1 Introduction

China's energy consumption will continue to grow. The coal-based energy resource endowment and the stage of economic and social development determine that China's development will still be inseparable from coal for a long time in the future (Xie et al. 2021; Xie 2022; Peng 2017; Lai and Fang 2022). With the increasing mining depth and production intensity, coal mines face more and more severe threats such as power disasters (Yuan 2021; Pan and Dai 2021; Jiang et al. 2023). Because of the problem of dynamic disaster, experts and scholars have put forward the strength theory, energy theory, deformation and instability theory (Qi et al. 2019; Jiang et al. 2014); Some monitoring methods, such as drilling cuttings, acoustic emission and microseism, have been developed (Dou et al. 2016, 2018; Cao et al. 2023; He et al. 2021; Ju et al. 2021); The control methods such as mining layout adjustment, mining protective layer, coal seam water injection, blasting pressure relief and drilling relief with large diameter have been formed (Liu et al. 2021; Lan et al. 2016; Dou et al. 2014; Chen et al. 2019; Cheng et al. 2023). Water injection softening and large-diameter drilling pressure relief are the two most commonly used pressure relief methods for coal seams. Some coal seams have a better effect by water injection softening, some by large-diameter drilling, and some by coupling the two methods. There are few researches on the specific mechanical mechanism of the effects of water injection and large-diameter drilling pressure relief on coal rock failure.

The mining roadway of working face is a frequent area of dynamic disasters, especially the coal body in the area affected by the advance pressure of working face is the key area to pay attention to the occurrence of dynamic disasters and take preventive measures. Because the lateral horizontal stress transfer is cut off by the existence of mining roadway, the stress environment of coal body in these areas is uniaxial stress environment dominated by vertical stress. It is of great significance to study the mechanical response mechanism and failure characteristics of saturated coal with a single pre-existing hole under uniaxial compression load, which is of great importance to take more targeted pressure relief measures for coal seam.

At present, much research has been carried out on the mechanical failure of rock samples with holes. On the one hand, the evolution law of cracks around holes is mainly studied from the perspective of damage mechanics (Li et al. 2015; Xw et al. 2021; Zhang et al. 2021; Cui et al. 2022); on the other hand, the crack initiation, propagation and mechanical properties of rock mass with holes are analyzed from the point of view of fracture mechanics (Ma et al. 2006; Yang et al. 2009; You et al. 2007; Jin-Chao et al. 2006), A series of significant research results have also been obtained on the mechanical properties of coal samples with water saturation, and the mechanical and acoustic emission characteristics of coal samples with different water content have been obtained (Lai et al. 2020; Yao et al. 2021; Su et al. 2014; Wang et al. 2018; Gu et al. 2023; Yang et al. 2022). There are still relatively few studies on the mechanical response mechanism, failure mode and energy evolution characteristics of saturated coal with a single pre-existing hole. This paper carries out uniaxial compression test of coal, uses high-speed camera and acoustic emission equipment to monitor various mechanical characteristics of coal sample failure process, and analyzes the mechanical characteristics and energy evolution laws of different types of coal samples. The aim is to guide for further improving the pertinence and effectiveness of coal seam dynamic disaster prevention measures.

2 Test equipment and scheme

2.1 Preparation and processing of test coal samples

The sampling site of the coal sample used in this experiment is B3+6 working face at + 450 m level in the south area of Wudong Coal Mine of Xinjiang Energy Company. Wudong coal mine is divided into south, North and west areas, and the dynamic disaster is mainly concentrated in the south area. B1+2 and B3+6 coal seams are mainly mined in the south area, of which the average thickness of B1+2 coal seam is 28 m, and the average thickness of B3+6 coal seam is 40 m. The average inclination of the coal seam is 87°, which belongs to the typical near vertical ultra-thick coal seam group mining (Fig. 1).

Fig. 1
figure 1

Occurrence area of near vertical extra thick coal seam group in Wudong coal mine

Full size image

Field sampling shall be carried out according to the general regulations of coal and rock sampling. Considering the convenience of monitoring during the test process, the coal sample shall be processed into a 70 * 70 * 70 mm square sample according to relevant standards (Hudson 2002; Zheng et al. 2010), and the cut coal sample shall be polished to make the non-parallelism and non-perpendicularity of the loading end face of the coal sample meet the technical requirements of international standards. The industrial analysis and elemental analysis tests of coal samples were carried out to determine the content of each component and elemental content of coal samples. The metamorphism degree of coal samples was classified as 1/3 coking coal, and the content of C element was the most, followed by O element.

