Development of Specimen Curing Procedures that Account for the Influence of Effective Stress During Curing on the Strength of Cemented Mine Backfill

Paste backfill used to provide ground support in underground mining is generated from full-stream tailings and is almost always placed underground with cement. For the backfill, both the rate of strength development and the final strength are important considerations for design, particularly when the backfill is subsequently exposed in the stope-mining sequence. There is strong evidence that strengths measured on specimens obtained from coring the in situ cemented backfill are much greater than laboratory-cured specimens with the same cement content. The paper reviews some of the experimental evidence showing that one of the major reasons for the different strength is the difference in effective stress acting on the backfill during curing. Laboratory specimens are (almost) always cured under zero total stress, so no effective stress develops. In contrast, backfill in a stope may cure under high effective stress, which develops due to either “conventional” consolidation in free-draining backfills, or to the so-called “self-desiccation” mechanism in fine-grained fills. Evidence is presented showing how the final strength is affected by applying stress to specimens at different stages of curing and at different rates. It is shown that a fully-coupled analysis of the filling behaviour is required to determine the appropriate effective stress regime to apply in curing laboratory specimens, where “fully-coupled” in this context means taking account of the interaction of consolidation/drainage rate, filling rate and cement hydration rate. Curing protocols for laboratory specimens are proposed, which would ensure that the strengths obtained are representative of in situ conditions.     

Modeling the heat development in hydrating CPB structures

Cemented paste backfill (CPB) is a mixture of dewatered tailings, hydraulic binders and water. In addition to contributing to the stability of mine workplaces, CPB greatly benefits the environment by minimizing surface tailings disposal. Hence, it has become one of the most commonly used ways in mine backfilling around the world. Temperature can significantly affect the mechanical properties of cemented backfill. A source of heat in CPB is produced by binder hydration. Hence, a FLAC based numerical model is developed to predict and analyse the heat developed by hydrating CPB structures. To validate the model, results of the developed model are compared with three case studies (mathematical, laboratory, and field investigations). The validation results show a good agreement between the developed model and these cases. The effects of stope geometry, thermal properties of both rock and CPB, filling rate, binder content and initial boundary conditions are also investigated.     

Design and Application of Underground Mine Paste Backfill Technology

This paper reviews the design and application of paste backfill in underground hard rock mines used as ground support for pillars and walls, to help prevent caving and roof falls, and to enhance pillar recovery for improved productivity. Arching after stope filling reduces vertical stress and increases horizontal stress distribution within the fill mass. It is therefore important to determine horizontal stress on stope sidewalls using various predictive models in the design of paste backfill. Required uniaxial compressive strength (UCS) for paste backfill depends on the intended function, such as vertical roof support, development opening within the backfill, pillar recovery, ground or pillar support, and working platform. UCS design models for these functions are given. Laboratory and backfill plant scale designs for paste backfill mix design and optimization are presented, with emphasis on initial tailings density control to prevent under-proportioning of binder content. Once prepared, paste backfill is transported (or pumped) and placed underground by pipeline reticulation. The governing elements of paste backfill transport are rheological factors such as shear yield stress, viscosity, and slump height (consistency). Different models (analytical, semi-empirical, and empirical) are given to predict the rheological factors of paste backfill (shear yield stress and viscosity). Following backfill placement underground, self-weight consolidation settlement, internal pressure build-up, the arching effect, shrinkage, stope volume, and wall convergence against backfill affect mechanical integrity. An erratum to this article can be found at     

Coupled Modeling of Temperature Distribution and Evolution in Cemented Tailings Backfill Structures that Contain Mineral Admixtures

Cemented paste backfill (CPB, a mixture of tailings, water and binder) is widely utilized to fill underground mine voids. To achieve a good, economical performance, one approach is to proportionally use mineral admixtures such as fly ash and slag as partial substitutes for Portland cement. Binder hydration is one of the most significant factors that can generate heat within hydrating CPB structures, which in turn, influences the mechanical and hydraulic properties of CPB, as well as the pore structure within CPB. However, the temperature evolution due to the hydration of Portland cement that contains fly ash or slag is different from that of hydration with solely Portland cement. Hence, in consideration of the heat generated by both binder hydration and transferred between CPB and its surrounding media, a numerical model is developed to predict and determine the temperature development within CPB that contains mineral admixtures. After that, data from field and laboratory studies are employed to validate the developed model. The validation results demonstrate a good consistency between the model and the field and laboratory studies. Consequently, the proposed model is applied to simulate and determine the temperature evolution with time via mineral admixtures, binder content, initial rock and CPB temperatures, stope geometry, backfilling rate, curing time and backfilling strategy. The obtained results will contribute to better designs and preparation of CPB mixtures, as well as predict the temperature distribution within CPB structures.     

