Geological tectonic movements, as well as complex and varying coal-forming conditions, have led to the formation of rock partings in most coal seams. Consequently, the coal in coal-rock composites is characterised by different mechanical properties than those of pure coal. Uniaxial compression tests were performed in this study to determine the mechanical properties and bursting liability of specimens of coal-rock composites (hereinafter referred to as “composites”) with rock partings with different dip angles
Most thick coal seams contain rock partings as a result of complex coal-forming geological conditions [
Numerous researchers have extensively studied the mechanical properties of coal-rock composites. Yang et al. [
In this study, specimens of a coal-rock composite with a rock parting (hereinafter referred to as “composites”) with different dip angles
Experimental coal-rock samples were collected from the No. 112201 working face of a mine in Shaanxi Province. These rock samples had a sandstone lithology. All the samples were collected from intact and highly homogeneous coal-rock blocks to eliminate interference factors and ensure the reliability of the test results. Cuboidal specimens (50 mm × 50 mm × 100 mm) were fabricated from the coal-rock samples according to the specifications of the International Society for Rock Mechanics. The ends of each specimen were polished to ensure that the surface roughness of the ends did not exceed 0.2 mm and the non-parallelism did not exceed ±0.1%. The specimens were divided into groups A and B. The specimens in group A had
Composite specimens with different rock-parting dip angles (
Composite specimens with different rock-parting thicknesses (
Figure
The experimental monitoring system.
An MTS-C64.106 electrohydraulic servo control system was selected as the load control system for the uniaxial compression tests. The strain control mode was used in this study. Each specimen was loaded at a rate of 0.01 mm/s until failure.
A PCI-2 AE system was used to monitor the AE activity. Four AE sensors were adhered to the surfaces of each composite specimen to collect data. The noise suppression threshold, peak definition time, hit lockout time, sampling frequency, and preamplifier gain of the AE monitoring system were set to 30 dB, 50
A GX-1/3 high-speed camera (NAC Inc, Japan) was used to acquire digital photographs to observe the loading process and record the deformation and failure patterns of the composite specimens.
In the composite specimen images, the yellow lines indicate the expansion fractures, and the pink areas indicate the coal body flakes.
The stress-strain curve of the composite could be divided into four regimes: pore compaction (OA), a linear elastic section (AB), prepeak crack development (BC), and postpeak fracture development (CD). During the pore compaction stage, the preexisting cracks and pores in the coal and the rock parting were closed, and the AE activity was weak and fluctuated slightly. The AE activity in the composite remained basically stable over the linear elastic regime. However, there was some propagation of the preexisting cracks in the composite under compressive loading, and a small quantity of coal spalled off in the form of flakes. Consequently, a few AE peaks appeared, albeit with relatively small amplitudes. During prepeak crack development, the cracks inside the composite developed steadily, accompanied by the caving of small coal blocks. A small quantity of rock powder fell off, the coal failed in local areas, there were violent fluctuations in the AE activity, and multiple AE peaks appeared. During postpeak fracture development, small coal blocks erupted accompanied by a notable cracking sound and increasing AE activity. Each steplike sudden increase observed in the cumulative AE count curve corresponded to the initiation and development of microcracks in the composite, as well as the spalling of coal in the form of flakes. The cumulative AE count curve demonstrably corresponded to the stress-strain curve and could be approximately divided into four regimes: pore compaction and closure (during which the AE count increased slightly); a slowly ascending linear elastic section (during which the AE count increased steadily overall but suddenly increased to a relatively small extent at isolated points as a result of the propagation of the preexisting cracks); prepeak steady crack propagation (during which the propagation of the preexisting cracks intensified, new cracks were initiated, the AE activity increased, and the cumulative AE count curve exhibited a large rate of increase); and peak unsteady crack propagation (during which cracks developed unsteadily, and there was a sharp increase in the cumulative AE count curve).
Increasing
Stress-strain curve and acoustic emission monitoring curve of composite specimens with a 10° dip angle (A10-1).
Stress-strain curve and acoustic emission monitoring curve of composite specimens with a 20° dip angle (A20-1).
Stress-strain curve and acoustic emission monitoring curve of composite specimens with a 30° dip angle (A30-1).
Stress-strain curve and acoustic emission monitoring curve of composite specimens with a 40° dip angle (A40-1).
High-speed photographs of the failure process of the composite showed that the coal failed first in the composite. This is primarily because of the large difference between the strengths of the coal and rock in the composite. The development of internal microcracks resulted in a relatively low coal strength. Consequently, under compressive loading, crack initiation and propagation occurred first in the coal, which spalled off in the form of flakes.
Figure
Test results of the mechanical parameters of composite specimens with different dip angles. (a) Uniaxial compressive strength. (b) Elastic modulus. (c) Bursting energy index.
Mechanical parameters of composite specimens with different dip angles.
