Analysis of mechanical response mechanism and energy evolution characteristics of saturated coal with a single pre-existing hole

doi:10.21203/rs.3.rs-3124274/v1

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Analysis of mechanical response mechanism and energy evolution characteristics of saturated coal with a single pre-existing hole

https://doi.org/10.21203/rs.3.rs-3124274/v1

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It is very important to elucidate the mechanical mechanism of the effect of water injection and large-diameter borehole pressure relief on coal rock deformation and failure for improving the pertinence and effectiveness of coal mine dynamic disaster prevention measures. In order to obtain the mechanical response mechanism and energy evolution characteristics of saturated coal with a single pre-existing hole, uniaxial compression test was carried out, and the failure process and acoustic emission characteristics of coal were monitored by using high-speed camera and acoustic emission monitoring system. The results show that the strength of coal samples deteriorated by 23.49%, 9.47% and 47.95% respectively by water saturation, single pre-existing holemeasures and the coupling effect of the two measures. The final failure mode of coal samples with different measures is shear failure mode. The law of fracture development and expansion and energy accumulation and release in different coal samples is roughly the same, but the speed of energy accumulation and release in different stages is different. Water saturation and single pre-existing hole measures can affect the impact risk of coal samples, and water saturation can significantly reduce the impact tendency of coal samples, and reduce the impact energy index of complete coal samples by 56.59%. The impact tendency index of natural coal sample is slightly increased by single pre-existing hole measures, and the impact energy index of intact coal sample is reduced by 46.78% by the coupling of saturated water and cavity measures. It can provide technical support for coal mine power disaster prevention and control.

Dynamic disaster prevention

Water saturation effect

Hole pressure relief

Energy evolution characteristics

Mechanical properties

Acoustic emission

China's energy consumption will continue to grow in the future for a long time. 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^[1–4]. With the increasing mining depth and production intensity, coal mines face more and more serious threats such as power disaster^[5–7]. In view of the problem of dynamic disaster, experts and scholars have put forward the strength theory, energy theory, deformation and instability theory^[8–9]; Some monitoring methods, such as drilling cuttings, acoustic emission and microseismic, have been developed^[10–14]; The control methods such as mining layout adjustment, mining protective layer, coal seam water injection, blasting pressure relief and large bore hole pressure relief have been formed^[15–19]. Among them, 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 better effect by water injection softening, some by large-diameter drilling, and some by coupling the two methods. The specific mechanical mechanism of the impact of water injection and large-diameter drilling pressure relief on coal rock damage is rarely studied.

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 significance to take more targeted pressure relief measures for coal seam.

At present, a lot of 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^[20–23], 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^[24–27], 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^[28–33]. 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 provide guidance for further improving the pertinence and effectiveness of coal seam dynamic disaster prevention measures.

2.1 Preparation and processing of test coal samples

The sampling site of coal sample used in this experiment is B₃₊₆ working face at +450m 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 28m, and the average thickness of B₃₊₆ coal seam is 40m. The average inclination of coal seam is 87°, which belongs to the typical near vertical ultra-thick coal seam group mining.

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*70mm square sample according to relevant standards^[34-35], and the cut coal sample shall be polished to make the non-parallelism and non-perpendicality 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.

In order to avoid the interference of other factors, the square coal samples were all naturally air-dried for 24h and defined as natural state. In order to approximate the real engineering environment as much as possible, this paper takes the common methods on site as an example, involving water softening and large diameter pressure relief drilling. The test was divided into 4 groups with 3 samples in each group, of which 3 were used as tests and 1 was used as backup. The test treatment of specific coal samples was as follows:

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

(2) Group b is the intact coal sample under Water-saturated Condition. In order to make the coal sample

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 Figure 1. The displacement controlled loading method was adopted in this test, and the loading rate was constant at 0.5mm/min. The acoustic emission probe is coupled with coal sample by petroleum jelly. The axial load-deformation curve of coal sample is tested, and the acoustic emission monitoring system and high-speed camera are turned on simultaneously.

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

In this test, uniaxial loading was carried out on 4 groups of coal samples, and 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 were shown in Table 1.

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 sample 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 sample. 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 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 change of macro and micro structure and the deterioration of mechanical parameters of coal samples is the most obvious. In order to further understand the role of water saturation and single pre-existing hole measures in the coupling process of coal sample, 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, and the single pre-existing hole measure plays a supporting role in the deterioration of mechanical parameters such as coal sample strength. The coupling effect of the two factors has a far greater effect on the mechanical deterioration of coal sample than the single measure, which is more conducive to the weakening of coal rock strength.

