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Failure characteristics of large unconfined cemented gangue backfill structure in partial backfill mining
Construction and Building Materials 194 (2019) 257–265
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Failure characteristics of large unconfined cemented gangue backfill structure in partial backfill mining Xianjie Du a,b, Guorui Feng a,b,⇑, Tingye Qi b,c, Yuxia Guo a,b, Yujiang Zhang a,b, Zehua Wang a,b a
College of Mining Engineering, Taiyuan University of Technology, Taiyuan 030024, China Research Center of Green Mining Engineering Technology of Shanxi Province, Taiyuan 030024, China c Institute of Mining Technology, Taiyuan University of Technology, Taiyuan 030024, China b
h i g h l i g h t s The curves of internal pressure and deformation can be divided into four stages. A hard core is formed in the central part of CGBM block under uniaxial compression. The stability of UBS can be monitored by embedded pressure and deformation sensors.
a r t i c l e
i n f o
Article history: Received 19 December 2017 Received in revised form 20 May 2018 Accepted 3 November 2018
Keywords: Partial backfill Cemented gangue backfill material Unconfined backfill structure Uniaxial compression Embedded sensors Digital image correlation Internal pressure Failure characteristics
a b s t r a c t For the subsidence prevention purpose, partial backfill mining is employed. The mined-out area is supported by carefully designed backfill piers and strips. The stability of these unconfined backfill structures (UBS) is very important to the mining operation. In order to study the failure characteristics of UBS, the stress-strain curve of a large cemented gangue backfill material (CGBM) test block (800 800 800 mm) was derived through uniaxial compression test. The failure process of the test block was monitored by digital image correlation (DIC) and ultrasonic testing (UT) techniques. The internal pressures and deformations were monitored by sensors embedded at different depths in the test block. It is found that: 1) The curves of internal pressure and deformation can be divided into four stages. 2) A hard core is formed in the central part of the test block under uniaxial compression load. 3) The three manifestations of the curves of internal pressure or deformation can be used as the failure precursor point, critical instability point, residual bearing capacity or residual deformation of the test block respectively. The experimental findings could be used to guide the design, stability monitoring and reinforcement of UBS in partial backfill operations. Ó 2018 Elsevier Ltd. All rights reserved.
1. Introduction The cemented paste backfill (CPB) has been widely used in the field of backfill mining because of its high early and cured strengths and its good pumping performance [1,2]. The stressstrain behavior of CPB is strongly influenced by the components, the age and the confinement [3]. Yilmaz et al. investigated the effect of curing time on the one-dimensional consolidation characteristics of CPB samples with different types of binder and their content [4]. Ke et al. assessed the effect of particle gradation of tailings on properties of fresh and hardened CPB [5]. Lee et al. and Wu ⇑ Corresponding author at: College of Mining Engineering, Taiyuan University of Technology, 79 West Yingze Street, Taiyuan, Shanxi 030024, China. E-mail addresses: [email protected], [email protected] (G. Feng). https://doi.org/10.1016/j.conbuildmat.2018.11.038 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.
et al. investigated the effects of fly ash content on the compressive strength and microscopic characteristics of CPB [6,7]. Zheng et al. studied the effects of water-reducing admixture dosages on the properties of CPB [8]. The mechanical and thermal properties of CPB are strongly influenced by the initial backfill temperature, curing stress and drainage conditions, or their interactions, since these can significantly affect the cement hydration, capillary pressure development and pore structure [9,10]. Yilmaz et al. studied the effect of curing under pressure on compressive strength development of CPB [11]. The applied pressure could attribute to the removal of excess pore water during the effective curing process, which could improve consolidation process of the CPB material. The sulphate attacks [12,13] and heat [14,15] have significant influence on the mechanical properties and the pore structure of CPB. A large similar stope
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model (SSM) was designed for simulating the consolidation of CPB in a stope, the UCS test on specimens cored in the different position of CPB sample in SSM was conducted [16]. In terms of stability management, Ercikdi et al. presented the effect of the natural pozzolana as mineral additives on the shortand long-term strength and stability performance of CPB samples [17]. A 28-days UCS of 1.0 MPa and the maintenance of stability over 224 days of curing were selected as the design criteria for the evaluation of CPB performance based on the effect of desliming to sulphide-rich mill tailings [18]. Liu showed that the stiffness and elastic modulus of rock mass with lower strength were more likely to lead to system instability of CPB [19]. Cemented gangue backfill material (CGBM) is a paste-like material made of the coal gangue, fly ash and the cementitious materials [20,21]. The amounts of raw materials of gangue and fly ash for backfill mining have been reducing with the rapid development of backfill mining and the cost of backfill mining is gradually increasing. To reduce the cost while effectively managing the coal waste, partial backfill mining method has been proposed [22]. In Fig. 1a, the roof in a partial backfill panel is supported by spaced backfill piers and strips [23]. It leaves a large amount of underground space in the panel after partial backfill which can be used for other purposes, such as storage. The unconfined backfill structures (UBS) formed in the panel are not laterally confined around them and are only affected by the vertical pressure of the roof as in Fig. 1b, it is essential to maintain the stability of the UBS for the success of the partial backfill mining. There are few reports on evaluating the failure characteristics of the UBS. Xu et al. investigated stress-strain behavior, resistivity and thermal infrared (TIR) characteristics of backfill mass under uniaxial compression load, and collected the precursory information for impending failure of cemented backfill mass [24]. Gao et al. analyzed the failure characteristic and damage constitutive equations of composite cemented backfill under uniaxial compression load based on complex defects influence [25]. In order to analyze the failure characteristics of UBS in partial backfill, the uniaxial compression test of a large CGBM test block (800 800 800 mm) was conducted. The failure process of the test block under uniaxial compression load was monitored by digital image correlation (DIC) technique and ultrasonic testing (UT) technique. The internal pressures and deformations during uniaxial compression process were measured using embedded sensors at different depths in the test block. The relationship between the measured data and the stability of the test block was analyzed. The bearing capacity and the failure depth of the test block are assessed by the internal pressures and deformations at different
(a) layout of UBS
depths. The experimental findings could be used to guide the design, stability monitoring and reinforcement of UBS in partial backfill operations. 2. Material and methods 2.1. Materials To meet the requirements of mixing, pumping, placement and cost, the reasonable proportion range of the CGBM was obtained through a large number of experiments [26–28]. The materials used in this paper include coal gangue collected from Xinyang coal mine (950 kg/m3), secondary fly ash obtained from the power plant of Fenxi Mining Group (380 kg/m3), ordinary Portland cement Grade 42.5 (190 kg/m3), water reducer composed of poly methane naphthalene sulfonic acid sodium salt (0.95 kg/m3), tap water (380 kg/m3). The ratio of solid materials to the total weight of CGBM is 80%. The coal gangue is classified as 0–5 mm, 5–10 mm and 10–15 mm after two stages crushing, accounting for 40%, 30% and 30% of the total coal gangue respectively. The chemical components and physical properties of the main raw materials of CGBM are same as those in reference [29]. It can be seen that the fly ash (SiO2 + Al2O3 + Fe2O3 = 88.52%) used in this paper is F class fly ash. The F class fly ash has pozzolanic properties and filler effect in cementitious mixtures. It can not only fills the pores or cracks within the CGBM, but also reacts with calcium hydroxide and form hydration products (e.g., C-S-H gel and ettringite) [7,26]. 2.2. Methods 2.2.1. Test block designing Materials such as rock, concrete and CPB have size effect because of the inhomogeneity of the components and the existence of the pores and cracks [30]. When the size of the specimen is small, the strength of the specimen is dispersed. However, when the size of the specimen is larger than 800 mm, the dispersion of the strength is almost disappeared [31]. Considering the actual size of the UBS in partial backfill mining operations, the CGBM test block was designed to be 800 800 800 mm for avoiding the size effect of rock mechanics tests. The test block was layer casted, each layer was 200 mm [32]. A vibrating bar was used in the pouring process so the materials in each layer are mixed evenly. Three pressure sensors (u15 mm 6 mm) and three deformation sensors (u10 mm 100 mm) were embedded in the different positions (50, 200 and 400 mm from the block surface) of the test block (Fig. 2a). The bearing surface of each sensor was upward. The
(b) load on UBS
Fig. 1. Schematic diagram of UBS in partial backfill [29].
