Effects of layering on the mechanical properties of cemented tailings backfill under unconfined compression

This study investigated the mechanical behaviour of layered cemented tailings backfill. Indeed, paste backfill in low stopes can be poured completely at once. However, this strategy is difficult in large stopes since it exerts excessive pressure on the barricade leading to the failure of the barricade. As result, layer-by-layer backfilling poured within a 24-hour interval is adopted. This practice leads to the stratification of the backfill structure which then results in a layered backfill body. Several studies on backfill bodies have neglected the effect of layering on the mechanical strength of the backfill structures. This study attempted to close this gap by incorporating the shear force between the layers in the estimation of the safety factor of the backfill structure. In order to study the mechanical behaviour of layered cemented backfill, laboratory tests, numerical simulations and mathematical modelling were conducted. Uniaxial compressive strength tests were performed using an unconfined compression machine. The OPTUM G3 software package, on the other hand, was utilised for numerical analysis. The limit equilibrium wedge solution was used to evaluate the stability of layered cemented backfill while results from the laboratory tests were used to develop a deformation model of layered cemented backfill. It was concluded from the deformation model that the strength of cemented backfill decreases with increasing layers of backfill at early age (14 days). Nonetheless, the strength of layered cemented backfill gradually increases when more layers are added. The limit equilibrium solution was also improved by incorporating the shear forces acting along the backfill-backfill interfaces and the sliding plane. The limit equilibrium solution also demonstrated a decreasing safety factor when the shear force along the backfill interfaces is included. In contrast, the safety factor decreased with increasing stope height. It is however recommended in future that advanced numerical analysis is explored to develop stability graphs that would predict the waiting period for the backfill body to set till the next blast. This would pave the way for improved safety and productivity of mines.Mining EngineeringPh.D. (Mining Engineering

Analysis of strength and microstructural characteristics of mine backfills containing fly ash and desulfurized gypsum

The utilization of solid wastes (SWs) as a potential resource for backfilling is not only conducive to environmental protection but also reduces the surface storage of waste. Two types of SWs, including fly ash (FA) and desulfurized gypsum (DG), were used to prepare cementitious backfilling materials for underground mined-out areas. Ordinary Portland cement (OPC) was used as cement in mine backfill. To better investigate the feasibility of preparing backfill materials, some laboratory tests, such as uniaxial compressive strength (UCS), scanning electron microscopy (SEM), and energy dissipation theory, were conducted to explore both strength and microstructural properties of backfilling. Results have demonstrated that the main components of FA and DG in this study are oxides, with few toxic and heavy metal components. The ideal ratio of OPC:FA:DG is 1:6:2 and the corresponding UCS values are 2.5 and 4.2 MPa when the curing time are 7 days and 14 days, respectively. Moreover, the average UCS value of backfilling samples gradually decreased when the proportion of DG in the mixture increased. The main failure modes of various backfilling materials are tensile and shearing cracks. In addition, the corresponding relations among total input energy, dissipated energy and strain energy, and stress-strain curve were investigated. The spatial distribution of oxygen, aluminum, silicon, calcium, iron and magnesium elements, and hydration product are explored from the microstructure's perspective. The findings of this study provide both invaluable information and industrial applications for the efficient management of solid waste, based on sustainable development and circular economy.National Natural Science Foundation of China (NSFC) 51804017 Fundamental Research Funds for the Central Universities FRF-TP-20-001A

Initial pore distribution characteristics and crack failure development of cemented tailings backfill under low impact amplitude

The stability of the cemented paste backfill is threatened by the dynamic disturbance during the excavation of the surrounding ore body. In this paper, the computerized tomography (CT) and Split Hopkinson Pressure Bar (SHPB) tests were conducted to explore the initial pore distribution characteristics of the cemented tailings backfill (CTB) and the development of the crack under low impact amplitude. SHPB tests were conducted with impact amplitudes of 34, 37, and 39 mV, respectively. Results show that the initial pores of CTB were steadily distributed with the height of CTB. The CTB contained many initial pores with similar pore size distribution characteristics, and the largest number of pores is between 0.1 and 0.3 mm. Most of the cracks in CTB after low impact amplitude develop and expand along the initial pores, and the damage of CTB mainly exists in shear cracks. A dependence has been established that the dynamic uniaxial compressive strength of the CTB increases, the total crack volume first increases and then decreases, and the number of cracks increases as the impact amplitude increases. The research results can provide a valuable reference for the dynamic performance of CTB under low impact amplitude and the design of mining backfill

Assessment of an expediency of binder material mechanical activation in cemented rockfill