Table 1 Mechanical parameters of different types of coal samples

Test piece status		Average compressive strength /MPa	Average peak strain	Average modulus of elasticity /MPa
Natural state	intact	16.10	0.0201	1729.25
Water-saturated	intact	12.318	0.0197	1506.10
Natural state	hole	14.576	0.0223	1422.24
Water-saturated	hole	8.38	0.0178	1184.523

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

According to the obtained full-stress-strain curves, the deformation of the coal samples can be divided into pore fracture compaction stage (OA section), elastic deformation stage (AC section), microelastic fracture stable development stage (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 whole stress-strain process of the rock are determined strictly according to the method^[37]. A~F is taken as the key characteristic points of the corresponding whole stress-strain curve, where A is the starting point of linear elastic deformation (from micropore compaction to the inflection 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) Typical intact coal samples in natural state

Figure 4 shows the whole stress-strain 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 obvious 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 microelastic crack (CD section), the surface crack of coal sample continues to expand and penetration occurs; Unstable fracture development stage (DE section) : surface cracks of coal sample continue to develop and penetrate, and spalling will occur, and the spalling area will form a region with obvious 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 natural state mainly presents the shear failure mode with large and obvious single inclined plane cracks penetrating through the coal sample.

(2) Typical intact coal sample under Water-saturated Condition

Figure 5 shows the whole stress-strain damage and failure process of the intact coal sample with full water content. The intact coal sample under 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 microelastic cracks (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 of them 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 natural state. The shear failure mode of one large and obvious main crack and several smaller secondary cracks through each other is presented.

(3) Typical coal sample with a single pre-existing hole in natural state

Figure 6 shows the whole stress-strain damage and failure process of the coal sample with a single pre-existing hole in 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 microelastic fracture (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 expand and penetrate with hole in the right region, and eventually destroy the entire coal sample. The existence of hole will affect the development and penetration of cracks on the surface of coal sample, making the area around hole an easy area for fracture development and expansion. In the natural state, the failure mode of coal sample with a single pre-existing hole is shear failure of single inclined plane cracks through each other.

(4) Typical coal sample with a single pre-existing hole under Water-saturated Condition

Figure 7 shows the full stress-strain damage and failure process of the coal sample with a single pre-existing hole under Water-saturated Condition. 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 microelastic cracks (CD segment), the cracks in the left and right regions of the coal sample rapidly develop and penetrate to the top and bottom, and the cracks and 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.

The stress-strain curves of four different types of coal samples are compared comprehensively, and the intact coal samples in 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 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 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, in order to further explore the influence of different measures on the five stages of pore fracture compaction, elastic deformation and microelastic fracture stability development during the deformation and failure process of coal samples. The peak intensity of stress values at key characteristic points in the total stress-strain process of four different 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 total stress-strain process is adopted to eliminate the difference caused by the comparison of single stress values.

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 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 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, and the measures of saturated water and hole mainly change the time when the peak strength of coal samples falls to the residual strength.

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

Test part		Key feature point The peak value of the peak value (%)
Test part		A	C	D	F
Natural state	intact	17.114	84.574	95.103	3.828
Water-aturated	intact	12.608	88.846	96.824	4.079
Natural state	hole	17.492	83.022	93.151	7.106
Water-aturated	hole	8.258	93.476	95.654	3.934

According to the relevant energy calculation method in literature[38], the total energy absorbed during the loading process of four different types of coal samples, the energy dissipated by coal samples, and the elastic energy stored in coal samples were calculated respectively. The evolution curves of stress and energy density in the failure process of different types of coal samples are shown in Figure 8.

It can be seen from figure 8 that the total input energy density curve and elastic energy density curve of the intact coal sample in 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 microelastic cracks, 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 the elastic energy still occupies the main position. In this stage, the dissipative energy density curve climbs rapidly, the internal cracks of the coal sample develop and expand rapidly, and the damage degree of the coal sample increases rapidly. 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 peak is released in the form of macroscopic cracks accompanied by kinetic energy.

The intact coal sample under Water-saturated Condition, coal sample with a single pre-existing hole in natural state and coal sample with a single pre-existing hole under Water-saturated Condition 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, but 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 natural state is roughly the same as that of intact coal samples in 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 natural state is before the stable failure stage (OC stage), the start point of the rapid growth stage of dissipative energy density moves forward, and the rapid 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 Condition is roughly the same as that of the intact coal sample under Water-saturated Condition, showing a significant downward trend and then an upward trend. The coal sample with a single pre-existing hole under Water-saturated Condition 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.