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(a) Schematic diagram
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(b) Scene diagram
Fig. 2. The large backfill test block and test equipment. 1-Pressure Sensor, 2-Deformation sensor, 3-Ultrasonic probe, 4-Camera.
sensors were fixed with fine wire to ensure the location and direction. The fine wire was fixed on the wood template. Butter was coated evenly at sensors’ surfaces to prevent the CGBM from sticking on the surfaces of the sensors, and minimize the impact on the mechanical behavior of the test block as much as possible. The test block was wet cured for 14 days after casting with a temperature of 20 ± 5 °C. 2.2.2. Uniaxial compression test and sensors data acquisition The test block was loaded by the YAW-5000 kN Electrohydraulic servo press with a loading speed of 1 kN/s (Fig. 2b). When the load was up to 3600kN with a mean stress of 5.63 MPa, the press is changed to the displacement control with a speed of 0.01 mm/s. There was friction on the upper and lower faces of the hydraulic servo press. The data of the pressure sensors were simultaneously collected by TST3822E-20 static resistance strain testing system with an acquisition frequency of 10 times/ min. The deformation sensors were read every 4 min by a resistive intelligent reader. 2.2.3. DIC monitoring and UT detection To analyze the failure characteristics of the test block under uniaxial compression load, the surface failure characteristics of one surface were measured by DIC monitoring system and the internal crack propagation characteristics were measured by a non-metallic UT detector. The DIC monitoring frequency was 2 times/min. The sampling frequency of UT detector was 15 times/h. The two probes of UT detector were placed on the middle of two opposite sides (Fig. 2). The UT probes were connected to the surface of the test block by Vaseline to ensure the UT signals were well received. 3. Results 3.1. Macroscopic failure characteristics The uniaxial compressive strength (UCS) of the lager CGBM test block is 6.40 MPa at 14 days (Fig. 3). The macroscopic failure characteristic of test block is X-shaped conjugate slant shear damage (Fig. 4). The failure process can be divided into four stages: initial compaction stage, elastic compression stage, plastic failure stage and residual bearing stage. The porosities of the test block are compacted in the initial compaction stage; there is no obvious damage on the surface of the test block (Fig. 3a). The mean stress increases slowly with the increase of mean strain, and the slope of the curve increase. The solid materials of the test block are compressed in the elastic compression stage. The mean stress increases rapidly and linearly with the increase of mean strain, which obeys Hooke’s
law. The first crack occurs at the right side of the central at 2460 s with a mean stress of 2.15 MPa in the elastic compression stage (Fig. 3b). Damage is gradually occurs in the plastic failure stage. The mean stress rises slowly and then decreases rapidly. A through crack forms at the right side of the test block at 4530 s with a mean stress of 4.31 MPa (Fig. 3c). The new cracks are formed at the two sides at 6570 s with a mean stress of 6.12 MPa (Fig. 3d). The mean strain at maximum mean stress is 0.019, it means that the compression ratio of large test block is small, which is propitious to support the roof. Further damage occurs in residual bearing stage, but the mean stress decreases slowly. The through crack forms at the left side and the large cracks on the right side extend at 7140 s with a mean stress of 3.94 MPa (Fig. 3e). Then the surface of test block begins to fall off (Fig. 3f), and finally forms the residual area as upper and lower combined cone in the central part (Fig. 4). The average residual bearing capacity is larger than 2.77 MPa, which is about 43.3% of the UCS. The large average residual bearing capacity is propitious to the long-term stability of UBS. 3.2. DIC surface strain characteristics DIC is an image correlation algorithm, which can be used to calculate the surface strain characteristics [33–35] and crack propagation [36–38] of materials. It can be seen from Fig. 5: In the initial compression stage, the vertical strain and horizontal strain of the test block are 1.40.8% and 11% respectively (Fig. 5a,b). The strain varies slightly, but the distribution is rather disordered, which means that the distribution of material is adjusted due to pore compression [39]. The test block enters the elastic compression stage with the pressure continues increasing. When the mean stress reaches 2.15 MPa, the overall variation of the vertical strain and the horizontal strain remain less than 1% (Fig. 5c,d). But the maximum horizontal strain locates on a line of the right part in the middle of the test block, which means a small crack is formed on the surface of the test block. The maximum horizontal strain locates at the lower right when the mean stress reaches 4.31 MPa, which indicates that the small crack extends to the lower right of the test block. When the mean stress reaches 6.12 MPa, the vertical strain reaches 25% at the middle of the top (Fig. 5e). The horizontal strain reaches 12% in the left top corner and reaches 30% in the right top corner of the test block (Fig. 5f). The range of strain variation is large. It means that large cracks occur at two top corners after the test block enters the plastic failure stage. When the mean stress drops to 3.94 MPa, the test block enters the residual bearing stage. The maximum values of vertical strain and horizontal strain locate at the corners of both sides, and the range of variation is 5%–8% (Fig. 5g,h). When the mean stress drops to 2.77 MPa, The maximum values of vertical strain and horizontal strain locate at the edges of both sides, and
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Fig. 3. Stress-strain curve and surface failure characteristics at different stages. 1–initial compaction stage, 2–elastic compression stage, 3–plastic failure stage, 4–residual bearing stage.
Fig. 4. Ultimate failure morphology.
the range of variation is 7%–6% (fig. 5i,j). It means that the large cracks of the test block continue to expand and some parts of the surface fall off in the residual bearing stage. DIC monitoring shows that the surface strain variation law and cracks propagation are consistent with the surface macroscopic observation. The DIC technique can be used to quantify the surface failure characteristics of the CGBM test block.
materials [46,47]. As you can see from the Fig. 6a, the UT waveform is changed obviously at the 15th group and the 23rd group, which indicates that the structural failure occurred within the test block after the mean stress reaches 2.29 MPa. From the ultrasonic pulse velocity (UPV) curve (Fig. 6b), it can be see that the original UPV of the test block is 2.743 km/s. It is thought that the higher UCS of CGBM and higher water content of the large test block lead to a higher UPV values [48,49]. In the initial compaction stage, the UPV is slightly increased to 2.751 km/s at 0.49 MPa. It means that the test block becomes more compact as a result of the compression of the pores. Then the UPV fluctuates slightly, which indicates that the test block has no structural failure in the early stage of elastic compression stage. The UPV has a slight decrease by 2.6% with a mean stress of 2.47 MPa, which indicates that the test block produced some small cracks. The UPV changes abruptly from 2.67 km/s to 2.392 km/s with a mean stress of 2.83 MPa, with a decrease by 10.4%, which indicates that large cracks were formed in the test block at later elastic compression stage. Then the UPV begins to fluctuate largely at the plastic failure stage before the peak value of the mean stress, with the opening and closing of the cracks. The overall trend of UPV is decreasing, which indicates that the internal cracks continue to expand at the plastic failure stage. The UPV fluctuates reduces to 2.144 km/s at the peak value of the mean stress, which is reduced by 21.8% compared with the original UPV. The result of UT monitoring is close to that of surface macroscopic observation. The UT technique can be used to quantify the crack propagation characteristics of the CGBM test block.
