Publications

The presence of cracks within the cemented paste backfill (CPB) matrix can stem from multiple factors, such as excessive pressures from CPB overburden, stresses induced by the closure of adjacent rock walls, rockbursts, and shrinkage. These cracks significantly compromise structural integrity and mechanical strength while increasing permeability and impacting safety, durability, and environmental performance. While the self-healing technique has been extensively explored to repair cracks and reduce maintenance costs in conventional cementitious materials (eg, concrete, mortar), limited studies have delved into CPB’s self-healing capability. This study aims to explore the self-healing (autogenous healing) behavior within CPB materials, evaluating the healing efficiency in restoring mechanical strength and hydraulic conductivity.

CPB samples were pre-cracked at different levels of pre-cracking to initiate a variety of crack widths after initial curing. Subsequently, these samples underwent self-healing periods ranging from 7–90 days. The results revealed a significant self-healing capacity within CPB materials. After 7 days of self-healing, precracked specimens restored mechanical and permeation properties to levels comparable to uncracked control specimens without external intervention. Moreover, CPB specimens with high pre-cracking levels even exhibited up to a 22% increase in mechanical strength compared to control specimens after a 90 day self-healing period. This suggests that cracks within the CPB matrix can enhance hydration reactions, thereby favoring improved self-healing performance. These findings provide valuable insights into CPB’s self-healing behaviour, holding significant implications for engineering CPB structures, evaluating their safety, durability, and serviceability, and offering sustainable solutions for the mining industry.

The increase of mining tonnage throughput and the variation of tailings mineralogy in a mature mining complex can bring new challenges to underground operations that depend on cemented paste backfill (CPB). The LaRonde mining complex in the Abitibi region (Quebec, Canada) is facing this challenge, as the mill throughput includes an increasing proportion of satellite zone material with the Volcanic-Massive-Sulphides (VMS) ore. The inclusion ore from the LZ5, 11-3 and Frange zones introduced new geometallurgical tailings signatures that can vary daily. These variations affect the backfill operation and the overall CPB strength development.

Historically, LaRonde VMS tailings mineralogy consisted of various tectosilicate (quartz, anorthite, microcline), sulphates (gypsum) and a large proportion of sulphides, with up to 27.5% of pyrite. This tailings mineralogy did not significantly affect CPB quality prior to 2018. This mineralogy was inert at early curing age and did not affect the hydration dynamics of cementitious reactions. In mid 2018, the LaRonde mill started introducing a low proportion of LZ5 ore in its millfeed,; following this, there was a significant increase in phyllosilicate mineral (mica and clay) content whose impact was not caught initially. This new tailing mineralogy containing a larger concentration of muscovite, clinochlore and chlorite affected CPB strength development. Depending on the ratio of LaRonde and LZ5 material making up the tailings, the required binder percentage had to be modified to achieve the same UCS strength for the same curing time.

This paper considers how tailings mineralogy variability in a mature mining complex can impact CPB quality, operational lessons-learned, and how data analytics are used to maintain backfill quality.

In the mine backfill industry, the typical replacement ratio, a number that measures the ratio between the mass of the ore mined and the mass of tailings placed underground, is ~ 50%. In other words, approximately half of the tailings cannot be returned underground. The remainder of the tailings not returned underground are most commonly stored on the surface within a tailings management facility (TMF). NexGen Energy has proposed an innovative solution for its Rook I Project (the “Project”) which would enable placing 100% of the process waste solids underground. Through constructing purpose-built underground chambers, the replacement ratio can be further increased beyond the traditional limits. This method also requires designing a backfill system that can reliably handle all streams of waste solids from the process plant regardless of the particle size and mineralogy. Utilizing a systematic set of material experiments and engineering studies, multiple paste backfill mixes have been designed in consideration of the geotechnical and geochemical requirements for supporting mining operations, ground stability and permanent storage. This paper discusses the considerations, the approach, and the lessons learned from advancing a Project with zero process waste on surface.

