J. Cent. South Univ. (2017) 24: 423-431

DOI: 10.1007/s11171-017-3444-1

Effects of water intrusion and loading rate on mechanical properties of and crack propagation in coal–rock combinations

CHEN Tian(陈田), YAO Qiang-ling(姚强岭), WEI Fei(卫斐), CHONG Zhao-hui(种照辉),

ZHOU Jian(周健), WANG Chang-bin(王常彬), LI Jing(李静)

School of Mines, Key Laboratory of Deep Coal Resource Mining of Ministry of Education of China,

China University of Mining & Technology, Xuzhou 221116, China

Central South University Press and Springer-Verlag Berlin Heidelberg 2017

Abstract: Tackling the problems of underground water storage in collieries in arid regions requires knowledge of the effect of water intrusion and loading rate on the mechanical properties of and crack development in coal–rock combinations. Fifty-four coal–rock combinations were prepared and split equally into groups containing different moisture contents (dry, natural moisture and saturated) to conduct acoustic emission testing under uniaxial compression with loading rates ranging from 0.1 mm/min to 0.6 mm/min. The results show that the peak stress and strength-softening modulus, elastic modulus, strain-softening modulus, and post-peak modulus partly decrease with increasing moisture content and loading rate. In contrast, peak strain increases with increasing moisture content and fluctuates with rising loading rate. More significantly, the relationship between stiffness and stress, combined with accumulated counts of acoustic emission, can be used to precisely predict all phases of crack propagation. This is helpful in studying the impact of moisture content and loading rate on crack propagation and accurately calculating mechanical properties. We also determined that the stress thresholds of crack closure, crack initiation, and crack damage do not vary with changes of moisture content and loading rate, constituting 15.22%, 32.20%, and 80.98% of peak stress, respectively. These outcomes assist in developing approaches to water storage in coal mines, determining the necessary width of waterproof coal–rock pillars, and methods of supporting water-enriched roadways, while also advances understanding the mechanical properties of coal–rock combinations and laws of crack propagation.

Key words: water intrusion; loading rate; mechanical properties; coal-rock combination; crack propagation; stress threshold

1 Introduction

Water–rock interactions are a pervasive and challenging problem in engineering rock mechanics. Understanding the behavior of coal–rock samples under conditions of water intrusion is helpful in addressing underground engineering problems and facilitates investigations of crack damage mechanisms [1–3]. In western China, there are substantial coal reserves, but, owing to the severe dearth of water resources, many scholars have proposed that underground reservoirs in collieries be constructed to preserve and recycle the underground water produced by mining. Investigating the mechanical characterization of coal can provide guidance on how to set width of boundary coal pillars at the edge of such underground reservoirs or protect an aquifer by setting reasonable width of a coal pillar [4, 5]. Without a doubt, the mechanical properties of coal-rock combinations can usually be applied to evaluate the rock mass characters by Q (Barton rock mass quality classification) and RMR (Rock mass rating) [6–8]. In addition, with deeper mining depths and the perturbation caused by excavated roadways and ground stress, the boundary coal and rock pillars bear different loads. Understanding the mechanical properties and crack propagation of water-intruded and loading coal are therefore invaluable and useful in underground engineering.

Relationships between rock mechanics and moisture content or loading rate have received some research attention. HUANG and LIU [9] found that a higher loading rate more easily prompts coal and rock eruption. VERSTRYNGE et al [10] analyzed the impact of moisture content on ferruginous sandstone, determining that water had a definite effect on acoustic emission (AE) activities and promoted creep of the specimens. VISHAL et al [11] found that increasing amounts of gas and water can diminish the strength of coal samples and the corresponding AE counts. YAO et al and EBERHARDT et al [12] found that peak stress and elastic modulus of rock samples (including coal samples) increase with increasing moisture content, but peak strain has the opposite response. LI et al [13], WU et al [14] and LIU et al [15] determined that AE activity is positively related to loading rate and the sum of the AE counts is negatively related to loading rate. YU et al [16], FENG and DING [17] tracked the movement of trace elements and measured ion concentration to analyze water–rock interactions and permeability.