3.3. UT characteristics 3.4. Internal pressure UT technique is often used to detect the crack propagation characteristics of rock [40,41], concrete [42,43], and other materials [44,45]. It can also be used to characterize cemented backfill
The internal pressures are measured by pressure sensors at the different depths of the test block. The curves of internal pressure at
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(a) EYY–270s(0.18Mpa)
(b) EXX –270s(0.18Mpa)
(c) EYY -2460s(2.15MPa)
(d) EXX -2460s(2.15MPa)
(e) E YY–4530s(4.31MPa)
(f) EXX–4530s(4.31MPa)
(g) EYY–6570s(6.12MPa)
(h) E XX–6570s(6.12MPa)
(i) EYY–7140s(3.94MPa)
(j) EXX–7140s(3.94MPa)
(k) EYY–7680s(2.77Mpa)
(l) EXX–7680s(2.77Mpa)
Fig. 5. Surface strain at different loading time of the test block (EYY-vertical strain, EXX-horizontal strain).
(a) Variation of UT waveform
(b) Variation of UPV
Fig. 6. UT characteristics at different loading times.
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the different depths are coinciding with the stress-strain curve (Fig. 7). The internal pressures increase slowly in the initial compaction stage of the test block. The slopes of the curves become larger after the test block enters the elastic compression stage. The internal pressures increase rapidly and reach the peak after the maximum mean stress of the test block. Then the internal pressures rapidly drop to the constant levels. Therefore, the curves of internal pressure are divided into four stages: slow increase stage, rapid growth stage, rapid decline stage and slow decline stage. The curves of internal pressure have three manifestations. As shown in Fig. 7, the internal pressures of the test block are close to zero in the early period. The internal pressures increase abruptly when the stress of the test block enters the elastic compression stage (Fig. 7a). The internal pressures change from the slow increase stage to the rapid growth stage. It is called the early appearance phenomenon of internal pressure. It is thought that the reason is the closer contact between the material and the pressure sensors after the pore in the test block is compacted. The mean stresses of the test block at the time of early appearance phenomenon at 50, 200 and 400 mm are 0.64, 1.72 and 4.23 MPa respectively, which are 10, 26.9 and 66.1% of the UCS of test block at 14 d respectively. It means that the pressure early appearance phenomenon of the test block is from outside to inside, which is 10–66% of UCS at different depths. When the test block enters the plastic failure stage, the values of internal pressures increase to the peak and then decrease quickly, and fall from the rapid growth stage to the rapid decline stage. It is called the peak appearance phenomenon of internal pressure. It is thought that the reason is the destruction of the material leads to the relaxation of the contact between material and pressure sensors. The peak appearance phenomenon of the internal pressure is from outside to inside. The value of maximum internal pressures at 50, 200 and 400 mm are 3.46, 6.02 and 13.3 MPa, which are 54.1, 94.1 and 207.8% of the UCS of the test block respectively. It means that the peaks of the internal pressure are 0.54–2.08 times of the UCS and increase sequentially from outside to inside. When the internal pressures drop to a certain level rapidly, the internal pressures enter the slow decline stage. It is called the residual appearance phenomenon of internal pressure. It is thought that the reason is the material and pressure sensors still in contact and have certain pressure between each other after the instability of the test block. The value of residual internal pressures at 50, 200 and 400 mm are 0.61, 5.39 and 5.28 MPa, which are 9.5, 84.2 and 82.5% of the UCS respectively. It means that the residual bearing capacity of the center area still larger than 80% of the UCS of the
test block. It is thought that a composite cone-shaped hard core is formed in center of the large test block because of the end friction between the test block and the press. The bearing capacity of material inside the hard core is larger than outside [29,50,51]. 3.5. Internal deformation The internal deformations are measured by deformation sensors at different depths of the test block. The change characteristics of internal deformations are basically consistent with the change characteristics of internal pressures. Compared with the internal pressures, there is slight difference in the internal deformations at different positions (Fig. 8). At the initial compaction stage, the curves of internal pressure at the different depths fall behind the stress-strain curve slightly. During the elastic compression stage, the internal deformations increase rapidly and reach the peak before the maximum mean stress. Then the internal deformations drop to the constant levels rapidly. Therefore, the curves of internal deformation are also divided into four stages: slow increase stage, rapid growth stage, rapid decline stage and slow decline stage. The curves of internal deformation also have three deformation manifestations: early appearance, peak appearance and residual appearance. The early appearance phenomenon at 50 mm is early than the 200 and 400 mm (Fig. 8a), but the peak appearance phenomenon is behind the 200 and 400 mm (Fig. 8b). It is thought that the damage first occurs on the surface at the early stage of loading, the surface deformation is larger than the internal deformation. Then the test block is damaged from outside to inside, and the surface deformation is gradually overtaken by the internal deformation. The internal deformations decrease rapidly after the test block is destroyed, but it still remains at constant levels in the residual bearing stage. The maximum micro-strain at 50 mm is 1699, with a mean strain of 0.0162. The maximum micro-strain at 200 mm is 1747, with a mean strain of 0.0150. The maximum micro-strain at 400 mm is 2087, with a mean strain of 0.0151. It means that the ability of the test block to withstand deformation increases sequentially from outside to inside. 4. Discussion 4.1. Change mechanism of internal pressures and internal deformations The gangue particles in the CGBM are cemented together by hydration reaction of the cementitious material, and form some
Fig. 7. Internal pressure-strain curves at different depths and the stage division. I–slow increase stage, II– rapid growth stage, III– rapid decline stage, IV– slow decline stage.
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Fig. 8. Internal deformation-strain curves at different depths. I–slow increase stage, II– rapid growth stage, III– rapid decline stage, IV– slow decline stage.
ettringite [52]. At the initial compaction stage and the early elastic compression stage, the materials and the sensors are not contact closely enough since there are many pores in the test block [53]. The internal pressures and deformations are not change obviously. With the increase of mean stress, the materials and the sensors contact more closely, the internal pressures and deformations increase rapidly. Then the cementing surface in outer part of the test block is broken and the small cracks are formed [54,55]. The outer part of the test block goes into the plastic failure stage, while the internal part is still in the elastic compression stage and forms a central elastic area. The bearing capacity of the outer part of the test block is weakened while the mean stress is still increasing, which causes the central pressure and deformation of the test block increase faster than the outside pressures and deformations. The early appearance phenomena of internal pressures and deformations occur from outside to inside. Therefore, the early appearance of internal pressures and deformations can be used as the failure precursor point of the test block. The plastic damage of the test block occurs gradually from outside to inside. The elastic area is reducing as the plastic damage expands inward. The internal pressures and deformations increase and reach the peak value when the residual area extends to the point. When the central area enters the residual bearing stage, the test block loses its stability. Therefore, peak appearance of internal pressures and deformations can be taken as the critical instability point of the test block. The residual appearance of internal pressures and deformations are the residual bearing capacity and deformation of the test block residual area.
