TY - JOUR
AU1 - He,, Sheng-quan
AU2 - Jin,, Long-zhe
AU3 - Ou,, Sheng-nan
AU4 - Ming,, Xiao-hong
AB - Abstract The adsorption, permeability properties, and damage evolution of internal cracks are often not considered in simulation tests for coal and gas outbursts. Therefore, in previous studies of similar materials, the materials used may not be similar in the properties mentioned above. To solve these issues, the adsorption, permeability, mechanical, and acoustic emission (AE) response characteristics of a soft coal solid–gas coupling similar material (SCSCSM) were studied in an orthogonal experiment where gas adsorption, permeability, uniaxial compression, and AE tests were performed. The analysis of the experimental data revealed that the SCSCSM has mechanical, gas adsorption and permeability properties similar to those of soft coal. The mass fraction of humic acid sodium solution (HASS) was the main parameter, affecting physical and mechanical properties. As the mass fraction of HASS increased, the adsorption volume and permeability varied logarithmically and exponentially, respectively. Meanwhile, the failure pattern transitioned from extrusion damage to wedge splitting and brittle shear failure. AE response from the beginning to the peak stress can be divided into a slow increase stage and a rapid increase stage. A damage parameter experienced an initial rise, then a rapid decline, then a quick rise, and finally a slow increase. Through segmentation fitting, expressions of damage evolution mechanisms for each section were obtained. Through the study of the SCSCSM’s internal gas storage, transport, and mechanical properties, AE response characteristics, and multiscale evaluation of mechanical response, this study has laid a foundation for the subsequent development of solid–gas coupling similar simulation materials and physical simulation experiments of coal and gas outburst for soft coal seams. simulation material, adsorption and permeability properties, acoustic emission responses, damage evolution, coal and gas outburst 1. Introduction The number of soft coal mines in China is large and gassy, and high-outburst mines are common (Yao et al2010, Liu 2014). Because of low permeability and poor stability of gas drainage boreholes in soft coal seams, gas extraction is inefficient and difficult to control. Soft coal mines have become a high-risk area of major disasters (Yuan 2010, Qu et al2011). Due to increased mining depths, it is difficult to study the law of coal and gas outbursts at underground mines. Furthermore, it is difficult to make standard specimens of soft coal. The technology of similar material simulation has become an effective means to explore the mechanisms of coal and gas outburst, with its advantages of simplicity, intuition, and reappearance (Niu et al2018). To reveal the mechanism of coal and gas outburst in soft coal seam, we need to reproduce the dynamic evolution process based on a physical similarity model test. It is difficult to meet the experimental requirements with the original soft coal material (pulverized coal cannot be bonded together). Therefore, the selection and ratio of appropriate materials become key to creating a successful soft coal solid–gas coupling similar material (SCSCSM). According to previous studies, similar materials for coal and rock are mainly composed of aggregate and a cementing agent (Xu et al2008, Li et al2010, Ou 2012, Chen et al2015, Kang et al2015, Cheng et al2016, Niu et al2018). Zhang et al (2015) using cement as the cementing agent and pulverized coal as the aggregate, developed material for a coal and gas outburst test. Kang et al (2015) used coal or sand as aggregate and gypsum and cement as cementing agent for comparative tests. Their results indicated that pulverized coal as aggregate for similar material of coal was better than sand. Li et al (2010) developed a solid–fluid coupling similar material using sand and talc powder as aggregate and paraffin as cementing agent. Chen et al (2015) developed a solid–fluid coupling similar material of deep aquifuge. Paraffin and Vaseline were used as cementing agents, while river sand and calcium carbonate were used as aggregate. These similar materials in solid and solid–fluid coupling simulations play an important role, but are not suitable for solid–gas coupling simulation experiments. Due to the greater strength and lower adsorption characteristics of these materials, gas adsorption, permeability, stress transfer, and crack evolution of soft coal seams are not well reflected. In past research, the optimum makeup of a similar material and the degree to which a similar material reflects the real material were evaluated by stress–strain curves, compressive strength, and failure mode (Li and Lin 2015). However, these parameters can only judge macroscopic appearance. It is seldom studied whether the adsorption, permeability characteristics, and evolution laws of internal cracks are similar to real materials. Acoustic emission (AE) testing is one way to measure the evolution of internal cracks. AE is caused by transient elastic waves caused by deformation and the creation of micro-cracks. The AE technique, where the number and intensity of AE events are measured, has been applied to coal, rock, concrete, and buildings for detection of damage evolution of internal cracks. The technique measures damage continuously in real time, to reflect the microscopic deformation and failure of materials (Shkuratnik et al2004, Hirata et al2007, Watanabe et al2010, Carpinteri et al2013, 2016, Aker et al2014, Han et al2015, Vishal et al2015, Li et al2017, Su et al2017, Vilhelma et al2017, Li et al2018). Kong et al (2017) studied acoustic emission signals’ frequency-amplitude characteristics in sandstone. After thermal treatment