The Failure Characteristics and Energy Evolution Pattern of Compound Coal–Rock under the Action of Cyclic Loading

Abstract

Based on the entire loading process of compound coal–rock, test pieces with three different coal/rock ratios (1:3, 1:1, and 3:1) have been constructed and the corresponding cyclic loading experiments have been carried out. Through the experiment, the deformation and failure characteristics of the compound coal–rock samples have been explored and the stage evolution characteristics of energy density have been subsequently analyzed. Ultimately, the relation between deformation failure and the energy evolution mechanism has been established, and thus the reasons behind rock bursts in the coal–rock compounds have been discussed. The experimental results indicate that with the increase in cyclic loading, the stress–strain curve of the compound coal–rock demonstrates a positive shift, whereas the change in the hysteretic curve from dense to sparse results in a “hysteresis expansion”. The increase in the coal body height increases the chance of brittleness failure of the compound coal–rock. The coal body, as the main controlling factor of compound coal–rock failure, generates cracks that expand to the rock body along the juncture of the coal and rock, leading to instability. The energy density evolution curve can be described by a quadratic function. The evolution process is initiated from the slow increase in input energy density and elastic energy density. A large amount of energy is stored through the rapid increase in the density mentioned above. At last, the evolution is completed by a surge in dissipated energy. The energy evolution drives the crack expansions in the compound coal–rock under load. The energy accumulation in the compound coal–rock is increased by the exploitation of the clamping effect of the thick and hard top and bottom plate. The risk of rock burst is intensified by the failure of the coal body because of the energy in the coal–rock system. The study results help to comprehend the energy evolution pattern in the surrounding rock of deep mining roadways and expand the prevention methods for impact ground pressure.

1. Introduction

Up to now, coal is still the main pillar of global energy, and shallow resources have been gradually depleted with the development of the economy. In the context of deep coal mining becoming the new norm, the research on the prevention and control of disasters such as coal and gas outbursts [1], roof caving [2], water inrush [3,4], and impact ground pressure [5,6] has gradually become a hot topic.

Impact ground pressure disasters are increasingly significant as the deep exploitation of coal mines continues [7]. As shown in Figure 1a,b, during the deep exploitation of coal mines, the main sources of destabilization of the stress field of the surrounding rock are loading, drilling, blasting, periodic activity on the roof, and mine earthquakes. The resultant cyclic stress load acts on the compound coal–rock unit, potentially causing catastrophes.

With the advancement of the working face, the periphery of the stope gradually forms a stable pressure area, a pressure boost area, and a pressure reduction area. As shown in Figure 1c, the positive stress, σ₁, equals the side stress, σ₃, in a stable pressure area, which is approximate to the stress of primary rock. In pressure boost areas, σ₁ and σ₃ increase, while these stresses decrease in pressure reduction areas. In the corresponding stress change, the original three-dimensional static combined coal–rock microelement gradually shifts to a cyclic dynamic. According to incomplete statistics [8,9,10,11,12,13], impact ground pressure is more likely to occur in the mining roadway. The mining roadway is located at the junction of the coal–rock combination, and its mechanical characteristics are quite different. The stress transfer and energy conduction show discontinuous and heterogeneous complex characteristics. The non-uniform load transfer increases the stress of the front coal–rock combination area of the working face, where the accumulated energy gathers at the weak face of the coal–rock structure. Such energy will be abruptly released by the disturbance, forming impact ground pressure. It is of important significance in the prevention and control of impact ground pressure to study the instability of coal–rock and the energy characteristics under a dynamic load state.

Lu Gaoming et al. [14] carried out cycle fatigue cyclic loading tests on a test piece of yellow sandstone to explore the effects of cyclic loading on the instability failure of the rock mass. The test results indicated that the failure of yellow sandstone is a result of two-way accumulation of irreversible deformation. Ma Linjian [15] conducted a study on the mechanical properties of a specific confining pressure cyclic loading mode and discovered that the failure process can be accelerated at the development stage of transformation of stable deformation of coal and rock masses. Feng Xiaodong et al. [16] compared and analyzed the mechanical properties of sandstone with different water contents. The results revealed the strength characteristics of sandstone under various saturated states. They also uncovered that the number of cycles experienced by a test piece will decrease with the increase in water saturation. Song Jie et al. [17] carried out two cyclic loading tests on rock, discovering that the deterioration of the mechanical properties of rock will cause an increase in electrical resistivity, and the extent of damage to the test piece can be determined by the surge in electrical resistivity. Zhou Lei [18] conducted cyclic loading and unloading mechanical tests on marble and obtained a mechanical damage model. The above studies discussed the damage characteristics of coal and rock under cyclic loading and unloading conditions; however, they failed to verify the damage mechanism of compound coal–rock under dynamic loads.

