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A light barricade for tailings recycling as cemented paste backfill
Journal Pre-proof A light barricade for tailings recycling as cemented paste backfill
Hongjian Lu, Chongchong Qi, Chenghe Li, Deqing Gan, Yingnan Du, Sheng Li PII:
S0959-6526(19)34258-1
DOI:
https://doi.org/10.1016/j.jclepro.2019.119388
Reference:
JCLP 119388
To appear in:
Journal of Cleaner Production
Received Date:
08 May 2019
Accepted Date:
18 November 2019
Please cite this article as: Hongjian Lu, Chongchong Qi, Chenghe Li, Deqing Gan, Yingnan Du, Sheng Li, A light barricade for tailings recycling as cemented paste backfill, Journal of Cleaner Production (2019), https://doi.org/10.1016/j.jclepro.2019.119388
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
Amount of words: 7368
Journal Pre-proof
A light barricade for tailings recycling as cemented paste backfill
Hongjian Lu 1, Chongchong Qi 2*, Chenghe Li 3, Deqing Gan 1, Yingnan Du 3, Sheng Li 3 1
Hebei Province Key Laboratory of Mining Development and Safety Technique, North
China University of Science and Technology, Tangshan 063210, China. 2
School of Resources and Safety Engineering, Central South University, Changsha 410083,
China 3
Shirengou Iron Mine, HBIS Group Mining Company, Tangshan 064200, China.
*
Corresponding author: Chongchong Qi, Email: [email protected], Tel: +86
17361393317
1
Journal Pre-proof
A light barricade for tailings recycling as cemented paste backfill
Abstract: The cemented paste backfill (CPB) technology that recycles waste tailings to fill underground voids is crucial for cleaner production of mineral resources. A main challenge in its application is the generally high cost and long time-period incurred to ensure the stability of the barricades (retaining wall structures). To circumvent these trends, a new light barricade with improved construction efficiency, mitigated material usage, and reduced operational cost is presented in this paper. The proposed barricade comprises recycled waste products from mining operations as main construction materials. These include rebar skeletons, mesh reinforcement, wood board, earthwork cloth, and steel support. The new barricade concept has been discussed in detail. This is followed by the description of an engineering application of the barricade in Shirengou Iron Mine (SIM). The backfill height in a single operation (h) and the distance from the barricade to the stope drawpoint (l) can be optimized using numerical simulations. These parameters were determined to be 1.5 m and 4–6 m, respectively, for SIM. The SIM implementation of the proposed barricade illustrates that it could reduce construction time from the 72 h for a brick barricade to 40 h, reuse steel pipes and mesh reinforcement for 6–8 cycles and wood board for 3–4 cycles, and decrease the barricade cost from 1,050 RMB/m3 to 727 RMB/m3. Therefore, the present study offers an efficient, reusable, and economical strategy for barricade construction that can be applied to the CPB practice in underground mines. Keywords: tailings recycling; cemented paste backfill; light barricade; construction method; parameters determination; efficient, reusable and cost-effective.
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Journal Pre-proof Nomenclature BST
Brazilian split tests
C
Cohesiom
CPB
Cemented paste backfill
CTR
Cement-tailings ratio
Cu and Cc
Coefficient of uniformity and curvature
Gs
Specific gravity of tailings
GSI
Geological Strength Index
SIM
Shirengou Iron Mine
TCT
Triaxial compressive tests
UCS
Uniaxial compressive strength
UTS
Uniaxial tensile strength
E
Elastic modulus
h
The backfill height in a single operation
H and W
Height and width of the barricade
Hb
Height of the CPB slurry
k
Coefficient of permeability
l
The distance from the barricade to the drawpoint
Mmax
Maximum bending moment
φ
Internal friction angle
γs or γ
Specific gravity of the CPB slurry or mixture
3B
Specific gravity of CPB after 3 days curing
P
Total pressure
Rc
Uniaxial compressive strength
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Journal Pre-proof Rt
Uniaxial tensile strength
Poisson’s ratio
s
Yield stress
Permissible stress
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Journal Pre-proof 1. Introduction The waste tailings produced in mineral excavation have received increasing attention from both the academy and industry. A widely accepted solution for tailing management is to recycle them as cemented paste backfill (CPB) material in underground voids. This procedure can increase mining productivity, reduce surface-disposed tailings, and improve safety in mining operations (Chen et al., 2018; Cihangir et al., 2015; Fall and Nasir, 2010; Kesimal et al., 2005; Lu et al., 2018; Qi et al., 2018a; Yilmaz and Fall, 2017). In addition to its operational, economical, and environmental benefits, stringent laws have also promoted the wide applications of CPB. For example, the Environmental Protection Tax Law of the People’s Republic of China (CLI.2.307655) levies 15 RMB/ton for surface-disposed tailings that exceed the maximum permissible capacity of the tailing dams. In contrast, a 50% reduction in resource tax is applied when mineral resources under buildings, railway, and water in China are excavated using the backfill mining method (Zhao and Pei, 2019). Prior to backfilling, barricades (or retaining wall structures) need to be constructed in each of the down-level gallery entrances to prevent the in-rush of fresh CPB. Suitable barricade designs can yield safety and economic benefits by increasing the backfill rate and minimizing discontinuities between successive backfill operations (Hughes et al., 2010). However, barricade design is a complex task that involves a comprehensive understanding of barricade materials as well as of their mechanical performance and operational stresses (Berndt et al., 2007). An inappropriate barricade design could trigger a barricade failure, which further results in catastrophic or tragic consequences such as equipment breakdown and personal injury (Kuganathan, 2001; Yang et al., 2016; Yumlu and Guresci, 2007). In practice, every barricade can be made safe with adequate construction materials and improved construction techniques. However, the trade-off between safety, efficiency, and cost is the main concern during the selection of a barricade. 4
Journal Pre-proof A diverse range of barricades have been proposed, which can be categorized into four main types: concrete barricade, timber barricade, brick barricade, and cable sling barricade (Hughes, 2008). Each of these barricade types has been widely adopted by mine sites with due considering of their advantages and disadvantages. However, it should be emphasized that the attempt to improve the barricade structure cannot be abandoned to make it more suitable for each specific mining condition. Moreover, the proposed barricade should be validated for the engineering application before it can be adopted and enhanced by practitioners from other mine sites. From the authors’ experience, a better barricade is an urgent need for underground mines using the CPB technology. This work is aimed at proposing a novel barricade with enhanced efficiency, reusability, and cost-effectiveness, while with safety characteristics similar to those of available barricades. To be comparable, the efficiency, reusability, and cost-effectiveness are represented by the construction time, material reuse cycle, and barricade cost. The proposed barricade includes the selection of barricade materials, determination of the construction parameters, and adjustment of CPB practice. The feasibility of the proposed method has been validated through a case study, the Shirengou Iron Mine (SIM). can promote a broader application of the CPB technology and cleaner production of mineral resources. In Section 2, a brief introduction of contemporary barricade design methods is provided. The materials and methods including study site, barricade materials, lab experiments, determination of barricade and backfill parameters, and the barricade construction procedure are explained in Section 3. Then, a case study is presented in detail to validate our proposed barricade. Finally, the main observations are summarized, and future works are indicated.
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Journal Pre-proof 2. Literature review Concrete barricade to retain hydraulic backfill material was first proposed in the 1970s (Hughes, 2008) and was applied to paste backfill for the first time in 1975 (Mitchell et al., 1975). Since then, it has been widely adopted (even with continuous modification for each engineering case) by underground mines that adopt CPB practice. A concrete barricade is generally constructed with a width of 1 m and with mousetrap drains for water dissipation. Timber barricade consists of vertical timbers, laminated beams, and draining fabric. It was previously used as barricade with a high draining capability, for cemented hydraulic backfill. Construction details of timber barricade are provided in Ref (Smith and Mitchell, 1982). They illustrate that a timber barricade could withstand up to 95.8 kPa before yielding and is suitable for headings of less than 4.3 m. Brick barricade has been gaining popularity from the 1980s for replacing the traditional wooden barricade. It is constructed as an arch from a mortar made of water, cement, sand, and gravel. The utilization of porous bricks imparts a brick barricade with adequate permeability for use in CPB and hydraulic backfill. A concern with regard to the adoption of brick barricades is that their strength is highly dependent on the construction scenarios and thus can vary significantly (Kuganathan, 2001). Berndt et al. (2007) provided a thorough literature review on brick barricades in underground mines, including barricade design and construction and their advantages. Although the cable sling barricade is not the first choice in most cases, it is still adopted in several operations worldwide, e.g., in the South Gold Mine region in South Africa and the Campbell Gold Mine in Canada (Hughes, 2008). The construction of the cable sling barricade starts with the bolting of horizontal and vertical cables into the neighboring rock mass. Then, burlap is used to cover the bolted cable to function as the fence for the backfill material. An 6
Journal Pre-proof extensive introduction to the cable sling barricade has been provided by Lang et al. (1999), which was designed for backfill retention in longhole stopes. Table 1 summaries the materials, advantages, and disadvantages associated with each barricade type. As illustrated, there are several drawbacks in the above methods, which limit their more extensive applications. For example, concrete barricades are limited by their high cost and long curing time, whereas timber barricades are restricted as it is challenging to control their qualities. Therefore, a better barricade is desired for a broader application of the CPB technology. 3. Materials and methods 3.1. Study site The proposed barricade would be applied to SIM, which is located 90 km from Tangshan City, Hebei province, China. The mine was established in 1975 using open-pit mining to excavate the near-surface ore deposits. With ore deposits extending to “great depth,” sublevel open stoping has been adopted since 2004 owing to its advantages such as high production rate and simultaneous unit operations (Qi et al., 2018b). After decades of excavations, 129 normal stopes and several illegal stopes have been created in the first underground level (-60 m). Then, sublevel open stoping with subsequent backfill is used to excavate the ore deposits at the level between -60 m and -180 m. According to the thickness of orebody, two stope layouts were considered, which were in the direction perpendicular to or along the strike of orebody (Fig. 1). Fig. 1a illustrates the layout in the direction perpendicular to the orebody. In such a stope layout, the length of each of first mining rooms (20 m wide and 105 m high) and second mining rooms (30 m wide and 105 m high) was equal to the thickness of the orebody. In the case of the layout in the direction parallel to the orebody (Fig. 1b), the width of each of first 7
Journal Pre-proof mining rooms (15 m long and 105 m high) and second mining rooms (35 m long and 105 m high) was equal to the thickness of the orebody. For either of the cases, the sublevel height was 15 m. In addition, the first and second mining rooms were arranged at intervals with top pillars of 10–15 m in height. After the ore was mined from the first mining rooms, the goaf were backfilled with CPB as artificial pillars. The cement-tailing ratio (CTR) of the CPB within a height of 6 m from the bottom was selected to be 1:4. The CTR for the remaining first mining rooms was 1:8. For the second mining rooms, the CTR was 1:4 and 1:20 for within and above 6 m, respectively. The higher cement usage within 6 m is to recycle the ore pillar. The excavation and backfill of the first mining room and second mining room is referred to as a mining unit in the following. Prior to the backfill practice, barricades need to be constructed to retain the CPB in the stopes. The barricade construction arrangements for a mining unit in the perpendicular and parallel stope layouts are illustrated in Fig. 2. As shown, 14 barricades and 17 barricades are required in a mining unit in the perpendicular and parallel stope layouts, respectively. For SIM, it is estimated that 200 barricades need to be constructed annually to fulfil the mining output of two million tons. The average barricade size is 4.5 (width) × 3.75 (height) × 0.75 (thick) m. Brick barricades with a construction cost of 1,050 RMB/m3 and an average construction time of 72 h per barricade were selected (Fig. 3). 3.2. Barricade materials Considering the advantages and disadvantages of traditional barricades (the horizontal stresses caused by CPB, cost, and construction practice), an efficient, usable, and costeffective barricade was proposed for the SIM practice. The main barricade materials for the proposed barricade design include rebar skeleton, mesh reinforcement, wood board, earthwork cloth, and steel support. These materials were previously used as supporting
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Journal Pre-proof materials in underground roadways and recycled as barricade materials. Each of the materials can be described as follows: Rebar skeleton is the outermost layer of the proposed barricade and consists of bolts and steel pipes. The diameter and length of the bolts are 36 mm and 500 mm, respectively. These bolts are bolted into the rock mass for at least 300 mm and fixed with the anchoring agent. The rest of the bolts are fitted in the steel pipe of diameter 50 mm and length 150 mm. Steel pipes of outer diameter 70 mm and inner diameter 65 mm are used as the primary material in the rebar skeleton, which are connected to the steel pipe of diameter 50 mm. The rebar spacing needs to be less than 0.5 0.6 m when the barricade area is larger than 16 m2. Angular tendons are used in all the intersections of the steel pipes to increase the overall strength of the rebar skeleton. Two filter tubes are constructed to improve the draining of the barricade. Mesh reinforcement is the second layer of the barricade. It has a diameter of 6.5 mm and mesh size of 50 50 mm, which is bandaged to the rebar skeleton. Wood boards of size 4000 200 30 mm are used. The space between adjacent boards is less than 5 mm, and the board needs to cover the whole barricade area except the observation hole. Earthwork cloth is a type of ϕ6.5 mm nonwoven geotextile. The earthwork cloth is fixed to the wood board using the steel nail and is at least 20 cm longer than the barricade in all directions. The redundant earthwork cloth at the top and both sides is reinforced by the brick structure, and the redundant earthwork cloth is placed in the slot (width = 500 mm and height = 300 mm). Shotcrete is used at the periphery of the contact area between the barricade and drift.