5.1 Quantitative evaluation and analysis 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 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 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 natural state, both of which change within 2%. The single pre-existing hole measure slightly increases the impact tendency index of coal samples in natural state, which increases the impact energy index from 2.05 to 2.08. The coupling effect of the two measure reduce the total input strain energy density of coal sample before peak by 62.78%, and the fracture strain energy density after peak by 30.06%. The impact energy index of intact coal sample decreases by 46.78% from 2.08 to 1.09.

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.

Table 3 Comparison of impact risk indexes of different types of coal samples

Test part		Average peak stress /MPa	The total input of the average peak peaks /MJ·m^-3	Destroy the density of strain energy after the average peak /MJ·m^-3	Impact energy indexW_B
Natural state	intact	16.10	0.10252	0.04996	2.05204
Water-saturated	intact	12.318	0.06663	0.07480	0.89078
Natural state	hole	14.576	0.10181	0.04884	2.08456
Water-saturated	hole	8.38	0.03816	0.03494	1.09216

5.2 Joint analysis of acoustic emission 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. FIG.9 shows the relationship between acoustic emission characteristics, dissipated energy and stress of intact coal samples in natural state over time. The four stages of initial compaction to unstable failure of intact coal samples in natural state go through 120s, 118s, 22s and 47s, 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 obvious.

Figure 10 shows the relationship between acoustic emission characteristics, dissipated energy and stress of intact coal samples in water-saturated state with time. The initial compaction stage and the elastic deformation stage of the intact coal sample with water saturation lasted for 145s and 86s respectively, and the stable and unstable failure stages lasted for 34s in total. The AE ringing count and energy of the intact coal sample with water saturation in the elastic deformation stage showed sporadic surge phenomenon, and relatively high ringing count and energy appeared at individual moments. In both stable and unstable failure stages, the AE ringing count and energy level are 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 7MPa, the AE ringing count and energy increase rapidly, and the AE signal characteristics are more intense.

Figure 11 shows the relationship between acoustic emission characteristics, dissipated energy and stress of coal sample with a single pre-existing hole in 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 natural state lasted about 138s, 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 obvious acoustic emission signal characteristics. The elastic deformation stage lasts for about 75s. 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 12s, during which the AE ringing count and energy were more intense, and the dissipative energy increased more rapidly. The unstable failure stage lasted for 45s, 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, and there was a short acoustic emission quiet period, during which the dissipative energy increased rapidly. In the post-peak failure stage, it takes 177s for the AE ringing count and energy level to fall from the peak strength to the residual strength, especially when the post-peak stress is reduced to 3MPa, the AE ringing count and energy increase rapidly.

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

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 natural state are mainly concentrated in the unstable failure stage and post-peak failure stage, and the energy release is relatively concentrated. The maximum damage energy is 10⁶. In the pre-peak stage, the intact coal samples with 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 natural state, and the highest failure energy was 10⁵, which was one energy level lower than that of intact coal samples with 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 severe at each stage, and there are relatively severe high-level energy events in each stage, especially in the unstable failure stage and the early stage of post-peak failure stage, the high-level ringing count and energy are relatively concentrated, and the energy release is also 10⁶, but the highest energy value is about 4 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 release during the failure process of coal sample, and will aggravate the failure intensity of coal sample. The AE ringing count at each stage of coal sample with a single pre-existing hole under Water-saturated Condition is more intense, but 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, the existence of 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 large energy release of coal samples.

(1) It is very important to understand the specific mechanical mechanism of coal seam water softening and large-diameter drilling pressure relief methods on coal rock failure. The effects of Water saturation and hole on mechanical parameters of coal samples are different. The strength of intact coal sample is degraded by 23.49% by Water saturation measure, and by 9.47% by single pre-existing hole measure. Compared with the mechanical parameters of intact coal sample in natural state, the coupling effect of Water saturation and single pre-existing hole measure has the most obvious deterioration on mechanical parameters of coal sample, which results in the strength deterioration of coal sample by 47.95%. In terms of the influence of different measures on the deterioration of mechanical parameters such as coal sample strength, the Water saturation measure plays a more basic role, and the cavity measure plays a supplementary role. The coupling effect of the two measures on the mechanical deterioration of coal sample is far greater than that of a single measure, which is more conducive to the weakening of coal strength.