under uniaxial compression, the AE signals’ waveforms contained plentiful precursor information of sandstone deformation and fracture behavior in time series. Lacidogna et al (2011) investigated the mechanical behavior of concrete and rock samples loaded up to their failure by the analysis of AE. They conclude the presence of AE signals during the damage process. Kong et al (2016) studied the fractal characteristics and AE features of coal under triaxial compression experiments. Their results indicated that AE response and fractal dimension can reflect the evolution and propagation of cracks in the loading process. For similar simulation experiments, similar materials must be used. To improve the reliability of test results, similar materials must be selected to meet the requirements of the simulation test. Therefore, a combination of pulverized coal with different particle sizes and humic acid sodium solution (HASS) as the cementing agent is proposed as a similar material, which can simulate the coal and gas outbursts of soft coal seams. The proposed material has good cementing properties, good adsorption and permeability properties for gas, stable mechanical properties and failure characteristics similar to those of soft coal. In this study, based on the orthogonal experimental method, samples were prepared with different molding moisture (M), mass fraction of HASS (H), and mass ratio of pulverized coal (C) with different particle sizes. The effects of M, H, and C on the compressive strength, elastic modulus, gas adsorption, and permeability were studied. The effect of H on the adsorption, permeability, physical and mechanical properties, and the failure mode of SCSCSM samples were also analyzed. The specimens of SCSCSM were accompanied by AE events in the process of loading, revealing the damage evolution law inside the specimen. The AE response characteristics of the specimens were tested and analyzed. The crack damage evolution law and relationship between the cumulative AE pulses and strain of similar materials were also studied. We believe that this study would lay the theoretical and experimental foundation for subsequent simulation tests of coal and gas outbursts in soft coal seam. 2. Experiment system and preparation of sample specimens 2.1. Experiment system and methods The experimental device consists of a gas adsorption system (WY-98A adsorption constant analyzer), permeability system (STY-6 gas permeability tester), loading system (SANS microcomputer controlled servo press machine), and acoustic emission collection system (DS5-16C AE monitoring system with 16 channels, capable of real-time acquisition of AE parameters; matching AE probe is RS-2A), as shown in figure 1. Figure 1. View largeDownload slide Schematic of experimental system. Figure 1. View largeDownload slide Schematic of experimental system. CH4 adsorption isotherms of SCSCSM samples were measured by the volumetric method, following the MT/T752-1997 method of China for the determination of methane adsorption capacity in SCSCSM. Approximately 21 g of fragmented SCSCSM samples with 60–80 mesh (0.177–0.250 mm in size fraction) were placed into each adsorption cell. The methane adsorption experiments were performed at the pressures of 0.840, 1.876, 2.779, 3.612, 4.430, 5.058, and 5.606 MPa at 303 K. Permeability was tested at 0.2, 0.4, 0.6, 0.8, and 0.9 MPa; the confining pressure was 1.4 MPa. To detect AE response, 6 sensors were employed on the surface of the specimen. To reduce the signal attenuation, it was necessary to apply a special coupling agent (high vacuum grease) to the AE sensor. After the probe arrangement was completed, a lead-off test was required before the experiment until the magnitude of the signal reached at least 90 dB. Load control mode was adopted in the experiment, and the loading rate was 10 N s-1. According to the China National Standard GB/T 20307-2006, the surface morphological characteristics of humic acid sodium were observed using a Hitachi SU8010 scanning electron microscope. 2.2. Design of orthogonal experiment Pulverized coal with particle sizes ranging from 0–1 mm to 1–3 mm were sieved as aggregate. The mass ratios of pulverized coal with particle size of 1–3 mm and 0–1 mm were 1:5, 1:4, 1:3, 2:3, and 1:1. Herein, a brief explanation of the selection of the mass ratio of pulverized coal in different particle sizes is given. According to the expression of Gaudin–Schuman, when index m of the particle size distribution is about 0.25 (i.e., the mass ratio of pulverized coal with particle sizes of 1–3 mm and 0–1 mm is 24:76), the strength of the briquette is the highest. When the index m deviates from the optimum value, the strength of the briquette decreases. Therefore, five different ratios were designed around the 1:3 ratio. The proportions of molding moisture M were 6%, 7%, 8%, 9%, and 10%. For soft coal, when M is less than 6%, the wetting of pulverized coal is uneven, leading to poor bonding and low strength. When the M is more than 10%, pulverized coal will cake and bridge, and water will spill out under pressure, affecting the strength and homogeneity. HASS was selected as the cementing agent (He 2007, Wang et al2015b). Humic acid sodium is extracted from lignite coal with higher humic acid content (He 2007). Therefore, HASS does not affect the physical and chemical properties of coal. Mass fractions were 2.5%, 5%, 10%, 15% and 20%. When the mass fraction is less than 2.5%, the cohesion of pulverized coal is poor. When the mass fraction is over 20%, the strength of the briquette is too high. Based on the above considerations, a ‘three factors, five levels’, L25 (53) orthogonal test program was designed as shown in table 1. Twenty-five different matching schemes were used. Table 1. Factors and levels of