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Journal Pre-proof Steel support of diameter 70 mm is used to further improve the strength of the barricade. The end of the steel support is anchored to the drift floor. 3.3. Lab experiments A successful barricade is dependent on a comprehensive understanding of the material properties. Table 2 shows the main laboratory tests conducted for CPB, rock mass, orebody, and barricade materials. A brief introduction to each type of experiment is provided in the following sections. Detailed explanations can be as conveniently obtained from relevant references (Chen et al., 2017; Liu et al., 2018; Lu et al., 2018). The authors note here the lab experiments are mainly performed for the stability analysis of barricade rather than for the property of CPB. Therefore, a few properties of fresh or hardened CPB, such as bleeding, segregation, and air content, are not explained here. The feasibility of CPB in SIM has been reported in (Lu et al., 2018). Specific gravity was obtained followed the experimental procedure in (ASTM C127, 2007). Grain size distribution of the mine tailings was determined by a Malvern Mastersizer 2000 laser diffraction analyzer. X-ray diffraction analyses were conducted on the mine tailings to determine their chemical compositions. A Bruker AXS D8 Advance Diffractometer was used with a 5 – 90 ° 2θ range and a 0.005 step size. The chemical compositions were qualified by the Rietveld method. Slump tests were conducted to test the consistency of the CPB slurry using a slump cone (internal top diameter = 100 mm and internal bottom diameter = 200 mm). The slump cone was filled with CPB slurry before it was vertically removed. The slump test value was calculated to be the vertical distance between the top of the slumped 10
Journal Pre-proof CPB and that of the slump cone. Wu et al. (2015) demonstrated that a slump value of 170–250 mm is essential for the pumping transport of CPB slurry. Setting time detection tests were employed to detect the initial and final setting time of the CPB slurry. In this paper, a Vicat apparatus of the IOS-standard type (Dahong, Hebei) was used. UCS, TCS, BST, and UTT were conducted using an MTS 815.03 material testing system according to recommendations in ACI 229R-13 (Jang et al., 2018). UCT, TCT, and BST were performed on specimens from the CPB, rock mass, and orebody, whereas UTT was performed on specimens from the barricade materials, namely the rebar, steel pipe, and wood board. The diameter and height of the UCT and TCT specimens were 50 mm and 100 mm, respectively, whereas those of the BST specimens were 50 mm and 50 mm, respectively. For UTT, the rebar and steel pipe specimens were prepared to be of length L = 5d + 150 mm (d = diameter), whereas the wood board specimens were prepared to be of length L = 5.63 A + 150 mm (A is the board area). The force applied on the earthwork cloth and mesh reinforcement is relatively small as they are used here primarily to prevent the leakage of the CPB slurry and rotation of the wood boards, respectively. Therefore, their mechanical properties were not considered during the stability analysis in the numerical modeling. At least three experiments were carried out for each experimental scheme, and their average values were used for further analysis. The Geological Strength Index (GSI) system was used to transform intact mechanical properties to in-situ properties so that they could be used in numerical modeling (Deisman et al., 2013; Rafiei Renani et al., 2016).
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Journal Pre-proof 3.4. Determination of barricade and backfill parameters To ensure the stability of the barricade, the barricade and backfill parameters need to be determined properly for SIM by considering the interaction among the different materials. In this work, we focused on two important parameters: the backfill height in a single operation (h) and the distance from the barricade to the stope drawpoint (l). These two parameters can be determined through stability analysis using numerical modeling. When CPB slurry is transported to underground stopes, it transforms from the “homogeneous fluid” state (C = 0,
= 0) to the “solid” state (C > 0, > 0). Here, C represents cohesion, and represents internal friction angle (Cui and Fall, 2017; Yang and Li, 2015; Yang et al., 2016). A detailed explanation of the stability analysis through numerical modelling is provided in (Yang and Li, 2015; Yang et al., 2016). The authors note here that whereas the following numerical modeling was prepared for the backfill practice in SIM, the parameters for other mine sites can be determined as conveniently as for SIM. MIDAS-GST was used to model the mechanical response of the barricade and its interaction with the surrounding rock mass. Fig. 4 shows a typical stope in SIM of length 33 m, width 20 m, height 15 m, and dip angle 60°. The length, width, and height of the drift was 12 m, 4.5 m, and 3.75 m, respectively. To eliminate the boundary influence on the accuracy of numerical modeling, the whole model was 220 m, 140 m, and 105 m in length, width, and height, respectively. A traditional mesh conforming technique was used for the mesh generation, and each numerical model contained 10.962 to 11,508 meshes (Fig. 4). The natural, in-situ stresses were calculated by considering the overburden weight. The steel pipe and rebar were modeled as a homogeneous, isotropic material obeying the linear elastic criterion. The rock mass and orebody were modeled using the Mohr–Coulomb criterion. The Drucker–Prager criterion was applied to the CPB materials including the CPB
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Journal Pre-proof slurry and CPB body after curing. The von Mises criterion was used to model the wood board. Plate element, beam element, and solid element were used for the wood board, steel pipe and rebar, and the other materials, respectively. The model selection was based on engineering experiences and recommendations in the literature (Hui et al., 2016; Liu et al., 2017; Luo et al., 2017; Zhang et al., 2017). The model parameters were obtained considering laboratory results and the GSI system. Fig. 5 illustrates the calculation steps for each numerical simulation, which are also listed below: 1) The boundary conditions were set, and the numerical model was meshed. The initial stresses were calculated after the whole model was brought into equilibrium (Fig. 5a). 2) The stope was excavated in a single step to simulate the drill and blast processes (Fig. 5b). 3) Each time, the stope was progressively backfilled with a backfill height of h (Fig. 5c– d). The time interval between two backfill practices was three days. The material properties of CPB after curing for at least three days were altered to the mechanical properties at three days. Section 4.2.1 illustrates that further increases in the mechanical properties after three days of curing exerted negligible influence on the barricade pressures. 4) The stope was backfilled in one step once the progressive-backfilled height was larger than the barricade height (Fig. 5e). The numerical model was validated using an analytical solution. In the modeling validation, the stress distribution in the barricade immediately after backfilling was compared. In such cases, the CPB slurry can be considered to be a Bingham plastic fluid (Boger et al., 2006). Therefore, the influence of l on the stress distribution can be omitted (Yang and Li, 2015;
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Journal Pre-proof Yang et al., 2016). During the numerical modeling, multiple l values were used for each hb and the average of the numerical results was used for comparison. The stress state of the barricade is shown in Fig. 6. Considering equilibrium conditions, the total pressure (P) and maximum bending moment (Mmax) can be calculated as follows:
1 P s hb 2 W 2 M max
s 6H
hb 3 ( H hb
2hb hb ) 3 3H
(2)
(3)
where H and W are the height and width of the barricade, respectively; hb is the height of the CPB slurry; and s is the specific gravity of the CPB slurry. 3.5. Barricade construction procedure Fig. 7 illustrates the implementation procedure of the proposed barricade. As shown, the implementation of the proposed barricade is performed through the following steps: bolt installation, steel pipe installation, mesh reinforcement installation, wood board installation, and earthwork cloth installation. The authors note here that the steel support is optional in the proposed barricade; it is required where high horizontal pressures are observed. During the case study, the steel support is not used considering the surrounding rock conditions and based on engineering experience. 4. Results and discussion In this section, the implementation of the proposed barricade in SIM is described. First, the materials were characterized using the lab experiments results and the GSI system. Secondly, the h and l were determined through an extensive numerical study. Thirdly, the proposed barricade was implemented in the mine site and compared to the previous brick barricade.
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Journal Pre-proof 4.1. Characterization of materials Fig. 8 shows the experimental slump value with different cement-tailings ratios (CTRs) and solid contents. The CTRs of CPB were selected to be 1:8, 1:10, and 1:20 and those of solid contents were selected to be 68%, 72%, 75% 78%, and 81%. Note that a relatively high cement content was used in this study because the Portland cement was based on China's standard (No.GB175-2007). As is evident, the slump value increased as the solid content decreased. The slump values were less than 17.5 cm at 81% solid content. In contrast, the slump values were at least 26 cm when the solid content was increased to 72–75%. Based on the slump test results, CTRs of 1:4, 1:8, and 1:20 and solid content of 72% were selected for further analysis. Table 3 shows the setting time results of the CPB slurry under different CTRs. As a cementbased composite, CPB depends substantially on cement hydration for strength development (He et al., 2019; Wang et al., 2018; Xie et al., 2019). Table 4 illustrates the physical and mechanical properties of CPB after three days of curing with different CTRs. In Table 4, s represents the specific gravity of CPB slurry, 3B represents the specific gravity of CPB after three days of curing, Rc represents the uniaxial compressive strength, C represents the cohesion, represents the internal friction angle, E represents the elastic modulus, and
represents the Poisson’s ratio. Table 5 presents the physical and mechanical properties of the rock mass and orebody. Here, represents specific gravity, and Rt represents the uniaxial tensile strength. Similarly, properties of the steel pipe, rebar, and wood board are presented in Table 6. In this Table, s is the yield stress, and is the permissible stress. As illustrated, the safety factor was set to 3.5 for the wood board and 1.5 for the steel pipe and rebar.