Uniaxial compressive strength | Elastic modulus | Bursting energy index | ||||
---|---|---|---|---|---|---|
Specimen group | Tested | Average | Tested | Average | Tested | Average |
A10-1 | 14.236 | 13.726 | 1.172 | 1.206 | 7.059 | 7.694 |
A10-2 | 13.254 | 1.252 | 6.896 | |||
A10-3 | 13.689 | 1.193 | 9.126 | |||
A20-1 | 12.240 | 11.449 | 1.064 | 1.077 | 6.870 | 6.618 |
A20-2 | 11.123 | 1.143 | 7.569 | |||
A20-3 | 10.985 | 1.025 | 5.415 | |||
A30-1 | 10.234 | 9.404 | 0.942 | 1.003 | 4.325 | 5.384 |
A30-2 | 9.853 | 1.073 | 5.698 | |||
A30-3 | 8.126 | 0.995 | 6.129 | |||
A40-1 | 5.756 | 5.775 | 0.899 | 0.831 | 6.125 | 4.799 |
A40-2 | 6.443 | 0.792 | 4.365 | |||
A40-3 | 5.125 | 0.802 | 3.906 |
Similarly, the stress-strain curve of each of the composite specimens with different
For the composite specimen with
Stress-strain curve and acoustic emission monitoring curve of 10 mm-thick composite specimens (B10-1).
Stress-strain curve and acoustic emission monitoring curve of 20 mm-thick composite specimens (B20-1).
Stress-strain curve and acoustic emission monitoring curve of 30 mm-thick composite specimens. (a) B30-1. (b) B30-2.
Stress-strain curve and acoustic emission monitoring curve of 40 mm-thick composite specimens.
Figure
Test results of the mechanical parameters of composite specimens with different thicknesses. (a) Uniaxial compressive strength. (b) Elastic modulus. (c) Bursting energy index.
Mechanical parameters of composite specimens with different thicknesses.
Uniaxial compressive strength | Elastic modulus | Bursting energy index | ||||
---|---|---|---|---|---|---|
Specimen group | Tested | Average | Tested | Average | Tested | Average |
B10-1 | 12.148 | 12.662 | 1.167 | 1.286 | 2.968 | 3.131 |
B10-2 | 12.685 | 1.325 | 3.556 | |||
B10-3 | 13.154 | 1.367 | 2.869 | |||
B20-1 | 13.644 | 14.382 | 1.325 | 1.422 | 9.091 | 8.597 |
B20-2 | 14.851 | 1.485 | 8.873 | |||
B20-3 | 14.652 | 1.456 | 7.826 | |||
B30-1 | 15.936 | 15.779 | 1.513 | 1.583 | 13.521 | 9.236 |
B30-2 | 14.543 | 1.583 | 0.371 | |||
B30-3 | 16.859 | 1.653 | 13.815 | |||
B40-1 | 20.237 | 19.906 | 1.782 | 1.818 | 14.071 | 14.455 |
B40-2 | 20.125 | 1.856 | 15.026 | |||
B40-3 | 19.356 | 1.815 | 14.269 |
As
The experimental results were used to formulate two technologies for controlling the stability of the surrounding rock (Figure
Control technology for surrounding rock.
Mining is currently underway in the No. 2 coal seam of a mine in the Yushen mining area in Shaanxi. This coal seam, with an average burial depth of 370 m, is at low risk of rock burst. The No. 112201 working face of this coal seam, 350 m in length and 4,556.6 m in strike length, contains one to two partings overall but three partings in some local areas. The parting thickness is 0.6–2.0 m. The high hardness coefficient of the partings hinders excavation and makes cutting difficult. In addition, coal blocks are ejected during the cutting process, and the rocks surrounding the roadway have low stability and significantly spall off. To reduce the energy consumption of coal cutters and the dynamic manifestation at the working face, high-pressure pulsed water-jet fracturing technology was employed to reduce the strength of the partings at the working face. Figure
The parting before and after fracturing at the working face. (a) Before fracturing. (b) After fracturing.
The cutting speed of the coal cutters before and after fracturing of the parting.
Comparison of roadway formation with different support parameters. (a) Support spacing of 900 mm × 900 mm. (b) Support spacing of 900 mm × 800 mm.
The roadway formation with different support parameters.
As
The rock parting was the key area in which elastic energy accumulated. The initiation and propagation of cracks in the coal caused the rock parting to release elastic energy, which precipitated violent failure of the composite. As
The experimental results for the coal-rock composite with a seam parting were used to formulate two technologies to control the stability of the surrounding rock, namely, high-pressure fracturing technology to weaken partings at working faces and asymmetric strengthening support technology for rocks surrounding roadways. These two technologies can effectively ensure safe and efficient production at working faces.
Some of the data used to support the findings of this study are included within the article, and the other supporting data are available from the corresponding author upon request.
The authors declare that they have no known conflicts of interest or personal relationships that could appear to influence the results reported in this paper.
The authors acknowledge the financial support provided by the National Natural Science Foundation Project of China (51564044).