Many hazards encountered during coal mining can be caused by the instability and failure of the composite structure of the coal seam and the surrounding rock strata. The defects present in the coal affect the structural stability of the composite structure. In this study, uniaxial compression tests were conducted on sandstone-coal composite samples with pre-existing cracks in the coal, combined with tests performed with an acoustic emission (AE) device and a digital video camera. The strength, macrofailure initiation (MFI), and failure characteristics of composite samples, as influenced by the coal’s pre-existing cracks, were analysed. The coal’s pre-existing cracks were shown to reduce the strength, promote the occurrence of MFI, and affect the failure characteristics of the samples. Vertical penetration cracks had much more pronounced effects on strength and MFI occurrence, especially vertical penetration cracks that penetrated through the centre of the coal. Horizontal penetration cracks had a much reduced effect on strength and MFI occurrence. The MFI caused a step shape in the stress-strain curve accompanied with a peak energy index signal and occurred around the original coal cracks. The MFI models predominantly exhibited crack initiation from the pre-existing coal cracks and surface spalling caused by crack propagation. The intact composite sample failure presented as an instantaneous failure, whereas the composite samples containing the pre-existing cracks showed a progressive failure. The failures of composite samples occurred predominantly within the coal and displayed an X-typed shear failure accompanied by a small splitting failure. Both the coal and sandstone were destroyed in the composite sample with vertical penetration cracks through the centre of the coal. Failure of the coal occurred through a splitting failure accompanied by a small X-typed shear failure, while the sandstone showed a splitting failure induced by crack propagation in the coal.
The area being mined in a coal-mining operation is essentially a composite structure consisting of coal seam and surrounding rock strata. The instability and failure of the composite structure can cause many hazards during coal mining, such as rock bursts and coal bumps, resulting in potential safety hazards to coal production [
A lot of investigations have been performed in the past on the mechanical behaviour of composite structures composed of coal seam and surrounding rock strata [
The abovementioned studies and their attendant results and conclusions are important in order to understand the mechanical behaviour of composite structures consisting of a coal seam and surrounding rock strata. However, these investigations focused on relatively intact composite samples. Both preliminary testing results and theoretical analysis indicated obvious correlations between the initial coal defects and the mechanical properties of the composite samples. There are few studies on the effects of initial coal defects on the mechanical behaviour of the composite structure. Yin et al. investigated the effects of joint length and angle in coal on the strength and failure characteristics of rock-coal and rock-coal-rock composite samples using Particle Flow Code (PFC) [
Generally, in the edge of the composite structure firstly occurs instability and failure [
As the structure of internal microcracks within coal can be quite complicated, in this study, only rock-coal composite samples with penetrative cracks or surface cracks in the coal were selected for the uniaxial compression tests. The sample preparation and test conditions are presented in detail in this section.
The rock and coal in the composite samples were taken from the immediate roof and coal seam in the 3306 working face of Daizhuang coal mine located in Shandong Province of China. The average mining depth is about 323 m (below the surface). The immediate roof is sandstone, and its average thickness is 4.07 m. The average thickness of the coal seam is 1.45 m, and its average dip angle is 12°. The uniaxial compression tests were conducted on standard pure coal and sandstone samples to obtain their mechanical properties, as shown in Table
Mechanical properties of standard pure coal and rock samples [
Group | Uniaxial compression strength (UCS) | Elastic modulus | Poisson’s ratio |
---|---|---|---|
Standard pure coal sample | 109.3 MPa | 6.64 | 0.194 |
Standard sandstone sample | 39.4 MPa | 2.67 | 0.232 |
The rock and coal blocks taken from the coal mine were core drilled to form cylindrical samples with a diameter of 50 mm. The cylindrical samples of coal and rock were cut into 50 mm long sections with 50 mm diameter (
The distributions of the pre-existing coal cracks in composite samples are presented in Table
Distributions of pre-existing coal cracks [
No. | Composite sample photos | Main distribution of pre-existing coal cracks |
---|---|---|
A-1 |
|
Four small vertical surface cracks adjacent to the rock-coal interface (minimum persistence) (intact composite sample) |
|
||
A-2 |
|
Two large vertical surface cracks, one adjacent to the rock-coal interface and the other in the middle and lower part of the coal |
|
||
A-3 |
|
Two vertical penetrative side cracks through the coal near the lateral boundary |
|
||
A-4 |
|
One horizontal penetrative crack and some surface cracks |
A-5 |
|
One main vertical penetrative crack through the central part of the coal and the other cracks being half penetrative through the coal. One horizontal penetrative crack in the lower part of the coal near the coal-sandstone interface |
The testing system consisted of a loading frame, an acoustic emission (AE) monitoring system, and a digital video camera (DVC), as shown in Figure
Test equipment and monitoring system.
