1 Introduction

Coal resource mining develops into the deep as needed (Li et al. 2017), the coal-rock mass is usually under quasi-static and dynamic disturbance load in the production (Li et al. 2023). The dynamic disasters caused by high stress and mining-induced disturbance occur frequently with mining depth increasing, which poses a great hazard to safety production in coal mine (He et al. 2005; Pytlik et al. 2016). The failure characteristics of deep coal-rock combination are affected by the structure of surrounding rock (Zuo et al. 2017; Pan et al. 2020; Dou et al. 2014), except its own mechanical properties. The layered structure leads to the deterioration of mechanical properties of coal rock mass and the change of failure mode. The angle of bedding plane is the main factor (Tan et al. 2018; Li et al. 2021; He et al. 2020). The sidewall of the coal-rock roadway is a coal-rock combination, which is a common form of surrounding rock in roadway. Therefore, studying the mining-induced response of deep coal-rock combination is of great significance to promote the safe and efficient mining of coal mine.

Lots of scholars have made many useful explorations in the failure characteristics of coal-rock combinations the single static or dynamic loading test. A critical distance theory for predicting rock failure strength under dynamic loading was proposed (Pelekisa and Susmel 2017). Liu et al. (2004) proposed a coal-rock interaction model and found that the elastic modulus of the combination increased with the increase of the proportion of rock. Zuo et al. (2011c, 2021) found that the failure was mainly splitting in coal-rock combination under uniaxial condition, and that was mainly shear in triaxial test. Zhang et al. (2020) pointed out that the coal proportion was the main factor influencing mechanical responses of combinations. Zhang et al. (2012) found that the tensile failure was concentrated in the coal part of the combination under uniaxial loading. Liu et al. (2014) found that the coal part was compressed between the upper and lower rock to induce radial tensile stress. Zhao et al. (2008) studied the precursory information of deformation and failure of combination, and pointed out that coal-rock combination is more prone to instability.

The damage of coal rock mass under stress is accompanied by the accumulation and dissipation of energy (Tahmasebinia et al. 2018; Dok et al. 2021). It is more conducive to reflect the essential characteristics of the overall failure of rock by energy (Wen et al. 2019). Zuo et al. (2011a) analyzed the mechanical nature of layered failure in deep roadway surrounding rock under combined static and dynamic load. Zhu et al. (2007) revealed the instability process of roadway triggered by dynamic load by numerical method. Zuo et al. (2011b), Zuo and Song (2022) pointed out that the coal was the main body of the elastic energy accumulation in a combined sample. Xie et al. (2005) believed that the input energy of rock was equal to the sum of dissipation energy and release elastic strain energy. Feng et al. (2021) pointed out that the strain rate affects the change of energy and the fractal characteristics of fragments. Liu et al. (2021) proposed a model of wing-crack propagation for tensile under static and dynamic coupled loading. Zhang et al. (2006) studied the AE characteristics of granite failure under uniaxial multi-stage loading conditions, and pointed out that the number of AE events is directly related to crack propagation. Wang et al. (2019) concluded that there was a positive correlation between cumulative damage and impact times. Wang et al. (2020) studied the AE energy characteristics of during the rock fracture process, and the sharp increase of dissipation energy at peak stress led to the fundamental change of rock internal structure. He et al. (2015) pointed out the different stages of AE energy accumulation and the correlation between fracture mode and frequency. Dong and Zhao (2015) established models for rock energy dissipation and AE based on AE characteristics and stress–strain relationship. Li et al. (2010) and Ding et al. (2023) found the AE energy could reflect the rock failure information and load state. Wang et al. (2017) collected the AE energy characteristic parameters of rock damage process and found the relationship between AE energy and stress strain curve.

The existing research shows that the structure of coal-rock combination is more prone to dynamic disasters under high stress and disturbance conditions (excavation blasting, roof caving, fault sliding, etc.) (Dou et al. 2021; Cai et al. 2019; He et al. 2017; Yu et al. 2022; Ma et al. 2022). However, the existing research results are obtained under the test conditions of single static load or single impact load, which does not match the actual situation. The occurrence of deep rock disasters is closely related to the change of stress. High static stress and stress disturbance are not independent to each other. And the stress disturbance is multiple continuous, instead of a single short-term. As per the above, based on the self-developed experimental equipment that can realize creep and dynamic disturbance impact loading, aiming at the problem of easy instability of coal-rock roadway, this paper intends to study the mechanical behavior of coal-rock combination under the static stress and disturbance. It provides a reference for the prevention of dynamic disasters such as large deformation and rock burst in deep roadway.