In order to investigate the relationship between crack propagation and water, AE and volumetric strain have been employed to study the thresholds of crack closure and crack propagation for brittle rock [18, 19] found that the strength of wetted concrete samples was lower than that for dry samples. In addition, the elastic modulus and Poisson's ratio increased with increasing moisture content in uniaxial compression tests. Crack initiation and crack damage were explored using cumulative AE counts. DENG et al [20] demonstrated that fracture has a significant effect on the strength of coal–rock combinations. Rock brittleness was found to impact the gradient of the post-peak curve and AE technology can be used to explain post-peak crack development. KHAZAEI et al [21] defined the propagation of fracture planes by analysis of b-values. There are, however, few studies that have explored the effect of loading rate and moisture content on the mechanical properties of and crack propagation in coal–rock combinations.

In this work, we performed AE experiments under uniaxial compression and varied loading rate on coal–rock samples containing different moisture contents. We initially investigated the effect of moisture content and loading rate on overall stress–strain curves of the coal–rock combinations. Using these stiffness-stress curves combined with cumulative AE curves, we then analyzed the development of cracks and the stress thresholds of crack closure, crack initiation, and crack damage. We also studied the impact of loading rate and moisture content on the mechanical properties of coal–rock combinations (peak stress, peak strain, elastic modulus, post-peak modulus, and strain-softening modulus) and on the stress thresholds of crack closure, crack initiation, and crack damage. The results of these experiments are useful in developing approaches to address issues of water storage in coal mines, to determine the width of waterproof coal pillars, and methods of supporting water-enriched roadways, as well as improve understanding of the mechanical characterization of coal-rock combinations and the laws of its crack propagation.

2 Experimental scheme

2.1 Collection and preparation of coal–rock specimens

To study the mechanics and AE characteristics during the rupture of coal–rock combinations with various moisture contents under different loading rates, 25 coal samples, 25 immediate roof samples, and 25 immediate floor samples (each 300 mm in length, 300 mm in width, and 200 mm in height) were collected from Xinqiao Colliery of the Yongcheng coal group. Fifty-four coal–rock combinations were processed into standard coal–rock samples, each with a diameter of 50 mm and height of 100 mm, according to the requirements of the International Society for Rock Mechanics [22, 23]. To ensure representativeness, it is suggested that the ratio of the height of the immediate roof, the coal, and immediate floor be 1:1:1 in the composed coal–rock samples [10, 24]. Therefore, the immediate roof (sandy mudstone), coal, and immediate floor (sandy mudstone) were processed into 50 mm (diameter)×33 mm (height), 50 mm (diameter)×34 mm (height), and 50 mm (diameter)×33 mm (height) specimens, respectively, by dense drilling. These specimens were then appropriately combined into a standard composed coal–rock specimen of 50 mm (diameter)×100 mm (height) (see Fig. 1) using superglue.

Fig. 1 Composition of coal–rock combinations used to prepare specimens for testing (Unit: mm)

2.2 Experimental equipment

The main components of the testing equipment are a purpose-built wetting apparatus, a loading system, and a digital strain data recorder. A schematic diagram of the testing system is shown in Fig. 2. The wetting apparatus is a humidifier connected by a plastic hose to a sealed specimen box. When the humidifier is turned on, it creates water mist that enters the sealed box through the plastic hose, creating a high-moisture environment. Using this method, the coal-rock specimens in the sealed box remain intact during water infusion, thus avoiding the need to immerse the specimens in water. Immersing the samples in water can cause them to disintegrate. The loading system used was a microcomputer-controlled electronic universal testing machine (model No. CMT5305) developed by San Si Co., Shenzhen in China. Strain is recorded by static resistance strain indicators (model No. TS3890). The strain acquisition system utilises digital strain gauges with microprocessor chips. The strain gauges are glued to the surface of the specimen and connected to the main control computer (MCC). The MCC controls, processes, displays, and transmits the measured data. This strain acquisition system collects multi-point strain values at a high rate.