Unsaturated hydraulic properties of cemented tailings backfill that contains sodium silicate

Recycling the mine waste (tailings) into cemented tailings backfill has economical and environmental advantages for the mining industry. One of the most recent types of cemented tailings backfill is gelfill (GF), a backfill that contains sodium silicate as chemical additive. GF is typically made of tailings, water, binder and chemical additives (sodium silicate gel). It is a promising mine tailings backfill technology. From a design point of view, the environmental performance or durability of GF structures is considered as a key factor. Due to the fact that GF structures are cementitious tailings, their durability and environmental performance depend on their ability to resist the flow of aggressive elements (water and oxygen). Thus, understanding the unsaturated hydraulic properties of GF is essential for a cost-effective, environmentally friendly and durable design of GF structures. However, there is a lack of information with regards to unsaturated hydraulic properties of GF, the factors that affect them and their evolution with time. Hence, the unsaturated hydraulic properties (water retention curve (WRC) or water characteristic curve, air entry value (AEV), residual water content, unsaturated hydraulic conductivity) of GF are investigated in this paper. GF samples of various compositions and cured in room temperature for different times (3, 7, 28, and 90 days) are considered. Saturated hydraulic conductivity and microstructural tests have been conducted; WRCs are measured by using a WP4-T dewpoint potentiameter and the saline solution method. Unsaturated hydraulic conductivity is predicted using the van Genuchten (1980) equation. The water retention curve (WRC) is determined as the relationship between volumetric water content and suction for each GF mix and curing time. The van Genuchten (1980) equation is used to simulate the WRC to best-fit the experimental data. AEV and residual water content are also computed for each mix and curing time. Furthermore, functions are developed to predict the evolution of AEVs, residual water content and fitting parameters of the van Genuchten model with degree of hydration. Important outcomes have been achieved with regards to unsaturated hydraulic properties. The unsaturated hydraulic conductivity of GF was calculated to decrease when the suction, binder content, and degree of hydration increase. The effects of binder content and degree of hydration are more obvious at low suction ranges. The obtained results would contribute to a better design and assessment of the durability and environmental performance of GF structures.     

A coupled chemo‐viscoplastic cap model for simulating the behavior of hydrating cemented tailings backfill under blast loading

Although the use of blasting has become a routine in contemporary mine operations, there is a lack of knowledge on the response of cement tailings backfills subjected to sudden dynamic loading. To rationally describe such a phenomenon, a new coupled chemo‐viscoplastic cap model is proposed in the present study to describe the behavior of hydrating cemented tailings backfill under blast loading. A modified Perzyna type of visco‐plasticity model is adopted to represent the rate‐dependent behavior of the cemented tailings backfill under blast loading. A modified smooth surface cap model is consequently developed to characterize the yield of the material, which also facilitates hysteresis and full compaction as well as dilation control. Then, the viscoplastic formulation is further augmented with a variable bulk modulus derived from a Mie–Gruneisen equation of state, in order to capture the nonlinear hydrostatic response of cemented backfills subjected to high pressure. Subsequently, the material properties required in the viscoplastic cap model are coupled with a chemical model, which captures and quantifies the degree of cement hydration. Thus, the behavior of hydrating cemented backfills under the impact of blast loading can be evaluated under any curing time of interest. The validation results of the developed model show a good agreement between the experimental and the predicted results. The authors believe that the proposed model will contribute to a better understanding of the performance of cemented backfills under mine blasting and contribute to evaluating and managing the risk of failure of backfill structures under such a dynamic condition. Copyright © 2015 John Wiley & Sons, Ltd.     