To avoid the interference of other factors, the square coal samples were all naturally air-dried for 24 h and defined as the natural state. To approximate the real engineering environment as much as possible, this paper takes the standard methods on site as an example, involving water softening and large-diameter pressure relief drilling. The test was divided into four groups with three samples in each group, of which three were used as tests and one as backup. The test treatment of specific coal samples was as follows:

  1. (1)

    Group a is the intact coal sample in its natural state. Sealed with plastic film in its natural state;

  2. (2)

    Group b is the intact coal sample under Water-saturated Condition. To make the coal sample have a better water saturation effect, refer to the steps of rock sample Water SATURATION in reference (Wang et al. 2010). First, water is injected to 1/3 of the height of the sample. After 12 h, 2/3 of the length of the sample is immersed again. After soaking for 30 days, the water-saturated sample was prepared, wiped dry and sealed;

  3. (3)

    Group c is the coal sample with a single pre-existing hole in its natural state. According to the finite element theory, the influence range around the hole is three to five times the radius, and five times the radius is taken here. To better observe the damage of the hole, a hole with a diameter of 10–12 mm is selected. Due to the limitation of the experimental drill, a hole with a diameter of 10 mm is finally determined.

  4. (4)

    Group d is the coal sample with a single pre-existing hole under water-saturated conditions. The prepared coal sample with is a single pre-existing hole repeated the saturation step of group b coal sample (Figs. 2, 3).

Fig. 2
figure 2

SEM image of test coal sample

Full size image
Fig. 3
figure 3

Detailed information of the coal used in the experiment

Full size image

2.2 Test device and scheme

The test loading equipment used in this test is a microcomputer-controlled electro-hydraulic servo pressure testing machine system, MISTRAS Micro-11 rapid digital acoustic emission system, and high-speed cameras are used to collect acoustic emission signals in the process of coal sample rupture and film the critical damage of coal sample, respectively. The test loading and monitoring equipment is shown in Fig. 4. The displacement-controlled loading method was adopted in this test, and the loading rate was constant at 0.5 mm/min. The acoustic emission probe is coupled with a coal sample by petroleum jelly. The axial load-deformation curve of the coal sample is tested, and the acoustic emission monitoring system and high-speed camera are turned on simultaneously.

Fig. 4
figure 4

Test coal sample and equipment

Full size image

3 Mechanical response characteristics of loaded coal samples

3.1 Statistics of mechanical parameters of saturated coal with a single pre-existing hole

Uniaxial loading was carried out on four groups of coal samples. Mechanical parameters of four different types of coal samples were calculated respectively, and their average values were obtained. The statistical statistics of mechanical parameters of four different types of coal samples are shown in Table 1.

Table 1 Mechanical parameters of different types of coal samples
Full size table

Comparing and analyzing the variation of mechanical parameters of different types of coal samples, it is found that the strength and elastic modulus of coal samples are degraded by water saturation measures, single pre-existing hole measures. The strength, elastic modulus and peak strain of the intact coal sample are degraded by 23.49%, 12.904% and 1.99%, respectively. The degradation of mechanical parameters of the coal sample by the water-saturated measure is mainly due to the change of the microstructure of the coal sample through lubrication. The measure of single pre-existing hole degrades the strength of coal samples by 9.47% and the modulus of elasticity by 17.75%, but increases the peak strain slightly. The single pre-existing hole measure mainly changes the macro-structure of coal rock by creating round hole defects, which degrades the strength, elastic modulus and other parameters of coal samples. The slight increase of peak strain is due to the existence of holes, which increases the brittleness of coal sample, and the failure instantly increases the deformation of coal sample. The coupling effect of the two measures changes the microstructure and macro-structure of coal samples at the same time. The strength of intact coal samples in the natural state deteriorates by 47.95%, the elastic modulus deteriorates by 31.5%, and the peak strain decreases by 11.44%.