Fig. 2 Schematic diagram of testing system:

2.3 Experimental method

The initial weight of the samples was measured and then they were oven dried. All dry specimens were kept in an oven to protect them from exchanging moisture with the atmosphere. The first test group, designated as DS, comprised of 18 dry coal–rock combinations; the second group, WS, comprised of 18 coal–rock combinations containing a natural quantity of moisture; the third group, SS, comprised of 18 fully saturated coal–rock combinations.

AE data from the coal specimens were monitored during the uniaxial compression tests. The experimental AE equipment was produced by Physical Acoustics Corporation in Princeton of United states. The AE sensor used was an Nano30 with an operating frequency range of 35–100 kHz, a resonant frequency of 55 kHz, a threshold value of 45 dB, a gain of 40 dB, and analogue filter lower and upper limits of 1 and 400 kHz. The sampling frequency was 20 Hz.

The experiments were performed using the universal testing machine at loading rates of 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6 mm/min. These loading rates are used as suffixes in serial numbers to identify the various samples: for example, the serial number DS1 represents the dry coal–rock combination under a loading rate of 0.1 mm/min. The AE system and universal testing machine recorded the data simultaneously during the tests.

3 Results and analysis

3.1 Effect of water intrusion and loading rate on peak point and slope of stress–strain curves

The overall stress–strain curves for coal–rock combinations with various water contents under a loading rate of 0.2 mm/s are shown in Fig. 3(a). Pursuant to the crack development, these curves can be divided into five phases: crack closure, elastic deformation, stable crack propagation, unstable crack propagation, and post-peak failure. From a micromechanical perspective, water damage can be partly attributed to hydroxyl ions that soften and couple the rock and coal specimens and consequently change their microstructure and weaken the mechanical properties [1, 16, 25, 26]. The mineral ingredients, granular size, stress conditions, and integrity of the samples also partly determine their mechanical characterization [27]. In comparison with the curves of the dry coal–rock combinations, those of the wetted specimens bend much more and the time to elastic deformation is shortened. The slopes of the regions of elastic deformation, stable crack propagation, and unstable crack propagation decline significantly. The peak strength changes from 8.44 MPa to 2.63 MPa for fully wetted samples, which is 68.8% less than the strength of dry samples. Water therefore has rather damaging and softening effects on the mechanical properties of coal–rock samples. Post-peak failure is normally evaluated by means of the strain-softening modulus and post-peak modulus that are discussed in Section 3.3.

It is clear from Fig. 3(b) that the slope of the stress–strain curve generally increases with increasing loading rate: the larger the loading rate, the slower the propagation of new and initial cracks. With an increase in loading rate, damage to the specimen increases, but the structure of the coal–rock combinations can recombine with time and attain a certain subsequent resistance to absorb strain energy before the stress drops again: higher loading rates can therefore contribute to higher peak stress.

Fig. 3 Effect of moisture content (a) and loading rate (b) on stress–strain curves

3.2 Analysis of four phases of crack propagation for coal–rock combinations

According to Fig. 3, it is difficult to precisely estimate the initiation of the four phases (crack closure, elastic deformation, stable crack propagation, and unstable crack propagation) prior to the onset of peak stress. Nevertheless, it is essential to estimate the stress thresholds of crack closure, crack initiation, and crack damage to determine the elastic modulus, elastoplasticity, and other mechanical properties. Moreover, these properties are important in underground engineering in determining the effects of water intrusion and loading rate on crack development. Hence, the stiffness–stress and sum of AE counts curves are used to determine the specific point of crack development [27, 28]. Stiffness can be derived from the following equation:

                                     (1)

where k is the stiffness of the coal–rock combination; p is the axial force; △l is the displacement increment of the coal–rock combination.