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Journal Pre-proof 4.2. Numerical validation, simulation scenarios, and results 4.2.1. Numerical validation Table 7 illustrates the comparison between the analytical solutions (Eqs 1 and 2) and the present numerical simulation results. As shown, the maximum deviations between the analytical and numerical P and Mmax were 8.9% and 1.0%, respectively, when hb =1 m. This illustrates that the numerical modeling agreed well with the analytical solutions. Fig. 9 shows the beam forces of the barricade with backfill height when the mechanical properties of CPB after three days of curing was used during the simulation. As shown, the maximum beam forces were quite small when the three-day mechanical properties were used. This implies that the CPB body can be self-standing with the three-day mechanical properties and that further adjustment of mechanical properties is not necessary for CPB materials in SIM. 4.2.2. Simulation scenarios Various numerical simulations were conducted for determining h and l. In practice, the CTR for each stope backfilling is different based on the stope stability requirements. Therefore, the determination of h and l needs to consider the CTR. In SIM, three scenarios were used: 1) CTR = 1:4 for the bottom 6 m and CTR = 1:8 for the remaining 9 m; 2) CTR = 1:8 for the bottom 6 m and CTR = 1:8 for the remaining 9 m; 3) CTR = 1:20 for the bottom 6 m and CTR = 1:20 for the remaining 9 m. A preliminary selection of h was performed using the analytical solution (Eqs 2 and 3) as the stresses on the barricade are more evident when the CPB is in a slurry state (Cui and Fall, 2017). Fig. 10 illustrates the variation of P and Mmax when h was increased from 0.5 m to 3.5 m. Note that the CTR in a stope was maintained constant during the preliminary selection. As shown, P and Mmax increased non-linearly with h. A rapid increase in P and Mmax was 16
Journal Pre-proof triggered when h = 1.5 m or h = 2.0 m. Therefore, h = 1.5 m and h = 2.0 m were selected for further analysis. In mining operations, barricades are generally constructed 0–12 m away from the stope drawpoint, i.e., 0 m < l < 12 m (Yang and Li, 2015). Therefore, 1 m, 3 m, 5 m, 7 m, 9 m, and 11 m were selected for l. An orthogonal numerical experiment was conducted on three CTR scenarios, two h values, and six l values, resulting in a total of 36 numerical models in the stability analysis for determining h and l. 4.2.3. Numerical results Figs. 11–13 show the maximum stresses of the plate and beam elements in the simulation when the CTR was 1:4 and h was 1.5 m. The authors note here that CTR = 1:4 and h = 1.5 was used as an example and that the stress analysis for other scenarios can be performed as conveniently following this procedure. As discussed before, the plate element was used to simulate the wood board, whereas the beam element was used to simulate the steel pipe and rebar. During the simulation, the stresses in the steel pipe and rebar were analyzed in combination because they were all beam elements and with similar strength (Table 6). For each l value, the stope was backfilled successively in several steps, and the stress was monitored for each step. The hx values in Figs. 11–13 show the backfill step when the maximum stress was observed. For example, h2 implies that the maximum stress was observed after the backfilling of the second 1.5 m. That is, the maximum stress was observed after 3 m of backfilling. Only the maximum stresses were analyzed for each simulation scenario (each combination of h, l, and CTR) as this is when a barricade failure is most likely. As shown, the maximum plate stress was observed at different hx for various l. For example, the maximum plate stress for l = 1 m was observed after 4.5 m of backfilling (h3) whereas it was observed after 1.5 m backfilling (h1) for l = 3. Thus, no relationship was evident between
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Journal Pre-proof hx and l. Similar results were obtained for the horizontal and vertical beam stresses. It is noteworthy that the maximum plate stress was not always observed at the same backfilling height with the maximum beam stresses. Moreover, different hx can be observed even between the maximum horizontal and vertical beam stresses (i.e. the maximum horizontal beam stress occurred at h1 for l = 9, whereas the maximum vertical beam stress occurred at h3 for l = 9). Therefore, the maximum stress as well as its corresponding hx values cannot be obtained without a thorough simulation of the whole backfilling process. The maximum plate stress was generally observed to be 1.4–2.9 m above the barricade floor. The horizontal beam stress was more localized on several beams near the middle of the barricade, resulting in a larger maximum horizontal beam stress than the maximum vertical beam stress. Moreover, the maximum vertical beam stress was observed to be 0.8–1.5 m above the barricade floor. Fig. 14 summarizes the maximum plate and beam stress for all the 36 numerical models. As shown, the variation pattern of maximum stresses versus l was controlled by h rather than by CTR. To be more specific, similar variation patterns of maximum stresses versus l were observed for all the CTR values once h was fixed. When h = 1.5 m, stable maximum stresses were obtained at l > 3 m. The maximum plate stresses, maximum horizontal beam stresses, and maximum vertical beam stresses were 4.0–5.0 MPa, 85.0–99.0 MPa, and 72.0–80.0 MPa, respectively. In contrast, the maximum plate stresses, maximum horizontal beam stresses, and maximum vertical beam stresses at h = 2.0 m were 9.3–10.5 MPa, 140.0–150.0 MPa, and 160.0–200.0 MPa, respectively. Compared with the material properties in Table 6 and in-situ construction environments, h and l were determined to be 1.5 m and 4.0–6.0 m, respectively, for SIM.
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Journal Pre-proof 4.3. Barricades implementation The barricade proposed in this paper was implemented in SIM. As discussed before, backfilling was performed in several operations, with each backfill height being 1.5 m (when the total backfill height is less than 6.0 m). The barricade was constructed 4.0–6.0 m away from the stope drawpoint considering the construction requirements. To facilitate the barricade construction, the construction site generally needs to exhibit the following characteristics: 1) intact surrounding rock properties; 2) without significant fractures; 3) small barricade area; and 4) flat drift roof and floor. The in-situ construction process is shown in Fig. 15. Curing for 8 h was required for the concrete during the seaming and slot filling. Fifteen days after the backfilling, the barricade could be removed and reused for constructing the barricades described below. The in-situ images after the barricade removal are shown in Fig. 16. As shown, the CPB body exhibited smooth surfaces and remained intact subsequently. These characteristics are desirable for barricade construction. In this section, we compared the efficiency, reusability, and cost-effectivity of the proposed barricade with those of the brick barricade adopted previously in the SIM. For efficiency, we focused on the construction time of the barricade. The construction time comparison between the proposed barricade and the brick barricade is illustrated in Table 8. As shown, the construction time for a brick barricade was 72 h. This construction time has been reduced to 40 h by employing the proposed barricade. Therefore, there is a significant improvement in the construction efficiency of barricade. With regard to the reusability, it is well accepted that a brick barricade cannot be reused (at least in most cases) during CPB application. However, the engineering application of the proposed barricade in SIM reveals that its main construction materials (steel pipe, wood 19
Journal Pre-proof board, and mesh reinforcement) can be reused for several cycles. The reusability tests were conducted to determine the number of cycles for which the main barricade materials can be reused. In the reusability test, was calculated using the safety factor presented in Table 6 and was compared to the numerical results (the needs to be larger than or approximately equal to the required strength from the numerical modelling). Table 9 presents results of the reusability tests in SIM. As shown, the steel pipe and mesh reinforcement can be reused for 6–8 cycles, and the wood board can be reused for 3–4 cycles. The authors note that the reusability could vary marginally considering the difference in the in-situ application. Overall, a reuse rate exhibiting high potential has been achieved by the proposed barricade. For the cost, Table 10 and Table 11 compare the material and operational costs of the brick barricade and proposed barricade. It is evident that the barricade cost can be decreased from 1,050 RMB/m3 to 727 RMB/m3 (calculated based on a brick barricade volume of 12.66 m3). For SIM, barricade construction using the proposed barricade design method will save 4,100 RMB, resulting in an annual saving of 0.8 million RMB in barricade construction (calculated based on 200 barricades per year). 4.4. Limitations The first limitation of the present study is that all the numerical simulations were performed assuming complete drainage of the barricade. The CPB material being modeled with the Drucker–Prager criterion is evidently another limitation. The application of more advanced constitutive models such as in (Cui and Fall, 2017) will result in more accurate results. However, the authors consider that the model selection will not influence the illustration of the proposed barricade. A final limitation is that the numerical models were only validated using the analytical solution, and no quantitative in-situ measurements are reported. In-situ
20
Journal Pre-proof measurements are presently being conducted. The authors conjecture that more representative model parameters can be obtained using advanced back-analysis methods (Qi et al., 2018b). 5. Conclusions and future works Recycling waste tailings as CPB is fundamental to the cleaner production of mineral resources, and their successful application requires a suitable barricade. Considering the advantages and disadvantages of traditional barricade, this paper proposes an efficient, reusable, and cost-effective barricade for improved recycling of waste as CPB. The proposed barricade uses rebar skeleton, mesh reinforcement, wood board, earthwork cloth, and steel support as the main construction materials. It was verified on an engineering instance, the SIM, based on which the following conclusions can be drawn: 1. The h and l were determined to be 1.5 m and 4.0–6.0 m for successfully applying the barricade and CPB technology. 2. The construction time can be reduced from 72 h (for brick barricades) to 40 h with the proposed barricade. 3. The proposed barricade has a highly potential reuse rate, with the steel pipes and mesh reinforcement having been reused for 6–8 cycles and wood board for 3–4 cycles. 4. The barricade cost has been reduced from 1,050 RMB/m3 to 727 RMB/m3 with the application of the proposed barricade. This can save up to 0.8 million RMB per year for SIM. For future works, the methods to improve the strength of the proposed barricade should be tested so that the proposed barricade can be used under high pressures. Furthermore, more precise methods, such as the multi-criteria decision-making process, should be employed to select the appropriate barricade for a specific mine site. Finally, better incorporation of the 21
Journal Pre-proof proposed barricade into the mining and backfilling system will further improve the efficiency of underground operations.
Acknowledgement This study was supported by the National Natural Science Foundation of China (No. 51774134), the Natural Science Foundation of Hebei province, China (No. E2016209220, No. E2016209277, No. E2018209129). The corresponding author was supported by China Scholarship Council (No. 201606420046). Conflict of interest: None.
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Journal Pre-proof References: Berndt, C.C., Rankine, K.J., Sivakugan, N., 2007. Materials properties of barricade bricks for mining applications. Geotechnical and Geological Engineering 25(4), 449-471. Boger, D., Scales, P., Sofra, F., 2006. Rheological concepts. Paste and Thickened Tailings-A Guide (Second Edition), Jewell and Fourie (eds), Australian Centre for Geomechanics, Perth, Australia, 25. Chen, Q., Zhang, Q., Fourie, A., Chen, X., Qi, C., 2017. Experimental investigation on the strength characteristics of cement paste backfill in a similar stope model and its mechanism. Construction and Building Materials 154, 34-43. Chen, Q., Zhang, Q., Qi, C., Fourie, A., Xiao, C., 2018. Recycling phosphogypsum and construction demolition waste for cemented paste backfill and its environmental impact. Journal of Cleaner Production 186, 418-429. Cihangir, F., Ercikdi, B., Kesimal, A., Deveci, H., Erdemir, F., 2015. Paste backfill of highsulphide mill tailings using alkali-activated blast furnace slag: Effect of activator nature, concentration and slag properties. Minerals Engineering 83, 117-127. Cui, L., Fall, M., 2017. Modeling of pressure on retaining structures for underground fill mass. Tunnelling and Underground Space Technology 69, 94-107. Deisman, N., Khajeh, M., Chalaturnyk, R.J., 2013. Using geological strength index (GSI) to model uncertainty in rock mass properties of coal for CBM/ECBM reservoir geomechanics. International Journal of Coal Geology 112, 76-86. Fall, M., Nasir, O., 2010. Mechanical behaviour of the interface between cemented tailings backfill and retaining structures under shear loads. Geotechnical and Geological Engineering 28(6), 779-790. He, Z., Zhu, X., Wang, J., Mu, M., Wang, Y., 2019. Comparison of CO2 emissions from OPC and recycled cement production. Construction and Building Materials 211, 965-973. 23
Journal Pre-proof Hughes, P., Pakalnis, R., Hitch, M., Corey, G., 2010. Composite paste barricade performance at Goldcorp Inc. Red Lake Mine, Ontario, Canada. International Journal of Mining, Reclamation and Environment 24(2), 138-150. Hughes, P.B., 2008. Performance of paste fill fences at red lake mine. University of British Columbia. Hui, Z., Yang, G., Zhang, C., Xiang, T., 2016. Numerical simulation of reinforced concrete lining considering the interaction with thesurrounding rock. Journal of Hydraulic Engineering 47(6), 763-771. Jang, J.G., Park, S.-M., Chung, S., Ahn, J.-W., Kim, H.-K., 2018. Utilization of circulating fluidized bed combustion ash in producing controlled low-strength materials with cement or sodium carbonate as activator. Construction and Building Materials 159, 642-651. Kesimal, A., Yilmaz, E., Ercikdi, B., Alp, I., Deveci, H., 2005. Effect of properties of tailings and binder on the short-and long-term strength and stability of cemented paste backfill. Materials Letters 59(28), 3703-3709. Kuganathan, K., 2001. Design and construction of shotcrete bulkheads with engineered drainage system for mine backfilling, International seminar on surface support liners, Australian Centre for Geomechanics. pp. 1-15. Lang, B., White, P., Pakalnis, R., 1999. Design and Construction of Tunnel Plugs and Bulkheads. Short Course Presented at CIM-AGM, Calgary. Leonards, GA (1962). Foundation …. Liu, H.-l., Li, S.-c., Li, L.-p., Zhang, Q.-q., 2017. Study on deformation behavior at intersection of adit and major tunnel in railway. KSCE Journal of Civil Engineering 21(6), 2459-2466.