The single effect of water saturation and single pre-existing hole measures and the coupling effect of the two measures on the change of mechanical parameters of coal samples are comprehensively compared. The coupling effect of the two measures causes the simultaneous shift in macro and micro structure and the deterioration of mechanical parameters of coal samples is the most obvious. To further understand the role of water saturation and single pre-existing hole measures in the coupling process of coal samples, by further comparing the mechanical changes of water saturation measures on intact and single pre-existing hole coal samples, and the changes of mechanical parameters of natural and saturated coal samples, it can be found that in the coupling process of the two measures on coal samples, The change of microstructure of coal sample caused by water saturation measure plays a more fundamental role in the deterioration of mechanical parameters such as coal sample strength. The single pre-existing hole measure plays a supporting role in the decline of mechanical parameters such as coal sample strength. The coupling effect of the two factors has a more significant impact on the mechanical deterioration of coal samples than the single measure, which is more conducive to the weakening of coal rock strength.

3.2 Deformation and failure characteristics of saturated coal with a single pre-existing hole

According to the obtained complete stress–strain curves, the deformation of the coal samples can be divided into pore fracture compaction stage (OA section), elastic deformation stage (AC section), The stable development stage of micro-fractures (CD section), unstable fracture development stage (DE section) and post-peak failure stage (EF section). The typical stress–strain curve of the coal sample is selected for analysis, and the key characteristic points of the complete stress–strain curves process of the rock are determined strictly according to the method (Qin et al. 2018). A–F is the key characteristic points of the corresponding complete stress–strain curve, where A is the starting point of linear elastic deformation (from micropore compaction to the inflexion point of elastic deformation). B is the semi-peak intensity point; C is the limit point of linear elastic damage; Point D is the turning point of elastoplastic deformation (yield point). E is the peak intensity; Point F is the residual strength.

  1. (1)

    Typical intact coal samples in the natural state

Figure 5 shows the complete stress–strain curves damage and failure process of the intact coal sample in its natural state. The coal sample is in the pore crack compaction stage (OA section): the surface of the coal sample is relatively complete with no apparent cracks. When the coal sample enters the elastic deformation stage (AC section), small cracks will appear in different areas of the coal sample surface, and initial fracture areas will be formed on the coal sample surface, and the cracks will expand with the increase of pressure. In the stable development stage of micro-fractures (CD section), the surface crack of the coal sample continues to grow, and penetration occurs; In Unstable fracture development stage (DE section): surface cracks of the coal sample continue to develop and penetrate, and spalling will appear, and the spalling area will form a region with apparent damage; The post-peak failure stage (EF stage): the surface cracks of the coal sample penetrate each other, and the failure is more severe, and finally the shear failure mode appears. The intact coal sample in its natural state mainly presents the shear failure mode with significant and prominent single inclined plane cracks penetrating the coal sample.

Fig. 5
figure 5

Complete stress–strain damage and failure characteristics of typical intact coal samples in the natural state

Full size image
  1. (2)

    Typical intact coal sample under water-saturated condition

Figure 6 shows the complete stress–strain curves damage and failure process of the intact coal sample with full water content. The intact coal sample under the water-saturated condition enters the elastic deformation stage (AC section): long initial cracks appear at the right edge of the coal sample, irregular cracks appear in the left area, and the initial long cracks in the right area continue to expand to the bottom. When the coal sample enters the stable development stage of micro-fractures (section CD), the initial long strip cracks on the surface of the coal sample and the cracks in the bottom region run through, and the cracks continue to expand in the left edge region of the coal sample surface. Unstable rupture development stage (DE stage): Cracks develop rapidly in the left region of the coal sample surface, and some peel off from the surface of the coal sample to form a more obvious failure region than the broken region. After the peak and post-peak failure stage, cracks on the coal sample surface penetrate each other, and the coal sample is further damaged. The final failure pattern of the intact coal sample under water-saturated condition is more complicated than that of the intact coal sample in the natural state. The shear failure mode of one large and apparent main crack and several more minor secondary cracks through each other is presented.

Fig. 6
figure 6

Complete stress–strain damage and failure characteristics of typical intact coal samples with saturated water

Full size image
  1. (3)

    Typical coal sample with a single pre-existing hole in the natural state

Figure 7 shows the complete stress–strain curves damage and failure process of the coal sample with a single pre-existing hole in its natural state. In the elastic deformation stage (AC section) of the coal sample, long strip initial cracks appear successively at the right and left edges of the coal sample, and the long strip cracks in the right region expand under the loading of the force. In the stable development stage of micro-fractures(CD segment), micro cracks appear around the holes, and long strip cracks further expand in the right region of the coal sample. In the development stage of unstable fracture (DE segment), cracks continue to grow and penetrate with hole in the right region, eventually destroying the entire coal sample. The hole will affect the development and penetration of cracks on the surface of the coal sample, making the area around the hole an accessible area for fracture development and expansion. In the natural state, the failure mode of the coal sample with a single pre-existing hole is shear failure of single inclined plane cracks through each other.