In the Abitibi-Témiscamingue mining region in Canada, underground mines utilize cemented paste backfill (CPB) for secondary ground support. The conventional CPB formulation employs a binder known as the reference binder (RB), consisting of 20% general use Portland cement (GU) and 80% ground granulated blast furnace slag (GGBFS). This reference binder (RB = 20 GU/80 GGBFS) demonstrates excellent mechanical and hydro-geotechnical properties for CPB. However, its high cost, limited availability of GGBFS, and the environmental impact associated with GU production highlight the critical need for research into alternative, cost-effective, and environmentally friendly binders.This study explores the use of electrical arc furnace slag (EAFS) and circulating dry scrubber dust (CDSD) from the steelmaking industry to partially replace GGBFS and act as a substitute for GU cement. The EAFS undergoes a processing stage involving screening and grinding, resulting in a product termed ground EAFS (GEAFS). Various formulations of the ternary blended binder (GGBFS/GEAFS/CDSD) are evaluated for their unconfined compressive strength (UCS) at 7 and 28 days to assess their suitability as binders for CPB. Our results demonstrate the feasibility of completely replacing type GU cement and substituting up to 30% of GGBFS in the RB without compromising the UCS at 28 days (UCS28d). These highly promising findings suggest the potential to lower the cost and carbon footprint of CPB while promoting the recycling of metallurgical waste within the mining industry, aligning with a circular economy approach.

The sudden failure of the dormant iron ore tailings storage facility (TSF) near Brumadinho, Brazil, on January 25, 2019, the tragic loss of 270 lives, and subsequent regulatory, societal, and mining industry responses hold important implications for the planning and operation of tailings harvesting at surface, for mining backfill. This paper describes some of the new responsibilities for tailings harvesting schemes, with particular focus on the risks, safety, and stability of TSFs as harvesting takes place. GISTM 2020, ICMM 2021 and several associated publications require more detailed scrutiny of risk, free prior and informed consent (FPIC) for stakeholders, renewed attention to assessment of potential liquefaction, slope stability, management of surface water, risk-informed design and new environmental, health and safety precautions for operators.

Effective and efficient use of binders is critical to optimizing underground backfill performance while minimizing operating costs. Controlled laboratory studies demonstrate the strong influence of binder efficiency (ie, strength development) on bulk properties (eg, density or porosity). While it might be hoped that backfill self-weight consolidation during placement will increase binder efficiency, there are several high-quality field studies that indicate this self-weight consolidation mechanism may be inconsequential. The underlying causes for the lack of consolidation are studied in detail in the laboratory using Cemented Paste Backfill (CPB) from Williams mine. A specialized hydraulic conductivity test quantifies permeability reductions due to binder hydration on samples with as-prepared bulk properties. A servocontrolled oedometer quantifies stiffness changes with variable time delays to the onset of effective stress development and with variable effective stress loading rates. A 1.8 m high column test with carefully controlled boundary conditions and representative backfilling rates is used to simulate the full-scale response expected in the topmost layers of deposited backfill. The column test results are interpreted using Biot-type analysis for accreting sediments, modified to incorporate time-dependent material properties. The backfill void ratios determined from the column tests are consistent with field observations, and the integrated interpretation of laboratory physical and numerical test results is that the enhanced backfill strength and stiffness due to hydration occurs faster than the onset and rate of effective stress development as pore water pressures dissipate.

While the results are specific to the Williams mine CPB, the result probably have broad implications for other mines, because the Williams fieldwork demonstrated that its CPB is one of the fastest to develop effective stresses during placement. Therefore, all mines should carry out fieldwork to quantify any selfweight effects that may occur at their mine if they intend to rely on such effects to increase binder efficiency.

The paper describes the design, realization and 10 years of operation of the filtration part of a mine backfill project/operation in Europe. It begins with the first step, the filter sizing based on a sample provided by the customer, followed by the commissioning of the filter units and reports on the development of the filter performance during the following 10 years of operation.

The paper first describes the physical properties of the sample and the process targets of the client for the backfill plant. It shows the logic steps taken in the laboratory to first determine the pressure difference (vacuum) and the filter setting required to reach the moisture target. It further describes how the filtration area gets optimized using flocculant and filter rotational speed while still keeping the moisture below target level. In the last sizing step, the ancillary units such as vacuum pumps were sized based on the test results and finally the performance guarantee was given as well based on these lab test results.

After supply and installation of the filter, the commissioning started. The paper discusses some issues with filter operation during the commissioning phase, possible solutions discussed at that time and final measures that have been taken. Finally, the paper concludes by highlighting the gradual change of the filter feed observed over the more than 10 years of operation and how this change affected filter performance and operating costs.

The success of a paste underground distribution system (UDS) is dependent on the proper installation, operation and maintenance of its boreholes. The boreholes that are required over the life of mine, in particular surface boreholes, represent both the greatest cost and risk to the UDS.