As a result, scholars have initiated studies on compound coal–rock in an attempt to establish the relationship between the structure failure and the impact ground pressure [19,20]. Liu Bo [21] calculated the failure judgement criteria of compound coal–rock by conducting a study on the dynamic failure characteristics of a test piece under deep impact. Zuo Jianping and Chen Yan [22,23] pointed out the influence of the confining pressure on damage by carrying out mechanical tests. In addition, they further explored the evolution process of the cracks on the boundary of coal and rock and built an extended crack closure model. Combined with numerical simulations, Yang Ke [24] investigated the impact of the coal–rock height on the damage of compound coal–rock and determined the relation between noise emission and the failure characteristics of compound coal–rock. Furthermore, they pointed out the double mutual feed mechanism of coal and rock failures. The above studies initiated research on the failure patterns of compound coal–rock under load; however, they failed to indicate the significance of energy in compound coal–rock failure and failed to reveal the nature of the impact of ground pressure.

Energy theory has been widely recognized by the public to uncover the nature of coal–rock damage. Zhao Chuang et al. [25] determined the relation between energy and test piece damage under different confining pressures and disclosed the energy evolution pattern in the damage and destruction process of rock. Han Suping [26] studied the energy conversion relation for the coal–rock body in the destruction process by employing energy dissipation theory. They explicated that when the energy efficiency from the external input equals the energy dissipation rate, the coal–rock energy can reach dynamic equilibrium. Xiao Fukun [27] probed the relation between elastic energy and the hysteretic area of a stress–strain curve by carrying out cyclic loading tests on coal–rock test pieces. With the increase in cycles, the area of the hysteresis curve and the elastic energy also increase. The test piece was destroyed when the irreversible deformation reached its limit. The above studies indicate the effect of energy on the damage of coal and rock; however, they fail to study the mechanism of action among dynamic load, overall discrepancy of coal–rock, and energy.

A great number of research works have been conducted previously in terms of cyclic loading, compound coal–rock, and coal–rock energy. However, these studies are relatively isolated and cannot better expound the reasons behind dynamic pressure disasters in deep mining roadways under the mining effect. To enrich the above studies, we designed test pieces with three ratios of coal to rock to carry out cyclic loading and unloading tests. These tests aim to study the failure characteristics of compound coal–rock so that the energy evolution pattern of input energy density, elastic energy density, and dissipation energy density can be further analyzed. Furthermore, the relation between deformation failure and the energy evolution mechanism of compound coal–rock can be explored. The process of the generation of impact ground pressure in deep mining roadways can be discussed and the corresponding prevention and control methods can be built upon.

2. Cyclic Loading and Unloading Tests of Compound Coal–Rock

2.1. Project Overview

The coal and rock samples for this test were collected from a mining roadway of a deep impact mine in Heilongjiang Province. The designed production capacity of the mine is 4 million t/a and the actual production capacity is 1.955 million t/a. At present, the main mining face is the second section of the No.17 layer, with a buried depth of 719.9~732.3 m, an average tilt length of 178 m, a return air drift length of 1056 m, a transport chute length of 996 m, and a coal seam thickness of 1.99 m to 15.83 m in the mining area. The average dip angle is 32.5° and the occurrence is relatively stable. Most of the roof is moderately stable coarse sandstone with a thickness of 3.0~5.0 m.

The k in Figure 2 is the ratio of rock mass height to coal body height. According to the actual measurements at 16 coal points in the return air drift, the thickness of the roof in the seriously damaged area is about 1.2~1.5m. With the change in the thickness of the coal seam, the drift gradually deforms and destabilizes and has certain signs of a rockburst. When k = 1:3, the side has fallen in in a 200 m section of the working face, and a large amount of coal chips have fallen into the roadway. When k = 1:1, the anchor rod, anchor cable, and other supports are severely ineffective for a section with a length of 310 m. When k = 3:1, the roof has fallen in in a 170 m section of the mining line. In order to discuss the cause of the impact failure in the above areas, this paper will assemble combined coal and rock samples according to their k value and carry out research on the failure characteristics and energy evolution pattern of compound coal–rock under the action of cyclic loading.