Fig. 7
figure 7

Complete stress–strain damage and failure characteristics of typical coal sample with a single pre-existing hole in the natural state

Full size image
  1. (4)

    Typical coal sample with a single pre-existing hole under water-saturated condition

Figure 8 shows the complete stress–strain curves damage and failure process of the coal sample with a single pre-existing hole under water-saturated conditions. In the elastic deformation stage (AC section) of the coal sample, initial tiny cracks appear in the left edge region of the coal sample surface. The micro-cracks spread under the loading of the force, and irregular stripe cracks appear on the right part of the coal sample surface in the late stage of elastic deformation. In the stable development stage of micro-fractures (CD segment), the cracks in the left and right regions of the coal sample rapidly develop and penetrate to the top and bottom. The cracks and the hole in the right region are connected. The development stage of unstable fracture (DE section): the area with severe crack penetration spalling off from the surface of the coal sample, forming a large failure area. Under the joint action of the water saturation and single pre-existing hole measures. The failure of the coal sample is more complicated, and the cracks on the surface of the coal sample are more developed.

Fig. 8
figure 8

Complete stress–strain damage and failure characteristics of typical coal sample with a single pre-existing hole under water-saturated condition

Full size image

The complete stress–strain curves of four different types of coal samples are compared comprehensively, and the intact coal samples in the natural state show relatively rapid fall failure. The post-peak part of the stress–strain curve of the intact coal sample with saturated water shows a multi-peak and multi-step decline, and the stress drops to the residual strength slowly. The coal sample with a single pre-existing hole in the natural state fall rapidly after peak, showing brittle failure pattern. The post-peak part of the stress–strain curve of the coal sample with a single pre-existing hole under the water-saturated condition shows a slight multi-step decline.

At present, the quantitative description of the influence of various measures on the deformation and failure process of coal rock is relatively few. To further explore the influence of different measures on the five stages of pore fracture compaction, elastic deformation and micro-fractures stability development during the deformation and failure process of coal samples. The peak intensity of stress values at key characteristic points in the complete stress–strain process of four types of typical coal samples is calculated, as shown in Table 2. The peak intensity ratio of stress values at key characteristic points in the complete stress–strain process is adopted to eliminate the difference caused by comparing single stress values.

Table 2 Peak strength ratio of stress values at key characteristic points in total stress–strain process of different types of typical coal samples
Full size table

The elastic deformation stage of natural intact coal samples occupies 67.46% of the pre-peak, and the stable failure stage occupies 10.529%. The elastic deformation stage of intact coal sample under water-saturated condition occupies 76.238% of the pre-peak and the stable failure stage occupies 7.978% of the pre-peak. Water saturation measures prolong the elastic deformation stage by 8.778% and shorten the stable failure stage by 2.551%. The elastic deformation stage of coal sample with a single pre-existing hole in the natural state occupies 65.53% of pre-peak, and the stable failure stage occupies 10.129% of pre-peak. The single pre-existing hole measure has little influence on the proportion of peak stress intensity in each stage of coal sample in the natural state. Compared with the intact coal samples of the natural state, the starting point of linear elastic deformation A, the limiting point of linear elastic damage C and the turning point of elastoplastic deformation D change within 2%. The elastic deformation stage of coal sample with a single pre-existing hole under water-saturated condition occupies 85.218% and the stable failure stage occupies 2.178%. The joint action of saturated water and hole measures makes the peak intensity ratio of stress at the starting point of linear elastic deformation A decrease, and the peak intensity ratio of stress at the limiting point of linear elastic damage C increase more obviously. The coupling effect of the two measures prolong the elastic deformation stage by 17.758%, and shorten the stable failure stage by 8.351%.

By comparing the ratio of peak stress value of residual strength of different types of coal samples, the change difference is less than 4%, indicating that the measures of saturated water and hole have little influence on the residual strength of coal samples. The measures of water saturation and hole mainly change the time when the peak strength of coal samples falls to the residual strength.