It can be seen from Eq. (2), as the relative depth increases, the relative minimum bearing capacity increases linearly. The relative minimum bearing capacity at the central of test block is about 64% of the UCS. It can be seen from Eq. (3), as the relative depth increases, the relative maximum bearing capacity increases linearly. The relative maximum bearing capacity at the central of test block is about 1.93 times of the UCS. The bearing capacity of the test block increases linearly from outside to inside. It is coinciding with the limit equilibrium condition of material under three direction stress state [56]. According to Eqs. (2) and (3), the relative depth of the plastic failure area (x1) and the residual bearing area (x2) of the test block under different relative pressure (y) are obtained
x1 ¼ 1:53347y þ 0:0115 ð0 x1 1Þ x2 ¼ 0:50236y 0:04078 ð0 x2 1Þ
R2 ¼ 0:97973
R2 ¼ 0:95669
ð4Þ ð5Þ
When y = 0.645, x1 = 1, x2 = 0.283. It means that when the relative pressure reaches 64.5% of the UCS, the test block enters the plastic failure stage as a whole, and 28.3% of the outer part enters the residual bearing stage. When y = 1, x1 > 1, x2 = 0.462. It means that when
4.2. Bearing capacity at different depths The ratio of the depth from the surface to the size of the test block is defined as the relative depth (x). The ratio of the mean stress to the UCS of test block is defined as the relative pressure (y). The ratio of the mean stress at the time of early appearance phenomenon to the compressive strength of the test block is defined as the minimum relative bearing capacity (y1). The ratio of the maximum internal pressure to the UCS of the test block is defined as the maximum relative bearing capacity (y2). The relationship between x and y1, y2 of the test block can be obtained in Fig. 9
y1 ¼ 0:64331 x 0:00392 ð0 x 1Þ y2 ¼ 1:93312x þ 0:10453 ð0 x 1Þ
R2 ¼ 0:97973
R2 ¼ 0:95669
ð2Þ ð3Þ
Fig. 9. The relationship between the relative depth and the relative bearing capacity.
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the relative pressure reaches the UCS, the 46.2% region of the outer part of the test block enters the residual bearing stage, and the 53.8% region of the center still in the plastic failure stage. It means a residual bearing core is formed in the test block. This coincides with the stress-strain curve and failure characteristics. 4.3. Stability monitoring of UBS It can be seen from the above analyses that a hard core is formed in the central area of the test block under uniaxial compression load. The maximum bearing capacity of the hard core of the test block is much larger than the UCS of the test block. The residual bearing capacity of the hard core is also large than 80% of the UCS. If appropriate reinforcement measures are taken, such as configuring the internal reinforcement or increasing the surface envelope, to maintain the integrity of the UBS, the bearing capacity of the UBS can be increased about 1.9 times. The residual capacity of the UBS also can be improved. It is meaning for the long-term stability of the UBS in partial backfill. DIC technology and UT technology can be used to monitor the failure of the materials, but to implement them in the underground backfill project also has some difficulties. The internal pressures and deformations are corresponding to the failure progress and less sensitive to external interference [57]. Therefore, the stability of the UBS in partial backfill can be monitored by the internal pressures and deformations. Appropriate measures can be taken according to the internal pressures and deformations to reinforce the UBS. The curves of internal pressure and deformation have three manifestations: early appearance, peak appearance and residual appearance. They are the failure precursor point, critical instability point, residual bearing capacity or deformation respectively. The UBS with the internal pressures and deformations before the failure precursor point is in the initial compaction stage and the elastic compression stage. There is no crack formed in the UBS. The stability of UBS is good and no reinforcement is needed to be done. The UBS with the internal pressures and deformations between the failure precursor point and the critical instability point is in the plastic failure stage, the internal cracks develop in different degrees at different depths. The hard core is in the elastic stage with a high carrying capacity and the UBS remains stable. Some reinforcement measures need be taken to keep the UBS integrity and to prevent its overall instability. The UBS with the internal pressures and deformations after the residual appearance loses of overall stability. The residual bearing capacity of the UBS is gradually lost. The corresponding measures should be taken to restore the stability of the UBS according to the residual bearing capacity and deformation. 5. Conclusions In this paper, the stress-strain relationship of a large CGBM test block (800 800 800 mm) at 14 days was obtained by uniaxial compression test. DIC technique and UT technique are used to study the failure process of the large CGBM test block. The internal pressures and deformations were measured using the embedded sensors at different depths. The bearing capacity at the different depths and the failure depth of the test block under different loading are analyzed. The main conclusions are: (1) The failure process of the test block can be divided into four stages: initial compaction stage, elastic compression stage, plastic failure stage and residual bearing stage. (2) The curves of internal pressure and deformation of the test block are similar at different depths. They are divided into four stages: slow increase stage, rapid growth stage, rapid
decline stage and slow decline stage. The curves have three manifestations: early appearance, peak appearance and residual appearance. (3) The relative minimum bearing capacity of the test block is about 64% of the UCS while its relative maximum bearing capacity is about 1.93 times of the UCS. A hard core is formed in the central area of the test block under uniaxial compression load. The residual bearing capacity of the central area is also larger than 80% of the UCS. (4) The three manifestations of the curves of internal pressure and deformation can be taken as the failure precursor point, critical instability point, residual bearing capacity or residual deformation respectively. Appropriate measures can be taken according to the internal pressures and internal deformations to reinforce the UBS. The experimental findings could be used to guide the design, stability monitoring and reinforcement of UBS in partial backfill operations. Conflict of interest The authors declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article. Acknowledgements This research is supported by the National Science Fund for Excellent Young Scholars of China, China (No. 51422404), the National Natural Science Foundation of China, China (No. 51574172), Postgraduate Innovate Project of Shanxi Province, China (No. 2017BY043), the Youth Fund Project for Applied Basic Research of Shanxi Province, China (No. 201601D021089) and the Key Scientific and Technological Coal Projects of Shanxi Province, China (No. MQ2014-12). The authors would like to express their sincere thanks and appreciation to Assist. Prof. Dr. Yi Luo for improving the quality of writing. References [1] Q. Zhang, X. Wang, Performance of cemented coal gangue backfill, J. Cent. South Univ. Tech. 14 (2007) 216–219, https://doi.org/10.1007/s 11771-0070043-y. [2] Q. Chang, J. Chen, H. Zhou, J. Bai, Implementation of paste backfill mining technology in chinese coal mines, Sci. World J. 2014 (2014), https://doi.org/ 10.1155/2014/821025. [3] M. Fall, T. Belem, S. Samb, M. Benzaazoua, Experimental characterization of the stress-strain behaviour of cemented paste backfill in compression, J. Mater. Sci. 42 (11) (2007) 3914–3922, https://doi.org/10.1007/s10853-006-0403-2. [4] E. Yilmaz, T. Belem, B. Bussière, M. Mbonimpa, M. Benzaazoua, Curing time effect on consolidation behaviour of cemented paste backfill containing different cement types and contents, Constr. Build. Mater. 75 (2015) 99–111, https://doi.org/10.1016/j.conbuildmat.2014.11.008. [5] X. Ke, H. Hou, M. Zhou, Y. Wang, X. Zhou, Effect of particle gradation on properties of fresh and hardened cemented paste backfill, Constr. Build. Mater. 96 (2015) 378–382, https://doi.org/10.1016/j.conbuildmat.2015.08.057. [6] J.K. Lee, J.Q. Shang, S. Jeong, Thermo-mechanical properties and micro fabric of fly ash-stabilized gold tailings, J. Hazard. Mater. 276 (2014) 323–331, https:// doi.org/10.1016/j.jhazmat.2014.04.060. [7] D. Wu, Y. Zhang, Y. Liu, Mechanical performance and ultrasonic properties of cemented gangue backfill with admixture of fly ash, Ultrasonics 64 (2016) 89– 96, https://doi.org/10.1016/j.ultras.2015.08.004. [8] J. Zheng, Y. Zhu, Z. Zhao, Utilization of limestone powder and water-reducing admixture in cemented paste backfill of coarse copper mine tailings, Constr. Build. Mater. 124 (2016) 31–36, https://doi.org/10.1016/j.conbuildmat.2016.07.055. [9] A. Wu, Y. Wang, B. Zhou, J. Shen, Effect of initial backfill temperature on the deformation behavior of early age cemented paste backfill that contains sodium silicate, Adv. Mater. Sci. Eng. 2016 (2016), https://doi.org/10.1155/ 2016/8481090. [10] L. Cui, M. Fall, Mechanical and thermal properties of cemented tailings materials at early ages: Influence of initial temperature, curing stress and
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Failure characteristics of large unconfined cemented gangue backfill structure in partial backfill mining Xianjie Du a,b, Guorui Feng a,b,⇑, Tingye Qi b,c, Yuxia Guo a,b, Yujiang Zhang a,b, Zehua Wang a,b a
College of Mining Engineering, Taiyuan University of Technology, Taiyuan 030024, China Research Center of Green Mining Engineering Technology of Shanxi Province, Taiyuan 030024, China c Institute of Mining Technology, Taiyuan University of Technology, Taiyuan 030024, China b
h i g h l i g h t s The curves of internal pressure and deformation can be divided into four stages. A hard core is formed in the central part of CGBM block under uniaxial compression. The stability of UBS can be monitored by embedded pressure and deformation sensors.