2.2. The Assembling of Combined Test Piece

According to Standard Test Method for Engineering Rock (GB/T 50266-2013), the test pieces were processed into a cylinder with a diameter of 50 mm and a height of 100 mm, which is known as a test piece replicating the “roof-coal bed”. There was no adhesive at the contact area of the coal and rock so that the interactive state of the coal–rock bed in the mining working face could be simulated. The heights of the coal body in the test piece models were 25 mm, 50 mm, and 75 mm. The assembly design scheme and the processed test piece are shown in Figure 3. Sound wave screening was applied to the processed test pieces to avoid the interference of original cracks and newly generated cracks on the coal and rock. The parameters of the test pieces are shown in

Table 1. Parameter table of model test pieces.

No.	Ratio of Coal and Rock	Size/mm × mm	Travel Time/Μs	Wave Velocity/M·S−1
RC-25-1	1:3	φ50.02 × 98.21	32.5	3021.85
RC-25-2		φ51.22 × 100.05	33.7	2968.84
RC-25-3		φ50.43 × 100.59	35.6	2825.56
RC-50-1	1:1	φ49.86 × 98.50	52.3	1883.37
RC-50-2		φ48.98 × 99.96	53.1	1882.49
RC-50-3		φ51.11 × 101.41	55.1	1840.47
RC-75-1	3:1	φ50.79 × 101.50	75.8	1339.05
RC-75-2		φ51.38 × 99.08	74.7	1326.37
RC-75-3		φ50.6 × 98.83	73.7	1340.98

. The results indicate that as the height of the coal body increases, the wave velocity of the combined test pieces decreases, whereas the test pieces with same height show insignificant differences.

2.3. Experimental Scheme

First, a TAW-2000 servo-controlled testing machine was adopted to conduct uniaxial compression tests on the standard test pieces of coal and rock. The uniaxial compressive strength for coarse sandstone and coal is 61.48 MPa and 8.18 MPa, respectively.

Next, the load was tested. A load of 45 kN (70% of the estimated peak of coal body) was introduced at a rate of 1 kN/s and this was taken as the starting point of the initial cycle. Then, the load was reduced to 2 kN, the lower limit of cycle, at the same rate. The upper limit of the load was increased by 2 kN in each cycle and then the load was reduced to the lower limit of the cycle (2 kN) until the entire test piece was destroyed. The process of loading is shown in Figure 4.

3. Failure Characteristics of Compound Coal–Rock

Deformation is crucial to evaluate the stable state of coal and rock. The failure characteristics can be obtained through an analysis of the deformation processes of compound coal–rock in the loading and unloading cycles, which is of great significance to prevent dynamic pressure disasters in the surrounding rocks in mining roadways.

Analysis on the Deformation Characteristics

Under the effect of cyclic loading and unloading, the destruction shape and loading process of compound coal–rock are synchronous. When the stress peak was reached, the stress–strain curve of compound coal–rock with different coal body heights plunged instantaneously, showing significant brittle failure characteristics. With the increase in the height of the coal body, the peak breaking point became gradually less sharp, signifying that the compound coal–rock was increasingly deformed before failure. Brittle failure became less significant, whereas ductile failure became more significant.

Table 2. Mechanical parameters and failure shape of compound coal–rock samples.

No.	Ratio of Coal and Rock	Failure Strength/Mpa	Cycles/N	Elastic Modulus/Gpa	Failure Shape
RC-25-1	1:3	24.13	18	4.98	Coal body bursts and rock shows a lightning-shaped crack
RC-25-2		23.19	17	4.83
RC-25-3		22.10	16	4.64
Average value		23.14	17	4.87
RC-50-1	1:1	16.28	13	4.47	Coal body bursts and rock surface peeling off
RC-50-2		15.79	12	4.38
RC-50-3		13.92	11	4.23
Average value		23.14	17	4.87
RC-75-1	3:1	11.24	8	3.80	Coal body breaks and rock shows micro cracks
RC-75-2		9.92	6	3.72
RC-75-3		8.55	4	3.31
Average value		9.92	6	3.61