4 Analysis of energy evolution law of saturated coal with a single pre-existing hole

4.1 Analysis of stress and energy evolution characteristics during the failure process of saturated coal with a single pre-existing hole

According to the energy calculation method proposed by Xie et al. (2005) the total energy U absorbed during the loading process of four different types of coal samples, the energy Ud dissipated by coal samples, and the elastic energy Ue stored in coal samples were calculated respectively. The evolution curves of stress and energy density in the failure process of different coal samples are shown in Fig. 9.

Fig. 9
figure 9

Evolution curves of stress and energy density during failure of different types of coal samples

Full size image

It can be seen from Fig. 8 that the total input energy density curve and elastic energy density curve of the intact coal sample in its natural state almost coincide in the pore compaction stage, which is because the internal micro-cracks of the coal sample are closed under load, and various energy density indexes in this stage are relatively low. Elastic deformation stage: the total input energy and elastic energy density curve of the coal sample increase steadily, while the dissipated energy density curve increases slowly. In this stage, the total energy input to the coal sample is converted into elastic energy and stored inside the coal sample. If the unloading is carried out at this stage, the elastic energy stored inside the coal sample will be released without damaging the coal sample. In the stable development stage of micro-fractures, the dissipative energy density curve increases rapidly, which is caused by the rapid and stable development of microcracks in the coal sample. However, energy storage is still the main factor in this stage, and elastic energy still occupies the main position. In this stage, the dissipative energy density curve climbs quickly, the internal cracks of the coal sample develop and expand rapidly, and the damage degree of the coal sample increases quickly. In this stage, the energy storage capacity of the coal sample is weakened, and most of the energy input to the coal sample is converted into dissipative energy to destroy the coal sample. Post-peak failure stage: the dissipative energy increases sharply, and the elastic energy stored in the coal sample before the peak is released as macroscopic cracks accompanied by kinetic energy.

The intact coal sample under water-saturated conditions, the coal sample with a single pre-existing hole in its natural state and the coal sample with a single pre-existing hole under water-saturated conditions also go through the above stages, and the law of internal fissure development and expansion and energy accumulation and release is roughly the same as that of intact coal sample. The speed of energy accumulation and release at each stage is significantly different. The dissipative energy density curve of the intact coal sample with saturated water shows a slow growth trend in OB stage, and a significant decline trend in BD stage. When the coal sample enters the unstable failure stage, the dissipative energy density curve increases rapidly. The growth trend of dissipative energy density curve of coal samples in the natural state is roughly the same as that of intact coal samples in the natural state. The dissipative energy density curve first experiences slow growth and then fast climbing. The slow growth stage of coal sample with a single pre-existing hole in the natural state is before the stable failure stage (OC stage), the start point of the fast growth stage of dissipative energy density moves forward, and the fast growth stage is after the elastic deformation stage (CF stage). The growth trend of the dissipated energy density curve of the coal sample with a single pre-existing hole under water-saturated conditions is roughly the same as that of the intact coal sample under water-saturated conditions, showing a significant downward trend and upward trend. The coal sample with a single pre-existing hole under water-saturated conditions is in a slow growth stage before the half-peak strength point (OB stage), and is in a significant downward trend between the half-peak strength point and the yield point (BD stage). When entering the unstable failure stage (DF stage) quickly climb.

4.2 Joint analysis of AE characteristics and energy dissipation of saturated coal with a single pre-existing hole

The acoustic emission signals during the loading process of coal samples provide convenience for analyzing the internal damage evolution information. Figure 10 shows the relationship between acoustic emission characteristics, dissipated energy and stress of intact coal samples in the natural state over time. The four stages of initial compaction to unstable failure of intact coal samples in the natural state go through 120 s, 118 s, 22 s and 47 s, respectively. The AE signal of each stage presents more intense characteristics than that of the previous stage, especially in the stable and unstable failure stages. The dissipated energy density curve increases rapidly, and the AE ringing count and energy also show a higher level. In the post-peak failure stage, the acoustic emission characteristics of coal samples are more prominent.

Fig. 10
figure 10

The relationship of acoustic emission characteristics, dissipated energy and stress with time in natural intact coal samples

Full size image

Figure 11 shows the relationship between acoustic emission characteristics, dissipated energy and stress of intact coal samples in the water-saturated state with time. The initial compaction stage and the elastic deformation stage of the intact coal sample with water saturation lasted for 145 s and 86 s, respectively. The stable and unstable failure stages lasted for 34 s. The AE ringing count and energy of the intact coal sample with water saturation in the elastic deformation stage showed a sporadic surge phenomenon. Relatively high ringing count and energy appeared at individual moments. In both stable and unstable failure stages, the AE ringing count and energy level is low. Only before the load reaches the peak intensity, the AE ringing count and energy surge phenomenon, in the late post-peak failure stage of water-saturated intact coal samples, that is after the post-peak stress is reduced to 7 MPa, the AE ringing count and energy increase rapidly, and the AE signal characteristics are more intense.