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Journal Pre-proof Liu, L., Fang, Z., Qi, C., Zhang, B., Guo, L., Song, K.-I., 2018. Experimental investigation on the relationship between pore characteristics and unconfined compressive strength of cemented paste backfill. Construction and Building Materials 179, 254-264. Lu, H., Qi, C., Chen, Q., Gan, D., Xue, Z., Hu, Y., 2018. A new procedure for recycling waste tailings as cemented paste backfill to underground stopes and open pits. Journal of Cleaner Production 188, 601-612. Luo, Y., Chen, J., Wang, H., Sun, P., 2017. Deformation rule and mechanical characteristics of temporary support in soil tunnel constructed by sequential excavation method. KSCE Journal of Civil Engineering 21(6), 2439-2449. Mitchell, R.J., Smith, J.D., Libby, D.J., 1975. Bulkhead Pressures Due to Cemented Hydraulic Mine Backfills. Canadian Geotechnical Journal 12(3), 362-371. Qi, C., Fourie, A., Chen, Q., 2018a. Neural network and particle swarm optimization for predicting the unconfined compressive strength of cemented paste backfill. Construction and Building Materials 159, 473-478. Qi, C., Fourie, A., Zhao, X., 2018b. Back-Analysis Method for Stope Displacements Using Gradient-Boosted Regression Tree and Firefly Algorithm. Journal of Computing in Civil Engineering 32(5), 04018031. Rafiei Renani, H., Martin, C.D., Hudson, R., 2016. Back Analysis of Rock Mass Displacements Around a Deep Shaft Using Two- and Three-Dimensional Continuum Modeling. Rock Mechanics and Rock Engineering 49(4), 1313-1327. Smith, J.D., Mitchell, R., 1982. Design and control of large hydraulic backfill pours, International Journal of Rock Mechanics and Mining Sciences and Geomechanics Abstracts. Elsevier Science, pp. 134-134. Wang, J., Mu, M., Liu, Y., 2018. Recycled cement. Construction and Building Materials 190, 1124-1132.
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Journal Pre-proof Wu, A., Wang, Y., Wang, H., Yin, S., Miao, X., 2015. Coupled effects of cement type and water quality on the properties of cemented paste backfill. International Journal of Mineral Processing 143, 65-71. Xie, J., Wang, J., Rao, R., Wang, C., Fang, C., 2019. Effects of combined usage of GGBS and fly ash on workability and mechanical properties of alkali activated geopolymer concrete with recycled aggregate. Composites Part B: Engineering 164, 179-190. Yang, P., Li, L., 2015. Investigation of the short-term stress distribution in stopes and drifts backfilled with cemented paste backfill. International Journal of Mining Science and Technology 25(5), 721-728. Yang, P., Li, L., Aubertin, M., Brochu-Baekelmans, M., Ouellet, S., 2016. Stability analyses of waste rock barricades designed to retain paste backfill. International Journal of Geomechanics 17(3), 04016079. Yilmaz, E., Fall, M., 2017. Paste tailings management. Springer. Yumlu, M., Guresci, M., 2007. Paste backfill bulkhead monitoring: A case study from Inmet’s Cayeli Mine, Turkey, Proceedings of the 9th International Symposium in Mining with Backfill. Montréal, Quebec, Canada. Zhang, W., Li, W., Yang, N., Wang, Q., Li, T., Wang, G., 2017. Determination of the bearing capacity of a Concrete-filled Steel Tubular arch support for tunnel engineering: Experimental and theoretical studies. KSCE Journal of Civil Engineering 21(7), 2932-2945. Zhao, B., Pei, X., 2019. The Evolution of Environmental Protection Tax in China. Modern Management 9(3), 389-392.
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Journal Pre-proof Figures caption list: Fig. 1. Stope layout: a perpendicular to orebody strike, b parallel to orebody strike. Fig. 2. Barricade construction arrangement: a perpendicular to orebody strike, b parallel to orebody strike. Fig. 3. Brick barricade design and engineering construction. Fig. 4. Numerical model during stability analysis. Fig. 5. Calculation steps for numerical modelling: a model equilibrium, b stope excavation, c–f stope backfilling. Fig. 6. Barricade stress state analysis. Fig. 7. Implementation procedure of proposed barricade: a bolt installation, b steel pipe installation, c mesh reinforcement installation, d wood board installation, e earthwork cloth installation, f cross-section of proposed barricade. Fig. 8. Experimental slump value of CPB slurry at different cement-tailing ratios and solid contents. Fig. 9. Beam forces of barricade under different backfilling heights using three-day mechanical properties. Fig. 10. Influence of h and CTR on P and Mmax from analytical solution: a CTR = 1:4; b CTR = 1:8; c CTR = 1:20. Fig. 11. Maximum plate stresses in CTR = 1:4 and h = 1.5 simulation scenarios. Fig. 12. Maximum horizontal beam stresses in CTR = 1:4 and h = 1.5 simulation scenarios. Fig. 13. Maximum vertical beam stresses in CTR = 1:4 and h = 1.5 simulation scenarios.
27
Journal Pre-proof Fig. 14. Influence of h, l, and CTR on maximum stresses in barricade. Fig. 15. Engineering application of proposed barricade: a bolt and steel pipe installation, b mesh reinforcement and wood board installation, c earthwork cloth installation, d concrete seam and slot fill, e–f steel support installation. Fig. 16. In-situ images of CPB after barricade removal.
28
Journal Pre-proof Table 1 Materials, advantages and disadvantages of barricades. Type
Materials
Advantages
Disadvantages
Concrete
Reinforced concrete
High capacity and
Difficult to construct; large and
barricade
and mousetrap drains.
low permeability.
specialized labour required; long curing time; high cost.
Timber
Filter cloth; wood
Easy construction;
Hard to control barricade quality;
barricade
board; steel support;
low cost; free
labour intensive; low stiffness;
mesh reinforcement.
draining; reusable.
difficult to transport materials.
Brick
Filter cloth and
High stresses; free
Non-reusable; long curing time;
barricade
concrete bricks.
draining; standard
prone to variability; long
construction method.
construction time.
Cable
Cables; burlap; chain
Easy to construct
Low strength at the beginning of
sling
link; fabric.
and limited amount
backfill; difficult to retain backfill
of material used.
material behind screen.
barricade
29
Journal Pre-proof Table 2 Laboratory tests conducted for each type of materials. Materials type
Laboratory tests
CPB
Specific gravity, grain size distribution, x-ray diffraction, slump tests, setting time detection tests, uniaxial compressive strength (UCS), triaxial compressive tests (TCT), and Brazilian split tests (BST).
Rock mass and orebody
Specific gravity, UCT, TCT, and BST.
Barricade materials
Specific gravity and uniaxial tensile strength (UTS).
Table 3 Setting time of CPB slurry.
CTR 1:4 1:8 1:20
Initial setting time (hh:mm) Sample 1 Sample 2 Average 23:10 23:50 23:30 39:00 38:50 38:55 50:00 50:10 50:05
Final setting time (hh:mm) Sample 1 Sample2 Average 24:00 24:40 24:20 40:50 39:48 39:49 51:30 51:50 51:40
Table 4 Physical and mechanical properties of CPB after 3-days curing. CTR
S t / m 3
3 B t / m 3
Rc/MPa
C/MPa
/()
E/GPa
1:4 1:8 1:20
1.84 1.81 1.61
1.99 1.95 1.73
0.92 0.43 0.05
0.25 0.18 0.01
24 24 22
0.22 0.16 0.02
0.28 0.28 0.36
Table 5 Physical and mechanical properties of the orebody and rock mass. Material type Orebody Rock mass
t / m 3
Rc / MPa
Rt / MPa
C/MPa
/()
E/GPa
3.00 2.71
13.00 9.00
2.40 0.684
38.00 36.00
4.80 4.31
0.21 0.22
30
Journal Pre-proof Table 6 Physical and mechanical properties of the barricade materials. Materials
t / m3
Steel pipe 7.80 Rebar 7.60 Wood board 0.63
s /MPa
/MPa
E/GPa
215 220 44
143 146 12.6
51.3 208 1.0
0.20 0.20 0.06
Table 7 Comparison between analytical solutions and numerical modelling.
hb (m) 1.0 2.0 3.0
Analytical solution values P (t) Mmax (t·m) 4.14 0.24 16.56 1.51 37.26 3.94
Numerical modelling values P (t) Mmax (t·m) 3.77 0.25 17.18 1.51 37.30 3.94
Deviation P (%) 8.9 3.7 1
Mmax (%) 1 0 0
Table 8 Construction time comparison between the proposed barricade and the brick barricade. Barricade
Construction items
Time (hours)
Brick
Bottom slot, brick wall and horizontal bolt.
24
Surrounding concrete and shotcrete.
8
Curing.
40
Total
72
Bottom slot and bolt.
8
Rebar skeleton, mesh reinforcement, wood
8
barricade
Proposed barricade
boards, earthwork cloth and steel support. Shotcrete and curing
24
Total
40
31
Journal Pre-proof Table 9 Reusability tests of the main barricade materials. Steel pipe /MPa
Wood board /MPa
Mesh reinforceme
Cycle Sampl Sampl Sampl s
e
e
e
Sampl Sampl Sampl Averag
e
e
e
e 1
2
3
1
141
140
142
2
140
139
3
137
4
nt
Averag e
deterioratio
1
2
3
141
13
12
11
12
None
138
139
12
11
10
11
None
136
138
137
10
10
10
10
None
133
133
136
134
9
7
8
8
Mild
5
132
130
128
130
5
7
6
6
Mild
6
125
124
126
125
--
--
--
--
Mild
7
122
118
120
120
--
--
--
--
Mild
8
111
113
115
113
--
--
--
--
Mild
9
99
101
100
100
--
--
--
--
Medium
n
Note: the reusability of mesh reinforcement is characterized by the number of breaking near cross-over points. None represents less than 1, Mild represents between 1-5, medium represents more than 5. The mesh reinforcement will not be reused when the number of breaking is more than 5.
32
Journal Pre-proof Table 10 Material and operation costs of the brick barricade Unit price Total price Items
Size
Quantity
Unit (RMB)
(RMB)
Bricks
4.5m×3.75m×0.75m 12.66
m3
870
11014
Plastering
4.5m×3.75m×2
33.75
m2
40
1350
0.13m/each
2
--
100
200
Φ10mm steel
5m/each
7
m
21
735
In total
--
--
--
--
13299
Iron drain-pipes with flange
Note: the unit price includes both material and operation cost.
33
Journal Pre-proof Table 11 Material and operation costs of the proposed barricade Items
Φ70mm steel
Size
Quantity
Unit
Unit
Total
price
price
(RMB)
(RMB)
5 m/each
18
m
40
3600
0.5m/each
30
m
30
450
Wood board
(4000mm×200mm×30mm)/each
20
25
500
Φ6mm mesh
5m×4m
m2
20
400
Earthwork cloth
5m×4m×3mm
m2
5
100
Iron drain-pipes
0.13m/each
2
100
200
Steel nail
--
2
bag
42
84
Cement
--
2
bag
50
100
Electrode
--
2
bag
35
70
Bolt
--
10
5
50
Joint sleeve
(Φ50mm×150mm)/each
30
10
300
Steel card
--
30
2
60
--
3
30
90
pipes Φ36mm rockbolt
reinforcement
with flange
Anchoring agent
bag
Operation cost
3200
In total
9204
Note: the recycled materials price has been reduced considering the price of new materials. 34
Journal Pre-proof Highlights:
A light barricade is proposed for tailings recycling as cemented paste backfill.
The structure, material and key construction technology are introduced.
The required experiments and process parameters are introduced.
Its capability is verified at an engineering case, the Shirengou Iron Mine (SIM).