a r t i c l e
i n f o
Article history: Received 19 December 2017 Received in revised form 20 May 2018 Accepted 3 November 2018
Keywords: Partial backfill Cemented gangue backfill material Unconfined backfill structure Uniaxial compression Embedded sensors Digital image correlation Internal pressure Failure characteristics
a b s t r a c t For the subsidence prevention purpose, partial backfill mining is employed. The mined-out area is supported by carefully designed backfill piers and strips. The stability of these unconfined backfill structures (UBS) is very important to the mining operation. In order to study the failure characteristics of UBS, the stress-strain curve of a large cemented gangue backfill material (CGBM) test block (800 800 800 mm) was derived through uniaxial compression test. The failure process of the test block was monitored by digital image correlation (DIC) and ultrasonic testing (UT) techniques. The internal pressures and deformations were monitored by sensors embedded at different depths in the test block. It is found that: 1) The curves of internal pressure and deformation can be divided into four stages. 2) A hard core is formed in the central part of the test block under uniaxial compression load. 3) The three manifestations of the curves of internal pressure or deformation can be used as the failure precursor point, critical instability point, residual bearing capacity or residual deformation of the test block respectively. The experimental findings could be used to guide the design, stability monitoring and reinforcement of UBS in partial backfill operations. Ó 2018 Elsevier Ltd. All rights reserved.
1. Introduction The cemented paste backfill (CPB) has been widely used in the field of backfill mining because of its high early and cured strengths and its good pumping performance [1,2]. The stressstrain behavior of CPB is strongly influenced by the components, the age and the confinement [3]. Yilmaz et al. investigated the effect of curing time on the one-dimensional consolidation characteristics of CPB samples with different types of binder and their content [4]. Ke et al. assessed the effect of particle gradation of tailings on properties of fresh and hardened CPB [5]. Lee et al. and Wu ⇑ Corresponding author at: College of Mining Engineering, Taiyuan University of Technology, 79 West Yingze Street, Taiyuan, Shanxi 030024, China. E-mail addresses: [email protected], [email protected] (G. Feng). https://doi.org/10.1016/j.conbuildmat.2018.11.038 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.
et al. investigated the effects of fly ash content on the compressive strength and microscopic characteristics of CPB [6,7]. Zheng et al. studied the effects of water-reducing admixture dosages on the properties of CPB [8]. The mechanical and thermal properties of CPB are strongly influenced by the initial backfill temperature, curing stress and drainage conditions, or their interactions, since these can significantly affect the cement hydration, capillary pressure development and pore structure [9,10]. Yilmaz et al. studied the effect of curing under pressure on compressive strength development of CPB [11]. The applied pressure could attribute to the removal of excess pore water during the effective curing process, which could improve consolidation process of the CPB material. The sulphate attacks [12,13] and heat [14,15] have significant influence on the mechanical properties and the pore structure of CPB. A large similar stope
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model (SSM) was designed for simulating the consolidation of CPB in a stope, the UCS test on specimens cored in the different position of CPB sample in SSM was conducted [16]. In terms of stability management, Ercikdi et al. presented the effect of the natural pozzolana as mineral additives on the shortand long-term strength and stability performance of CPB samples [17]. A 28-days UCS of 1.0 MPa and the maintenance of stability over 224 days of curing were selected as the design criteria for the evaluation of CPB performance based on the effect of desliming to sulphide-rich mill tailings [18]. Liu showed that the stiffness and elastic modulus of rock mass with lower strength were more likely to lead to system instability of CPB [19]. Cemented gangue backfill material (CGBM) is a paste-like material made of the coal gangue, fly ash and the cementitious materials [20,21]. The amounts of raw materials of gangue and fly ash for backfill mining have been reducing with the rapid development of backfill mining and the cost of backfill mining is gradually increasing. To reduce the cost while effectively managing the coal waste, partial backfill mining method has been proposed [22]. In Fig. 1a, the roof in a partial backfill panel is supported by spaced backfill piers and strips [23]. It leaves a large amount of underground space in the panel after partial backfill which can be used for other purposes, such as storage. The unconfined backfill structures (UBS) formed in the panel are not laterally confined around them and are only affected by the vertical pressure of the roof as in Fig. 1b, it is essential to maintain the stability of the UBS for the success of the partial backfill mining. There are few reports on evaluating the failure characteristics of the UBS. Xu et al. investigated stress-strain behavior, resistivity and thermal infrared (TIR) characteristics of backfill mass under uniaxial compression load, and collected the precursory information for impending failure of cemented backfill mass [24]. Gao et al. analyzed the failure characteristic and damage constitutive equations of composite cemented backfill under uniaxial compression load based on complex defects influence [25]. In order to analyze the failure characteristics of UBS in partial backfill, the uniaxial compression test of a large CGBM test block (800 800 800 mm) was conducted. The failure process of the test block under uniaxial compression load was monitored by digital image correlation (DIC) technique and ultrasonic testing (UT) technique. The internal pressures and deformations during uniaxial compression process were measured using embedded sensors at different depths in the test block. The relationship between the measured data and the stability of the test block was analyzed. The bearing capacity and the failure depth of the test block are assessed by the internal pressures and deformations at different
(a) layout of UBS
depths. The experimental findings could be used to guide the design, stability monitoring and reinforcement of UBS in partial backfill operations. 2. Material and methods 2.1. Materials To meet the requirements of mixing, pumping, placement and cost, the reasonable proportion range of the CGBM was obtained through a large number of experiments [26–28]. The materials used in this paper include coal gangue collected from Xinyang coal mine (950 kg/m3), secondary fly ash obtained from the power plant of Fenxi Mining Group (380 kg/m3), ordinary Portland cement Grade 42.5 (190 kg/m3), water reducer composed of poly methane naphthalene sulfonic acid sodium salt (0.95 kg/m3), tap water (380 kg/m3). The ratio of solid materials to the total weight of CGBM is 80%. The coal gangue is classified as 0–5 mm, 5–10 mm and 10–15 mm after two stages crushing, accounting for 40%, 30% and 30% of the total coal gangue respectively. The chemical components and physical properties of the main raw materials of CGBM are same as those in reference [29]. It can be seen that the fly ash (SiO2 + Al2O3 + Fe2O3 = 88.52%) used in this paper is F class fly ash. The F class fly ash has pozzolanic properties and filler effect in cementitious mixtures. It can not only fills the pores or cracks within the CGBM, but also reacts with calcium hydroxide and form hydration products (e.g., C-S-H gel and ettringite) [7,26]. 2.2. Methods 2.2.1. Test block designing Materials such as rock, concrete and CPB have size effect because of the inhomogeneity of the components and the existence of the pores and cracks [30]. When the size of the specimen is small, the strength of the specimen is dispersed. However, when the size of the specimen is larger than 800 mm, the dispersion of the strength is almost disappeared [31]. Considering the actual size of the UBS in partial backfill mining operations, the CGBM test block was designed to be 800 800 800 mm for avoiding the size effect of rock mechanics tests. The test block was layer casted, each layer was 200 mm [32]. A vibrating bar was used in the pouring process so the materials in each layer are mixed evenly. Three pressure sensors (u15 mm 6 mm) and three deformation sensors (u10 mm 100 mm) were embedded in the different positions (50, 200 and 400 mm from the block surface) of the test block (Fig. 2a). The bearing surface of each sensor was upward. The
(b) load on UBS
Fig. 1. Schematic diagram of UBS in partial backfill [29].