This study is aimed to assess the expediency of applying the binder material mechanical activation in a cemented rockfill (CRF), consisting of ground smelter slag, waste of limestone and rock refuse at one of the largest mines, as well as at any other mines which use these components for CRF. The polynomial dependences have been obtained of strength variation of the CRF, which is used in the conditions of studied mine, on the time of consolidation and the ratio of backfill materials. In the CRF mixtures, the mechanical activation was carried out of the granulated blast-furnace slag, and the compliance has been assessed of CRF with the design strength of the backfill massif. In the studied conditions of the ore mine, with ratio of a binder material to filler of 0.5 and the existing cost of backfill materials, the use of mechanical activation of the binder material according to the two-stage grinding scheme turned out to be insufficiently expedient, since the production cost (materials + grinding) of the most economical backfill mixture is only 2.8% less compared with a basic composition. It is noted that the expediency of using the mechanical activation depends on the remoteness of the mineral raw material base, especially the main inert filler that significantly increases the cost of backfilling works. It is shown that in the operating conditions of other mines with a similar component proportion and a close rich mineral raw base, the mechanical activation of the binder material can be enough effective. It has been determined that with an increase in ratio of Cbin/Cin from 1.0 to 4.6, the difference in costs for the backfill mixture production in the considered compositions, where mechanical activation was performed, increases in a positive direction, but for the most economical backfill mixture, if compared to the basic one, it will be changed from 16.8 to 46.0%. An attention is focused on possible ways to increase the expediency of applying the mechanical activation of the binder material by means of forming the backfill massif with different strength along the height of the stope chamber

Experimental evaluation of cemented tailings backfill strength characteristics and acid mine drainage potential

Following the research & development of Sustainable Mining by Drilling (SMD) technique in the narrow vein mineral deposits, a conceptual framework for backfilling large-diameter drilled holes has been developed, utilizing a mixture of cemented tailings paste. In the paste mixture, the tailings constitute 75% to 85%, the binder makes up 3% to 9%, and the water content represents 15% to 25% of the total weight of dry materials. This thesis contains a comprehensive assessment of various aspects related to cemented tailings backfill (CTB), including the strength characteristics, measurement of dynamic elastic properties using the ultrasonic method, rheological analysis, and evaluation of acid mine drainage potential. To minimize the cement cost in backfill operation, comparative analysis was conducted in the laboratory for the samples of 6% and 4% cement content by weight along with the concerned proportional mixture of tailings and water to achieve the design strength of 1 MPa. The strength characteristics were evaluated following the compressive strength test, stress-strain behaviour and dynamic elastic properties measurement through ultrasonic wave velocity method. Moreover, on the collected backfilled samples from field location, strength measurement and particle size distribution were conducted to understand the effect of particle gradation on strength development. Then, comparative analysis has been described from lab experimental results to evaluate the acid mine drainage potential. From the static test of tailings, the experimental results shows that the tailings has high potential of acid generation. The long-term kinetic test of cemented tailings mixture with variable cement percentage showing the neutralization behaviour of acid generating tailings for the presence of cement components

NUMERICAL ANALYSIS OF STRESS DISTRIBUTIONS FOR MULTIPLE BACKFILLED STOPES

Over the past three decades, technological innovations with respect to cemented paste backfill (CPB) as a means of ground support has allowed for increased production within the mining industry, management mine waste costs, as well as the improvement of the overall health and safety of underground mining operations. Despite the extensive use of this relatively new ground support material, many fundamental factors affecting the design of safe and economical CPB structures are still not well understood.Recently, a significant amount of academic and industry research has been conducted to better understanding the distribution of stress with respect to primary-secondary extraction sequencing for stope-and-fill mining operations. While current, as well as past research, as provided a wealth of knowledge on the distribution of stress through the fill material itself, it lacks in providing an examination into the mechanism by which stress is able to redistribute itself through the backfill material as well as within the surrounding rockmass. The scope of this work is to optimize stope-and-fill extraction sequencing through the analysis of stress distributions as well as local and global stability of multiple narrow verticalfully-drained backfilled stopes. Scientific investigations into the behavior of the CPB material and surrounding rockmass will result in animproved understanding of how to better implement engineered paste-fill materials as a means of ground support for underground mining operations. Numerical simulations (FLAC3D and RocScience) were utilized in analyzing hypothetical (literature) as well as site-specific (field) case studies. While these simulations confirm generalized stress behaviors within the backfill material for single and adjacent stopes, stress redistributions within the surrounding rockmass as well as the rock-pillarindicate the development of tensile and compressive zones. From these results, one is able to better approximate ground and CPB instability with respect to site-specific conditions, geometries, and material properties. These simulations have been validated with respect to published analytical solutions, numerical simulations, and site-measurements for single (isolated) and adjacent narrow vertical fully-drained backfilled stopes

Loading rate effect on the uniaxial compressive strength (UCS) behavior of cemented paste backfill (CPB)

