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Cemented paste backfill (CPB) has been widely used as local and regional underground support to reduce host rock wall closure in mined-out areas and also to reduce rockfall and rockburst incidents. However, analyzing the rock mass—CPB interactions in the first month after backfill placement must account for the CPB’s time-dependent strength, stiffness, and volume change characteristics during binder hydration. This article presents the first such comprehensive study made for CPB from the Williams mine in Ontario, Canada. Monotonic isotopically consolidated drained compressive triaxial tests were conducted on cured CPB specimens using the lubricated-ends test technique. The specimens had 3.0–7.5% Cement Content (CC); Curing Times (CTs) were from 3 to 28 days; and the confining stress ranged from 25 to 350 kPa. During shearing, all tests exhibited an initial contractive phase leading to a Characteristic State (CHS or point of volumetric strain reversal) followed by dilation with a maximum dilation rate corresponding to peak stress at the Failure State (FS). Both CHS and FS are adequately described by the Mohr–Coulomb criterion and a framework formulation was proposed to predict the evolution of CHS and FS based on CC and CT. Furthermore, the volumetric strains at CHS and FS can be defined as linear functions of the respective axial strains at the CHS and FS. Quantification of the observed behaviors through these functional relationships can help develop future constitutive models that better represent CPB’s transient response to triaxial stress loading while curing, which is essential to understanding as-placed backfill properties and its interaction with surrounding rock mass.

Cemented paste backfill (CPB) plays an important role in the mining industry due to safety, cost efficiency, and environmental benefits. Studies on CPB have improved the design and application of paste backfill in underground mines. Direct shear is one of the most fundamental parameters for assessing backfill strength. This study harnesses direct shear tests to explore the low confining stress behavior of CPB. We perform all the tests in a standard apparatus on the combination of three binder contents of 4.2%, 6.9%, and 9.7% CPB with four curing times of 3, 7, 14, and 28 days, respectively. The applied confining stress levels vary in a range according to the in situ regime. Results are presented by strength envelope, stress-strain property, and shear strength with curing time and binder content. The data suggest that the shear strength follows the Mohr–Coulomb envelope in which the shear strength and behavior are time and binder content dependent. In addition, the results show that shear strength is strongly related to the binder content than the curing time, namely, the higher the degree of binder hydration, the higher the cementation binding force between CPBs.

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.

Pore water pressure and effective stress development within cemented paste backfill (as one of the most popular local and regional underground mining supports) depends on rates of change of hydraulic conductivity and stiffness, which in turn are functions of cement hydration and backfilling rates. Previous laboratory studies attempted to investigate these interactions; however, the loading conditions they used are not representative of effective stress paths recorded in field monitoring programs during mine backfilling. In this work, typical effective stress paths occurring in mining operations are characterized in terms of an initial period of zero effective stress ranging from 3 to 48 h, and subsequently developed effective stress rates ranging from 5 to 20 kPa/hr. Servo-controlled consolidation machines apply the prescribed stress paths with adjustments every minute, thereby achieving an essentially continuous stress rate. The stress paths are applied to samples with 3.0%, 5.3% and 7.5% binder contents, and electrical conductivity monitoring on control samples is used to correlate the stress levels to stages of cement hydration. The secant constrained modulus is used to quantify the degree to which different stress paths may damage developing hydration products, resulting in softer backfill. For instance, the secant constrained modulus at 2.5% axial strain of samples with 3.0% binder content and loaded at the fastest rate with 48 h delay time was almost half of the ones loaded at the slowest rate and 12 h delay time after 65 h of curing indicating cement hydration products damage due to faster loading rate. The test samples' void ratios are compared with similar CPB’s in-situ void ratios and stress paths. Void ratios obtained from these experiments were very close to the average in-situ values under similar loading conditions. The test results help interpret the as-placed CPB’s bulk properties and will lead to better sample preparation procedures for other tests intended to determine CPB's engineering properties.

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 (Et), tensile strength (σt), 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 mi=UCS/σt the range of available testing data is found to be mi= 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 mi= 4 is found for the materials tested, which is much lower than the value mi= 10 commonly assumed and implies a limiting UCS 60% lower compared to the conventional assumption. It is also found that the relationship between Et 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 Et 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.

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.

External corrosion is a major factor contributing to the deterioration of cast iron water mains; it weakens the pipe wall, which increases the risk of failure. External corrosion is a function of the interaction between the pipeline and the soil that surrounds it. The aggressiveness of soil towards cast iron is affected by soil properties such as resistivity, pH, and the presence of sulphate reducing bacteria. Water main sections and accompanying soil samples were collected from locations across Toronto within the framework of a comprehensive research project over a 2 year period. After careful examination of the effect of each of the soil properties, it appears that soil resistivity has the largest effect on the observed maximum average pitting rate. Limitations to the practical application of the American Water Works Association soil corrosiveness scoring system are also presented. A preliminary spatial analysis of the data indicates that water mains in the district of Etobicoke have had a higher average rate of external corrosion than those in the district of Toronto. Microbiological corrosion could be an aggravating factor in the district of Etobicoke, since areas exhibiting increased levels of sulphide concentration were identified in soils that had originated from this district.Key words: water mains, external corrosion, soil properties, Toronto, cast iron, pipes.

Deep and high-stress mining results in stress transfers onto the previously placed backfill, and mines have recorded several MPa induced backfill stresses. Understanding the backfill-rock mass interaction is therefore critical. Previous work considered tabular ore bodies undergoing primarily one-dimensional compression and showed how the backfill reaction curves could be estimated from oedometer laboratory test results. This work considers massive orebodies and develops a similar approach based on isotropic compression curves. Isotropic compression tests exceeding 6 MPa are carried out on samples with 3.0 to 11.1% binder content, tested at 1-day cure time to 28-day cure time. The compression curve is characterized in three stages: initial elastic compression up to a yield point, followed by a transition stage to the start of a final stage with a linear post-yield compression line in ε v - log p ′ space. Because these isotropic compression tests are rare (the reported results are the first for Cemented Paste Backfill), attempts are made to relate the isotropic compression test parameters to parameters from the more commonly used Unconfined Compression Strength (UCS) tests. Unifying equations as functions of binder content and cure time are found to determine the initial yield stress and the peak strength from UCS tests. These are then related to the corresponding parameters in isotropic compression. Finally, the slope of the post-yield compression line is found as a function of UCS of similar CPB with the same binder content and cure time. Then the isotropic compression behavior of CPB is reconstructed as a function of binder content and curing time using UCS values of similar CPB. Although the calibrated parameters are specific to the studied mine’s materials, the framework is general and applicable to other mines’ CPBs.

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.