The proposed barricade is efficient, reusable and cost-effective.
Hongjian Lu, Chongchong Qi, Chenghe Li, Deqing Gan, Yingnan Du, Sheng Li PII:
S0959-6526(19)34258-1
DOI:
https://doi.org/10.1016/j.jclepro.2019.119388
Reference:
JCLP 119388
To appear in:
Journal of Cleaner Production
Received Date:
08 May 2019
Accepted Date:
18 November 2019
Please cite this article as: Hongjian Lu, Chongchong Qi, Chenghe Li, Deqing Gan, Yingnan Du, Sheng Li, A light barricade for tailings recycling as cemented paste backfill, Journal of Cleaner Production (2019), https://doi.org/10.1016/j.jclepro.2019.119388
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
Amount of words: 7368
Journal Pre-proof
A light barricade for tailings recycling as cemented paste backfill
Hongjian Lu 1, Chongchong Qi 2*, Chenghe Li 3, Deqing Gan 1, Yingnan Du 3, Sheng Li 3 1
Hebei Province Key Laboratory of Mining Development and Safety Technique, North
China University of Science and Technology, Tangshan 063210, China. 2
School of Resources and Safety Engineering, Central South University, Changsha 410083,
China 3
Shirengou Iron Mine, HBIS Group Mining Company, Tangshan 064200, China.
*
Corresponding author: Chongchong Qi, Email: [email protected], Tel: +86
17361393317
1
Journal Pre-proof
A light barricade for tailings recycling as cemented paste backfill
Abstract: The cemented paste backfill (CPB) technology that recycles waste tailings to fill underground voids is crucial for cleaner production of mineral resources. A main challenge in its application is the generally high cost and long time-period incurred to ensure the stability of the barricades (retaining wall structures). To circumvent these trends, a new light barricade with improved construction efficiency, mitigated material usage, and reduced operational cost is presented in this paper. The proposed barricade comprises recycled waste products from mining operations as main construction materials. These include rebar skeletons, mesh reinforcement, wood board, earthwork cloth, and steel support. The new barricade concept has been discussed in detail. This is followed by the description of an engineering application of the barricade in Shirengou Iron Mine (SIM). The backfill height in a single operation (h) and the distance from the barricade to the stope drawpoint (l) can be optimized using numerical simulations. These parameters were determined to be 1.5 m and 4–6 m, respectively, for SIM. The SIM implementation of the proposed barricade illustrates that it could reduce construction time from the 72 h for a brick barricade to 40 h, reuse steel pipes and mesh reinforcement for 6–8 cycles and wood board for 3–4 cycles, and decrease the barricade cost from 1,050 RMB/m3 to 727 RMB/m3. Therefore, the present study offers an efficient, reusable, and economical strategy for barricade construction that can be applied to the CPB practice in underground mines. Keywords: tailings recycling; cemented paste backfill; light barricade; construction method; parameters determination; efficient, reusable and cost-effective.
1
Journal Pre-proof Nomenclature BST
Brazilian split tests
C
Cohesiom
CPB
Cemented paste backfill
CTR
Cement-tailings ratio
Cu and Cc
Coefficient of uniformity and curvature
Gs
Specific gravity of tailings
GSI
Geological Strength Index
SIM
Shirengou Iron Mine
TCT
Triaxial compressive tests
UCS
Uniaxial compressive strength
UTS
Uniaxial tensile strength
E
Elastic modulus
h
The backfill height in a single operation
H and W
Height and width of the barricade
Hb
Height of the CPB slurry
k
Coefficient of permeability
l
The distance from the barricade to the drawpoint
Mmax
Maximum bending moment
φ
Internal friction angle
γs or γ
Specific gravity of the CPB slurry or mixture
3B
Specific gravity of CPB after 3 days curing
P
Total pressure
Rc
Uniaxial compressive strength
2
Journal Pre-proof Rt
Uniaxial tensile strength
Poisson’s ratio
s
Yield stress
Permissible stress
3
Journal Pre-proof 1. Introduction The waste tailings produced in mineral excavation have received increasing attention from both the academy and industry. A widely accepted solution for tailing management is to recycle them as cemented paste backfill (CPB) material in underground voids. This procedure can increase mining productivity, reduce surface-disposed tailings, and improve safety in mining operations (Chen et al., 2018; Cihangir et al., 2015; Fall and Nasir, 2010; Kesimal et al., 2005; Lu et al., 2018; Qi et al., 2018a; Yilmaz and Fall, 2017). In addition to its operational, economical, and environmental benefits, stringent laws have also promoted the wide applications of CPB. For example, the Environmental Protection Tax Law of the People’s Republic of China (CLI.2.307655) levies 15 RMB/ton for surface-disposed tailings that exceed the maximum permissible capacity of the tailing dams. In contrast, a 50% reduction in resource tax is applied when mineral resources under buildings, railway, and water in China are excavated using the backfill mining method (Zhao and Pei, 2019). Prior to backfilling, barricades (or retaining wall structures) need to be constructed in each of the down-level gallery entrances to prevent the in-rush of fresh CPB. Suitable barricade designs can yield safety and economic benefits by increasing the backfill rate and minimizing discontinuities between successive backfill operations (Hughes et al., 2010). However, barricade design is a complex task that involves a comprehensive understanding of barricade materials as well as of their mechanical performance and operational stresses (Berndt et al., 2007). An inappropriate barricade design could trigger a barricade failure, which further results in catastrophic or tragic consequences such as equipment breakdown and personal injury (Kuganathan, 2001; Yang et al., 2016; Yumlu and Guresci, 2007). In practice, every barricade can be made safe with adequate construction materials and improved construction techniques. However, the trade-off between safety, efficiency, and cost is the main concern during the selection of a barricade. 4
Journal Pre-proof A diverse range of barricades have been proposed, which can be categorized into four main types: concrete barricade, timber barricade, brick barricade, and cable sling barricade (Hughes, 2008). Each of these barricade types has been widely adopted by mine sites with due considering of their advantages and disadvantages. However, it should be emphasized that the attempt to improve the barricade structure cannot be abandoned to make it more suitable for each specific mining condition. Moreover, the proposed barricade should be validated for the engineering application before it can be adopted and enhanced by practitioners from other mine sites. From the authors’ experience, a better barricade is an urgent need for underground mines using the CPB technology. This work is aimed at proposing a novel barricade with enhanced efficiency, reusability, and cost-effectiveness, while with safety characteristics similar to those of available barricades. To be comparable, the efficiency, reusability, and cost-effectiveness are represented by the construction time, material reuse cycle, and barricade cost. The proposed barricade includes the selection of barricade materials, determination of the construction parameters, and adjustment of CPB practice. The feasibility of the proposed method has been validated through a case study, the Shirengou Iron Mine (SIM). can promote a broader application of the CPB technology and cleaner production of mineral resources. In Section 2, a brief introduction of contemporary barricade design methods is provided. The materials and methods including study site, barricade materials, lab experiments, determination of barricade and backfill parameters, and the barricade construction procedure are explained in Section 3. Then, a case study is presented in detail to validate our proposed barricade. Finally, the main observations are summarized, and future works are indicated.
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Journal Pre-proof 2. Literature review Concrete barricade to retain hydraulic backfill material was first proposed in the 1970s (Hughes, 2008) and was applied to paste backfill for the first time in 1975 (Mitchell et al., 1975). Since then, it has been widely adopted (even with continuous modification for each engineering case) by underground mines that adopt CPB practice. A concrete barricade is generally constructed with a width of 1 m and with mousetrap drains for water dissipation. Timber barricade consists of vertical timbers, laminated beams, and draining fabric. It was previously used as barricade with a high draining capability, for cemented hydraulic backfill. Construction details of timber barricade are provided in Ref (Smith and Mitchell, 1982). They illustrate that a timber barricade could withstand up to 95.8 kPa before yielding and is suitable for headings of less than 4.3 m. Brick barricade has been gaining popularity from the 1980s for replacing the traditional wooden barricade. It is constructed as an arch from a mortar made of water, cement, sand, and gravel. The utilization of porous bricks imparts a brick barricade with adequate permeability for use in CPB and hydraulic backfill. A concern with regard to the adoption of brick barricades is that their strength is highly dependent on the construction scenarios and thus can vary significantly (Kuganathan, 2001). Berndt et al. (2007) provided a thorough literature review on brick barricades in underground mines, including barricade design and construction and their advantages. Although the cable sling barricade is not the first choice in most cases, it is still adopted in several operations worldwide, e.g., in the South Gold Mine region in South Africa and the Campbell Gold Mine in Canada (Hughes, 2008). The construction of the cable sling barricade starts with the bolting of horizontal and vertical cables into the neighboring rock mass. Then, burlap is used to cover the bolted cable to function as the fence for the backfill material. An 6
Journal Pre-proof extensive introduction to the cable sling barricade has been provided by Lang et al. (1999), which was designed for backfill retention in longhole stopes. Table 1 summaries the materials, advantages, and disadvantages associated with each barricade type. As illustrated, there are several drawbacks in the above methods, which limit their more extensive applications. For example, concrete barricades are limited by their high cost and long curing time, whereas timber barricades are restricted as it is challenging to control their qualities. Therefore, a better barricade is desired for a broader application of the CPB technology. 3. Materials and methods 3.1. Study site The proposed barricade would be applied to SIM, which is located 90 km from Tangshan City, Hebei province, China. The mine was established in 1975 using open-pit mining to excavate the near-surface ore deposits. With ore deposits extending to “great depth,” sublevel open stoping has been adopted since 2004 owing to its advantages such as high production rate and simultaneous unit operations (Qi et al., 2018b). After decades of excavations, 129 normal stopes and several illegal stopes have been created in the first underground level (-60 m). Then, sublevel open stoping with subsequent backfill is used to excavate the ore deposits at the level between -60 m and -180 m. According to the thickness of orebody, two stope layouts were considered, which were in the direction perpendicular to or along the strike of orebody (Fig. 1). Fig. 1a illustrates the layout in the direction perpendicular to the orebody. In such a stope layout, the length of each of first mining rooms (20 m wide and 105 m high) and second mining rooms (30 m wide and 105 m high) was equal to the thickness of the orebody. In the case of the layout in the direction parallel to the orebody (Fig. 1b), the width of each of first 7
Journal Pre-proof mining rooms (15 m long and 105 m high) and second mining rooms (35 m long and 105 m high) was equal to the thickness of the orebody. For either of the cases, the sublevel height was 15 m. In addition, the first and second mining rooms were arranged at intervals with top pillars of 10–15 m in height. After the ore was mined from the first mining rooms, the goaf were backfilled with CPB as artificial pillars. The cement-tailing ratio (CTR) of the CPB within a height of 6 m from the bottom was selected to be 1:4. The CTR for the remaining first mining rooms was 1:8. For the second mining rooms, the CTR was 1:4 and 1:20 for within and above 6 m, respectively. The higher cement usage within 6 m is to recycle the ore pillar. The excavation and backfill of the first mining room and second mining room is referred to as a mining unit in the following. Prior to the backfill practice, barricades need to be constructed to retain the CPB in the stopes. The barricade construction arrangements for a mining unit in the perpendicular and parallel stope layouts are illustrated in Fig. 2. As shown, 14 barricades and 17 barricades are required in a mining unit in the perpendicular and parallel stope layouts, respectively. For SIM, it is estimated that 200 barricades need to be constructed annually to fulfil the mining output of two million tons. The average barricade size is 4.5 (width) × 3.75 (height) × 0.75 (thick) m. Brick barricades with a construction cost of 1,050 RMB/m3 and an average construction time of 72 h per barricade were selected (Fig. 3). 3.2. Barricade materials Considering the advantages and disadvantages of traditional barricades (the horizontal stresses caused by CPB, cost, and construction practice), an efficient, usable, and costeffective barricade was proposed for the SIM practice. The main barricade materials for the proposed barricade design include rebar skeleton, mesh reinforcement, wood board, earthwork cloth, and steel support. These materials were previously used as supporting
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Journal Pre-proof materials in underground roadways and recycled as barricade materials. Each of the materials can be described as follows: Rebar skeleton is the outermost layer of the proposed barricade and consists of bolts and steel pipes. The diameter and length of the bolts are 36 mm and 500 mm, respectively. These bolts are bolted into the rock mass for at least 300 mm and fixed with the anchoring agent. The rest of the bolts are fitted in the steel pipe of diameter 50 mm and length 150 mm. Steel pipes of outer diameter 70 mm and inner diameter 65 mm are used as the primary material in the rebar skeleton, which are connected to the steel pipe of diameter 50 mm. The rebar spacing needs to be less than 0.5 0.6 m when the barricade area is larger than 16 m2. Angular tendons are used in all the intersections of the steel pipes to increase the overall strength of the rebar skeleton. Two filter tubes are constructed to improve the draining of the barricade. Mesh reinforcement is the second layer of the barricade. It has a diameter of 6.5 mm and mesh size of 50 50 mm, which is bandaged to the rebar skeleton. Wood boards of size 4000 200 30 mm are used. The space between adjacent boards is less than 5 mm, and the board needs to cover the whole barricade area except the observation hole. Earthwork cloth is a type of ϕ6.5 mm nonwoven geotextile. The earthwork cloth is fixed to the wood board using the steel nail and is at least 20 cm longer than the barricade in all directions. The redundant earthwork cloth at the top and both sides is reinforced by the brick structure, and the redundant earthwork cloth is placed in the slot (width = 500 mm and height = 300 mm). Shotcrete is used at the periphery of the contact area between the barricade and drift.