(2) The final failure pattern of coal samples with different pressure relief measures is shear failure mode, and the surface cracks of coal sample with a single pre-existing hole under Water-saturated Condition are more developed, and finally the irregular failure pattern of cracks and hole connecting with each other is presented. The index of peak intensity ratio of stress value is introduced to quantitatively describe the influence of different measures on each stage of coal deformation and failure process. The elastic deformation stage of the intact coal sample is extended by 8.778% and the stable failure stage is shortened by 2.551%. The single pre-existing hole measure has little influence on the peak intensity ratio of stress value in each stage of coal sample in natural state. The coupling effect of saturated water and hole content can prolong the elastic deformation stage by 17.758%, and shorten the stable failure stage by 8.351%. The measures of Water saturation and hole have little effect on the ratio of peak strength of residual strength, and the main change of measures of Water saturation and hole is the time when peak strength falls to residual strength.

(3) The law of internal fissure development and expansion and energy storage and release is roughly the same in different types of coal samples, but the speed of energy storage and release at each stage is significantly different. The impact energy index of the complete coal sample decreased by 56.59% due to water saturation. The impact tendency index of coal samples in natural state is slightly increased by hole measure. The impact energy index of the complete coal sample is reduced by 46.78% by the coupling method of saturated water and hole. Water saturation can significantly reduce the impact tendency of coal samples, and hole measures can make the impact energy index of coal samples slightly increase.

(4) The high energy events of intact coal samples in natural state are mainly concentrated in the unstable failure stage and post-peak failure stage, and the energy release is relatively concentrated, and the highest failure energy is 10⁶. The high energy event of the intact coal sample with saturated state is mainly in the post-peak failure stage, and the highest failure energy is 10⁵. Each stage of the natural hole coal sample has a relatively severe high-level energy event, and the highest destruction energy is 10⁶, but the highest energy value is about 4 times that of the natural intact coal sample. The high level energy events of the coal sample with a single pre-existing hole under Water-saturated Condition are scattered, and the energy release is more uniform, and the highest energy level is lower than that of the intact coal sample in the natural state, but higher than that of the intact coal sample under Water-saturated Condition. Saturated water can increase the generation of cracks in the coal sample, reduce the energy release level when the coal sample is destroyed, and play a role in slowing down the energy release. The hole defect measures will affect the energy release and aggravate the failure strength of coal samples. The coupling effect of saturated water and hole measures will reduce the large energy release of coal sample failure.

Ethics Approval

The authors declare that this article does not involve human or animal experiments and does not require ethics approval.

Data Availability

The data presented in this study are available on request from the corresponding author.

Author Contributions Statement

Xingping Lai: ideas and methods, determined the title of this paper, completed the compilation of the summary, Supervision, Writing– review& editing

Xianwei Fang: Investigation, Data curation, Formal analysis, Writing –original draft.

Pengfei Shan: Provide guidance for the experiment, Funding acquisition, Project administration

Helong Gu: Participate in rock mechanics experiments

Shuai Zhan: Participate in rock mechanics experiments

Xudong Liu: investigation

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Funding

Financial support from National key basic research and development plan for science and technology(973 program): Basic Theory Research Project of Water Resources Protection in Coal mining in Northwest China（2015CB251600）, National Natural Science Foundation of China (52274138,51904227).

Consent to publish

All authors have read and agreed to the published version of the manuscript.

Acknowledgements

Acknowledgement for the coal samples support from National Energy Group Xinjiang Energy Co., Ltd. Wudong Coal Mine.

A special acknowledgement should be shown to the anonymous reviewers for their constructive and valuable comments. Thank them for their guidance in their busy schedule.

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No competing interests reported.

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Editorial decision: Revision requested
03 Jan, 2024
Reviews received at journal
02 Sep, 2023
Reviewers agreed at journal
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Analysis of mechanical response mechanism and energy evolution characteristics of saturated coal with a single pre-existing hole

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Version 1

Abstract

Figures

1. Introduction

2. Test equipment and scheme

2.1 Preparation and processing of test coal samples

2.2 Test device and scheme

3. Mechanical response characteristics of loaded coal samples

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

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

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

5. Discussion

5.1 Quantitative evaluation and analysis of the impact risk of saturated coal with a single pre-existing hole

5.2 Joint analysis of acoustic emission characteristics and energy dissipation of saturated coal with a single pre-existing hole

6. Conclusion

Declarations

Ethics Approval

Data Availability

Author Contributions Statement

Conflicts of Interest

Funding

Consent to publish

Acknowledgements

References

Additional Declarations

Status:

Version 1