The hydraulic and operational aspects of paste boreholes have been presented in multiple publications. However, the design, installation and maintenance aspects are not widely discussed. The decision to proceed with a certain borehole design is typically based on the local project team’s knowledge, or the local equipment available to drill and install the borehole infrastructure. Often time, a particular borehole design is selected without a complete understanding of the later operational and maintenance challenges that will arise from the design decisions.

This paper will cover the major design aspects of paste boreholes. Installation and maintenance aspects will be covered in another paper. The purpose of this paper is to present both advantages and disadvantages of the various design options so that the reader may make informed decisions before implementing a particular solution for their paste project

Cemented rockfill (CRF) is commonly used in conjunction with underhand cut-and-fill mining methods to provide ground support in weak rock conditions, particularly in the underground gold mines in Nevada. Because miners work directly beneath the backfill at these operations, a thorough understanding of the material properties of the in-place CRF is needed in order to design safe undercut spans beneath the fill. To assess the quality and strength of the backfill, unconfined compression tests are typically conducted with standard-sized samples of CRF as 6 × 12 in (15 × 30 cm) cylinders. However, depending on the preparation and testing of the CRF sample, the resultant unconfined compressive strength (UCS) may not be representative of the in-stope fill. To address this issue, the National Institute for Occupational Safety and Health (NIOSH) has been conducting research in cooperation with several underground mines in the United States to develop a better means of relating the material properties of in-stope CRF to test results obtained from standard-sized samples.

This paper presents the results of tests conducted with CRF samples from Nevada Gold Mines’ Goldstrike Operations near Carlin, NV, and compares these findings with similar tests conducted with CRF samples from other mines. Strength and elastic properties are reported for tests conducted with CRF samples ranging in size from 6 ×12 in (15 × 30 cm) to 18 × 36 in (46 × 91 cm). The effects of sample size, density, and maximum aggregate screen-size are discussed along with the consequences of unintended changes in mix design during sampling. Developing better methods of relating the strength of CRF samples to the properties of the in-stope material should lead to more clearly defined target strengths, more appropriate factors of safety, and, thus, safer backfill mine designs.

Vale Base Metals' Manitoba Operations currently use hydraulic backfill, comprised of classified mill tailings, to fill underground stopes at their T1 and T3 mines. A new sandfill plant was built to service the planned future mining of the T3 mine. Due to the distance of the T1 sand plant from the future T3 mining horizons, studies showed that a new backfill plant was better suited to service the T3 stopes. Based on material availability, mill tailings dewatering capabilities, location of backfilling, and costs, a hydraulic backfill plant was selected as the preferred method of backfill. This plant is designed to use a combination of alluvial sand and classified tailings for backfill.

This paper provides a case study with regards to the recipe selection, plant design/selection, and commissioning of a new hydraulic backfill plant at Vale Base Metals’ Manitoba Operations. The laboratory testing, design, and commissioning experience will be presented, along with the key design parameters of the new plant.

It is now uncommon for a cemented paste backfill underground distribution system to not have some sort of pipeline instrumentation. The common types of instrumentation are pressure and flow meters. While these instruments are useful and can be used to both control and design the distribution system, it is hard to check the overall performance of the system with something external to the system.

This paper presents a case study using an in situ pipeline instrument called a Piper developed by INGU. This instrument is actually a small cluster of instruments that monitor pipeline pressure, acceleration, rotation, magnetic flux, and acoustic emissions. The Piper is used to measure the flow behaviour within the pipeline, as well as attempting to determine possible wear areas and determining potential leaks.

The case study area is the reticulation network at the Dead Bullock Soak Mine located at Newmont’s Tanami Operations. The paper includes discussion trial setup and approach, including deployment and retrieval. It also provide an analysis of the Piper data in comparisons with the reticulation network instrumentation and the operational flow model. Piper data will also be tied into operational observations.

To ensure a safe mining environment during pillar recovery in mines, a new mine backfill method known as the geofabriform cofferdam mine backfill method was proposed based on mining environment regeneration theory, and validated through field industrial experiments. The results indicate that the multipoint backfill mode can enhance the roof-contact effect of backfill when utilizing the cofferdam as the working platform for goaf backfill. The use of a geofabriform cofferdam can improve an unfavorable mining environment. Additionally, the geofabriform cofferdam mine backfill method could yield significant economic benefits, prevent ground subsidence, and contribute to enhanced ore recovery and environmentally sustainable mining practices.