orthogonal test. Factor Level M H C 1 6% 2.5% 1:5 2 7% 5% 1:4 3 8% 10% 1:3 4 9% 15% 2:3 5 10% 20% 1:1 Factor Level M H C 1 6% 2.5% 1:5 2 7% 5% 1:4 3 8% 10% 1:3 4 9% 15% 2:3 5 10% 20% 1:1 View Large Table 1. Factors and levels of orthogonal test. Factor Level M H C 1 6% 2.5% 1:5 2 7% 5% 1:4 3 8% 10% 1:3 4 9% 15% 2:3 5 10% 20% 1:1 Factor Level M H C 1 6% 2.5% 1:5 2 7% 5% 1:4 3 8% 10% 1:3 4 9% 15% 2:3 5 10% 20% 1:1 View Large 2.3. Mold design and sample preparation To remove the specimen from the mold easily, a double-valve cylindrical mold with a diameter of 50 mm and a height of 120 mm was designed (figure 2(a)). Figure 2. View largeDownload slide The schematic for the preparation process and different ratio of SCSCSM samples. (a) Mold; (b) pulverized coal; (c) HASS; (d) making specimens; (e) vacuum drying oven to dry specimens; (f) SCSCSM specimens. Figure 2. View largeDownload slide The schematic for the preparation process and different ratio of SCSCSM samples. (a) Mold; (b) pulverized coal; (c) HASS; (d) making specimens; (e) vacuum drying oven to dry specimens; (f) SCSCSM specimens. Different particle sizes of pulverized coal (figure 2(b)) and HASS (figure 2(c)) were stirred according to the requirements of table 1, for wetting and even distribution of the pulverized coal. Then, material was poured into the mold and a compaction force was imposed for 15 min before removing the specimen (figure 2(d)). The forming pressure was 15 MPa. The label was pasted to the sample, and the specimen was placed in a vacuum drying oven for 2 h at 105 °C (figure 2(e)). Following China National Standard GB/T50266-2013 and regulation SL264-2001, both ends of the specimen were polished smooth. The specimen size was Φ 50 × 100 mm, and each group contained six specimens. 150 specimens were used (figure 2(f)). 3. Experimental results 3.1. Results of orthogonal test and range analysis for SCSCSM 3.1.1. Similar materials experimental results of each group Table 2 lists the experimental results of SCSCSM of 25 groups with different ratios, including the compressive strength, elastic modulus, gas adsorption, and permeability. As shown in table 2, the changes in the content of the factors have an effect on the physical and mechanical properties of the similar materials. The compressive strength (σc), elastic modulus (E), gas adsorption (X), and permeability (k) were in the ranges 0.53–1.92 MPa, 28.4–159.22 MPa, 21.43–29.10 m3 t-1, and 0.15–5.61 ×10-16 m2, respectively. The distribution of each parameter is wide, and the properties are all similar to those of the soft coal. Therefore, the physical model can be constructed according to the actual parameters of soft coal seams to explore the law of coal and gas outburst in different soft coal seams and can be helpful for the prevention and control of coal and gas outburst in soft coal seams. Table 2. Orthogonal experiment results of SCSCSM. Factor Testing results Experimental groups M/% H/% C σc/MPa Ε/MPa X/m3 t-1 k/10-16 m2 Overall rating 1 6 2.5 1:5 0.93 58.91 22.09 2.59 21.13 2 6 5 1:4 1.23 88.23 23.09 2.05 28.65 3 6 10 1:3 0.81 61.35 24.59 5.61 23.09 4 6 15 2:3 1.02 88.90 26.87 3.23 30.00 5 6 20 1:1 1.92 153.09 26.36 3.40 46.19 6 7 2.5 1:4 1.06 98.27 26.50 2.43 32.07 7 7 5 1:3 0.65 51.64 21.43 0.91 18.66 8 7 10 2:3 1.55 159.22 21.77 4.24 46.70 9 7 15 1:1 1.76 134.85 29.10 0.15 41.47 10 7 20 1:5 1.35 99.76 27.23 0.21 32.14 11 8 2.5 1:3 0.85 89.08 28.88 1.66 30.12 12 8 5 2:3 0.88 57.98 22.67 0.59 20.53 13 8 10 1:1 1.21 94.65 23.61 0.17 29.91 14 8 15 1:5 1.36 106.34 23.97 0.52 33.05 15 8 20 1:4 1.30 91.68 23.80 0.96 29.43 16 9 2.5 2:3 0.59 42.42 22.35 2.09 16.86 17 9 5 1:1 0.53 90.63 22.17 0.39 28.43 18 9 10 1:5 1.03 93.80 22.39 0.29 29.38 19 9 15 1:4 0.54 36.83 22.34 0.31 15.00 20 9 20 1:3 1.47 138.72 23.06 0.19 40.86 21 10 2.5 1:1 1.82 139.81 23.34 0.25 41.30 22 10 5 1:5 0.59 28.40 22.12 2.99 13.52 23 10 10 1:4 0.85 84.46 22.28 0.28 26.97 24 10 15 1:3 1.02 94.74 23.94 1.18 30.22 25 10 20 2:3 1.14 95.22 22.11 0.46 29.73 Factor Testing results Experimental groups M/% H/% C σc/MPa Ε/MPa X/m3 t-1 k/10-16 m2 Overall rating 1 6 2.5 1:5 0.93 58.91 22.09 2.59 21.13 2 6 5 1:4 1.23 88.23 23.09 2.05 28.65 3 6 10 1:3 0.81 61.35 24.59 5.61 23.09 4 6 15 2:3 1.02 88.90 26.87 3.23 30.00 5 6 20 1:1 1.92 153.09 26.36 3.40 46.19 6 7 2.5 1:4 1.06 98.27 26.50 2.43 32.07 7 7 5 1:3 0.65 51.64 21.43 0.91 18.66 8 7 10 2:3 1.55 159.22 21.77 4.24 46.70 9 7 15 1:1 1.76 134.85 29.10 0.15 41.47 10 7 20 1:5 1.35 99.76 27.23 0.21 32.14 11 8 2.5 1:3 0.85 89.08 28.88 1.66 30.12 12 8 5 2:3 0.88 57.98 22.67 0.59 20.53 13 8 10 1:1 1.21 94.65 23.61 0.17 29.91 14 8 15 1:5 1.36 106.34 23.97 0.52 33.05 15 8 20 1:4 1.30 91.68 23.80 0.96 29.43 16 9 2.5 2:3 0.59 42.42 22.35 2.09 16.86 17 9 5 1:1 0.53 90.63 22.17 0.39 28.43 18 9 10 1:5 1.03 93.80 22.39 0.29 29.38 19 9 15 1:4 0.54 36.83 22.34 0.31 15.00 20 9 20 1:3 1.47 138.72 23.06 0.19 40.86 21 10 2.5 1:1 1.82 139.81 23.34 0.25 41.30 22 10 5 1:5 0.59 28.40 22.12 2.99 13.52 23 10 10 1:4 0.85 84.46 22.28 0.28 26.97 24 10 15 1:3 1.02 94.74 23.94 1.18 30.22 25 10 20 2:3 1.14 95.22 22.11 0.46 29.73 View Large Table 2. Orthogonal experiment results of SCSCSM. Factor Testing results Experimental groups M/% H/% C σc/MPa Ε/MPa X/m3 t-1 k/10-16 m2 Overall rating 1 6 2.5 1:5 0.93 58.91 22.09 2.59 21.13 2 6 5 1:4 1.23 88.23 23.09 2.05 28.65 3 6 10 1:3 0.81 61.35 24.59 5.61 23.09 4 6 15 2:3 1.02 88.90 26.87 3.23 30.00 5 6 20 1:1 1.92 153.09 26.36 3.40 46.19 6 7 2.5 1:4 1.06 98.27 26.50 2.43 32.07 7 7 5 1:3 0.65 51.64 21.43 0.91 18.66 8 7 10 2:3 1.55 159.22 21.77 4.24 46.70 9 7 15 1:1 1.76 134.85 29.10 0.15 41.47 10 7 20 1:5 1.35 99.76 27.23 0.21 32.14 11 8 2.5 1:3 0.85 89.08 28.88 1.66 30.12 12 8 5 2:3 0.88 57.98 22.67 0.59 20.53 13 8 10 1:1 1.21 94.65 23.61 0.17 29.91 14 8 15 1:5 1.36 106.34 