It is evident from Fig. 4(a) that, with continued loading, the stiffness-stress curve reaches linearity at the end of the crack-closure (A-B) phase and namely the start of elastic deformation. Stiffness peaks temporarily at point C (end of elastic deformation), after which point C represents secondary crack closure (C-D), attributed to the existence of large fractures or high porosities. Crack initiation occurs at point D, before propagation of a stable crack (D-E). The crack propagates stably and grows to point E, after which the propagation becomes unstable. Point E represents the transition from elasticity to ductility; crack damage during unstable crack propagation is irreversible until failure of the sample. From the start of unstable crack propagation until failure (point F), the loaded coal–rock samples emit sound and the cracks develop quickly and strongly, forming a macro plane of rupture, but still maintaining the integrity of the samples. As illustrated in Fig. 4(a), the combination of the stiffness–stress and stress–strain curves can accurately reflect the propagation of a crack before failure.

In accordance with Fig. 4(b), as the loading increases, the AE cumulative curve is consistent with the overall stress–strain curve and reflects the all phases of crack development. At the stage of crack closure (phase A-B), little AE activity is recorded because closures of existing micro and macro cracks cause little or no energy release. As the uniaxial load increases, the AE cumulative counts increases slightly, representing the start of elastic deformation (phase B-C). After point C, there is an exponential growth of AE counts because of secondary crack closure or temporary failure (phase C-D). Thereafter, the accumulated counts increase approximately linearly, marking crack initiation (point D) where stable crack propagation takes place. The gradient of the plot (phase D-E) gradually increases, indicating an increase in the release of strain energy stored in the samples (i.e., more AE counts). Crack damage occurs at the point of departure of the curve from linearity to an exponential function. In this zone (phase E-F), unstable crack propagation occurs until failure (F) of the rock specimen [12, 28, 29]. The cumulative of AE counts curve confirms the events depicted by the stiffness–stress curve; we find that the stress thresholds for crack development in the cumulative AE counts curve are approximately equal to those determined from the stiffness–stress curves.

Fig. 4 Stiffness–stress curve (a) and relationship between stress and sum of AE counts as testing time (b)

3.3 Effect of water intrusion and loading rate on principal mechanical properties of coal–rock combinations, incorporating peak strength and strain as well as three moduli

According to Fig. 5(a), the peak stress of coal–rock combinations with different moisture contents trends upward as the loading rate increases. At a low loading rate, the initial and micro fractures can fully develop, but when loading rate reaches a certain value, fractures can no longer propagate completely; higher loading rates can consequently increase the strength of coal–rock samples [30, 31]. The linear relationships between loading rate and peak strength, shown in Fig. 5(a), are represented by

                (2)

where x represents the loading rate; y_DS, y_WS, and y_SS represent the coal–rock combinations with various moistures;andare the respective correlation coefficients of the fitted curves.

These fitted equations show that the peak stress of dry samples increases more quickly than that of water-intruded samples. The samples may be more sensitive to changes in loading rate in the absence of damage caused by water. Notably, the peak stress declines at a loading rate of 0.6 mm/s compared with that at 0.5 mm/s, which is mainly caused by large internal fractures. The moisture content also has more significant effect on the peak stress than the loading rate.

From Fig. 5(b), as the loading rate increases, the peak strain of the coal–rock combinations trends upward before falling, after which it increases again, and then decreases. The relationship between peak strain and loading rate fluctuates; however, the dry samples apparently have higher peak strain and it is difficult to distinguish which wetted samples have larger peak strain.

Fig. 5 Peak strength (a) and peak strain (b) under different loading rates and moisture contents

To explain this fluctuation, we employed the following two equations:

                                 (3)

                                      (4)

where △l is the displacement of the coal–rock combination; l is the height of the specimen; ν is the loading rate; t is the loading time; ε' is the differential of ε with respect to t.

Using Eqs. (3) and (4), we can determine that, under constant loading rate, the strain increases linearly with time and the gradient of the strain is constant. At lower loading rate, the crack can fully widen and develop; up to a loading rate of 0.2 mm/s, the crack still has sufficient time to achieve deformation and so the strain increases during this period. Thereafter, a higher loading rate can initiate recombination of the samples and partially reduces the deformation energy [9, 11]. When the loading rate is below 0.4 mm/min, deformation of the samples is incomplete for a short time. At a loading rate of 0.6 mm/s, the samples fail sharply and quickly, because the micro and macro fractures cannot expand efficiently and the strain therefore decreases slightly.