and Figure 5 show the mechanical parameters and failure shape of the compound coal–rock samples. As the height of the coal body in the test piece varied from 25 mm to 75 mm, the peak load reduced by 34% and 35%, respectively. The cycles decrease by 29% and 50% and the elastic modulus drops by 9.7% and 17%, respectively. The results indicate that increasing the height of the coal body will deteriorate the strength and rigidity of the compound coal–rock. The coal body, as the part with the smallest stiffness in the composite coal–rock system, determines the deformation characteristics of the composite coal–rock. The increase in the coal body height directly leads to the deterioration of the composite coal–rock system, and the peak load and elastic modulus of the coal–rock system show a linear decrease. Under the same cycle period, the increase in the coal body height gradually reduces the elastic deformation stage of the composite coal–rock, and the composite coal–rock enters the plastic deformation stage more quickly. At the same time, the larger the elastic modulus, the greater the peak strength of the composite coal–rock, which verifies that the instability and failure of the composite coal–rock is related to its own resistance to deformation. The test results show that the peak load and elastic modulus of the composite coal–rock specimens with the same composition are almost unchanged, which further proves that the composite specimens with the same composition have little variety. In the following, one reliable datapoint will be taken from three composite coal–rock specimens with different coal body heights for further analysis.

4. Results

When the load exceeds the load peak of the previous stage, the curve will rise along with the loading curve, without being affected by the loading and unloading process and ratio of coal to rock. The viscosity [28] existing in the coal–rock body lead to the stress phase lagging behind the strain phase. The loading curve did not overlap with the unloading curve. Residual deformation remained in each loading and unloading process, forming a typical hysteresis curve. As the cycles proceed, the stress–strain curve constantly moves in a positive direction, as the deformation accumulates.

The transition of the hysteresis curve from dense to sparse allows the visualization of the “hysteresis expansion”. It shows the energy dissipated by the internal damage development and connection during the loading and unloading processes of the compound coal–rock. In these processes, the augmented difference between the upper and lower limit of the cycle accelerates the plastic deformation rate of the compound coal–rock. The higher the ratio of coal to rock, the larger the generated irreversible deformation and the more obvious the hysteresis expansion.

The density of the curve demonstrates the deformation degree of the compound. As we can see from the enlarged areas in Figure 6a–c, the compound RC-25 experienced 17 cycles and the internal cracks were repeatedly compacted and intensified, which makes the hysteresis curve denser. The compound RC-50 experienced 12 cycles and new cracks were generated during the closure and due to the intensified process of the internal cracks. With the attenuation of intensifying effect, the curve became relatively compact. As for the compound RC-75, it experienced six cycles of low-level load that caused new cracks to develop, accumulating plenty of irreversible deformation and reducing the intensifying effect. The compacting process improves the elastic modulus of the compound coal–rock and increases its ability to resist deformation.

Analysis of Failure Characteristics

Under cyclic loading, the failure shape of compound coal–rock corresponds to its stress–strain curve. The compound test piece RC-25 has relatively fewer internal defects and the cracks can be repeatedly compacted. With the clamping action of the press machine, the compound coal–rock can maintain a certain integrity. It has high bearing capacity and the brittle failure degree of the test piece is severe. With many internal defects, the compound test piece RC-75 showed large plastic deformations even with low cyclic loading. The coal body is worn down, showing a low degree of brittle failure. The internal defects of RC-50 are medium level, and under the cyclic loading, it has a certain integrated intensifying capability that can resist large plastic deformations. Brittle failure was a cause of the failure of the test piece.

As the loading and unloading cycles proceed, the compound coal–rock test pieces show the damage characteristic of division between the coal and rock. Expansion and cracking are the main causes of the failure of the coal body. RC-50 shows an “X-shape conjugate shear fracture” [29], as shown in Figure 6b, and the coal body of RC-75 burst and showed large area spalling, as shown in Figure 6c. Compared with the failure of the coal body, the rock body shows less failure, with tension cracks and a small portion of abrasion, as shown in Figure 6a. Under the cyclic loading, the original cracks on the inside of the coarse sandstone develop, but they appear to be more stable. There is no drift during the loading process. Bursting noises were only heard when the compound coal–rock sustained a high-level load.

With the increase in height of the coal body, the failure degree of the rock body constantly decreases. The rock body after failure still shows a high bearing capacity. However, the characteristics of the coal body are opposite, proving that failure of the coal body is the main controlling factor of the instability of compound coal–rock. Under the cyclic loading, the coal body in the compound coal–rock is more sensitive to the change in load. The cracks first appeared in the coal body and then extended to the rock body along the boundary of the coal and rock. Ultimately, the rapid extension of cracks or the spalling of the coal body has destroyed the stability of the compound coal–rock.