超细全尾砂材料胶凝成岩机理试验

矿山充填是保护土地资源、生态环境,实现矿山无废开采和消除重大安全隐患的理想途径。以某矿全尾砂为试验材料,在分析该矿全尾砂材料的化学成分、粒径级配组成等基本物理、化学特性基础上,借助XRD能谱分析和电镜扫描（SEM）方法,得到不同条件下的超细全尾砂材料胶凝成岩微观规律。对比以水泥、固结剂1#和固结剂2#分别为胶结剂时充填体的强度结果表明,在灰砂配比、浓度和龄期相同的条件下3种胶结材料充填体的强度大小为：固结剂1#＞固结剂2#＞水泥;固结剂1#可替代水泥作为矿山全尾砂胶结充填的胶结剂,且价格比水泥便宜,有利于降低矿山充填成本。对不同条件下的充填体强度曲线进行拟合,得到充填体的单轴抗压强度增长规律：在养护龄期28 d之内,充填体单轴抗压强度增长规律随龄期变化基本相同,皆遵循指数函数曲线增长规律;当以固结剂1#、2#分别作为胶结材料时,强度增长规律与水泥为胶结剂时相同;强度增长曲线趋势与灰砂配比、料浆浓度以及养护龄期正相关。     

A solution to estimate the total and effective stresses in backfilled stopes with an impervious base during the filling operation of cohesionless backfill

Mining backfill is commonly used in underground mines. A critical concern of this practice is to evaluate the pressures and total stresses in backfilled stopes to ensure a safe and economic design of barricades, constructed to retain the backfill. When a slurried backfill is placed in a mine stope, excess pore water pressure (PWP) can instantaneously generate and progressively dissipate. The dissipation of the excess PWP and consolidation lead to the development of effective stresses, which in turn lead to an arching effect in the backfilled stope. Until now, arching effect has been largely considered for stress estimation in dry or submerged backfill. The former corresponds to the final state at the end process of the drainage and consolidation of the backfill with a pervious while the latter with an impervious barricade. However, previous studies have shown that the most critical moment for the stability of barricades is during the stope filling. Therefore, the design of barricades requires a proper estimation of the pressure and total stresses during the filling operation. This in turn needs joint consideration of the arching effect and consolidation of the backfill. In this paper, a new solution is developed to evaluate the pressures and stresses in backfilled stopes during the filling operation of cohesionless backfill by considering the self-weight consolidation and arching effect. The proposed solution is validated by numerical modeling with Plaxis2D. It can thus be used to evaluate the pressures and stresses in backfilled stopes during the stope filling with an impervious barricade.     

Lateral Variation of the Vertical Stress in Underground Mine Stopes Filled with Granular Backfills

Marston’s theory and its modifications are widely used to predict the average vertical stress variation with depth within mine stopes. However, this does not model the lateral variation in vertical stress at a particular depth. In this study, a mathematical expression to simulate the vertical stress variation is developed using the experimental shear stress data of granular backfill. The developed model is validated against average vertical stress measured in the experiment. Therefore, the developed model has the advantage of determining both the average vertical stress and its distribution respectively, at a particular depth and a cross sectional area of the mine stope.     

A New Analytical Solution for the Stress State in Inclined Backfilled Mine Stopes

There are several good reasons for using backfill in underground stopes, including a reduction of mine wastes on the surface and the improvement of ground stability. Backfilling is now commonly used in underground operations worldwide, so practical methods are required to assess the stress state in stopes, on the surrounding rock mass and on support structures. The majority of existing analytical solutions for the stresses have been developed for vertical openings. In practice, stopes often have inclined walls, and this affects the stress state. Recent numerical studies have shown how the stresses distribution in inclined backfilled stopes is influenced by stope geometry and backfill strength. It has also been shown that existing analytical solutions do not capture the essential tendencies regarding these influence factors. In this paper, a new solution is proposed for the vertical and horizontal stresses in backfilled stopes with inclined walls. This solution takes into account the variation of the stresses along the opening height and width, including the difference between the hanging wall and footwall, for various inclination angles of the walls. Key results are presented and validated using recently performed numerical simulations.     