Fig. 11
figure 11

The relationship of acoustic emission characteristics, dissipated energy and stress with time in intact coal samples in the saturated state

Full size image

Figure 12 shows the relationship between acoustic emission characteristics, dissipated energy and stress of coal sample with a single pre-existing hole in the natural state with time. As can be seen from Fig. 9, the initial compaction stage of coal sample with a single pre-existing hole in the natural state lasted about 138 s, during which AE ringing count and energy were relatively intense, AE ringing count was at a high level, but dissipated energy was at a low level. This was because the holes were the initial defects, and coal samples were in a self-balancing process at the initial loading stage, resulting in apparent acoustic emission signal characteristics. The elastic deformation stage lasts for about 75 s. In this stage, the sporadic ringing count and acoustic emission energy are high, and the dissipative energy increases steadily. The stability failure stage lasted for 12 s, during which the AE ringing count and energy were more intense, and the dissipative energy increased more rapidly. The unstable failure stage lasted for 45 s, during which a relatively high level of ringing count and energy appeared at individual moments, and before the load reached the peak intensity, the ringing count and energy surged. There was a short acoustic emission quiet period, during which the dissipative energy increased rapidly. In the post-peak failure stage, it takes 177 s for the AE ringing count and energy level to fall from the peak strength to the residual strength, significantly when the post-peak stress is reduced to 3 MPa, the AE ringing count and energy increase rapidly.

Fig. 12
figure 12

The relationship of acoustic emission characteristics, dissipated energy and stress with time in the natural state coal sample with a single pre-existing hole

Full size image

Figure 13 shows the relationship between acoustic emission characteristics, dissipated energy and stress of the coal sample with a single pre-existing hole under water-saturated conditions over time. The initial compaction stage of the coal sample with a single pre-existing hole under water-saturated condition lasted for 125 s, and the ringing count was at a relatively high level, while AE energy and dissipated energy were at a relatively low level. The elastic deformation stage lasted for about 98 s, during which AE ringing count and energy level were down, and the dissipative energy increased steadily. The stable and unstable failure stages lasted for 12 s, while AE ringing count and energy level were down, but the dissipative energy grew rapidly. It takes 55 s for the coal sample to fall from peak to residual strength, and the AE ringing count and energy level are relatively high. Significantly when the post-peak stress is reduced to 1 MPa, the AE ringing count and energy increase rapidly.

Fig. 13
figure 13

The relationship of acoustic emission characteristics, dissipated energy and stress with time in the water-saturated coal sample with a single pre-existing hole

Full size image

The acoustic emission characteristics, dissipated energy and stress of different coal samples were compared with time. The high levels AE ringing count and energy of intact coal samples in the natural state are mainly concentrated in the unstable failure stage and post-peak failure stage, and the energy release is relatively focused. The maximum damage energy is 106 mv·us. In the pre-peak stage, the intact coal samples with the saturated state showed sporadic high-level AE ringing count and energy, and the high energy events were mainly in the late post-peak failure stage. The high-level ringing count was more than that of intact coal samples with the natural state, and the highest failure energy was 105 mv·us, which was one energy level lower than that of intact coal samples with the natural state. It shows that saturated water can increase the generation of cracks in coal samples, reduce the energy release level when coal samples are destroyed, and play a role in slowing energy release. The damage of coal samples in the natural state is relatively extreme at each stage, and there are rather severe high-level energy events in each stage, especially in the unstable failure stage and the early stage of the post-peak failure stage, the high-level ringing count and energy are relatively concentrated, and the energy release is also 106 mv·us, but the highest energy value is about four times that of the highest energy of the intact coal samples in the natural state. It shows that the existence of holes will affect the energy released during the failure process of coal sample, and will aggravate the failure intensity of the coal sample. The AE ringing count at each stage of the coal sample with a single pre-existing hole under the water-saturated condition is more intense, the high-level AE energy events are scattered and the energy release is more uniform. The highest energy level is higher than that of the water-saturated intact coal sample, but lower than that of the natural intact coal sample.