23.97 0.52 33.05 15 8 20 1:4 1.30 91.68 23.80 0.96 29.43 16 9 2.5 2:3 0.59 42.42 22.35 2.09 16.86 17 9 5 1:1 0.53 90.63 22.17 0.39 28.43 18 9 10 1:5 1.03 93.80 22.39 0.29 29.38 19 9 15 1:4 0.54 36.83 22.34 0.31 15.00 20 9 20 1:3 1.47 138.72 23.06 0.19 40.86 21 10 2.5 1:1 1.82 139.81 23.34 0.25 41.30 22 10 5 1:5 0.59 28.40 22.12 2.99 13.52 23 10 10 1:4 0.85 84.46 22.28 0.28 26.97 24 10 15 1:3 1.02 94.74 23.94 1.18 30.22 25 10 20 2:3 1.14 95.22 22.11 0.46 29.73 Factor Testing results Experimental groups M/% H/% C σc/MPa Ε/MPa X/m3 t-1 k/10-16 m2 Overall rating 1 6 2.5 1:5 0.93 58.91 22.09 2.59 21.13 2 6 5 1:4 1.23 88.23 23.09 2.05 28.65 3 6 10 1:3 0.81 61.35 24.59 5.61 23.09 4 6 15 2:3 1.02 88.90 26.87 3.23 30.00 5 6 20 1:1 1.92 153.09 26.36 3.40 46.19 6 7 2.5 1:4 1.06 98.27 26.50 2.43 32.07 7 7 5 1:3 0.65 51.64 21.43 0.91 18.66 8 7 10 2:3 1.55 159.22 21.77 4.24 46.70 9 7 15 1:1 1.76 134.85 29.10 0.15 41.47 10 7 20 1:5 1.35 99.76 27.23 0.21 32.14 11 8 2.5 1:3 0.85 89.08 28.88 1.66 30.12 12 8 5 2:3 0.88 57.98 22.67 0.59 20.53 13 8 10 1:1 1.21 94.65 23.61 0.17 29.91 14 8 15 1:5 1.36 106.34 23.97 0.52 33.05 15 8 20 1:4 1.30 91.68 23.80 0.96 29.43 16 9 2.5 2:3 0.59 42.42 22.35 2.09 16.86 17 9 5 1:1 0.53 90.63 22.17 0.39 28.43 18 9 10 1:5 1.03 93.80 22.39 0.29 29.38 19 9 15 1:4 0.54 36.83 22.34 0.31 15.00 20 9 20 1:3 1.47 138.72 23.06 0.19 40.86 21 10 2.5 1:1 1.82 139.81 23.34 0.25 41.30 22 10 5 1:5 0.59 28.40 22.12 2.99 13.52 23 10 10 1:4 0.85 84.46 22.28 0.28 26.97 24 10 15 1:3 1.02 94.74 23.94 1.18 30.22 25 10 20 2:3 1.14 95.22 22.11 0.46 29.73 View Large 3.1.2. Range analysis The compressive strength, elastic modulus, gas adsorption, and permeability of SCSCSM samples were evaluated by a comprehensive scoring method (Hung et al2007) to find out the impact factors of each parameter. The results are listed in table 3. The ranges of H, C, and M decreased in sequence, indicating that the compressive strength and elastic modulus were much more sensitive to H than to C or M. The compressive strength and elastic modulus of specimens increase as H increases. The change in the mass fraction M has less impact on the compressive strength and elastic modulus of the specimens. The reason is that the adhesion of HASS to the aggregates makes the pulverized coal with different particle sizes stick together, which is not easy to separate and the strength increases. The gas adsorptirder: RH >RM > RC. The permeability order is as follows: RM > RC > RH. This means that the effect of M on the adsorption and permeability properties of the similar material is more than C. The reason is that the M directly affects the wettability of pulverized coal and has an important effect on the inner pore structure of the specimen during the molding process and affects the gas flow. For comprehensive score, RH > RC > RM, indicating that H had the greatest effect on physical and mechanical properties of the similar material, while M had the least effect. Therefore, in the preparation of similar materials, the ratio of each factor can be reasonably adjusted according to the actual parameters of the specific coal seam. Table 3. Comprehensive score results of similar material. σc Ε X κ Comprehensive score M H C M H C M H C M H C M H C K1 4.16 5.25 5.26 450.48 428.49 387.21 123.00 123.15 117.81 3.27 9.02 6.60 149.07 141.48 129.22 K2 5.42 3.88 4.98 543.74 316.88 399.47 126.04 111.48 118.01 5.16 6.93 6.93 171.02 109.79 132.12 K3 5.91 5.45 4.80 439.73 493.48 435.53 122.92 114.65 121.90 16.88 10.59 9.55 143.04 156.04 142.94 K4 6.37 5.70 5.18 402.4 461.66 443.74 112.31 126.21 115.77 7.94 5.39 10.61 130.54 149.74 143.82 K5 5.60 7.18 7.24 442.63 578.47 613.03 113.79 122.57 124.58 3.90 5.22 4.36 141.75 178.36 187.30 R 2.21 3.30 2.44 141.34 261.59 225.82 13.73 14.73 8.82 13.61 5.37 6.25 40.49 68.57 58.08 σc Ε X κ Comprehensive score M H C M H C M H C M H C M H C K1 4.16 5.25 5.26 450.48 428.49 387.21 123.00 123.15 117.81 3.27 9.02 6.60 149.07 141.48 129.22 K2 5.42 3.88 4.98 543.74 316.88 399.47 126.04 111.48 118.01 5.16 6.93 6.93 171.02 109.79 132.12 K3 5.91 5.45 4.80 439.73 493.48 435.53 122.92 114.65 121.90 16.88 10.59 9.55 143.04 156.04 142.94 K4 6.37 5.70 5.18 402.4 461.66 443.74 112.31 126.21 115.77 7.94 5.39 10.61 130.54 149.74 143.82 K5 5.60 7.18 7.24 442.63 578.47 613.03 113.79 122.57 124.58 3.90 5.22 4.36 141.75 178.36 187.30 R 2.21 3.30 2.44 141.34 261.59 225.82 13.73 14.73 8.82 13.61 5.37 6.25 40.49 68.57 58.08 Note. Ki represents the sum of the results of each test with the same number of bits in the ith factor; the size of R indicates the significance of the corresponding factors in the experiment. The four indicators have the same weight, each take 0.25. The sum of the four indexes is the values of comprehensive score. View Large Table 3. Comprehensive score results of similar material. σc Ε X κ Comprehensive score M H C M H C M H C M H C M H C K1 4.16 5.25 5.26 450.48 428.49 387.21 123.00 123.15 117.81 3.27 9.02 6.60 149.07 141.48 129.22 K2 5.42 3.88 4.98 543.74 316.88 399.47 126.04 111.48 118.01 5.16 6.93 6.93 171.02 109.79 132.12 K3 5.91 5.45 4.80 439.73 493.48 435.53 122.92 114.65 121.90 16.88 10.59 9.55 143.04 156.04 142.94 K4 6.37 5.70 5.18 402.4 461.66 443.74 112.31 126.21 115.77 7.94 5.39 10.61 130.54 149.74 143.82 K5 5.60 7.18 7.24 442.63 578.47 613.03 113.79 122.57 124.58 3.90 5.22 4.36 141.75 178.36 187.30 R 2.21 3.30 2.44 141.34 261.59 225.82 13.73 14.73 8.82 13.61 5.37 6.25 40.49 68.57 58.08 σc Ε X κ Comprehensive score M H C M H C M H C M H C M H C K1 4.16 5.25 5.26 450.48 428.49 387.21 123.00 123.15 117.81 3.27 9.02 6.60 149.07 141.48 129.22 K2 5.42 3.88 4.98 543.74 316.88 399.47 126.04 111.48 118.01 5.16 6.93 6.93 171.02 109.79 132.12 K3 5.91 5.45 4.80 439.73 493.48 435.53 122.92 114.65 121.90 16.88 10.59 9.55 143.04 156.04 142.94 K4 6.37 