Coal-rock samples drilled in a colliery are anisotropic and nonhomogeneous because of numerous micro cracks and porosity, which contribute to presenting apparently different mechanical properties under loading conditions. The elastic deformation of coal–rock samples is not linear, as confirmed by the data of Fig. 3. The elastic modulus for anisotropic and nonhomogeneous materials can be expressed incorporating the tangent, secant, and average modulus [9, 13]. In this paper, we used the average modulus as the elastic modulus. If we assume the strain–stress formula as σ=f(ε), the elastic modulus can be derived from the following equation:

                            (5)

where ε₁ and ε₂ represent the strain of start point and the end point over the elastic phase respectively; E is the elastic modulus; f(ε₁) and f(ε₂) represent the initial and end points of the elastic phase, respectively.

As shown in Fig. 6(a), the elastic moduli of coal–rock combinations with similar moisture contents exhibit various values, mainly because the size, shape, surface roughness, and micro fractures of specimens have an important impact on elastic modulus [2, 32]. With increasing moisture content, the elastic modulus trends strongly downward, but higher loading rates increase the elastic modulus. The moduli of coal–rock samples experiencing substantial closure fractures will increase with increasing friction coefficient of fracture surfaces; if the friction of fracture surfaces becomes sufficiently large, the crack surfaces will have difficulty in slipping. The proportion of fracture surfaces also influence the slipping speed of cracks and crack roughness. When coal–rock combinations absorb more water or are subjected to a lower loading rate, this friction will decline and the movement of cracks will be relatively slower, resulting in a drop in the elastic modulus [12].

Connecting the points of peak strength and residual strength gives a straight line, the slope of which gives the post-peak modulus that generally is used to express the post-peak deformation [33–35]. From Fig. 6(c), the post-peak moduli decrease with increasing moisture content of the coal–rock combinations because water has a strong softening effect on the granules, changes the contact between granules, and even hydrolyses the sample or prompts chemical reaction [36]. We found that the overall trend of the post-peak moduli shows a positive relationship with loading rate, but the post-peak moduli of coal–rock samples are much more scatter due to the anisotropy and internal defects of these materials.

Fig. 6 Effect of loading rate and moisture content on elastic (a), post-peak (b), and strain-softening (c) moduli

Like elastic modulus, the strain-softening modulus is represented by the slope of the post-peak linear phase, and normally expresses the extent of brittle failure of samples. Unlike steel and other ductile materials, the deformation of soft rock (such as coal) will continue to rise after the specimen has reached peak stress and then the stress then falls sharply. This category of deformation caused by degradation of rock materials is known as strain softening [9, 37, 38]. From Fig. 5(b), the strain-softening modulus declines with increasing moisture content, which indicates that water intrusion has a marked influence on post-peak mechanical properties and that the loading rate can enhance the strain-softening modulus. This result can be confirmed with reference to Fig. 3. Note: in this work, measurements of strain-softening modulus and post-peak modulus refer to their absolute values.

From Fig. 6, it is clear that strain-softening modulus and post-peak modulus are an order of magnitude larger than elastic modulus, which demonstrates the releasing energy after peak stress much larger than the absorbing energy in elastic deformation. Also, it explains why the resistance to deformation in the post-peak stage lowers the figure for elastic deformation.

3.4 Effect of water intrusion and loading rate on stress thresholds of crack closure, crack initiation, and crack damage

Figure 7(a) shows that with increasing loading rate and decreasing moisture content, the stress threshold of crack closure increases. Water intrusion can significantly weaken the coal–rock structure and both its closure and strength can decline with rising moisture content. Nevertheless, a higher loading rate can cause that cracks do not have sufficient time to close and thus increase the crack closure stress.