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(a) Schematic diagram
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(b) Scene diagram
Fig. 2. The large backfill test block and test equipment. 1-Pressure Sensor, 2-Deformation sensor, 3-Ultrasonic probe, 4-Camera.
sensors were fixed with fine wire to ensure the location and direction. The fine wire was fixed on the wood template. Butter was coated evenly at sensors’ surfaces to prevent the CGBM from sticking on the surfaces of the sensors, and minimize the impact on the mechanical behavior of the test block as much as possible. The test block was wet cured for 14 days after casting with a temperature of 20 ± 5 °C. 2.2.2. Uniaxial compression test and sensors data acquisition The test block was loaded by the YAW-5000 kN Electrohydraulic servo press with a loading speed of 1 kN/s (Fig. 2b). When the load was up to 3600kN with a mean stress of 5.63 MPa, the press is changed to the displacement control with a speed of 0.01 mm/s. There was friction on the upper and lower faces of the hydraulic servo press. The data of the pressure sensors were simultaneously collected by TST3822E-20 static resistance strain testing system with an acquisition frequency of 10 times/ min. The deformation sensors were read every 4 min by a resistive intelligent reader. 2.2.3. DIC monitoring and UT detection To analyze the failure characteristics of the test block under uniaxial compression load, the surface failure characteristics of one surface were measured by DIC monitoring system and the internal crack propagation characteristics were measured by a non-metallic UT detector. The DIC monitoring frequency was 2 times/min. The sampling frequency of UT detector was 15 times/h. The two probes of UT detector were placed on the middle of two opposite sides (Fig. 2). The UT probes were connected to the surface of the test block by Vaseline to ensure the UT signals were well received. 3. Results 3.1. Macroscopic failure characteristics The uniaxial compressive strength (UCS) of the lager CGBM test block is 6.40 MPa at 14 days (Fig. 3). The macroscopic failure characteristic of test block is X-shaped conjugate slant shear damage (Fig. 4). The failure process can be divided into four stages: initial compaction stage, elastic compression stage, plastic failure stage and residual bearing stage. The porosities of the test block are compacted in the initial compaction stage; there is no obvious damage on the surface of the test block (Fig. 3a). The mean stress increases slowly with the increase of mean strain, and the slope of the curve increase. The solid materials of the test block are compressed in the elastic compression stage. The mean stress increases rapidly and linearly with the increase of mean strain, which obeys Hooke’s
law. The first crack occurs at the right side of the central at 2460 s with a mean stress of 2.15 MPa in the elastic compression stage (Fig. 3b). Damage is gradually occurs in the plastic failure stage. The mean stress rises slowly and then decreases rapidly. A through crack forms at the right side of the test block at 4530 s with a mean stress of 4.31 MPa (Fig. 3c). The new cracks are formed at the two sides at 6570 s with a mean stress of 6.12 MPa (Fig. 3d). The mean strain at maximum mean stress is 0.019, it means that the compression ratio of large test block is small, which is propitious to support the roof. Further damage occurs in residual bearing stage, but the mean stress decreases slowly. The through crack forms at the left side and the large cracks on the right side extend at 7140 s with a mean stress of 3.94 MPa (Fig. 3e). Then the surface of test block begins to fall off (Fig. 3f), and finally forms the residual area as upper and lower combined cone in the central part (Fig. 4). The average residual bearing capacity is larger than 2.77 MPa, which is about 43.3% of the UCS. The large average residual bearing capacity is propitious to the long-term stability of UBS. 3.2. DIC surface strain characteristics DIC is an image correlation algorithm, which can be used to calculate the surface strain characteristics [33–35] and crack propagation [36–38] of materials. It can be seen from Fig. 5: In the initial compression stage, the vertical strain and horizontal strain of the test block are 1.40.8% and 11% respectively (Fig. 5a,b). The strain varies slightly, but the distribution is rather disordered, which means that the distribution of material is adjusted due to pore compression [39]. The test block enters the elastic compression stage with the pressure continues increasing. When the mean stress reaches 2.15 MPa, the overall variation of the vertical strain and the horizontal strain remain less than 1% (Fig. 5c,d). But the maximum horizontal strain locates on a line of the right part in the middle of the test block, which means a small crack is formed on the surface of the test block. The maximum horizontal strain locates at the lower right when the mean stress reaches 4.31 MPa, which indicates that the small crack extends to the lower right of the test block. When the mean stress reaches 6.12 MPa, the vertical strain reaches 25% at the middle of the top (Fig. 5e). The horizontal strain reaches 12% in the left top corner and reaches 30% in the right top corner of the test block (Fig. 5f). The range of strain variation is large. It means that large cracks occur at two top corners after the test block enters the plastic failure stage. When the mean stress drops to 3.94 MPa, the test block enters the residual bearing stage. The maximum values of vertical strain and horizontal strain locate at the corners of both sides, and the range of variation is 5%–8% (Fig. 5g,h). When the mean stress drops to 2.77 MPa, The maximum values of vertical strain and horizontal strain locate at the edges of both sides, and
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Fig. 3. Stress-strain curve and surface failure characteristics at different stages. 1–initial compaction stage, 2–elastic compression stage, 3–plastic failure stage, 4–residual bearing stage.
Fig. 4. Ultimate failure morphology.
the range of variation is 7%–6% (fig. 5i,j). It means that the large cracks of the test block continue to expand and some parts of the surface fall off in the residual bearing stage. DIC monitoring shows that the surface strain variation law and cracks propagation are consistent with the surface macroscopic observation. The DIC technique can be used to quantify the surface failure characteristics of the CGBM test block.
materials [46,47]. As you can see from the Fig. 6a, the UT waveform is changed obviously at the 15th group and the 23rd group, which indicates that the structural failure occurred within the test block after the mean stress reaches 2.29 MPa. From the ultrasonic pulse velocity (UPV) curve (Fig. 6b), it can be see that the original UPV of the test block is 2.743 km/s. It is thought that the higher UCS of CGBM and higher water content of the large test block lead to a higher UPV values [48,49]. In the initial compaction stage, the UPV is slightly increased to 2.751 km/s at 0.49 MPa. It means that the test block becomes more compact as a result of the compression of the pores. Then the UPV fluctuates slightly, which indicates that the test block has no structural failure in the early stage of elastic compression stage. The UPV has a slight decrease by 2.6% with a mean stress of 2.47 MPa, which indicates that the test block produced some small cracks. The UPV changes abruptly from 2.67 km/s to 2.392 km/s with a mean stress of 2.83 MPa, with a decrease by 10.4%, which indicates that large cracks were formed in the test block at later elastic compression stage. Then the UPV begins to fluctuate largely at the plastic failure stage before the peak value of the mean stress, with the opening and closing of the cracks. The overall trend of UPV is decreasing, which indicates that the internal cracks continue to expand at the plastic failure stage. The UPV fluctuates reduces to 2.144 km/s at the peak value of the mean stress, which is reduced by 21.8% compared with the original UPV. The result of UT monitoring is close to that of surface macroscopic observation. The UT technique can be used to quantify the crack propagation characteristics of the CGBM test block.