5.70 5.18 402.4 461.66 443.74 112.31 126.21 115.77 7.94 5.39 10.61 130.54 149.74 143.82 K5 5.60 7.18 7.24 442.63 578.47 613.03 113.79 122.57 124.58 3.90 5.22 4.36 141.75 178.36 187.30 R 2.21 3.30 2.44 141.34 261.59 225.82 13.73 14.73 8.82 13.61 5.37 6.25 40.49 68.57 58.08 Note. Ki represents the sum of the results of each test with the same number of bits in the ith factor; the size of R indicates the significance of the corresponding factors in the experiment. The four indicators have the same weight, each take 0.25. The sum of the four indexes is the values of comprehensive score. View Large 3.2. Adsorption and permeability properties for soft coal and SCSCSM Coal and gas outbursts are heavily dependent on the storage and transportation of methane in coal seams (He and Liu 2017), and H is the main factor affecting the characteristics of SCSCSM. Therefore, when the similar material was used to study the law of coal and gas outburst of soft coal seams, investigating the effect of H on similar material gas adsorption and permeability was necessary. Materials with M = 7%, C = 1:1, and H of 2.5%, 5%, 10%, 15%, 20%, and 25% were used to study the effect of H on the adsorption, permeability, and mechanical properties. 3.2.1. Adsorption properties When the content of M and C remains unchanged, the effect of H on the gas adsorption of the similar material is shown in figure 3. The experimental results indicate that samples with high H have greater adsorption. When the H is in the range 2.5%–20%, the adsorption volume increases with increasing H, which is almost unchanged at >20%. The trend of increasing adsorption volume with increasing H was identical (pressure range from 0.84 to 5.61 MPa). According to the Langmuir adsorption equilibrium equation (Langmuir 1918), within a certain pressure range, greater pressure is associated with higher gas adsorption capacity. This concept is consistent with our results. This is because the greater the gas pressure, the more the number of gas molecules in the unit volume, and the greater the kinetic energy of the molecules. A large number of molecules collide on the surface of the specimen and occupy the adsorption site, increasing the adsorption volume. Therefore, according to the adsorption characteristics and gas pressure of different soft coal seams, the required materials can be configured by adjusting the H, in order to more accurately simulate the problems of coal and gas outburst. The experimental data were fitted to the logarithmic curve of equation (1): X=c-dln(H+e), 1 where c, d, and e are the fitting constants unique to each pressure (0.84, 1.88, 2.78, 3.61, 4.43, 5.06, and 5.61 MPa). For our results, R2 values were >0.82. Figure 3. View largeDownload slide Adsorption volume of SCSCSM under different H and gas pressure. Figure 3. View largeDownload slide Adsorption volume of SCSCSM under different H and gas pressure. The adsorption volumes of the soft coal in the above seven corresponding gas pressures, 0.84, 1.88, 2.78, 3.61, 4.43, 5.06, and 5.61 MPa, were 9.86, 13.51, 15.31, 16.29, 17.37, 18.46, 19.08 m3 t-1, respectively. 3.2.2. Permeability properties Figure 4 shows the permeability of specimens with different H under five different gas pressures, indicating that the permeability of the specimen decreases as H increases. When the mass fraction H is between 2.5% and 15%, the porosity decreases and the permeability of the specimen drops quickly. When the H is >20%, interstitial pores remain almost unchanged, and therefore, permeability changes very slightly as well. The permeability of SCSCSM specimens ranged from 0.26 ×10-16 m2 to 2.5 × 10-16 m2, low values that adequately simulate the permeability of soft coal (Cheng et al2011, Keim et al2011, Lei 2014). The trend of permeability volume which decreased with increasing H was identical (pressure range from 0.2 to 0.9 MPa). When the content of H, M, and C remains unchanged, the permeability increases with increasing gas pressure. This is because the greater the gas pressure, the more the number of gas molecules in the unit volume, and the greater the kinetic energy of the molecules, increasing the possibility of gas passing through the specimens. Figure 4. View largeDownload slide Permeability of SCSCSM under different H and gas pressure. Figure 4. View largeDownload slide Permeability of SCSCSM under different H and gas pressure. The experimental data were fitted to the exponential function given in equation (2). R2 values were >0.89. k=Aexp-Ht+B. 2 In equation (2), A, t, and B are the fitting constants unique to each pressure (0.2, 0.4, 0.6, 0.8, and 0.9 MPa). There was a positive correlation between the fitting constant A and H. 3.3. Mechanical properties and failure mode of SCSCSM specimens 3.3.1. Mechanical properties Figure 5(a) shows the complete stress–strain curves of the six SCSCSM specimens described in section 3.2. Varying HASS content makes the samples exhibit different deformation characteristics and damage processes. Samples with high H have greater strength. The SCSCSM specimens also contain typical compaction phase but with short duration; they are almost entirely linear elastic deformation before the peak stress; after the peak the curve become gentler and residual strength is smaller. The SCSCSM-20% samples were selected to analyze the whole process of uniaxial compression test in detail, as shown in figure 5(b), indicating the initial stage of uniaxial compression, the specimen is continuously compacted and the micro-cracks begin to appear. With increasing load, a large number of micro-cracks appear, leading to the mutual penetration of micro-cracks and the formation of macroscopic rupture. Figure 5. View largeDownload slide Complete stress–strain curves. (a) SCSCSM specimens (b) detailed analysis of