5. Energy Evolution Pattern of Compound Coal–Rock

The energy theory can expound the failure behavior of compound coal–rock samples, help establish an energy evolution model, and quantify its failure degree. The cycles can indirectly represent the load process in real situations and real-time of the compound coal–rock. As its failure under load is a dynamic attenuation process, the fitting of a variation curve of energy density and cycles can determine the relation between energy and damage. A complete cycle cannot be formed after the test pieces are destroyed; therefore, the energy evolution process before the peak load of the compound coal–rock will be discussed in this article.

5.1. Energy Calculation Method

During the experiment, the indoor environment should be strictly controlled. The press machine transfers the mechanical energy to the test piece of the compound coal–rock, initiating the energy evolution. Assuming that there is no energy exchange during the process, as known from the first law of thermodynamics, the relation between the input total energy, U, of cyclic loading, the restorable elastic energy, U^e, and the dissipation energy, U^d, shown in Figure 7, can be expressed as:

$$U=U^e+U^{\mathrm{d}}$$

(1)

The total input energy of the compound coal–rock from point A at a given cyclic state can be solved through the integral:

$$U={\displaystyle {\int }_0^{{\epsilon }_A}{\sigma }_i}d\epsilon$$

(2)

The elastic energy inside the compound coal–rock stored at point A at a given cyclic state can be calculated through elastic theory:

$$U^{{}_e}=\frac{1}{2}{\sigma }_A△{\epsilon }_e$$

(3)

where

$$△{\epsilon }^{{}_e}=\frac{1}{E}{\sigma }_A$$

(4)

Substitute Equation (4) into (3) to obtain:

$$U^{{}_e}=\frac{{\sigma }_A{}^2}{2E}$$

(5)

The unrecoverable dissipation energy, U^d, can be obtained by subtracting the elastic energy, U^e, from the total energy, U:

$$U^d=U-U^e={\displaystyle {\int }_0^{{\epsilon }_A}{\sigma }_i}d\epsilon -\frac{{\sigma }_A{}^2}{2E}$$

(6)

5.2. Energy Evolution Pattern

Under the cyclic loading, the energy evolution curve of compound coal–rock can be fitted to a quadratic function, as shown in Figure 8. With the increase in height of the coal body, the degree of fitting (R2) of the evolution curve gradually decreases. Such a phenomenon is caused by the increase in height of the coal body, the decrease in energy characteristic points of the cycle, and the augmentation of discreteness of the data. The energy evolution process shows distinctive partition characteristics, corresponding to the failure process of the coal and rock body. The evolution process initiates from the slow increase in input energy density and elastic energy density; after that, a rapid increase in these densities leads to the storage of a large amount of energy. The surge of dissipation energy demonstrates the completion of evolution process. According to the degree of fitting of the curve of the energy density, the evolution of energy can be described as an initial accumulation, followed by a rapid accumulation and a fast dissipation stage.

The initial energy accumulation phase: The input energy density and elastic energy density grow synchronously, and only a small amount of energy is applied to the dissipation. RC-25 experienced four cycles, which accounted for 22% of the entire cycles; RC-50 endured five cycles, which is 42% of the entire cycles; and RC-75 underwent three cycles, which is 50% of the entire cycles. The energy during the evolution process was applied to the adjustment of pores. With the increasing ratio of coal to rock, the amount of internal cracks increase as well. The compact integration of the structure is relatively long, which extends the initial energy accumulation phase.

The rapid energy accumulation phase: The energy evolution curve is larger and longer than the previous phase. The increase in elastic energy density is faster than that of the dissipation energy phase. The compound coal–rock mainly stores the elastic energy. The evolution process of RC-25 sustains eleven cycles, accounting for 65% of the entire cycles; RC-50 maintains six cycles, 50% of the entire cycles; as for RC-75, it only sustains one cycle, which is 17% of the entire cycles. The larger the coal body height, the shorter the time for rapid energy accumulation and evolution. In this phase, the energy increases the most, and it is the core phase of the entire evolution process. As shown in Figure 8a, there is a small fluctuation in the evolution curve of RC-25 at the end of the rapid energy accumulation. At this point, the load has almost reached the limit of the bearing capacity; as a result, some parts have generated plastic deformation. Overall, the compound coal–rock has a higher bearing capacity and the coal–rock system continues to store the elastic energy. Compared with the other two test pieces, the rock body of RC-25 develops more significant failure characteristics, indicating that the larger the accumulated elastic energy, the fiercer the impact of the released energy on failure.

