A longstanding mine backfill design challenge is determining the strength required if the (partially) cured backfill is subsequently undercut. Mitchell (1991) called the undercut backfill a sill mat and proposed an analytical solution that is still often used, at least for preliminary design, and has motivated subsequent empirical design methods. However, fully employing the Mitchell sill mat solution requires knowledge of the backfill material’s Unconfined Compressive Strength ($UCS$), tangent Young’s modulus ($E_t$), tensile strength (${\sigma }_t$), as well as estimates of stope wall closure. Conducting a high-quality $UCS$ test poses challenges but relating the test result to the remaining material parameters is more difficult. Some new material testing data is presented and compared to available published results. Using the parameter $m_i=UCS/{\sigma }_t$ the range of available testing data is found to be $m_i=$ 3 to 22, however, the most compelling data is obtained when the Mohr’s failure circle in tension is tangential to the corresponding Mohr–Coulomb failure envelope determined from other strength tests. In these cases, the value $m_i=$ 4 is found for the materials tested, which is much lower than the value $m_i=$ 10 commonly assumed and implies a limiting $UCS$ 60% lower compared to the conventional assumption. It is also found that the relationship between $E_t$ and $UCS$ is described by a power function that is close to linear, but the values for the constant and exponent in the power function depend on the material tested. However, for given tailings the power function is found to be independent of void ratio, binder type or concentration, curing time, and water salinity, within the ranges these parameters were investigated. Therefore, when $E_t$ is used in the Mitchell sill mat solution it should be correlated with the $UCS$ using the appropriate power function. These correlations are then used with the Mitchell sill mat solution and published measurements of backfill closure strains to estimate the Mitchell solution’s range of applicability based on its underlying assumptions, and a similar analysis is extended to an “empirical design method” motivated by the Mitchell sill mat solution. It is demonstrated that these existing approaches have limited applicability, and more generally a full analysis in support of rational design will require numerical modeling that incorporates the effect of confining stress on the material’s stiffness and mobilized strength.

This article presents the results of a laboratory study on the variation of specific gravity (Gs) and void ratio (e) in a cemented paste backfill (CPB) with different cement contents and curing times. CPBs consist of a mixture of mine tailings, portland cement, and mine process water. In this study, a helium stereopycnometer device was used to determine Gs for specimens having cement contents of 3.0, 5.3, 7.5, and 11.1 % by the weight of dry tailings. Curing times of 4 h, 1, 3, 7, 14, and 28 days were considered for testing purposes. In total, 240 helium stereopycnometer tests were conducted in this study, which shows that Gs decreases in the CPB specimens as specimens cure over time and the cement hydrates. Furthermore, 96 tests were conducted to determine e of the CPB specimens with different combinations of cement contents and curing times. The results of these tests indicate that similar to Gs, e decreases with an increase in curing time as the specimens hydrate. The reduction in Gs and e is attributed to the formation of hydration products, which have a lower density than the cement and fill the void space between the particles. The results show that the variation of Gs that is due to the hydration process should be considered in the calculation of e; otherwise, this physical property, especially for specimens with high cement contents, is overestimated.

Continuously poured paste backfill dramatically improves underground mining efficiency through reduced stope cycle time and simplified logistics. For longhole stopes, a backfill “plug” is poured to a few meters above the undercut brow and must gain sufficient strength to prevent failure through the plug when the “main” pour begins. A novel, rational engineering design approach that determines the required plug strength is developed. The potential failure mechanism during continuous pouring is identified and the theoretical solution and its numerical validation/calibration are discussed. Four field case histories are then used, three of them involving continuous pours, to demonstrate the theoretical solution’s validity in back-analysis. These case studies are unique in the extent and quality of total stress and water pressure measurements made throughout backfilling. Additionally, comprehensive laboratory data are available to characterize strength development during binder hydration in the first few days, which are critical to the back-analyses. Results indicate that continuous backfilling is feasible with reasonably attainable backfill strengths at most mines. However, mines must undertake comprehensive early strength laboratory testing, and must carry out field measurements during the pour to ensure the placed backfill behaviour is consistent with the analysis assumptions.

The tensile strength is an important parameter in the design and analysis of cemented paste backfill (CPB) for underground mining. Although traditional axisymmetric dogbone-shaped specimens have been used to directly determine the tensile strength of rocks and concretes, such methods are not practical for CPB because of its much lower strength and associated difficulties in shaping the specimens. To determine the tensile strength of CPB, a castable rectangular dogbone specimen is developed, the apparatus including a compression to tension load converter, and a four-part split mold. The designed apparatus is validated using numerical analysis and shown through testing applications to be relatively simple, practical, and reliable. Results illustrate that the primary design allows the direct determination of tensile strength and characterization of the stress-strain behavior of isotropic materials. The stress-strain behavior could be reliably correlated with unconfined compression strength test results. The data show how the tensile strength is dependent on curing time and cement content. These results can lead to better mine backfill designs that fundamentally improve the underground mine stability.