different stages of uniaxial compression for SCSCSM-20% samples. Figure 5. View largeDownload slide Complete stress–strain curves. (a) SCSCSM specimens (b) detailed analysis of different stages of uniaxial compression for SCSCSM-20% samples. Uniaxial compression experiments were carried out to test the mechanical parameters (compressive strength and elastic modulus) of SCSCSM specimens with different H values. The results are shown in figure 6. With increasing H, the compressive strength (0.59–3.17 MPa) and elastic modulus (49.48–281.05 MPa) increase, showing an overall upward trend. This indicates that H has a significant effect on the mechanical properties of similar materials. Therefore, in the preparation of the required strength of similar materials, we should focus on the factor of H. The specimen with H = 25% had a compressive strength >2 MPa, inconsistent with soft coal (Wang et al2016, Zhu et al2017). Figure 6. View largeDownload slide The influence of H on mechanical properties of SCSCSM specimens. Figure 6. View largeDownload slide The influence of H on mechanical properties of SCSCSM specimens. 3.3.2. Damage mode under uniaxial compression test Though uniaxial compressive deformation of SCSCSM with different H specimens varies, failure can be divided into three main types, designated as A, B and C. Figure 7 shows the photos of typical physical deformation and corresponding sketches. Type A is the extrusion damage, which occurred in specimens with low H (2.5%–5%). Features include no obvious major rupture surfaces, damage everywhere, crushing destruction, and noticeable lateral bulge (figures 7(A-1) and (A-2)). Type B is the split wedge damage, which primarily occurred in samples with intermediate H (10%, 15%). Type B is characterized by ‘U’ (figure 7(B-1)) and inverted ‘U’ (figure 7(B-2)) type damage patterns. Type C is the brittle shear failure, which occurred in specimens with higher H (20% and 25%). The ultimate destruction of type C is propagation of an existing fault along the fracture direction, mainly through the macroscopic shear fracture surface to the lower part of the specimen, most of the fracture surface and axial direction 45°. Figure 7(C-1) shows damage in a specimen with H = 20%. After the sample reached the peak stress, a crack at 45° appeared. In this specimen, the strength was high, and the bonding effect was great and the damage nature was consistent with soft coal. Figure 7. View largeDownload slide The failure modes of loaded SCSCSM specimens. (a) Physical pictures; (b) corresponding sketch pictures. Figure 7. View largeDownload slide The failure modes of loaded SCSCSM specimens. (a) Physical pictures; (b) corresponding sketch pictures. Based on the above analysis of the orthogonal test, and comparison of soft coal properties to similar material properties (adsorption, permeability properties, mechanical properties, and failure mode), H = 20% was found as the most suitable ratio. Therefore, the following study of AE response characteristics is based on the samples with 20% HASS (SCSCSM-20%-1 and SCSCSM-20%-2). 3.4. AE response of SCSCSM specimens Figure 8 shows the curves of AE events and mechanical properties for SCSCSM specimens (SCSCSM-20%-1 and SCSCSM-20%-2) during the uniaxial loading process. As stress increases, the AE pulses and energy values tend to rise until peak stress is reached. From the beginning of the loading to the peak stress, the cumulative AE pulse and energy can be divided into a slow growth stage (oa) and a rapid increase stage (ab). Like soft coal, the SCSCSM had some pre-existing pores and cracks in the specimen. At the initial stage of loading, the pores and cracks were compacted and closed, resulting in relatively high AE pulses and energy. As load increased, the specimens entered the linear elastic deformation stage of the stress–strain curve, as shown in figure 5. The original crack compacted and almost closed, and internal micro-crack propagation showed diversity and randomness. AE events occurred intermittently. This stage lasts for a long time, accounting about 54.1% of the total loading stage, but the number of AE pulses and cumulative energy only accounted for 7.12% and 7.17% of the whole process, respectively. When stress exceeded 60% of the peak stress, the micro-cracks in the specimen increased significantly, and the cumulative AE pulses and energy increased rapidly. The patterns of AE events of SCSCSM-20% were consistent with the studies of coal containing methane (Kong et al2016); therefore, the SCSCSM-20% samples can be used as the material of similar simulation of coal to conduct similar simulation experiment. Figure 8. View largeDownload slide The time characteristics of mechanical properties and AE response for SCSCSM specimens. Figure 8. View largeDownload slide The time characteristics of mechanical properties and AE response for SCSCSM specimens. 4. Analysis and discussion 4.1. Effects of H on adsorption and permeability properties of SCSCSM Humic acid sodium is water soluble and has a linear structure; it becomes a viscous colloid when dissolved in water. When pulverized coal and HASS are homogeneously mixed, long chain molecules of humic acid sodium cross each other in series, and form a well-developed network structure with a high surface area (He 2007). The micro-structure of sodium humic acid was observed by SEM. As shown in figure 9, the sodium humic acid has a large number of internal pores. The pore-fracture systems in sodium humic acid (extracted from lignite and weathered coal) provide plenty of adsorption sites (Clarkson and Bustin 1999, Harpalani and Zhao 2007, Flores et al2008, Pant et al2015), and sodium humic acid also has adsorption characteristics (Ramostejada et al2001, Sun 2011, Wang et al2015a). Hence, with increasing