The fast energy dissipation phase: The load has approached its peak and the compound coal–rock is still absorbing energy. The elastic energy curve gradually flattens, while the density of the dissipation energy skyrockets via the surge point, signifying that the compound coal–rock will be destroyed. The evolution process of RC-25 sustains three~six cycles, accounting for 18% of the entire cycle process; RC-50 maintains three~four cycles, accounting for 19~31% of the total; however, RC-75 only sustains three cycles, which is 43~50% of the total. With the increase in height of the coal body, the deformation resistance deteriorates. The elastic energy accumulation phase has been shifted to the fast energy dissipation phase and the failure degree of compound coal–rock gradually declines.

In conclusion, the energy evolution pattern of compound coal–rock under cyclic loading is: at the initial energy accumulation phase, the low-level of cyclic loading barely generates the new cracks. The dissipation energy makes the cracks compact. The plastic deformation decreases while the hysteresis curve become dense. At the rapid energy accumulation phase, a great number of elastic energy has been storage that the micro cracks are expanded stably and the area of hysteresis curve basically remain the same. At the fast dissipation phase, the compound test pieces approximate failure and the dissipation energy surges. As a result, the internal cracks accelerate the connection and irreversible deformation occurs, leading to the hysteresis expansion. With the height increase of coal body in the compound coal–rock, the elastic modulus constantly decreases, making the plastic deformation and hysteresis expansion more distinctive.

5.3. Energy Drive Mechanism

Under cyclic loading, the input energy, elastic energy, and dissipation energy of compound coal–rock present a non-linear growth. The elastic energy and the energy dissipation are the main drivers of the destruction of the compound coal–rock under cyclic loading during energy evolution.

The energy drive mechanism of the compound coal–rock: The growth rate is slow for the density of input energy, elastic energy, and dissipation energy in the early loading stage, which is similar to the compact phase for the deformation of the coal–rock body. A small amount of dissipation energy is used to compact the gap of the compound coal–rock and the hysteresis curve area becomes denser. In the middle stage of loading, the energy density shows linear growth, corresponding to the elastic stage. A large amount of energy has been accumulated and the cracks expand stably and the hysteresis curve area basically remains unchanged. In the late stage of loading, the dissipation energy surges, corresponding to the plastic phase. The compound coal–rock enters the unstable fracture development stage. The growth of internal cracks accelerates and hysteresis expansion occurs, indicating that the compound coal–rock is about to be destroyed. Under continuous and cyclic loading, the coal and rock deform. Energy is constantly accumulating in the compound coal–rock system. When it reaches the limit, the cracks in coal body expand or wear down. Energy is intensely released, forming the impact effect.

6. Discussion

The generation of rock bursts in mining roadways shall be discussed according to the failure characteristics and energy evolution pattern of compound coal–rock samples under cyclic loading. As shown in Figure 9, the roof and floor surround the coal bed to be mined, forming the compound coal–rock system. Such a system hampers the pressure relief and deformation of the coal–rock structure. Furthermore, during deposition, the “parallel layered structure” of coal forms friction and lateral resistance at the boundary, impeding the parallel slippage of the combined structure. The compactness of the combined system has constricted the coal bed. In the constricted area, the concentrated stress and high elastic energy have provided a natural environment for the accumulation and evolution of energy in the coal–rock system. Meanwhile, repeated loading and unloading during mining constantly provide input energy into the system. The compound coal–rock system experiences multiple initial and rapid energy accumulation phases, increasing the energy storage limit of such a system. When it hits the critical energy of the impact ground pressure, the process activates. The coal–rock system will be destroyed. The abrupt energy release expels the clast back to the mining space, resulting in impact ground pressure.

In the process of energy evolution, the coal body is firstly destroyed and a small portion of energy is released to break the equilibrium of the combined system. As a result, the energy storage in the roof is greater than that in the coal body. Energy will be released from the high-energy area to the surrounding low-energy area, allowing the energy released from the compound coal–rock to act on the coal bed. In this way, except for the elastic energy stored by itself, the coal bed must resist the elastic energy released from the rock body. The larger the proportion of rock, the higher the energy accumulated by coal–rock combination, which increases the risk of the occurrence of impact ground pressure.