It is clear that from Figs. 7(a) and (b) that the stress thresholds of crack initiation and crack damage have a positive linear relationship with loading rate and decline as the moisture content increases. Higher moisture content can strengthen lubrication and weaken the contact between the coal–rock granules. The thresholds for the dry samples are readily changed by altering the loading rate or moisture content.

The percentages of crack closure stress, crack initiation stress, and crack damage relative to the peak stress for all 54 coal–rock samples are shown in Fig. 8. Fitting the respective data (see Table 1) to linear relationships, we determined that crack closure stress constitutes 15.22% of peak stress, crack initiation stress is 32.20%, and crack damage stress is 80.98% of peak stress. More significantly, these three values bear no relationship to moisture content or loading rate. We can

therefore measure the uniaxial compression stress to evaluate the stress thresholds of crack closure, initiation, and damage to assess the extent of crack propagation.

Fig. 7 Stress thresholds of crack closure (a), crack initiation (b), and crack damage (c)

Fig. 8 Average crack closure, crack initiation, and crack damage stresses expressed as a percentage of average peak stress (Stress values were determined by uniaxial testing for samples. “y” is the average percentage stress for each moisture group)

Table 1 Mechanical and crack-development parameters of various coal–rock combinations

4 Conclusions

1) For this investigation, water will prompt the mudstone and coal to become disintegrative. We have invented a self-made apparatus to enhance the moisture content of coal-rock combinations to avoid the need to immerse the coal-rock specimens in water.

2) Stiffness–stress and cumulative AE counts curves for coal-rock combinations agree strongly with overall stress–strain curves and can be used to estimate the extent of crack propagation prior to peak stress. These two methods are useful to precisely calculate the moduli and thresholds of crack closure, crack initiation, and crack damage.

3) Peak strength of coal–rock combinations decreases strongly with increasing moisture content, but rises slightly with increasing loading rate. Peak strain of coal–rock combinations showed an overall fluctuating tendency: peak strain trends upward before falling, after which it increases again, and then decreases.

4) Elastic, post-peak, and strain-softening moduli for coal-rock combinations decrease with increasing moisture content and have a positive linear relationship with loading rate. In particular, the post-peak and strain softening moduli increase more slowly with increasing moisture content. Strain-softening modulus and post- peak modulus are an order of magnitude larger than the elastic modulus, which explains why the capacity for resistance to deformation in the post-peak stage lowers the elastic deformation.

5) Stress thresholds for crack closure, initiation, and damage have a negative correlation with increasing moisture content. These stress thresholds generally increase linearly with rising loading rate. From linear fits to the data presented in Fig. 8 and Table 1, crack closure stress is calculated to constitute 15.22% of peak stress; crack initiation stress and crack damage stress comprise 32.20% and 80.98%, respectively. More significantly, these values bear no relationship to moisture content or loading rate. We can therefore measure uniaxial compression stress to evaluate the stress thresholds of crack closure, crack initiation, and crack damage and thereby judge the development of cracks.

References

[1] HASHIBA K, FUKUI K. Effect of water on the deformation and failure of rock in uniaxial tension [J]. Rock Mech Rock Eng, 2014, 48: 1751–1761.

[2] Y. The effects of strain rate and saturation on a micro-cracked marble [J]. Eng Geol, 2006, 82: 137–144.

[3] YAO Qiang-ling, LI Xu-hua, ZHOU Jian, JU Ming-he, CHONG Zhao-hui, ZHAO Bin. Experimental study of strength characteristics of coal specimens after water intrusion [J]. Arab J Geosci, 2015, 8: 6779–6789.

[4] GU Da-zhao. Theory framework and technological system of coal mine underground reservoir [J]. J China Coal Soc, 2015, 40: 239–246. (in Chinese)

[5] LI Wang-lin, SHU Long-cang, YIN Zong-ze. Concept and design theory of groundwater reservoir [J]. Journal of Hydraulic Engineering, 2006, 37(5): 613–618. (in Chinese)

[6] AYDAN , ULUSAY R, KAWAMOTO T. Assessment of rock mass strength for underground excavations [J]. Int J Rock Mech Min Sci, 1997, 34(3, 4): 18.e1–18.e17.