H, the gas adsorption increases (figure 3). As H increases, more long-chain molecules of humic acid sodium exist in the solution, and therefore the coal particles become more closely bonded, resulting in the reduction of pore channels in the coal for gas flow. The release of gas is hindered, thus reducing the permeability of the gas in the specimen (figure 4). Figure 9. View largeDownload slide SEM image of sodium humic acid. Figure 9. View largeDownload slide SEM image of sodium humic acid. 4.2. Effects of H on mechanical properties of SCSCSM Humic acid sodium belongs to a polymer compound, a linear long chain structure linked by many tiny spherical particles (He 2007). Humic acid sodium is easy to dissolve in water and is a sponge-like structure with strong adsorbability that bonds pulverized coal particles. Due to the ‘bridging’ effect of linear molecules, in the process of pulverized coal into briquetting coal, the pulverized coal and HASS are homogeneously mixed. Long chain molecules of humic acid sodium cross each other in series, and form a well-developed network structure, making the pulverized coal bond closely together. When samples contained less humic acid sodium, cementing was less effective and therefore the strength and elastic modulus decreased. As H increased, pulverized coal cemented better, and the specimen surface became smooth, showing enhanced integrity and cohesion. Therefore, the average mechanical parameters of the SCSCSM specimens gradually improved, as shown in figures 5 and 6. 4.3. The damage evolution law of SCSCSM specimens In the process of uniaxial compression, SCSCSM specimens underwent microscopic deformation, macro fracture, and final failure process, and its internal damage has undergone a process from quantitative change to failure. In this study, the damage evolution of SCSCSM was studied by damage parameter (abbreviated as D), which was introduced by Tang and Xu (1990). D is calculated as follows. D=Sm-S′Sm=SSm, 3 where Sm is the initial cross-sectional area of specimen, S′ is the effective area, S is the area of the damaged section, and D is a damage parameter which ranges from 0 to 1. D = 0 is comparable to the state of material without any damage. This is only a condition for reference. D = 1 is comparable to the complete destruction of the material. Based on the principle of equivalent strain (Dhar et al2000) and Hooke’s law in mechanics of materials (Wen et al2016), D can be represented as: D=1-σεE, 4 where E is the elastic modulus of the similar material, σ is the stress, and ε is the strain. During the loading process, the evolution of D is shown in figure 10. Figure10. View largeDownload slide The evolution curves of stress–strain and damage parameter-strain for SCSCSM specimens. (a) SCSCSM-20%-1; (b) SCSCSM-20%-2. Figure10. View largeDownload slide The evolution curves of stress–strain and damage parameter-strain for SCSCSM specimens. (a) SCSCSM-20%-1; (b) SCSCSM-20%-2. As shown in figure 10, the evolution of D can be divided into four stages: an initial stage of increasing damage (OA), a subsequent stage of decreasing damage (AB), a growth stage of rapidly increasing damage (BC), and finally a saturated period of slowly growing damage (CD). SCSCSM specimens were made of pulverized coal with different particle sizes bonded by HASS, and therefore SCSCSM is a heterogeneous material. There are some original pores and cracks in the specimen. At the initial stage of loading, the pores and cracks are gradually compressed and tend to close, reducing effective bearing area of S′. According to equation (3), the damage parameter D should increase at this time. With the reduction of the original pores and cracks, the inside of the specimen is compacted, and the effective bearing area of S′ increases, decreasing D. As the loading continues, the formation speed and density of cracks will increase. As this process occurs, micro-cracks will eventually coalesce into primary and secondary fracture surfaces. Then, the effective bearing area decreases abruptly, and D grows rapidly. After peak stress is reached, some micro-cracks are still generated inside the specimen, and D continues to increase slowly until reaching a maximum value. As shown in figure 10, the damage evolution of specimen under uniaxial compression is a nonlinear process. Expressing the whole evolution process with a continuous equation is difficult, and it must be divided into different segments to describe (Wang et al2015). Therefore, we used a Exp3P2 function to fit the OA, AB, and CD segments. The BC segment is fitted with the doseresp function (Zhang 2014). All the fitting curves for both the specimens are shown graphically in figure 11 and expressed by equations (5) and (6), showing the results for specimens SCSCSM-20%-1 and SCSCSM-20%-2, respectively. The fitting coefficients (R2) of the different stages are high, indicating that the fitting results are ideal. D=exp(-63888.67ε2+319.91ε-0.77)(0≤ε<1.89×10-3)exp(-6853.37ε2-73.58ε-0.23)(1.89×10-3≤ε<1.07×10-2)0.2+0.6/[1+10(9.3-663.3ε)](1.07×10-2≤ε<1.70×10-2)exp(-1017.62ε2+60.65ε-0.93)(1.70×10-2≤ε<2.97×10-2), 5 D=exp(-272024.62ε2+1060.24ε-1.54)(0≤ε<2.51×10-3)exp(-8025.58ε2-29.03ε-0.38)(2.51×10-3≤ε<1.06×10-2)0.2+0.7/[1+10(10.7-698.6ε)](1.06×10-2≤ε<1.88×10-2)exp(-666.74ε2+37.96ε-0.56)(1.88×10-2≤ε<2.99×10-2). 6 Figure 11. View largeDownload slide The piecewise fitting curve of damage parameter-strain. Figure 11. View largeDownload slide The piecewise fitting curve of damage parameter-strain. 