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Journal Pre-proof Steel support of diameter 70 mm is used to further improve the strength of the barricade. The end of the steel support is anchored to the drift floor. 3.3. Lab experiments A successful barricade is dependent on a comprehensive understanding of the material properties. Table 2 shows the main laboratory tests conducted for CPB, rock mass, orebody, and barricade materials. A brief introduction to each type of experiment is provided in the following sections. Detailed explanations can be as conveniently obtained from relevant references (Chen et al., 2017; Liu et al., 2018; Lu et al., 2018). The authors note here the lab experiments are mainly performed for the stability analysis of barricade rather than for the property of CPB. Therefore, a few properties of fresh or hardened CPB, such as bleeding, segregation, and air content, are not explained here. The feasibility of CPB in SIM has been reported in (Lu et al., 2018). Specific gravity was obtained followed the experimental procedure in (ASTM C127, 2007). Grain size distribution of the mine tailings was determined by a Malvern Mastersizer 2000 laser diffraction analyzer. X-ray diffraction analyses were conducted on the mine tailings to determine their chemical compositions. A Bruker AXS D8 Advance Diffractometer was used with a 5 – 90 ° 2θ range and a 0.005 step size. The chemical compositions were qualified by the Rietveld method. Slump tests were conducted to test the consistency of the CPB slurry using a slump cone (internal top diameter = 100 mm and internal bottom diameter = 200 mm). The slump cone was filled with CPB slurry before it was vertically removed. The slump test value was calculated to be the vertical distance between the top of the slumped 10
Journal Pre-proof CPB and that of the slump cone. Wu et al. (2015) demonstrated that a slump value of 170–250 mm is essential for the pumping transport of CPB slurry. Setting time detection tests were employed to detect the initial and final setting time of the CPB slurry. In this paper, a Vicat apparatus of the IOS-standard type (Dahong, Hebei) was used. UCS, TCS, BST, and UTT were conducted using an MTS 815.03 material testing system according to recommendations in ACI 229R-13 (Jang et al., 2018). UCT, TCT, and BST were performed on specimens from the CPB, rock mass, and orebody, whereas UTT was performed on specimens from the barricade materials, namely the rebar, steel pipe, and wood board. The diameter and height of the UCT and TCT specimens were 50 mm and 100 mm, respectively, whereas those of the BST specimens were 50 mm and 50 mm, respectively. For UTT, the rebar and steel pipe specimens were prepared to be of length L = 5d + 150 mm (d = diameter), whereas the wood board specimens were prepared to be of length L = 5.63 A + 150 mm (A is the board area). The force applied on the earthwork cloth and mesh reinforcement is relatively small as they are used here primarily to prevent the leakage of the CPB slurry and rotation of the wood boards, respectively. Therefore, their mechanical properties were not considered during the stability analysis in the numerical modeling. At least three experiments were carried out for each experimental scheme, and their average values were used for further analysis. The Geological Strength Index (GSI) system was used to transform intact mechanical properties to in-situ properties so that they could be used in numerical modeling (Deisman et al., 2013; Rafiei Renani et al., 2016).
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Journal Pre-proof 3.4. Determination of barricade and backfill parameters To ensure the stability of the barricade, the barricade and backfill parameters need to be determined properly for SIM by considering the interaction among the different materials. In this work, we focused on two important parameters: the backfill height in a single operation (h) and the distance from the barricade to the stope drawpoint (l). These two parameters can be determined through stability analysis using numerical modeling. When CPB slurry is transported to underground stopes, it transforms from the “homogeneous fluid” state (C = 0,
= 0) to the “solid” state (C > 0, > 0). Here, C represents cohesion, and represents internal friction angle (Cui and Fall, 2017; Yang and Li, 2015; Yang et al., 2016). A detailed explanation of the stability analysis through numerical modelling is provided in (Yang and Li, 2015; Yang et al., 2016). The authors note here that whereas the following numerical modeling was prepared for the backfill practice in SIM, the parameters for other mine sites can be determined as conveniently as for SIM. MIDAS-GST was used to model the mechanical response of the barricade and its interaction with the surrounding rock mass. Fig. 4 shows a typical stope in SIM of length 33 m, width 20 m, height 15 m, and dip angle 60°. The length, width, and height of the drift was 12 m, 4.5 m, and 3.75 m, respectively. To eliminate the boundary influence on the accuracy of numerical modeling, the whole model was 220 m, 140 m, and 105 m in length, width, and height, respectively. A traditional mesh conforming technique was used for the mesh generation, and each numerical model contained 10.962 to 11,508 meshes (Fig. 4). The natural, in-situ stresses were calculated by considering the overburden weight. The steel pipe and rebar were modeled as a homogeneous, isotropic material obeying the linear elastic criterion. The rock mass and orebody were modeled using the Mohr–Coulomb criterion. The Drucker–Prager criterion was applied to the CPB materials including the CPB
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Journal Pre-proof slurry and CPB body after curing. The von Mises criterion was used to model the wood board. Plate element, beam element, and solid element were used for the wood board, steel pipe and rebar, and the other materials, respectively. The model selection was based on engineering experiences and recommendations in the literature (Hui et al., 2016; Liu et al., 2017; Luo et al., 2017; Zhang et al., 2017). The model parameters were obtained considering laboratory results and the GSI system. Fig. 5 illustrates the calculation steps for each numerical simulation, which are also listed below: 1) The boundary conditions were set, and the numerical model was meshed. The initial stresses were calculated after the whole model was brought into equilibrium (Fig. 5a). 2) The stope was excavated in a single step to simulate the drill and blast processes (Fig. 5b). 3) Each time, the stope was progressively backfilled with a backfill height of h (Fig. 5c– d). The time interval between two backfill practices was three days. The material properties of CPB after curing for at least three days were altered to the mechanical properties at three days. Section 4.2.1 illustrates that further increases in the mechanical properties after three days of curing exerted negligible influence on the barricade pressures. 4) The stope was backfilled in one step once the progressive-backfilled height was larger than the barricade height (Fig. 5e). The numerical model was validated using an analytical solution. In the modeling validation, the stress distribution in the barricade immediately after backfilling was compared. In such cases, the CPB slurry can be considered to be a Bingham plastic fluid (Boger et al., 2006). Therefore, the influence of l on the stress distribution can be omitted (Yang and Li, 2015;
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Journal Pre-proof Yang et al., 2016). During the numerical modeling, multiple l values were used for each hb and the average of the numerical results was used for comparison. The stress state of the barricade is shown in Fig. 6. Considering equilibrium conditions, the total pressure (P) and maximum bending moment (Mmax) can be calculated as follows:
1 P s hb 2 W 2 M max
s 6H
hb 3 ( H hb
2hb hb ) 3 3H
(2)
(3)
where H and W are the height and width of the barricade, respectively; hb is the height of the CPB slurry; and s is the specific gravity of the CPB slurry. 3.5. Barricade construction procedure Fig. 7 illustrates the implementation procedure of the proposed barricade. As shown, the implementation of the proposed barricade is performed through the following steps: bolt installation, steel pipe installation, mesh reinforcement installation, wood board installation, and earthwork cloth installation. The authors note here that the steel support is optional in the proposed barricade; it is required where high horizontal pressures are observed. During the case study, the steel support is not used considering the surrounding rock conditions and based on engineering experience. 4. Results and discussion In this section, the implementation of the proposed barricade in SIM is described. First, the materials were characterized using the lab experiments results and the GSI system. Secondly, the h and l were determined through an extensive numerical study. Thirdly, the proposed barricade was implemented in the mine site and compared to the previous brick barricade.
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Journal Pre-proof 4.1. Characterization of materials Fig. 8 shows the experimental slump value with different cement-tailings ratios (CTRs) and solid contents. The CTRs of CPB were selected to be 1:8, 1:10, and 1:20 and those of solid contents were selected to be 68%, 72%, 75% 78%, and 81%. Note that a relatively high cement content was used in this study because the Portland cement was based on China's standard (No.GB175-2007). As is evident, the slump value increased as the solid content decreased. The slump values were less than 17.5 cm at 81% solid content. In contrast, the slump values were at least 26 cm when the solid content was increased to 72–75%. Based on the slump test results, CTRs of 1:4, 1:8, and 1:20 and solid content of 72% were selected for further analysis. Table 3 shows the setting time results of the CPB slurry under different CTRs. As a cementbased composite, CPB depends substantially on cement hydration for strength development (He et al., 2019; Wang et al., 2018; Xie et al., 2019). Table 4 illustrates the physical and mechanical properties of CPB after three days of curing with different CTRs. In Table 4, s represents the specific gravity of CPB slurry, 3B represents the specific gravity of CPB after three days of curing, Rc represents the uniaxial compressive strength, C represents the cohesion, represents the internal friction angle, E represents the elastic modulus, and
represents the Poisson’s ratio. Table 5 presents the physical and mechanical properties of the rock mass and orebody. Here, represents specific gravity, and Rt represents the uniaxial tensile strength. Similarly, properties of the steel pipe, rebar, and wood board are presented in Table 6. In this Table, s is the yield stress, and is the permissible stress. As illustrated, the safety factor was set to 3.5 for the wood board and 1.5 for the steel pipe and rebar.