An electronic Shimadzu AUTOGRAPH (AG-X250) universal testing machine was selected as the loading system. A double screw loading structure was used for working flexibility. The testing system can execute conventional compression, tensile, or any other mechanical tests, as required. The maximum achievable testing load was 250 kN. To control fracture and crack growth, a displacement loading method was adopted in these tests at a loading rate of 0.0005 mm/s.
A MISTRAS AE instrument (Physical Acoustics Corporation, Princeton Jct, NJ, USA) was utilized to monitor the energy index characteristics of the composite sample over the entire duration of the test. The average gain of the main amplifier of the AE monitoring system was 40 dB with a maximum threshold value of 45 dB and a floating tolerance of 6 dB. The two monitoring sensors were both Nano 30 AE; operating at a harmonic frequency range of 100 to 400 kHz and a sampling rate of 10 MHz. The two AE sensors were installed on the surface of the sandstone and coal and fixed in place with appropriate tape. Vaseline was applied to the contact area between the sensors and the samples to ensure superior coupling conditions. The pencil-lead fracture method proposed by ASTM (2000) was used to calibrate the AE system. A SONY portable digital camera was used to record the failure process of the composite samples during exposure to uniaxial compression.
The uniaxial compressive stress-strain curves for the composite samples are presented in Figure
Uniaxial compressive stress-strain curves.
UCS comparison.
The UCSs of the composite samples containing pre-existing coal cracks were clearly lower than that of the intact composite sample, and the pre-existing coal cracks exerted a deteriorating influence on the structural strength of the composite sample. The strength of sandstone is far greater than that of coal in the composite sample. Therefore, in a composite sample, the coal with the lower strength determines the structural strength of the composite sample [
In order to analyse the strength of composite samples containing pre-existing coal cracks, the stress state of the coal in the composite sample was analysed as follows. According to the stress states of coal in the rock-coal-rock composite sample and rock-coal composite sample [
Stress state of the coal in the composite sample.
The coal near the upper and lower contact surface, which resisted destruction, was in triaxial compression. The coal in the central part of the test samples was still subject to uniaxial compression [
The horizontal penetrative crack in the coal was predominantly compacted under
One was that the region between the two vertical penetrative cracks may be the main load-bearing structure of the coal, and the vertical penetrative cracks thus had little effects on the coal’s strength. From the beginning of the loading to the macrofailure initiation (MFI) point, the fluctuation degree of the AE signal (energy index) of the A-2 composite sample was higher than that of the A-3 composite sample, as discussed in Section 3.4, and these illustrate that the initial internal damage in the coal in the A-2 composite sample was higher than that in the A-3 composite sample. The initial internal damage reduced the strength of A-2 composite sample. Therefore, the UCS of the A-2 composite sample might be lower than that of the A-3 composite sample.
The first macrofailure point in the composite sample is considered to be the MFI point, which represents the beginning of the macrofailure in the composite sample. The MFI point is well reflected in the stress-strain curve of the composite sample [
Macrofailure initiation of the rock-coal composite sample [
MFIS, energy index peak caused by the MFI, and MFIM.
No. | MFI, stress, and energy index | MFIS | MFIS/UCS | MFIM |
---|---|---|---|---|
A-1 |
|
45.21 | 97.13% | A macrotensile crack initiated from surface crack tip |
|
||||
A-2 |
|
31.41 | 84.46% | A macrocrack initiated from the middle part of original vertical surface crack |
|
||||
A-3 |
|
29.38 | 71.87% | A macromixed crack initiated from the lower part of the original vertical penetrative crack along with the formation of surface spalling |
A-4 |
|
34.69 | 77.93% | Surface spalling at the bottom of the coal |
|
||||
A-5 |
|
20.62 | 71.12% | A macrofailure initiated from the original vertical penetrative crack accompanied by surface spalling failure |
In Table
In equation (
MFIS comparison.
In addition, the ratio of MFIS to UCS for the intact composite sample was 97.13%, close to 100%. This indicated an instantaneous process from macrofailure to the complete failure of the composite sample. Moreover, the ratios of MFIS to UCS of the composite samples containing pre-existing coal cracks were between 50.73% and 84.46%, indicating a progressive process from the point of macrofailure to complete failure. The failure characteristics of the composite samples are further analysed and discussed in the following sections.