3.3. UT characteristics 3.4. Internal pressure UT technique is often used to detect the crack propagation characteristics of rock [40,41], concrete [42,43], and other materials [44,45]. It can also be used to characterize cemented backfill
The internal pressures are measured by pressure sensors at the different depths of the test block. The curves of internal pressure at
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(a) EYY–270s(0.18Mpa)
(b) EXX –270s(0.18Mpa)
(c) EYY -2460s(2.15MPa)
(d) EXX -2460s(2.15MPa)
(e) E YY–4530s(4.31MPa)
(f) EXX–4530s(4.31MPa)
(g) EYY–6570s(6.12MPa)
(h) E XX–6570s(6.12MPa)
(i) EYY–7140s(3.94MPa)
(j) EXX–7140s(3.94MPa)
(k) EYY–7680s(2.77Mpa)
(l) EXX–7680s(2.77Mpa)
Fig. 5. Surface strain at different loading time of the test block (EYY-vertical strain, EXX-horizontal strain).
(a) Variation of UT waveform
(b) Variation of UPV
Fig. 6. UT characteristics at different loading times.
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the different depths are coinciding with the stress-strain curve (Fig. 7). The internal pressures increase slowly in the initial compaction stage of the test block. The slopes of the curves become larger after the test block enters the elastic compression stage. The internal pressures increase rapidly and reach the peak after the maximum mean stress of the test block. Then the internal pressures rapidly drop to the constant levels. Therefore, the curves of internal pressure are divided into four stages: slow increase stage, rapid growth stage, rapid decline stage and slow decline stage. The curves of internal pressure have three manifestations. As shown in Fig. 7, the internal pressures of the test block are close to zero in the early period. The internal pressures increase abruptly when the stress of the test block enters the elastic compression stage (Fig. 7a). The internal pressures change from the slow increase stage to the rapid growth stage. It is called the early appearance phenomenon of internal pressure. It is thought that the reason is the closer contact between the material and the pressure sensors after the pore in the test block is compacted. The mean stresses of the test block at the time of early appearance phenomenon at 50, 200 and 400 mm are 0.64, 1.72 and 4.23 MPa respectively, which are 10, 26.9 and 66.1% of the UCS of test block at 14 d respectively. It means that the pressure early appearance phenomenon of the test block is from outside to inside, which is 10–66% of UCS at different depths. When the test block enters the plastic failure stage, the values of internal pressures increase to the peak and then decrease quickly, and fall from the rapid growth stage to the rapid decline stage. It is called the peak appearance phenomenon of internal pressure. It is thought that the reason is the destruction of the material leads to the relaxation of the contact between material and pressure sensors. The peak appearance phenomenon of the internal pressure is from outside to inside. The value of maximum internal pressures at 50, 200 and 400 mm are 3.46, 6.02 and 13.3 MPa, which are 54.1, 94.1 and 207.8% of the UCS of the test block respectively. It means that the peaks of the internal pressure are 0.54–2.08 times of the UCS and increase sequentially from outside to inside. When the internal pressures drop to a certain level rapidly, the internal pressures enter the slow decline stage. It is called the residual appearance phenomenon of internal pressure. It is thought that the reason is the material and pressure sensors still in contact and have certain pressure between each other after the instability of the test block. The value of residual internal pressures at 50, 200 and 400 mm are 0.61, 5.39 and 5.28 MPa, which are 9.5, 84.2 and 82.5% of the UCS respectively. It means that the residual bearing capacity of the center area still larger than 80% of the UCS of the
test block. It is thought that a composite cone-shaped hard core is formed in center of the large test block because of the end friction between the test block and the press. The bearing capacity of material inside the hard core is larger than outside [29,50,51]. 3.5. Internal deformation The internal deformations are measured by deformation sensors at different depths of the test block. The change characteristics of internal deformations are basically consistent with the change characteristics of internal pressures. Compared with the internal pressures, there is slight difference in the internal deformations at different positions (Fig. 8). At the initial compaction stage, the curves of internal pressure at the different depths fall behind the stress-strain curve slightly. During the elastic compression stage, the internal deformations increase rapidly and reach the peak before the maximum mean stress. Then the internal deformations drop to the constant levels rapidly. Therefore, the curves of internal deformation are also divided into four stages: slow increase stage, rapid growth stage, rapid decline stage and slow decline stage. The curves of internal deformation also have three deformation manifestations: early appearance, peak appearance and residual appearance. The early appearance phenomenon at 50 mm is early than the 200 and 400 mm (Fig. 8a), but the peak appearance phenomenon is behind the 200 and 400 mm (Fig. 8b). It is thought that the damage first occurs on the surface at the early stage of loading, the surface deformation is larger than the internal deformation. Then the test block is damaged from outside to inside, and the surface deformation is gradually overtaken by the internal deformation. The internal deformations decrease rapidly after the test block is destroyed, but it still remains at constant levels in the residual bearing stage. The maximum micro-strain at 50 mm is 1699, with a mean strain of 0.0162. The maximum micro-strain at 200 mm is 1747, with a mean strain of 0.0150. The maximum micro-strain at 400 mm is 2087, with a mean strain of 0.0151. It means that the ability of the test block to withstand deformation increases sequentially from outside to inside. 4. Discussion 4.1. Change mechanism of internal pressures and internal deformations The gangue particles in the CGBM are cemented together by hydration reaction of the cementitious material, and form some
Fig. 7. Internal pressure-strain curves at different depths and the stage division. I–slow increase stage, II– rapid growth stage, III– rapid decline stage, IV– slow decline stage.
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Fig. 8. Internal deformation-strain curves at different depths. I–slow increase stage, II– rapid growth stage, III– rapid decline stage, IV– slow decline stage.
ettringite [52]. At the initial compaction stage and the early elastic compression stage, the materials and the sensors are not contact closely enough since there are many pores in the test block [53]. The internal pressures and deformations are not change obviously. With the increase of mean stress, the materials and the sensors contact more closely, the internal pressures and deformations increase rapidly. Then the cementing surface in outer part of the test block is broken and the small cracks are formed [54,55]. The outer part of the test block goes into the plastic failure stage, while the internal part is still in the elastic compression stage and forms a central elastic area. The bearing capacity of the outer part of the test block is weakened while the mean stress is still increasing, which causes the central pressure and deformation of the test block increase faster than the outside pressures and deformations. The early appearance phenomena of internal pressures and deformations occur from outside to inside. Therefore, the early appearance of internal pressures and deformations can be used as the failure precursor point of the test block. The plastic damage of the test block occurs gradually from outside to inside. The elastic area is reducing as the plastic damage expands inward. The internal pressures and deformations increase and reach the peak value when the residual area extends to the point. When the central area enters the residual bearing stage, the test block loses its stability. Therefore, peak appearance of internal pressures and deformations can be taken as the critical instability point of the test block. The residual appearance of internal pressures and deformations are the residual bearing capacity and deformation of the test block residual area.
