratory scale but in the in situ condition for early strength characterization. It should be noted that the correlation to UCS must be considered empirical, and separate calibrations are required for each backfill material.

Underground mining operations need nondestructive test methods to assess placed backfill strength development at early age (especially up to 1 week of curing time) so that they can continue proximate mining quickly. Recent developments in electrical conductivity (EC) transducers for field applications offer this possibility, but the EC measurements must be correlated to backfill strength. To determine the feasibility of this approach, a laboratory test program used a mine’s backfill materials mixed with varying water and binder contents and tested these over a 7-day period. Strength was characterized using unconfined compressive strength (UCS) tests and correlated to EC. Comparisons were also made to complementary test results using Vicat and heat generation measurement techniques. Strong and consistent correlations were determined between EC and UCS for a given binder content and water content. But moreover, a lower-bound correlation was determined and could also be used in the field if as-placed properties vary to an unknown extent within the limits of parameters considered in the study. This provides the basis to use EC measurements in the field and confidently assess the backfill’s minimum attained strength in real time.

Mechanical characterization is important to the design and analysis of cemented paste backfill (CPB) structures. Unconfined compressive strength (UCS) tests have been widely used owing to their relative simplicity to characterize a material’s response to unconfined compressive loading. However, the UCS represents a single strength parameter and does not fully describe the material’s strength (or failure) envelope. In this study, we analyzed UCS tests with direct shear and uniaxial tensile strength tests conducted on the same CPB materials to provide mechanical characterization of CPB under a more complete range of loading conditions. The results demonstrate the Mohr–Coulomb failure envelope provides a consistent description of strengths arising from the three different test methods. Furthermore, a better estimate of the tensile strength is UCS/4, which is considerably higher than the conventional assumption that the tensile strength is equal to USC/10 or UCS/12. This has a significant impact on the assessed required strengths particularly for undercut designs using Mitchell’s sill mat analysis method and suggests that in future the conventional UCS tests should be complemented with direct tension and direct shear tests to improve underground designs using CPB.

Cemented paste backfill (CPB) plays an increasingly important role in the mining industry due to its operational and environmental benefits. CPB is placed in the mined-out stope to form a self-supporting structure. The strength and stability of the CPB is of great concern in its engineering applications. Indeed, CPB must remain stable during the extraction of adjacent stopes to ensure the safety of the mine operations. Although significant research has been conducted on the shear properties of CPB, there are limited studies on its post-failure behavior, in particular the yield characteristics of CPB. This paper presents the finding on the post-peak and yield property of CPB. The study is conducted on three cemented contents and six stress intervals based on the mining practice and field study. The results show that CPB exhibits dilative behavior under strain softening and contractive property under strain hardening conditions. Our study demonstrates that pure frictional resistance could exceed the cohesion strength at high stress levels.

The first one to four days constitutes a critical period in filling mining voids with cemented paste backfill (CPB, a mixture of mine tailings, binder, and water) and determining the saturated hydraulic conductivity (ksat) during this period is important to rational engineering design. However, most published studies started testing 24 h after specimen preparation, and those that started earlier did so by preconsolidating the specimens, which results in void ratios lower than those occurring in the field. This study uses a new test method that retains the backfill’s representative bulk properties and starts testing about one-half hour after specimen preparation. During backfilling, CPB undergoes three distinct ksat stages: a relatively constant high-value ksat stage associated with the portland cement’s (PC’s) dissolution phase; a rapidly declining ksat stage associated with PC’s acceleration phase; and then a slowly declining ksat stage associated with PC’s steady-stage and subsequent deceleration phase. The last two stages can be represented by modified Kozeny-Carman equations, in which the hydration effects are constant but different in each stage, and the ksat changes can be related to void ratio changes in each stage. In contrast, the distinct stages are not adequately represented by traditional PC “maturity” models, and, more importantly, the ksat values determined here are at least an order of magnitude higher than previously published results on similar materials.