This case study discusses a uniquely challenging design and implementation of a reclaimed tailings paste fill project. The mine operator identified the need for paste fill in a new ore zone and chose to refashion an incomplete hydraulic fill plant into a reclaimed tailings paste fill plant. A partially complete 850 m borehole was planned, however many challenges were encountered in the completion of the borehole. The borehole was abandoned at the latter stages in the project and an overland pumping and shaft pipe system was engineered to tie into the already complete underground paste reticulation. Underground distribution pushes the limits of gravity distribution with > 1.6 km of horizontal pipework and only 850 m of gravity head available.

Design and implementation was completed on a fast-track schedule which presented many challenges in undertaking design whilst implementing and preparing for operation in keeping with the stringent requirements of a top-tier mining company. The overall system design pushes the technical limits of generating quality paste fill from reclaimed tailings and transporting it through an arduous pumping and gravity reticulation network, where small changes in rheology have dire consequences on the flow of paste.

Mineral resource depletion drives the need for mining in increasingly remote areas using novel methods. Severe conditions in these remote areas can include permafrost. The Chidliak diamond mine of DeBeers, situated in the Arctic Circle, is one such operation. There are many challenges to Arctic mining projects, such as transportation limitations and severe climate conditions. Furthermore, the Chidliak mine has been planned to have the lowest possible carbon footprint. The mining operation applies a novel vertical boring technique from the surface, by which the vertical kimberlite pipe with a diameter of approx 125–150 m and to a depth of 400 m will be excavated. Each boring operation results in a vertical shaft of approx 5 m diameter which is subsequently backfilled. Cement or binder is omitted from the backfill mixture due to transportation cost and adverse effects of low temperature on cementation. Instead, a frozen backfill is employed where water serves as the binder as it freezes. As part of an overall investigation into backfill design, rate of backfill deposition, and mine planning (sequencing of shaft boring and deposition), this paper explores and discusses the thermal behavior and freezing time of the novel backfill technique. The temperature and ice saturation of two filled shafts over time are examined, providing an initial understanding of designing shafts across the entire mine area and optimizing their construction based on climate conditions.

Cemented paste fills (CPF) are composed of full-stream (usually) tailings, water, and a cementitious binder. The fines in the tailings and binder create a matrix which supports the coarser particles allowing the CPF to be transported to an underground stope via a mine’s underground reticulation network. However, what happens when the usual supply of tailings is disrupted and the only viable source of ‘tailings’ aggregate is significantly coarser than the usual tailings? This paper summarizes observations of a project conducted as part of an operational necessity to replace tailings fines with binder.

The project was conducted at Newmont Porcupine’s Hoyle Pond (HP) mine in Timmins, Canada. Tailings for HP are harvested during the summer and a surplus is stockpiled for use during the winter. Due to operational constraints, insufficient tailings were stockpiled. To keep up with mine production, HP utilized a CPF whose mix consisted of silty sand, binder, and water. This paper explores the development of the blend in comparison to the operation’s usual blend, operational observations during the CPF pours, and provides a comparison of the performance between its standard paste mix design and the project’s colloquially named “super paste.”

Cemented rockfill (CRF) is being used to backfill primary and secondary stopes at the Eagle Mine, an underground nickel and copper mine operated by Lundin Mining. The National Institute for Occupational Safety and Health (NIOSH) Spokane Mining Research Division (SMRD) is partnering with Eagle Mine to research the advancement of mine backfill QA/QC guidelines through batching and strength testing method studies, specifically through investigation of existing methods that best determine in situ properties of CRF. As part of this collaborative research, NIOSH researchers traveled to Eagle Mine and worked with mine staff to trial multiple 6 in QA/QC CRF cylinder preparation methods including ASTM C31, ASTM C1435, and a drop hammer compaction method to identify which method can best correlate to the in situ strength of CRF placed underground with the least operator bias and highest confidence. A total of 60 cylindrical samples (6 in) of CRF were cast at the mine and cured underground, in accordance with the three-cylinder preparation methods,. Samples were later transported to the SMRD laboratory to determine the strength and elastic properties of the CRF. Data collected from this study will help identify which test cylinder method provides consistent density and strength results that can better correlate with in situ density of CRF.