[7] HOEK E, BROWN E T. Practical estimates of rock mass strength [J]. Int J Rock Mech Min Sci, 1997, 34(8): 1165–1186.

[8] LIN Da-ming, SHANG Y, SUN Fu-jun. Study of strength assessment of rock mass and application [J]. Rock and Soil Mechanics, 2011, 32(3): 837–842. (in Chinese)

[9] HUANG Bing-xiang, LIU Jiang-wei. The effect of loading rate on the behavior of samples composed of coal and rock [J]. Int J Rock Mech Min Sci, 2013, 61: 23–30.

[10] VERSTRYNGE E, ADRIAENS R, ELSEN J, VAN B K. Multi-scale analysis on the influence of moisture on the mechanical behavior of ferruginous sandstone [J]. Constr Build Mater, 2014, 54: 78–90.

[11] VISHAL V, RANJITH P G, SINGH T N. An experimental investigation on behaviour of coal under fluid saturation, using acoustic emission [J]. J Nat Gas Sci Eng, 2015, 22: 428–436.

[12] EBERHARDT E, STIMPSON B, STEAD D. Effects of grain size on the initiation and propagation thresholds of stress-induced brittle fractures [J]. Rock Mech Rock Eng, 1999, 32: 81–99.

[13] LI Hua-min, LI Hui-gui, GAO Bao-bin, JIANG Dong-jie, FENG Jun-fa. Study of acoustic emission and mechanical characteristics of coal samples under different loading rates [J]. Shock Vibr, 2015, 2015: 458519.

[14] WU Yu-liang, CHEN Jie, ZENG Sen-mao. The acoustic emission technique research on dynamic damage characteristics of the coal rock [J]. Proc Eng, 2011, 26: 1076–1082.

[15] LIU Xi-ling, LI Xi-bing, HONG Liang, YIN Tu-bing, RAO Meng. Acoustic emission characteristics of rock under impact loading [J]. Journal of Central South University, 2015, 22: 3571–3577.

[16] YU Z, ZHANG L, JIANG P, PAPELIS C, LI Y. Study on water-rock interactions of trace elements in groundwater with leaching experiments [J]. Ground Water, 2015, 1: 95–102.

[17] FENG X T, DING W X. Coupled chemical stress processes in rock fracturing [J]. Mater Res Innov, 2011, 15(s1): s547–s550.

[18] EBERHARDT E, STEAD D, STIMPSON B, READ R. Identifying crack initiation and propagation thresholds in brittle rock [J]. Can Geotech J ,1998, 35: 222–233.

[19] RANJITH P G, JASINGE D, SONG J Y, CHOI S K. A study of the effect of displacement rate and moisture content on the mechanical properties of concrete: Use of acoustic emission [J]. Mech Mater, 2008, 40: 453–469.

[20] DENG Xiao-qian, LIU Xiao-fei, TIAN Shu-chong. Experimental study of original cracks features effecting fracture of coal samples under uniaxial compression [J]. Proc Eng, 2011, 26: 681–688.

[21] KHAZAEI C, HAZZARD J, CHALATURNYK R. Damage quantification of intact rocks using acoustic emission energies recorded during uniaxial compression test and discrete element modeling [J]. Computers Geotech, 2015, 67: 94–102.

[22] JAEGER J C, COOK N G, ZIMMERMAN R. Fundamentals of rock mechanics [M]. London: Blackwell, 2007.

[23] HUDSON J A, HARRISON J P. Engineering rock mechanics—An introduction to the principles [M]. New York: Elsevier Science Inc, 2000.

[24] WANG Xiao-nan, LU Cai-ping, XUE Jun-hua, YU Guo-feng, LUO Yong, LIU Hui, ZHANG Jun-wei. Experimental research on rules of acoustic emission and microseismic effects of burst failure of compound coal-rock samples [J]. Chin J Rock Mech Eng, 2013, 34(9): 2569–2575. (in Chinese)

[25] XU Hui-ning, LIU Jian-feng, LU Wang, YANG Bao-quan, YANG Hao-tian. The weakening effect of hydrostatic pressure on rock mass of different lithology [J]. Env Earth Sci, 2015, 74: 2489–2497.