Relevance of SEM to Long-Term Mechanical Properties of Cemented Paste Backfill

This paper is an attempt to relate the microstructure to long-term mechanical properties of the cemented paste backfill produced from a hard rock mine tailing from North Queensland in Australia bound with flyash-based geopolymer (geopolymer), flyash-blended cement (FBC), and general purpose cement (GPC). A relatively high slump (260 mm) paste backfill mix with 74 wt% solids has been used to prepare cylindrical paste backfill samples with a diameter of 50 mm and a height of 100 mm. The uniaxial compressive strength tests were conducted on all samples after curing for 112 days to obtain their strength, failure strain and Young’s modulus. Fractured samples were examined under scanning electron microscope to understand the failure mechanisms at the microstructural scale. The results show that binders significantly affected the mechanical properties of paste backfills (ANOVA, p < 0.05). The paste backfill bound with geopolymer gave the lowest strength and Young’s modulus, while the paste backfills bounded with FBC and GPC showed comparable higher strength and modulus values. This was attributed to the relatively well-packed paste backfills with less cracks and smaller pore sizes in these paste backfills bound with FBC and GPC binders. In particular, needle-shaped particles, which were originally identified in GPC, highly influenced the mechanical property of paste backfills. These results indicate that fly ash can be used to partially replace the cement as a binder for paste backfills to achieve economic and environmental benefits.     

添加絮凝药剂的尾矿砂浆充填材料的单轴抗压强度试验研究

尾矿充填主要包括3种类型：尾矿砂浆充填、尾矿糊状充填（paste fill）和废矿石充填（rock fill）,其中前面两种充填属于水力充填,第3种属于干式充填。3种充填方法在加拿大的矿山都有应用。压缩强度和渗透性是水力充填材料的关键力学特性,直接影响井下充填的效果。一方面,为了使充填材料具有一定的强度,必须在尾矿砂浆（slurry backfill）材料中添加水泥,其用量非常大,水泥在充填成本中占有很大的比例。另一方面,尾矿砂浆水力充填是利用水力旋流器来分级固体颗粒,颗粒较细的部分将通过溢流派到废矿池中,颗粒较粗的部分回收利用作为井下充填的材料使用,有时充填的尾矿数量不够,需要另外购置砂子混合作为充填材料。如何减少水泥的使用量和从水力旋流器底流增加尾矿砂的细颗粒的固体部分产量,以达到节约充填成本的目的,一直是矿山企业所面临的重要课题。为此,专门研究絮凝药剂对尾矿砂浆充填材料的单轴抗压强度的影响。研究结果表明,絮凝药剂能够大幅度地提高尾矿砂浆充填材料的强度,而且絮凝药剂使用量有一个最优值,使强度达到最大。当过量使用絮凝药剂,尾矿砂浆充填材料的强度则有所降低。     

尾砂胶结含盐冻结充填体力学特性研究

矿山充填开采正逐步向寒区甚至永久冻土区发展,这些区域含盐地下水分布非常广泛。同时,极端寒冷条件下往往需要向充填材料中加入一些防冻盐以防止料浆在输送过程中发生冻结。通过室内测试分析了?6 ℃环境下不同盐分（NaCl）膏体充填体力学特性（强度和初始弹性模型）随时间演化特征。利用单轴压力机测得了龄期为7、28、90 d膏体充填体的强度和初始弹性模量,结果表明NaCl的加入会降低冻结充填体的强度,其影响程度取决于胶结料类型。试样的强度随着养护时间的增加而增大。此外,不论盐分浓度、养护时间和胶结料类型,试样的强度与初始弹性模量之间存在显著的线性关系。该研究结果可以为寒区含盐膏体充填技术的开展提供一些依据。     

复杂应力下充填体破坏能耗试验研究

应用先进的MTS和INSTRON刚性伺服试验机对不同灰砂比的充填体进行了常规三轴、无侧限单轴动态、静态抗压强度试验,测得了各种相关力学参数,得到了对应的荷载-位移和应力-应变全曲线,根据所得试验力学参数和压缩试验的性质,将单个试件、单位体积、单位质量的破坏能耗作为考查指标,计算了三轴和单轴抗压动态、静态加载方式下的破坏能耗,计算过程中消除了负位移产生的不利影响,通过统计回归建立和分析了围压与能耗的函数关系和不同情况下破坏能耗的变化及规律,对矿山顺利回采矿柱、维护采场及巷道顶板稳定、控制地压、防止地表塌陷、环境保护等具有现实意义。     

A three-dimensional analytical solution to the arching effect in inclined backfilled stopes

The stress state in backfilled mine stopes is an important issue to assess the behavior of the interaction between the backfill and the surroundings or barricades. Most previous arching analyses have considered only the vertical backfilled stopes in both 3D and 2D conditions, and the 3D stress distribution that results from the arching effect in inclined mine stopes remains unclear. In this paper, based on the limit equilibrium theory, a 3D stress solution that is applicable to vertical and inclined backfilled stopes is proposed to further examine the arching effect. The solution is validated against an available centrifuge model by changing the inclination of the model. The proposed analytical solution is consistent with the numerical simulations, and it is suggested that neglecting the wall inclination causes one to underestimate the arching phenomenon. In other words, the vertical stresses at the bottom of the stope can decrease when the wall inclination is considered. Hence, when the stope is assumed in plain strain conditions, both the vertical and horizontal stresses exerted on the barricades are overestimated.     