4.4. The relationship between AE and strain parameter In uniaxial compression experiments, damage evolution of internal micro-cracks leads to failure of specimens. AE is caused by transient elastic waves caused by stress redistribution following fracture propagation (Pollock 1973, Ohtsu 1996, Carpinteri et al2007a, 2007b, 2008), and can be used as an external manifestation of internal crack evolution and verification of macroscopic fracture. AE events are consistent with the damage of revision loading samples, indicating that there is an internal relationship between AE and D. Therefore, based on the view of the damage mechanics, combining the strain parameter and cumulative AE pulse, we investigated the relationship between mechanical parameters and AE. Assuming that the strength of micro units of SCSCSM specimens obeys Weibull distribution (Tang and Xu 1990), its probability density function is as follows: φ(ε)=mαεαm-1exp-εαm, 7 m=1ln(Eεpk/σpk), 8 α=εpkmm, 9 where φ(ε) is the damage measurement value of the micro element of similar material during loading, m is the shape parameter and α is the scale parameter, computed from equations (8) and (9) (Cao et al2004). All the constants are non-negative. εpk is the peak strain, and σpk is the peak stress. In the process of uniaxial compression of SCSCSM specimens, the strain increased from 0 to ε under the force. Damage parameter D can be expressed as a function of α and m by exploiting the probability density function: D=∫0εφ(ε)dε=1-exp-εαm. 10 The relationship between the cumulative AE pulses and damage parameters of granular brittle materials is derived by Ji and Cai (2003), Zhang et al (2006), and Xue et al (2011). The formula is shown by equation (11): D=KN, 11 where N is the cumulative AE pulses and K is a constant given by K=Dmax/Nmax. 12 In this study, load control mode was used during the loading process, namely the linear relationship between the stress and time. σ=kt, 13 where k is a constant, representing the rate of loading. Substituting equations (11) and (13) into equation (4) yields: ε=σE(1-D)=ktE(1-KN). 14 Substituting equation (10) into equations (11) and (15) can be obtained: KN=D=1-exp-εαm. 15 Substituting equation (14) into equations (15) and (16) can be obtained: KN=1-exp-kE(1-KN)αmtm. 16 Then, simplifying equation (16) yields equation (17): t=[ln(1-KN)]1/mEα(1-KN)-(k). 17 Substituting equation (17) into equation (14), the relationship of AE and strain can be obtained. ε=αln11-KN1/m. 18 In the process of uniaxial compression of similar material samples, Dmax = 1; therefore, K=DmaxNmax=1Nmax. 19 Substitute equation (19) into equation (18), the relationship of AE and strain is obtained as follows. ε=-αln1-NNmaxm. 20 Equation (20) shows the relationship between the cumulative AE pulses and strain is independent of the experimental constant k, verifying that the accumulative AE parameters are a sign of internal micro-structure change of SCSCSM samples as well as strain. It is in agreement with the results of Xue et al (2011). AE events are consistent with the damage of loading samples. Therefore, AE events can be used to signal the evolution of internal micro-cracks of the soft coal solid–gas coupling of similar material as shown in figure 8. 5. Conclusions The three factors and five variable levels table of raw materials’ ratio was designed by an orthogonal design method. Adsorption, permeability, uniaxial compressive strength, elastic modulus, and damage evolution of the soft coal solid–gas coupling similar material samples were acquired through gas adsorption, permeability, uniaxial compression, and AE tests. Through orthogonal testing and comprehensive scoring method and range analysis, mass fraction of humic acid sodium solution was found to be the main factor affecting the properties of SCSCSM. As the mass fraction of humic acid sodium solution increased, adsorption volume increased logarithmically, while permeability decreased exponentially. The compressive strength (0.59–3.17 MPa) and elastic modulus (49.48–281.05 MPa) increased with the mass fraction of HASS. The main failure types of soft coal solid–gas coupling similar material are extrusion damage, split wedge damage, and brittle shear failure. The temporal variation law of AE parameters represents the evolution and degree of internal damage. From the beginning of the loading to peak stress, the cumulative AE pulse and energy can be divided into a slow growth stage and a rapid increase stage. The evolution of D of SCSCSM specimens can be divided into four stages: an initial stage of increasing damage, a stage of decreasing damage, a growth stage of rapidly increasing damage, and finally a saturated period of slowly growing damage. Using Exp3P2 and doseresp function fitting, segment expressions of damage evolution mechanism of SCSCSM were obtained. Through the study of SCSCSM’s internal gas migration, storage and mechanical properties, main failure mode, AE events, and multiscale mechanistic phenomena, this study has laid a foundation for the subsequent development of solid–gas coupling similar simulation materials and physical simulation experiment of coal and gas outburst for soft coal seam. Acknowledgments This study was supported by the National Natural Science Foundation of China (project No. 51574017) and the National Key Research and Development Program of China (No. 2016YFC0801706). The authors are grateful to the anonymous referees for their precious comments and suggestions. References Aker E , Kühn D , Vavryčuk V , Soldal M , Oye V . , 2014 Experimental investigation of acoustic emissions and their moment tensors in rock during failure , Int. J. Rock Mech. 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TI - Soft coal solid–gas coupling similar material for coal and gas outburst simulation tests
JF - Journal of Geophysics and Engineering
DO - 10.1088/1742-2140/aac098
DA - 2018-10-01
UR - https://www.deepdyve.com/lp/oxford-university-press/soft-coal-solid-gas-coupling-similar-material-for-coal-and-gas-om3N8MExgK
SP - 2033
VL - 15
IS - 5
DP - DeepDyve
ER -