It can be seen from the acoustic emission characteristics of different types of coal samples that saturated water can play a role in slowing energy release, a hole will intensify energy release and increase the damage degree of coal samples, and the coupling effect of saturated water and hole measures will reduce the significant energy release of coal samples.

5 Evaluation of the impact risk of saturated coal with a single pre-existing hole

The occurrence of dynamic disasters is accompanied by the release of energy, so it is reasonable to study the change of impulse tendency in the process of total stress and strain of coal samples from the perspective of energy. According to the measurement method of relevant indicators of coal seam impulse tendency, the area ratio under the curve before and after the peak of the coal sample is used to reflect the impulse tendency of coal. The ratio is called the impulse energy index. The integral area of the curve before the peak strength is the total input strain energy density before the peak strength, and the integral area of the curve after the peak strength is the fracture strain energy density after the peak strength.

The average pre-peak total input strain energy density and post-peak failure strain energy density of four different types of coal samples were calculated, and the impact energy index was calculated. The impact inclination index of different types of coal samples was compared with the change of the impact inclination index of different types of coal samples, as shown in Table 3. Water saturation reduces the total input strain energy density of the coal sample by 35.01% before peak, increases the fracture strain energy density by 33.21% after peak, and reduces the impact energy index of complete coal sample from 2.05 to 0.89, which decreases by 56.59%. The single pre-existing hole measure has little influence on the total input strain energy density before peak and the post-peak failure strain energy density of intact coal samples in the natural state, which change within 2%. The single pre-existing hole measure slightly increases the impact tendency index of coal samples in the natural state, which increases the impact energy index from 2.05 to 2.08. The coupling effect of the two measures reduce the total input strain energy density of the coal sample before peak by 62.78%, and the fracture strain energy density after peak by 30.06%. The impact energy index of the intact coal sample decreases by 46.78% from 2.08 to 1.09.

Table 3 Comparison of impact risk indexes of different types of coal samples
Full size table

The comprehensive comparative analysis shows that the impact tendency of coal samples can be significantly reduced by water saturation, and the impact energy index of coal samples can be slightly increased by cavity measures.

6 Conclusion

This paper conducted uniaxial compression test, using high-speed cameras and acoustic emission monitoring systems to monitor the failure process and acoustic emission characteristics of saturated coal with a single pre-existing hole. A comprehensive comparative analysis was conducted on the mechanical response characteristics and energy evolution laws of saturated coal with a single pre-existing hole, further revealing the mechanical mechanism of water injection and large-diameter drilling pressure relief measures on coal and rock. This provides essential theoretical guidance for improving the pertinence and effectiveness of coal mine dynamic disaster prevention and control measures. The main conclusions are as follows:

  1. (1)

    The degradation effects of different measures on mechanical parameters such as coal sample strength vary, water saturation, single pre-existing hole measures, and their coupling effect resulted in a deterioration in the coal sample's strength by approximately 23.49%, 9.47%, and 47.95% respectively. The water saturation measure plays a more fundamental role, while the single pre-existing hole measure plays an auxiliary role. The coupling effect of the two measures is much greater than the effect of a single measure, which is more conducive to the weakening of mechanical parameters such as coal rock strength.

  2. (2)

    Different types of coal samples ultimately exhibit shear failure modes. The different measures of water saturation and single pre-existing hole make the development and penetration of surface cracks on coal samples more complex. The coupling effect of water saturation and single pre-existing hole measures will prolong the time of the elastic deformation stage of coal samples, and also change the time from the peak strength to the residual strength of coal samples.

  3. (3)

    The development and expansion of internal fractures in different types of coal samples have similar patterns of energy accumulation and release, there are significant differences in the speed of energy accumulation and release in each stage. The energy release of intact coal samples in the natural state is relatively concentrated, and high-energy events are mainly focused in the unstable failure stage and post peak failure stage; the water saturation measure will reduce the energy release level during coal sample failure, playing a role in slow-release energy. The single pre-existing hole measures will intensify the strength of coal sample failure, and the coupling measures of the two measures will make the energy release more uniform, reducing the significant energy release of coal sample failure.

  4. (4)

    Different measures can affect the impact propensity index of coal samples. Water saturation measures reduce the impact energy index of intact coal samples by 56.59%, while single pre-existing hole measures slightly increase the impact propensity index of natural coal samples. The coupling effect of the two measures reduces the impact energy index of intact coal samples by 46.78%.