Therefore, to prevent the impact ground pressure, firstly, one must optimize the working face advancement speed and slow down the cyclic load of the energy input of the coal–rock system. Secondly, by using pressure relief measures, the degradation performance of the coal body should be fully utilized so that the energy storage phase can be shortened and the storage limit can be lowered. In this way, the energy can be confined within a safe threshold. Thirdly, one must reduce the proportion of rock in the coal–rock combination, control the thick hard roof in the mining process in a timely manner, and reduce the energy released by the rock when the coal body is unstable.

Based on a specific engineering background, the authors designed three coal–rock composite specimens and conducted uniaxial cyclic loading and unloading tests to try to reproduce the real state of the roof–coal seam and preliminarily explore the mechanism of the impact of ground pressure in deep mining roadways from an energy perspective. However, the actual situation is often more complex. Regarding the composition of coal–rock, the proportion of coal and rock, the stiffness of the individual coal–rock, and the combination mode of coal–rock will all affect the bearing capacity of the coal–rock composite body and change its energy storage structure. In addition, the surrounding pressure effect will become increasingly prominent in deep coal–rock disasters, and, to a certain extent, it will change the elastic limit and the strength limit of the coal–rock combination and affect the accumulation and release of energy. As analyzed in the above text, the change in dissipated energy contains rich information regarding the destruction of composite coal–rock [30,31], which should be monitored by various means. Therefore, based on the research in this paper, the authors will design a multi-element coal–rock composite specimen more representative of deep coal–rock roadways, and conduct triaxial cyclic loading and unloading tests. Through acoustic emission, electromagnetic radiation, and other means, the energy change in the destruction process of the composite coal–rock will be monitored in an all-round way to obtain richer destruction characteristics and energy evolution laws of composite coal–rock samples, and further explore the mechanisms of the impact ground pressure.

7. Conclusions

With the prevention of impact ground pressure and coal and rock dynamic disasters in deep mining roadways as the research goal, coal and rock samples have been collected from areas of varying thicknesses in a coal bed. Three types of coal–rock combinations were constructed to carry out cyclic loading and unloading tests. The purpose was to explore the failure modes and energy evolution patterns of compound coal–rock samples and to establish the failure mechanism of energy evolution. The generation of impact ground pressure in mining roadways has been discussed and the results are concluded as below:

(1)

Under cyclic loading, the failure of compound coal–rock presents a significant brittle fracture feature. With the increase in height of the coal body, the mechanical response of compound coal–rock gradually attenuates and the stress–strain curve moves in a positive direction. The hysteresis curve changes from dense to sparse, resulting in “hysteresis expansion”. The compaction process will improve the rigidity of the compound coal–rock.

(2)

The coal body is the main controlling factor of compound coal–rock failure. Under cyclic loading, cracks are first generated in the coal body, which extend to the rock body along the boundary of the coal and rock. Ultimately, the cracks will rapidly connect or the coal body will suddenly wear down, making them the major cause of compound coal–rock failure.

(3)

The energy evolution phases of compound coal–rock can better describe the failure process. They can be divided into the initial accumulation phase, the rapid accumulation phase, and the fast dissipation phase. Generally, energy shows a non-linear growth and can be described by a quadratic function. The growth rate is slow for the density of input energy, elastic energy, and dissipation energy in the early loading stage, which is similar to the compact phase for the deformation of a coal–rock body. In the elastic deformation phase, the energy density shows linear growth. A large amount of energy has been accumulated. With the surge in dissipation energy, the compound coal–rock enters the unstable fracture development stage. In the evolution process, coal and rock deform at the same time. The coal–rock system accumulates energy. When it reaches the limit, the compound coal–rock is destroyed because of the instability.

(4)

The roof and floor surround the coal bed to be mined, forming a locked area. In this area, the concentrated stress and high elastic energy provide a natural environment for the accumulation and evolution of energy in the coal–rock system. Meanwhile, cyclic fluctuations during mining constantly provide input energy into the system. When the critical energy of the impact ground pressure is reached, the process will be activated and impact ground pressure will be formed. Both the elastic energy in the coal bed and the coal–rock system will increase the risk of the impact ground pressure.