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Journal Pre-proof 4.2. Numerical validation, simulation scenarios, and results 4.2.1. Numerical validation Table 7 illustrates the comparison between the analytical solutions (Eqs 1 and 2) and the present numerical simulation results. As shown, the maximum deviations between the analytical and numerical P and Mmax were 8.9% and 1.0%, respectively, when hb =1 m. This illustrates that the numerical modeling agreed well with the analytical solutions. Fig. 9 shows the beam forces of the barricade with backfill height when the mechanical properties of CPB after three days of curing was used during the simulation. As shown, the maximum beam forces were quite small when the three-day mechanical properties were used. This implies that the CPB body can be self-standing with the three-day mechanical properties and that further adjustment of mechanical properties is not necessary for CPB materials in SIM. 4.2.2. Simulation scenarios Various numerical simulations were conducted for determining h and l. In practice, the CTR for each stope backfilling is different based on the stope stability requirements. Therefore, the determination of h and l needs to consider the CTR. In SIM, three scenarios were used: 1) CTR = 1:4 for the bottom 6 m and CTR = 1:8 for the remaining 9 m; 2) CTR = 1:8 for the bottom 6 m and CTR = 1:8 for the remaining 9 m; 3) CTR = 1:20 for the bottom 6 m and CTR = 1:20 for the remaining 9 m. A preliminary selection of h was performed using the analytical solution (Eqs 2 and 3) as the stresses on the barricade are more evident when the CPB is in a slurry state (Cui and Fall, 2017). Fig. 10 illustrates the variation of P and Mmax when h was increased from 0.5 m to 3.5 m. Note that the CTR in a stope was maintained constant during the preliminary selection. As shown, P and Mmax increased non-linearly with h. A rapid increase in P and Mmax was 16
Journal Pre-proof triggered when h = 1.5 m or h = 2.0 m. Therefore, h = 1.5 m and h = 2.0 m were selected for further analysis. In mining operations, barricades are generally constructed 0–12 m away from the stope drawpoint, i.e., 0 m < l < 12 m (Yang and Li, 2015). Therefore, 1 m, 3 m, 5 m, 7 m, 9 m, and 11 m were selected for l. An orthogonal numerical experiment was conducted on three CTR scenarios, two h values, and six l values, resulting in a total of 36 numerical models in the stability analysis for determining h and l. 4.2.3. Numerical results Figs. 11–13 show the maximum stresses of the plate and beam elements in the simulation when the CTR was 1:4 and h was 1.5 m. The authors note here that CTR = 1:4 and h = 1.5 was used as an example and that the stress analysis for other scenarios can be performed as conveniently following this procedure. As discussed before, the plate element was used to simulate the wood board, whereas the beam element was used to simulate the steel pipe and rebar. During the simulation, the stresses in the steel pipe and rebar were analyzed in combination because they were all beam elements and with similar strength (Table 6). For each l value, the stope was backfilled successively in several steps, and the stress was monitored for each step. The hx values in Figs. 11–13 show the backfill step when the maximum stress was observed. For example, h2 implies that the maximum stress was observed after the backfilling of the second 1.5 m. That is, the maximum stress was observed after 3 m of backfilling. Only the maximum stresses were analyzed for each simulation scenario (each combination of h, l, and CTR) as this is when a barricade failure is most likely. As shown, the maximum plate stress was observed at different hx for various l. For example, the maximum plate stress for l = 1 m was observed after 4.5 m of backfilling (h3) whereas it was observed after 1.5 m backfilling (h1) for l = 3. Thus, no relationship was evident between
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Journal Pre-proof hx and l. Similar results were obtained for the horizontal and vertical beam stresses. It is noteworthy that the maximum plate stress was not always observed at the same backfilling height with the maximum beam stresses. Moreover, different hx can be observed even between the maximum horizontal and vertical beam stresses (i.e. the maximum horizontal beam stress occurred at h1 for l = 9, whereas the maximum vertical beam stress occurred at h3 for l = 9). Therefore, the maximum stress as well as its corresponding hx values cannot be obtained without a thorough simulation of the whole backfilling process. The maximum plate stress was generally observed to be 1.4–2.9 m above the barricade floor. The horizontal beam stress was more localized on several beams near the middle of the barricade, resulting in a larger maximum horizontal beam stress than the maximum vertical beam stress. Moreover, the maximum vertical beam stress was observed to be 0.8–1.5 m above the barricade floor. Fig. 14 summarizes the maximum plate and beam stress for all the 36 numerical models. As shown, the variation pattern of maximum stresses versus l was controlled by h rather than by CTR. To be more specific, similar variation patterns of maximum stresses versus l were observed for all the CTR values once h was fixed. When h = 1.5 m, stable maximum stresses were obtained at l > 3 m. The maximum plate stresses, maximum horizontal beam stresses, and maximum vertical beam stresses were 4.0–5.0 MPa, 85.0–99.0 MPa, and 72.0–80.0 MPa, respectively. In contrast, the maximum plate stresses, maximum horizontal beam stresses, and maximum vertical beam stresses at h = 2.0 m were 9.3–10.5 MPa, 140.0–150.0 MPa, and 160.0–200.0 MPa, respectively. Compared with the material properties in Table 6 and in-situ construction environments, h and l were determined to be 1.5 m and 4.0–6.0 m, respectively, for SIM.
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Journal Pre-proof 4.3. Barricades implementation The barricade proposed in this paper was implemented in SIM. As discussed before, backfilling was performed in several operations, with each backfill height being 1.5 m (when the total backfill height is less than 6.0 m). The barricade was constructed 4.0–6.0 m away from the stope drawpoint considering the construction requirements. To facilitate the barricade construction, the construction site generally needs to exhibit the following characteristics: 1) intact surrounding rock properties; 2) without significant fractures; 3) small barricade area; and 4) flat drift roof and floor. The in-situ construction process is shown in Fig. 15. Curing for 8 h was required for the concrete during the seaming and slot filling. Fifteen days after the backfilling, the barricade could be removed and reused for constructing the barricades described below. The in-situ images after the barricade removal are shown in Fig. 16. As shown, the CPB body exhibited smooth surfaces and remained intact subsequently. These characteristics are desirable for barricade construction. In this section, we compared the efficiency, reusability, and cost-effectivity of the proposed barricade with those of the brick barricade adopted previously in the SIM. For efficiency, we focused on the construction time of the barricade. The construction time comparison between the proposed barricade and the brick barricade is illustrated in Table 8. As shown, the construction time for a brick barricade was 72 h. This construction time has been reduced to 40 h by employing the proposed barricade. Therefore, there is a significant improvement in the construction efficiency of barricade. With regard to the reusability, it is well accepted that a brick barricade cannot be reused (at least in most cases) during CPB application. However, the engineering application of the proposed barricade in SIM reveals that its main construction materials (steel pipe, wood 19
Journal Pre-proof board, and mesh reinforcement) can be reused for several cycles. The reusability tests were conducted to determine the number of cycles for which the main barricade materials can be reused. In the reusability test, was calculated using the safety factor presented in Table 6 and was compared to the numerical results (the needs to be larger than or approximately equal to the required strength from the numerical modelling). Table 9 presents results of the reusability tests in SIM. As shown, the steel pipe and mesh reinforcement can be reused for 6–8 cycles, and the wood board can be reused for 3–4 cycles. The authors note that the reusability could vary marginally considering the difference in the in-situ application. Overall, a reuse rate exhibiting high potential has been achieved by the proposed barricade. For the cost, Table 10 and Table 11 compare the material and operational costs of the brick barricade and proposed barricade. It is evident that the barricade cost can be decreased from 1,050 RMB/m3 to 727 RMB/m3 (calculated based on a brick barricade volume of 12.66 m3). For SIM, barricade construction using the proposed barricade design method will save 4,100 RMB, resulting in an annual saving of 0.8 million RMB in barricade construction (calculated based on 200 barricades per year). 4.4. Limitations The first limitation of the present study is that all the numerical simulations were performed assuming complete drainage of the barricade. The CPB material being modeled with the Drucker–Prager criterion is evidently another limitation. The application of more advanced constitutive models such as in (Cui and Fall, 2017) will result in more accurate results. However, the authors consider that the model selection will not influence the illustration of the proposed barricade. A final limitation is that the numerical models were only validated using the analytical solution, and no quantitative in-situ measurements are reported. In-situ
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Journal Pre-proof measurements are presently being conducted. The authors conjecture that more representative model parameters can be obtained using advanced back-analysis methods (Qi et al., 2018b). 5. Conclusions and future works Recycling waste tailings as CPB is fundamental to the cleaner production of mineral resources, and their successful application requires a suitable barricade. Considering the advantages and disadvantages of traditional barricade, this paper proposes an efficient, reusable, and cost-effective barricade for improved recycling of waste as CPB. The proposed barricade uses rebar skeleton, mesh reinforcement, wood board, earthwork cloth, and steel support as the main construction materials. It was verified on an engineering instance, the SIM, based on which the following conclusions can be drawn: 1. The h and l were determined to be 1.5 m and 4.0–6.0 m for successfully applying the barricade and CPB technology. 2. The construction time can be reduced from 72 h (for brick barricades) to 40 h with the proposed barricade. 3. The proposed barricade has a highly potential reuse rate, with the steel pipes and mesh reinforcement having been reused for 6–8 cycles and wood board for 3–4 cycles. 4. The barricade cost has been reduced from 1,050 RMB/m3 to 727 RMB/m3 with the application of the proposed barricade. This can save up to 0.8 million RMB per year for SIM. For future works, the methods to improve the strength of the proposed barricade should be tested so that the proposed barricade can be used under high pressures. Furthermore, more precise methods, such as the multi-criteria decision-making process, should be employed to select the appropriate barricade for a specific mine site. Finally, better incorporation of the 21
Journal Pre-proof proposed barricade into the mining and backfilling system will further improve the efficiency of underground operations.
Acknowledgement This study was supported by the National Natural Science Foundation of China (No. 51774134), the Natural Science Foundation of Hebei province, China (No. E2016209220, No. E2016209277, No. E2018209129). The corresponding author was supported by China Scholarship Council (No. 201606420046). Conflict of interest: None.
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Journal Pre-proof References: Berndt, C.C., Rankine, K.J., Sivakugan, N., 2007. Materials properties of barricade bricks for mining applications. Geotechnical and Geological Engineering 25(4), 449-471. Boger, D., Scales, P., Sofra, F., 2006. Rheological concepts. Paste and Thickened Tailings-A Guide (Second Edition), Jewell and Fourie (eds), Australian Centre for Geomechanics, Perth, Australia, 25. Chen, Q., Zhang, Q., Fourie, A., Chen, X., Qi, C., 2017. Experimental investigation on the strength characteristics of cement paste backfill in a similar stope model and its mechanism. Construction and Building Materials 154, 34-43. Chen, Q., Zhang, Q., Qi, C., Fourie, A., Xiao, C., 2018. Recycling phosphogypsum and construction demolition waste for cemented paste backfill and its environmental impact. Journal of Cleaner Production 186, 418-429. Cihangir, F., Ercikdi, B., Kesimal, A., Deveci, H., Erdemir, F., 2015. Paste backfill of highsulphide mill tailings using alkali-activated blast furnace slag: Effect of activator nature, concentration and slag properties. Minerals Engineering 83, 117-127. Cui, L., Fall, M., 2017. Modeling of pressure on retaining structures for underground fill mass. Tunnelling and Underground Space Technology 69, 94-107. Deisman, N., Khajeh, M., Chalaturnyk, R.J., 2013. Using geological strength index (GSI) to model uncertainty in rock mass properties of coal for CBM/ECBM reservoir geomechanics. International Journal of Coal Geology 112, 76-86. Fall, M., Nasir, O., 2010. Mechanical behaviour of the interface between cemented tailings backfill and retaining structures under shear loads. Geotechnical and Geological Engineering 28(6), 779-790. He, Z., Zhu, X., Wang, J., Mu, M., Wang, Y., 2019. Comparison of CO2 emissions from OPC and recycled cement production. Construction and Building Materials 211, 965-973. 23
Journal Pre-proof Hughes, P., Pakalnis, R., Hitch, M., Corey, G., 2010. Composite paste barricade performance at Goldcorp Inc. Red Lake Mine, Ontario, Canada. International Journal of Mining, Reclamation and Environment 24(2), 138-150. Hughes, P.B., 2008. Performance of paste fill fences at red lake mine. University of British Columbia. Hui, Z., Yang, G., Zhang, C., Xiang, T., 2016. Numerical simulation of reinforced concrete lining considering the interaction with thesurrounding rock. Journal of Hydraulic Engineering 47(6), 763-771. Jang, J.G., Park, S.-M., Chung, S., Ahn, J.-W., Kim, H.-K., 2018. Utilization of circulating fluidized bed combustion ash in producing controlled low-strength materials with cement or sodium carbonate as activator. Construction and Building Materials 159, 642-651. Kesimal, A., Yilmaz, E., Ercikdi, B., Alp, I., Deveci, H., 2005. Effect of properties of tailings and binder on the short-and long-term strength and stability of cemented paste backfill. Materials Letters 59(28), 3703-3709. Kuganathan, K., 2001. Design and construction of shotcrete bulkheads with engineered drainage system for mine backfilling, International seminar on surface support liners, Australian Centre for Geomechanics. pp. 1-15. Lang, B., White, P., Pakalnis, R., 1999. Design and Construction of Tunnel Plugs and Bulkheads. Short Course Presented at CIM-AGM, Calgary. Leonards, GA (1962). Foundation …. Liu, H.-l., Li, S.-c., Li, L.-p., Zhang, Q.-q., 2017. Study on deformation behavior at intersection of adit and major tunnel in railway. KSCE Journal of Civil Engineering 21(6), 2459-2466.