多因素组合影响下向进路充填顶板稳定性计算

下向分层进路充填采矿法中,进路充填顶板的稳定对回采过程安全性至关重要,而分层充填体叠加载荷计算一直是顶板稳定性分析的难点。在充分考虑采动岩体荷载、矿体倾角、相邻分层间回采进路的交错布置、充填体与围岩的接触等工程实际后,推导了进路顶板平衡微分方程,求解得到进路顶板静荷载的理论值。结合回采工艺建立了“多跨梁”力学模型,并得到了回采进路顶板拉应力的理论计算公式,分析得到影响进路顶板稳定性的4个重要理论因素：顶板上部载荷σ v、回采进路跨度l、1:4充填体的厚度h、充填体自身抗拉强度[σt]。为充分考虑进路顶板静载荷和回采爆破动载荷影响,利用FLAC^3D对多因素影响下的顶板稳定性进行了数值模拟正交计算。根据模拟结果,分析了各因素对顶板拉应力的影响规律,利用多元非线性回归的方法建立了多因素组合影响下顶板稳定性评价模型。该模型应用到某铜矿试验采场的实际生产,具有较好的指导作用。     

尾砂胶结充填体蠕变模型及在FLAC3D 二次开发中的实验研究

关于岩石蠕变特性的研究已有诸多成果,但对胶结充填体的蠕变特性还缺乏系统地研究。在室内充填体单轴蠕变试验的基础上,利用Hoek-Kelvin模型表征充填体的蠕变特性,运用粒子群优化算法,对选用的蠕变模型的参数进行辨识,并研究各蠕变参数对应力水平的敏感程度。研究结果表明,在不同分级应力水平加载下,弹性模量EH和黏滞系数η变化较小,而参数EK对应力水平变化较敏感。利用FLAC3D软件二次开发所建立的蠕变模型,用开发的模型进行分级加载蠕变的数值模拟,计算结果与试验数据吻合。     

Analytical Expression for Vertical Stress within an Inclined Mine Stope with Non-parallel Walls

Arching is a phenomenon that occurs in many situations in geotechnical engineering. When underground mine stopes are backfilled, a significant fraction of the self-weight of the backfill is carried by the side walls. As a result, the vertical stress at the bottom of the stope is significantly less than its overburden pressure. Few analytical expressions published in the literature can be used to determine the vertical stresses of stope with parallel walls. The objective of this paper is to extend the analytical solution previously developed by the authors to long plane-strain stopes with non-parallel walls with both slopes leaning to the same side. Different combinations of wall inclination are examined using the new analytical expression developed. To validate the analysis, the proposed results are compared with numerical model results. The results show that the proposed analytical expression is capable of estimating the vertical stress within mine stopes when the inclination of the hangingwall to the horizontal (α) is less than that of footwall (β). An important behavioural trend for the stress distribution is observed, where with the same overburden pressure and base width, the stress magnitude experienced by fill material significantly varies depending on the wall inclination.     

Arching in Inclined and Vertical Mine Stopes

Hydraulic fills are one of the most common backfills used by mining industries to backfill the stopes (voids) created after extracting the ore. It is important to estimate the stresses within to the stope to design the drainage and barricades. Most of the existing analytical models for the estimation of stresses within the stopes consider flat rectangular elements to include the effects of arching, although a continuous compression catenary arch of principal stresses using intersections of shear lines is the reality in field situations. In this paper, a circular compression arch of principal stresses has therefore been used to derive a general expression for stress within the inclined stopes. The results have been compared with the existing analytical and numerical models for vertical stopes as well as inclined stopes. A methodology has been presented to determine the vertical stress variation along the width of stope at different depths. The variation of stresses along the width of stope is also presented graphically.