It can be seen from Eq. (2), as the relative depth increases, the relative minimum bearing capacity increases linearly. The relative minimum bearing capacity at the central of test block is about 64% of the UCS. It can be seen from Eq. (3), as the relative depth increases, the relative maximum bearing capacity increases linearly. The relative maximum bearing capacity at the central of test block is about 1.93 times of the UCS. The bearing capacity of the test block increases linearly from outside to inside. It is coinciding with the limit equilibrium condition of material under three direction stress state [56]. According to Eqs. (2) and (3), the relative depth of the plastic failure area (x1) and the residual bearing area (x2) of the test block under different relative pressure (y) are obtained
x1 ¼ 1:53347y þ 0:0115 ð0 x1 1Þ x2 ¼ 0:50236y 0:04078 ð0 x2 1Þ
R2 ¼ 0:97973
R2 ¼ 0:95669
ð4Þ ð5Þ
When y = 0.645, x1 = 1, x2 = 0.283. It means that when the relative pressure reaches 64.5% of the UCS, the test block enters the plastic failure stage as a whole, and 28.3% of the outer part enters the residual bearing stage. When y = 1, x1 > 1, x2 = 0.462. It means that when
4.2. Bearing capacity at different depths The ratio of the depth from the surface to the size of the test block is defined as the relative depth (x). The ratio of the mean stress to the UCS of test block is defined as the relative pressure (y). The ratio of the mean stress at the time of early appearance phenomenon to the compressive strength of the test block is defined as the minimum relative bearing capacity (y1). The ratio of the maximum internal pressure to the UCS of the test block is defined as the maximum relative bearing capacity (y2). The relationship between x and y1, y2 of the test block can be obtained in Fig. 9
y1 ¼ 0:64331 x 0:00392 ð0 x 1Þ y2 ¼ 1:93312x þ 0:10453 ð0 x 1Þ
R2 ¼ 0:97973
R2 ¼ 0:95669
ð2Þ ð3Þ
Fig. 9. The relationship between the relative depth and the relative bearing capacity.
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the relative pressure reaches the UCS, the 46.2% region of the outer part of the test block enters the residual bearing stage, and the 53.8% region of the center still in the plastic failure stage. It means a residual bearing core is formed in the test block. This coincides with the stress-strain curve and failure characteristics. 4.3. Stability monitoring of UBS It can be seen from the above analyses that a hard core is formed in the central area of the test block under uniaxial compression load. The maximum bearing capacity of the hard core of the test block is much larger than the UCS of the test block. The residual bearing capacity of the hard core is also large than 80% of the UCS. If appropriate reinforcement measures are taken, such as configuring the internal reinforcement or increasing the surface envelope, to maintain the integrity of the UBS, the bearing capacity of the UBS can be increased about 1.9 times. The residual capacity of the UBS also can be improved. It is meaning for the long-term stability of the UBS in partial backfill. DIC technology and UT technology can be used to monitor the failure of the materials, but to implement them in the underground backfill project also has some difficulties. The internal pressures and deformations are corresponding to the failure progress and less sensitive to external interference [57]. Therefore, the stability of the UBS in partial backfill can be monitored by the internal pressures and deformations. Appropriate measures can be taken according to the internal pressures and deformations to reinforce the UBS. The curves of internal pressure and deformation have three manifestations: early appearance, peak appearance and residual appearance. They are the failure precursor point, critical instability point, residual bearing capacity or deformation respectively. The UBS with the internal pressures and deformations before the failure precursor point is in the initial compaction stage and the elastic compression stage. There is no crack formed in the UBS. The stability of UBS is good and no reinforcement is needed to be done. The UBS with the internal pressures and deformations between the failure precursor point and the critical instability point is in the plastic failure stage, the internal cracks develop in different degrees at different depths. The hard core is in the elastic stage with a high carrying capacity and the UBS remains stable. Some reinforcement measures need be taken to keep the UBS integrity and to prevent its overall instability. The UBS with the internal pressures and deformations after the residual appearance loses of overall stability. The residual bearing capacity of the UBS is gradually lost. The corresponding measures should be taken to restore the stability of the UBS according to the residual bearing capacity and deformation. 5. Conclusions In this paper, the stress-strain relationship of a large CGBM test block (800 800 800 mm) at 14 days was obtained by uniaxial compression test. DIC technique and UT technique are used to study the failure process of the large CGBM test block. The internal pressures and deformations were measured using the embedded sensors at different depths. The bearing capacity at the different depths and the failure depth of the test block under different loading are analyzed. The main conclusions are: (1) The failure process of the test block can be divided into four stages: initial compaction stage, elastic compression stage, plastic failure stage and residual bearing stage. (2) The curves of internal pressure and deformation of the test block are similar at different depths. They are divided into four stages: slow increase stage, rapid growth stage, rapid
decline stage and slow decline stage. The curves have three manifestations: early appearance, peak appearance and residual appearance. (3) The relative minimum bearing capacity of the test block is about 64% of the UCS while its relative maximum bearing capacity is about 1.93 times of the UCS. A hard core is formed in the central area of the test block under uniaxial compression load. The residual bearing capacity of the central area is also larger than 80% of the UCS. (4) The three manifestations of the curves of internal pressure and deformation can be taken as the failure precursor point, critical instability point, residual bearing capacity or residual deformation respectively. Appropriate measures can be taken according to the internal pressures and internal deformations to reinforce the UBS. The experimental findings could be used to guide the design, stability monitoring and reinforcement of UBS in partial backfill operations. Conflict of interest The authors declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article. Acknowledgements This research is supported by the National Science Fund for Excellent Young Scholars of China, China (No. 51422404), the National Natural Science Foundation of China, China (No. 51574172), Postgraduate Innovate Project of Shanxi Province, China (No. 2017BY043), the Youth Fund Project for Applied Basic Research of Shanxi Province, China (No. 201601D021089) and the Key Scientific and Technological Coal Projects of Shanxi Province, China (No. MQ2014-12). The authors would like to express their sincere thanks and appreciation to Assist. Prof. Dr. Yi Luo for improving the quality of writing. References [1] Q. Zhang, X. Wang, Performance of cemented coal gangue backfill, J. Cent. South Univ. Tech. 14 (2007) 216–219, https://doi.org/10.1007/s 11771-0070043-y. [2] Q. Chang, J. Chen, H. Zhou, J. Bai, Implementation of paste backfill mining technology in chinese coal mines, Sci. World J. 2014 (2014), https://doi.org/ 10.1155/2014/821025. [3] M. Fall, T. Belem, S. Samb, M. Benzaazoua, Experimental characterization of the stress-strain behaviour of cemented paste backfill in compression, J. Mater. Sci. 42 (11) (2007) 3914–3922, https://doi.org/10.1007/s10853-006-0403-2. [4] E. Yilmaz, T. Belem, B. Bussière, M. Mbonimpa, M. Benzaazoua, Curing time effect on consolidation behaviour of cemented paste backfill containing different cement types and contents, Constr. Build. Mater. 75 (2015) 99–111, https://doi.org/10.1016/j.conbuildmat.2014.11.008. [5] X. Ke, H. Hou, M. Zhou, Y. Wang, X. Zhou, Effect of particle gradation on properties of fresh and hardened cemented paste backfill, Constr. Build. Mater. 96 (2015) 378–382, https://doi.org/10.1016/j.conbuildmat.2015.08.057. [6] J.K. Lee, J.Q. Shang, S. Jeong, Thermo-mechanical properties and micro fabric of fly ash-stabilized gold tailings, J. Hazard. Mater. 276 (2014) 323–331, https:// doi.org/10.1016/j.jhazmat.2014.04.060. [7] D. Wu, Y. Zhang, Y. Liu, Mechanical performance and ultrasonic properties of cemented gangue backfill with admixture of fly ash, Ultrasonics 64 (2016) 89– 96, https://doi.org/10.1016/j.ultras.2015.08.004. [8] J. Zheng, Y. Zhu, Z. Zhao, Utilization of limestone powder and water-reducing admixture in cemented paste backfill of coarse copper mine tailings, Constr. Build. Mater. 124 (2016) 31–36, https://doi.org/10.1016/j.conbuildmat.2016.07.055. [9] A. Wu, Y. Wang, B. Zhou, J. Shen, Effect of initial backfill temperature on the deformation behavior of early age cemented paste backfill that contains sodium silicate, Adv. Mater. Sci. Eng. 2016 (2016), https://doi.org/10.1155/ 2016/8481090. [10] L. Cui, M. Fall, Mechanical and thermal properties of cemented tailings materials at early ages: Influence of initial temperature, curing stress and
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