Underground mined-out areas and tailings are the two major harmful consequences from metal mining industry. Cemented Paste Backfill (CPB) method, refilling the tailings back to the underground mined-out areas, has been widely used worldwide. Clear understanding on the uniaxial compressive strength (UCS) behavior of the CPB is very important for the stability analysis of the backfill structure and the parameters optimization of backfill plan. However, the UCS tests can be markedly affected by the testing loading rate. And few papers reported the effect of different loading rates on the UCS of CPB. In this study, the UCS behavior of 14-day cured CPB samples are investigated using five different loading rates: 0.1, 0.25, 0.5, 1, 2 mm/min. The digital image correlation (DIC) method and scanning electron microscope (SEM) analysis were also carried out to help clearly understand the different loading rates effect on the deformation behavior and failure surfaces. The findings show that, the increase of loading rate has a strengthening effect on the UCS. The correlation between the loading rates and the UCS is more in line with an exponential function when the loading rates are between 0.1 and 2 mm/min. During the loading process with different loading rates, the axial deformation characteristics of all the tested CPB samples can be divided into uniform and non-uniform phases. And the loading rates of 0.5 and 1 mm/min resulted the smallest standard deviation for both the axial displacement and the axial strain observed from the three preset monitoring points. As the loading rate increased, the Ca/Si parameter on the fracture surface of the samples also increased slightly. The findings of this research clearly demonstrated the behavior of UCS under different loading rates, and also provided a key research evidence in selecting appropriate loading rate in assessing USC of CPB sample for laboratory tests

Study on performance of Cement Paste Backfill (CPB) with different binder under various situation

The mining process involves the removal and recovery of economically valuable minerals from the earth’s crust. The resulting excavations are commonly revived by a process referred to as backfill. The fill performs as both a support system and a working platform; its different roles determine the requirements of its mechanical properties. Cemented Paste Backfill (CPB) is a high-density slurry composed of dewatered tailings, a cementitious binder and processed mine water, which is thickened to obtain a non-settling character for facile pumping into mined cavities resulted from underground mine operations. Over the past few decades, CPB technology has increasingly been used to revive mined cavities in underground mine operations, owing to its low operating cost. It minimises the adversities associated with tailings disposal, and it has higher mechanical performance compared with other backfilling methods. The present study intends to examine various factors influencing CPB mechanical and rheological properties to get a better understanding of backfill design. The experimental program consisted of the combined capacity of ordinary Portland cement (PC), a newly developed slag-blended cement (Minecem, MC), the binder, and fly ash (FA), the additive, as a sustainable solution towards improving the mechanical performance of a copper-gold underground mine CPB system. A series of unconfined compression tests were carried out on various binder and binder + additive mix designs to evaluate the effect of binder and additive contents and curing time on strength, toughness and stiffness of the CPB system. For some PC + FA mix designs, the failure patterns of the tested samples were also investigated using the three-dimensional DIC technique. Furthermore, the rheological properties of CPB slurry are studied with different concentrations Minecem (MC), Portland cement (PC) and fly ash (FA), under various water and temperature condition using the same tailings material as well. A series of rheometer tests (for yield stress) were carried out on different MC + FA and PC + FA designs for evaluating the effects of binder and additive contents, as well as water content, water type and temperature on the rheological properties of CPB slurry. After placing the CPB material in the stope, and during the setting and hardening processes, the weight of CPB stope applies an axial load over the CPB paste, from the upper layer to the bottom layer of the stope, may result in a consolidation of early-aged backfill material in the lower layers which is called self-consolidation. This self-applied load leads to the strength of CPB specimens obtained by coring from in situ backfill stope are 50% to 200% higher than CPB specimens prepared by using conventional plastic molds over the same curing period. The present study intends to investigate the impact of axial applied stress (As) during curing, which represents the various self-consolidation condition, on the mechanical properties of CPB material. Also, the complicated underground situation may lead to delayed drainage or re-saturation by external water after backfilling. The unsaturated condition will happen due to the irrigation and drain down of backfill material over time. A series of triaxial compression test for CPB consist of MC only were carried out on various saturation condition and confining pressure to evaluate the effect of saturation condition on the mechanical properties of the CPB system.Thesis (Ph.D.) -- University of Adelaide, School of Civil, Environmental and Mining Engineering, 202

A large goaf group treatment by means of Mine Backfill Technology

There are few studies on the management methods of large-scale goaf groups per the specific surrounding rock mass conditions of each goaf. This paper evaluates comprehensively the stability of the multistage large-scale goaf group in a Pb-Zn mine in Inner Mongolia, China, via the modified Mathews stability diagram technique. The volume of each goaf to be backfilled was quantitatively analyzed in the combination of theoretical analysis and three-dimensional laser scanning technology. The corresponding mechanical characteristics of the filling were determined by laboratory testing while formulating the treatment scheme of the large goaf group using the backfill method. The applicability of the treatment scheme using the backfill was verified by the combination of the numerical results of the distribution of the surrounding rock failure zone and the monitored data of the surface subsidence. The research results and treatment scheme using the backfill can provide a reference for similar conditions of mines worldwide.National Natural Science Foundation of China (NSFC) U1906208 China Postdoctoral Science Foundation 2021MD703874 2021M702015 Scientific Research Start-Up Project of University Talent Introduction 205012100