[26] ZHANG Zhi-zhen, GAO Feng. Experimental investigation on the energy evolution of dry and water-saturated red sandstones [J]. Int J Min Sci Technol, 2015, 25(3): 383–388.

[27] GALOUEI M, FAKHIMI A. Size effect, material ductility and shape of fracture process zone in quasi-brittle materials [J]. Computers Geotech, 2015, 65: 126–135.

[28] RANJITH P G, FOURAR M, PONG S F, CHIAN W, HAQUE A. Characterisation of fractured rocks under uniaxial loading states [J]. Int J Rock Mech Min Sci, 2004, 41: 43–48.

[29] RAO Q H, SUN Z Q, LI C L, STILLBORG B. Mechanism of pure shear (Mode II) fracture of brittle rock. Frontiers of rock mechanics and sustainable development in the 21st century [C]// Beijing China AA Balkema, 2001: 165–168.

[30] MASUDA K. Effects of water on rock strength in a brittle regime [J]. J Struct Geol, 2001, 23: 1653–1657.

[31] PERERA M S A, RANJITH P G, PETER M. Effects of saturation medium and pressure on strength parameters of Latrobe Valley brown coal: Carbon dioxide, water and nitrogen saturations [J]. Energy, 2011, 36: 6941–6947.

[32] SIDERIS K K, MANITA P, SIDERIS K. Estimation of ultimate modulus of elasticity and Poisson ratio of normal concrete [J]. Cement Concrete Comp, 2004, 26: 623–631.

[33] TATSUOKA F, SIDDIQUEE M S A, PARK C S, SAKAMOTO M, ABE F. Modelling stress-strain relations of sand [J]. Soils Found, 1993, 33(2): 60–81.

[34] LI Wen-ting, LI Shu-chen, FENG Xian-da, LI Shu-cai, YUAN Chao. Study of post-peak strain softeing mechanical properties of rock based on Mohr-coulomb criterion [J]. Chin J Rock Mech Eng, 2011, 7: 1460–1466. (in Chinese)

[35] J, BITTNAR Z. Experimental investigation and numerical simulation of post-peak behaviour and size effect of reinforced concrete columns [J]. Mater Struct, 2004, 37(3): 161–169.

[36] ZOU Jin-feng, LI Shuai-shuai. Theoretical solution for displacement and stress in strain-softening surrounding rock under hydraulic-mechanical coupling [J]. Sci China Techn Sci, 2015, 58: 1401–1413.

[37] HOSSEIN M, PAUL C H, SERKAN S. Modification to radial strain calculation in rock testing [J]. Geotech Test J, 2015, 38: 813–822.

[38] ZONG Yi-jiang, HAN Li-jun, WEI Jian-jun, WEN Sheng-yong. Mechanical and damage evolution properties of sandstone under triaxial compression [J]. Int J Min Sci Technol, 2016, 26: 601–607.

(Edited by DENG Lü-xiang)

Cite this article as: CHEN Tian, YAO Qiang-ling, WEI Fei, CHONG Zhao-hui, ZHOU Jian, WANG Chang-bin, LI Jing. Effects of water intrusion and loading rate on mechanical properties of and crack propagation in coal–rock combinations [J]. Journal of Central South University, 2017, 24(2): 423-431. DOI: 10.1007/s11171-017-3444-1.

Foundation item: Project(2014QNB31) supported by the Fundamental Research Funds for the Central Universities, China; Projects(51674248) supported by the National Natural Science Foundation of China; Project supported by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), China

Received date: 2016-01-13; Accepted date: 2016-09-09

Corresponding author: YAO Qiang-ling, Professor, PhD; Tel: +86-13805210624; E-mail: yaoqiangling@126.com