The term ‘arching’ has been used in the context of granular materials in (semi-)rigid containers, to describe the reduction of vertical stress compared with one-dimensional overburden calculations. The primary mechanism for the reduced vertical stress is shear resistance generated between the granular material and container sidewalls. This assumes that granular material shear resistance is fully mobilized along the contact surfaces, which is probably reasonable for uncemented material but may be unrealistic for cemented materials. A review of the best available mine backfill field data indicates Cemented Paste Backfill (CPB) can hydrate and gain significant strength and stiffness before pore water pressures dissipate and effective stresses develop within the backfill. Therefore, it is possible the response to developing effective stress is elastic, and so stresses will develop along the sidewalls during elastic loading, ie, pre-failure. The implications of elastic backfill behaviour are investigated using numerical simulations of stope filling and it is found that for tall stopes there is a maximum vertical centerline stress ≅γL (where γis the backfill unit weight and L is the stope span) and this is virtually fully developed at H= 2.5 L (where H is the stope height). A normalized equation is proposed to describe the centerline vertical stress for elastic backfills, and this is compared to several published case histories where highquality field monitoring data are available. The predictive equation reasonably captures the available field data within bounds of ±40%, whereas a conventional arching solution fit to match the “infinitely deep” vertical stress value γL captures only the lower-bound field measurements. The implications of these findings are particularly relevant to predicting the “main fill” stresses that may occur on CPB “plugs” that are subsequently undercut, and this design issue is considered in detail in terms of application to undercut backfill strength assessments.

Many practitioners use Mitchell’s (1991) method to assess backfill strength requirements for stability of undercut backfills, or other empirical methods motivated by Mitchell’s approach. There are, however, case histories of stable undercut backfill with strengths significantly less than the assessed strength requirements from these methods. Here, we consider the case of vertical orebodies and use a systematic numerical investigation to show that Mitchell’s method only captures the first stage of a stable progressive failure mechanism. Subsequent failure stages depend in part on the assumed backfill strength criterion, but recent laboratory investigations of direct tensile strength and compressive strengths under low confining stress are used to constrain the most likely subsequent failure modes and their corresponding imposed stress levels. The results can be normalized in terms of two ratios: 1) backfill Unconfined Compressive Strength (UCS) to driving stresses (self weight and imposed loads); and 2) the undercut span (L) to depth (d) of the backfill sill mat (also called the plug, or backfilled drift, depending on the design context). Numerical modelling, Mitchell’s equations, and empirical approaches give a similar required strength assessment for L/d ~ 1.0, but the results progressively diverge as L/d increases with the modelling approach giving the lowest strength assessments. Notably, the assessed strengths from modelling are consistent with the lowest values found in published case histories. Also, the modelling results indicate that flexure is always the critical failure mode; the other failure modes postulated by Mitchell (sidewall sliding and tensile caving) do not manifest in the numerical modelling results. While modelling in this study has some restrictive assumptions (ie, vertical orebodies, no rock mass closure), the results explain why previous design methods appear to over-estimate strength requirements, particularly for backfills with large undercut span to backfill depth (L/d) ratios.

Mine backfilling, such as cemented rockfill (CRF), is a critical step of the underground mining cycle, enabling increased resource recovery and a resilient tailings storage solution. The use of chemical binders is critical to achieve the desired strength and stability of CRF mix designs; however, traditional cement binders can be responsible for approximately 70% of the greenhouse gas emissions in the entire mine backfill process. Most large mining corporations have made commitments to achieve net-zero greenhouse gas emissions in the next 20 years, so the development of low carbon alterative binders for mine backfill is a necessary step to achieve this goal. This paper shows results obtained through the development of a versatile engineered lime-based binder for mine backfill systems that can replace up to 50% of cement, with no compromise in long-term performance, while reducing the greenhouse gas footprint by up to 75% in comparison to traditional cement. The performance of the newly developed binder showed its adaptability as it was successfully reformulated to be used with a non-standard cement formulation used by mining operator’s cemented rockfill operations.

The presence of discontinuities (cold, batch, flow, slick, slurry, or water joints) within cemented paste backfill (CPB) are known to have an impact on the stability of underhand stope exposures. Currently, there is limited understanding of the mechanical properties of these internal discontinuities. A geomechanical testing program has been completed that measures the tension response of synthetic discontinuities that have been created in cemented paste backfill in the laboratory. Three-dimensional modelling has been completed to demonstrate the effect on stability from the presence of discontinuities within a CPB fill mass that create backfill beams. It has been shown that when a backfill beam height is > 70% of the span width, its presence is unlikely to cause instability issues. As such, it is advised that this value be considered as the minimum thickness of a continuous plug pour to minimise dilution.