The macrofailure pattern photos of the composite samples are shown in Figure
Typical failure models of composite samples. (a) A-1. (b) A-2. (c) A-3. (d) A-4. (e) A-5.
In order to further discuss the effects of the pre-existing coal cracks on the failure characteristics of the composite samples, the deformation and failure processes of the composite samples were analysed, as shown in Figure
Deformation and failure processes of composite samples. (a) A-1. (b) A-2. (c) A-3. (d) A-4. (e) A-5.
The pre-existing coal cracks affected the deformation and failure process of the composite sample. Compared with the composite samples with pre-existing coal cracks, the Stage III process of the intact composite sample (A-1 composite sample) was too short to release the elastic energy predominantly stored in Stage II. The corresponding failure was instantaneous and violent, and thus could cause dynamic hazards. The pre-existing coal cracks promoted the MFI occurrence, shortened the Stage II process, and extended the Stage III process. The composite samples with pre-existing coal cracks exhibited progressive failure. The voids and cracks in the coal and rock and the compaction of the interface between them were further compacted under uniaxial loading in Stage I, and the energy index was stable and remained at a low level. The elastic energy was predominantly stored in the composite sample in Stage II, and slight fluctuations occurred in the energy index without points of abrupt increase due to the initial microcrack initiation and propagation, especially in composite sample A-2. In Stage III, macrocracks predominated, initiating and propagating within the coal. Original and new cracks propagated and coalesced, followed by the localized fractures and spalls. This process consumed a large amount of elastic energy and decreased the structural integrity of the composite sample. Therefore, the tendency for dynamic hazards to be caused under the conditions modelled with the composite samples with the coal’s pre-existing cracks was reduced. The corresponding energy index demonstrated significant fluctuations with a greater number of peak values.
During deep coal mining, close attention must be paid to the areas of coal seam with horizontal penetrative cracks. In these areas, some measures should be adopted to decrease the occurrence probability of rockburst disasters caused by the instability and failure of the composite system of coal seam and roof rock. These measures include coal seam infusion, advanced bore decompression in coal seam and cutting pressure-relief slot in coal seam.
In this study, uniaxial compression tests were conducted on sandstone-coal composite samples with penetrative cracks or surface cracks in the coal. The effects of the coal’s pre-existing cracks on the strength, macrofailure initiation (MFI), and failure characteristics of composite samples, were analysed. The following conclusions were obtained: The pre-existing coal cracks weakened the strength, promoted the occurrence of macrofailure initiation (MFI), and affected the failure characteristics of the composite samples. The vertical penetration cracks had much larger effects on strength and MFI occurrence, especially vertical penetration cracks that penetrated through the centre of the coal. Horizontal penetration cracks had much smaller effects on the strength and MFI occurrence. The MFI caused a step shape in the stress-strain curve, and the energy index reached a peak value. The values of macrofailure initiation stress (MFIS) of composite samples with pre-existing cracks were lower than that of the intact composite sample. The MFI of all composite samples originated at the pre-existing cracks, including the crack initiation of the pre-existing cracks, the formation of the newly-formed macroscopic cracks adjacent to the pre-existing cracks, and the surface spalls caused by crack propagation. The failures of the composite samples occurred predominantly within the coal and displayed an X-typed shear failure accompanied by a small splitting failure. Both the coal and sandstone were destroyed in the composite sample predominantly containing vertical penetration cracks through the centre of the coal. Failure of the coal occurred through a splitting failure accompanied by a small X-typed shear failure, while the sandstone showed a splitting failure induced by crack propagation in the coal. The intact composite sample failure involved an instantaneous failure with a significant amount of elastic energy that was rapidly released. There was a high tendency for the occurrence of hazards. Conversely, the composite samples with pre-existing coal cracks presented a progressive failure. From the MFI point to the peak stress point, original and new cracks propagated and coalesced, and then localized fractures and spalling occurred. This process consumed a large amount of elastic energy and decreased the supporting capacity of the composite sample. The tendency for the occurrence of hazards was thus reduced.
The data used to support the findings of this study are included within the article.
The authors declare that they have no conflicts of interest.
This study was supported by the National Natural Science Foundation of China (51904167, 51474134, 51774194, 51874189), Taishan Scholars Project, Taishan Scholar Talent Team Support Plan for Advantaged and Unique Discipline Areas, SDUST Research Fund, Shandong Provincial Natural Science Fund for Distinguished Young Scholars (JQ201612), and Shandong Provincial Key Research and Development Plan (2017GSF17112). The authors thank the Project of Open Research Fund for Key Laboratories of Ministry of Education for safe and efficient mining of coal mine (JYBSYS2019201).