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Journal Pre-proof Liu, L., Fang, Z., Qi, C., Zhang, B., Guo, L., Song, K.-I., 2018. Experimental investigation on the relationship between pore characteristics and unconfined compressive strength of cemented paste backfill. Construction and Building Materials 179, 254-264. Lu, H., Qi, C., Chen, Q., Gan, D., Xue, Z., Hu, Y., 2018. A new procedure for recycling waste tailings as cemented paste backfill to underground stopes and open pits. Journal of Cleaner Production 188, 601-612. Luo, Y., Chen, J., Wang, H., Sun, P., 2017. Deformation rule and mechanical characteristics of temporary support in soil tunnel constructed by sequential excavation method. KSCE Journal of Civil Engineering 21(6), 2439-2449. Mitchell, R.J., Smith, J.D., Libby, D.J., 1975. Bulkhead Pressures Due to Cemented Hydraulic Mine Backfills. Canadian Geotechnical Journal 12(3), 362-371. Qi, C., Fourie, A., Chen, Q., 2018a. Neural network and particle swarm optimization for predicting the unconfined compressive strength of cemented paste backfill. Construction and Building Materials 159, 473-478. Qi, C., Fourie, A., Zhao, X., 2018b. Back-Analysis Method for Stope Displacements Using Gradient-Boosted Regression Tree and Firefly Algorithm. Journal of Computing in Civil Engineering 32(5), 04018031. Rafiei Renani, H., Martin, C.D., Hudson, R., 2016. Back Analysis of Rock Mass Displacements Around a Deep Shaft Using Two- and Three-Dimensional Continuum Modeling. Rock Mechanics and Rock Engineering 49(4), 1313-1327. Smith, J.D., Mitchell, R., 1982. Design and control of large hydraulic backfill pours, International Journal of Rock Mechanics and Mining Sciences and Geomechanics Abstracts. Elsevier Science, pp. 134-134. Wang, J., Mu, M., Liu, Y., 2018. Recycled cement. Construction and Building Materials 190, 1124-1132.
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Journal Pre-proof Wu, A., Wang, Y., Wang, H., Yin, S., Miao, X., 2015. Coupled effects of cement type and water quality on the properties of cemented paste backfill. International Journal of Mineral Processing 143, 65-71. Xie, J., Wang, J., Rao, R., Wang, C., Fang, C., 2019. Effects of combined usage of GGBS and fly ash on workability and mechanical properties of alkali activated geopolymer concrete with recycled aggregate. Composites Part B: Engineering 164, 179-190. Yang, P., Li, L., 2015. Investigation of the short-term stress distribution in stopes and drifts backfilled with cemented paste backfill. International Journal of Mining Science and Technology 25(5), 721-728. Yang, P., Li, L., Aubertin, M., Brochu-Baekelmans, M., Ouellet, S., 2016. Stability analyses of waste rock barricades designed to retain paste backfill. International Journal of Geomechanics 17(3), 04016079. Yilmaz, E., Fall, M., 2017. Paste tailings management. Springer. Yumlu, M., Guresci, M., 2007. Paste backfill bulkhead monitoring: A case study from Inmet’s Cayeli Mine, Turkey, Proceedings of the 9th International Symposium in Mining with Backfill. Montréal, Quebec, Canada. Zhang, W., Li, W., Yang, N., Wang, Q., Li, T., Wang, G., 2017. Determination of the bearing capacity of a Concrete-filled Steel Tubular arch support for tunnel engineering: Experimental and theoretical studies. KSCE Journal of Civil Engineering 21(7), 2932-2945. Zhao, B., Pei, X., 2019. The Evolution of Environmental Protection Tax in China. Modern Management 9(3), 389-392.
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Journal Pre-proof Figures caption list: Fig. 1. Stope layout: a perpendicular to orebody strike, b parallel to orebody strike. Fig. 2. Barricade construction arrangement: a perpendicular to orebody strike, b parallel to orebody strike. Fig. 3. Brick barricade design and engineering construction. Fig. 4. Numerical model during stability analysis. Fig. 5. Calculation steps for numerical modelling: a model equilibrium, b stope excavation, c–f stope backfilling. Fig. 6. Barricade stress state analysis. Fig. 7. Implementation procedure of proposed barricade: a bolt installation, b steel pipe installation, c mesh reinforcement installation, d wood board installation, e earthwork cloth installation, f cross-section of proposed barricade. Fig. 8. Experimental slump value of CPB slurry at different cement-tailing ratios and solid contents. Fig. 9. Beam forces of barricade under different backfilling heights using three-day mechanical properties. Fig. 10. Influence of h and CTR on P and Mmax from analytical solution: a CTR = 1:4; b CTR = 1:8; c CTR = 1:20. Fig. 11. Maximum plate stresses in CTR = 1:4 and h = 1.5 simulation scenarios. Fig. 12. Maximum horizontal beam stresses in CTR = 1:4 and h = 1.5 simulation scenarios. Fig. 13. Maximum vertical beam stresses in CTR = 1:4 and h = 1.5 simulation scenarios.
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Journal Pre-proof Fig. 14. Influence of h, l, and CTR on maximum stresses in barricade. Fig. 15. Engineering application of proposed barricade: a bolt and steel pipe installation, b mesh reinforcement and wood board installation, c earthwork cloth installation, d concrete seam and slot fill, e–f steel support installation. Fig. 16. In-situ images of CPB after barricade removal.
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Journal Pre-proof Table 1 Materials, advantages and disadvantages of barricades. Type
Materials
Advantages
Disadvantages
Concrete
Reinforced concrete
High capacity and
Difficult to construct; large and
barricade
and mousetrap drains.
low permeability.
specialized labour required; long curing time; high cost.
Timber
Filter cloth; wood
Easy construction;
Hard to control barricade quality;
barricade
board; steel support;
low cost; free
labour intensive; low stiffness;
mesh reinforcement.
draining; reusable.
difficult to transport materials.
Brick
Filter cloth and
High stresses; free
Non-reusable; long curing time;
barricade
concrete bricks.
draining; standard
prone to variability; long
construction method.
construction time.
Cable
Cables; burlap; chain
Easy to construct
Low strength at the beginning of
sling
link; fabric.
and limited amount
backfill; difficult to retain backfill
of material used.
material behind screen.
barricade
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Journal Pre-proof Table 2 Laboratory tests conducted for each type of materials. Materials type
Laboratory tests
CPB
Specific gravity, grain size distribution, x-ray diffraction, slump tests, setting time detection tests, uniaxial compressive strength (UCS), triaxial compressive tests (TCT), and Brazilian split tests (BST).
Rock mass and orebody
Specific gravity, UCT, TCT, and BST.
Barricade materials
Specific gravity and uniaxial tensile strength (UTS).
Table 3 Setting time of CPB slurry.
CTR 1:4 1:8 1:20
Initial setting time (hh:mm) Sample 1 Sample 2 Average 23:10 23:50 23:30 39:00 38:50 38:55 50:00 50:10 50:05
Final setting time (hh:mm) Sample 1 Sample2 Average 24:00 24:40 24:20 40:50 39:48 39:49 51:30 51:50 51:40
Table 4 Physical and mechanical properties of CPB after 3-days curing. CTR
S t / m 3
3 B t / m 3
Rc/MPa
C/MPa
/()
E/GPa
1:4 1:8 1:20
1.84 1.81 1.61
1.99 1.95 1.73
0.92 0.43 0.05
0.25 0.18 0.01
24 24 22
0.22 0.16 0.02
0.28 0.28 0.36
Table 5 Physical and mechanical properties of the orebody and rock mass. Material type Orebody Rock mass
t / m 3
Rc / MPa
Rt / MPa
C/MPa
/()
E/GPa
3.00 2.71
13.00 9.00
2.40 0.684
38.00 36.00
4.80 4.31
0.21 0.22
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Journal Pre-proof Table 6 Physical and mechanical properties of the barricade materials. Materials
t / m3
Steel pipe 7.80 Rebar 7.60 Wood board 0.63
s /MPa
/MPa
E/GPa
215 220 44
143 146 12.6
51.3 208 1.0
0.20 0.20 0.06
Table 7 Comparison between analytical solutions and numerical modelling.
hb (m) 1.0 2.0 3.0
Analytical solution values P (t) Mmax (t·m) 4.14 0.24 16.56 1.51 37.26 3.94
Numerical modelling values P (t) Mmax (t·m) 3.77 0.25 17.18 1.51 37.30 3.94
Deviation P (%) 8.9 3.7 1
Mmax (%) 1 0 0
Table 8 Construction time comparison between the proposed barricade and the brick barricade. Barricade
Construction items
Time (hours)
Brick
Bottom slot, brick wall and horizontal bolt.
24
Surrounding concrete and shotcrete.
8
Curing.
40
Total
72
Bottom slot and bolt.
8
Rebar skeleton, mesh reinforcement, wood
8
barricade
Proposed barricade
boards, earthwork cloth and steel support. Shotcrete and curing
24
Total
40
31
Journal Pre-proof Table 9 Reusability tests of the main barricade materials. Steel pipe /MPa
Wood board /MPa
Mesh reinforceme
Cycle Sampl Sampl Sampl s
e
e
e
Sampl Sampl Sampl Averag
e
e
e
e 1
2
3
1
141
140
142
2
140
139
3
137
4
nt
Averag e
deterioratio
1
2
3
141
13
12
11
12
None
138
139
12
11
10
11
None
136
138
137
10
10
10
10
None
133
133
136
134
9
7
8
8
Mild
5
132
130
128
130
5
7
6
6
Mild
6
125
124
126
125
--
--
--
--
Mild
7
122
118
120
120
--
--
--
--
Mild
8
111
113
115
113
--
--
--
--
Mild
9
99
101
100
100
--
--
--
--
Medium
n
Note: the reusability of mesh reinforcement is characterized by the number of breaking near cross-over points. None represents less than 1, Mild represents between 1-5, medium represents more than 5. The mesh reinforcement will not be reused when the number of breaking is more than 5.
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Journal Pre-proof Table 10 Material and operation costs of the brick barricade Unit price Total price Items
Size
Quantity
Unit (RMB)
(RMB)
Bricks
4.5m×3.75m×0.75m 12.66
m3
870
11014
Plastering
4.5m×3.75m×2
33.75
m2
40
1350
0.13m/each
2
--
100
200
Φ10mm steel
5m/each
7
m
21
735
In total
--
--
--
--
13299
Iron drain-pipes with flange
Note: the unit price includes both material and operation cost.
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Journal Pre-proof Table 11 Material and operation costs of the proposed barricade Items
Φ70mm steel
Size
Quantity
Unit
Unit
Total
price
price
(RMB)
(RMB)
5 m/each
18
m
40
3600
0.5m/each
30
m
30
450
Wood board
(4000mm×200mm×30mm)/each
20
25
500
Φ6mm mesh
5m×4m
m2
20
400
Earthwork cloth
5m×4m×3mm
m2
5
100
Iron drain-pipes
0.13m/each
2
100
200
Steel nail
--
2
bag
42
84
Cement
--
2
bag
50
100
Electrode
--
2
bag
35
70
Bolt
--
10
5
50
Joint sleeve
(Φ50mm×150mm)/each
30
10
300
Steel card
--
30
2
60
--
3
30
90
pipes Φ36mm rockbolt
reinforcement
with flange
Anchoring agent
bag
Operation cost
3200
In total
9204
Note: the recycled materials price has been reduced considering the price of new materials. 34
Journal Pre-proof Highlights:
A light barricade is proposed for tailings recycling as cemented paste backfill.
The structure, material and key construction technology are introduced.
The required experiments and process parameters are introduced.
Its capability is verified at an engineering case, the Shirengou Iron Mine (SIM).
The proposed barricade is efficient, reusable and cost-effective.