2 Materials and methods

2.1 Testing equipment

The self-developed creep and dynamic disturbance impact loading test system (Wen et al. 2021, 2022b) in Fig. 1a was used to carry out the loading test of the sample under the coupled static and dynamic load, which is different from the way of drop-hammer impact and SHPB (Yang et al. 2023; Zhu et al. 2019). The system can realize custom waveform input methods such as sine wave, triangular wave and seismic wave. It can simulate the instability of coal-rock mass under the static load and dynamic disturbance such as blasting, roof break, fault slip and earthquake in underground engineering. The different combination of static load and dynamic load can be realized. Main parameters of system in Table 1.

Fig. 1
figure 1

Test system and its calibration, a testing system, b comparative test result

Full size image
Table 1 Main parameters of system
Full size table

A comparative test was carried out to verify the stability and reliability of the test system. The results obtained by the creep and dynamic disturbance impact loading test system and the Shimadzu AG-X250 test machine are shown in Fig. 1b. It indicates the test system can meet the experimental requirements.

2.2 Samples preparation

The coal, rock sample and coal-rock combination were used as the object in the research. The samples are from the coal-rock roadway on site. The parameters of samples are shown in Table 2. The single coal, single rock and combinations were made with a specification of Ф50 × 100 mm. The ratio of rock and coal in combination is 0:1 (C), 1:0(R), 1:1(RC-1), 2:1(RC-2), 3:1(RC-3), 1:1:1(RCR), as shown in Fig. 2. The non-parallelism and non-verticality of the ends of samples are less than 0.02 mm. The Marble glue is a mixed chemical binder and is widely circulated and applied to the bonding between various types of stone or used to fill the surface of stone and repair broken marks. It has the advantages of fast gel speed, short curing time and high bonding strength. The bond strength of glue reaches 103 MPa (Yu et al. 2022), which is sufficient to meet the strength requirements of the interface compared to the general rock, and the thickness of the adhesive is less than 1 mm when the specimen is bonded. There are no obvious defects in the samples.

Table 2 Mechanical parameters of the samples
Full size table
Fig. 2
figure 2

The samples in the test

Full size image

The quality, density and wave velocity of each sample were measured before the test to minimize the influence of heterogeneity on the test results. The samples with the same or similar parameters were selected for the test.

The 16-channel SH-II AE system is used to record the process of sample internal fracture initiation and energy release. The AE parameters are shown in Table 3. The AE sensor was directly attached to the surface of the sample using Vaseline to ensure good linkage. The digital image correlation (DIC) system is used to track the surface displacement and strain of the sample. Black and white points were applied to the surface of the sample to form a randomly distributed speckle pattern for DIC analysis (Fig. 3). The dual-camera resolution of the DIC system is 4096 × 3000 pixels, and the deformed digital image is recorded at a speed of 1 fps. Two high-intensity LED lights are used to provide diffuse, uniform and constant light.

Table 3 AE system setting parameters
Full size table
Fig. 3
figure 3

Speckle pattern and AE

Full size image

2.3 Testing scheme

  1. (1)

    The static load.

The uniaxial compressive strength (UCS) of the tests were carried out. The loading process was controlled by displacement until the sample destroyed. The whole test process was recorded in real time by servo control and data acquisition system.

  1. (2)

    The coupled static and dynamic load.

The UCS of the sample is used as reference for the design of the coupled static and dynamic load test scheme. The loading path includes static loading stage and dynamic loading stage. The static load simulates the static stress state of surrounding rock, and the dynamic load simulates the dynamic disturbance caused by excavation blasting, roof caving, fault sliding, He et al. (2014).

As shown in Fig. 4, firstly, the uniaxial loading test was carried out. The initial static stress of 1 MPa was applied to the sample. The static loading rate was 0.1 mm/min. In the second stage, the coupled static and dynamic load test was carried out (Liu et al. 2019), and the static stress level \(\sigma_{s}\), disturbance amplitude A, cycle number N, frequency \(f\) (\(T = {1/ f}\)) were used to characterize the cyclic sinusoidal waveform with wavelength \(\lambda\).

Fig. 4
figure 4

Loading path of the coupled static and dynamic load

Full size image

As in previous studies, in the second stage, different loading schemes were set by changing the static stress level and disturbance amplitude, as shown in Table 4. The disturbance is loaded until the sample is destroyed. If the disturbance loading is finished before failure, the loading is continued at a rate of 0.1 mm/min until the sample is destroyed.

Table 4 The design of the loading test scheme
Full size table

3 Result analysis

3.1 Mechanical properties and dynamic instability

The stress–strain curves of the samples under static load are shown in Fig. 5. It is found that the trend of stress–strain curves of all samples is relatively consistent, and the samples are less affected by heterogeneity, but the mechanical parameters are different in different groups of samples.

Fig. 5
figure 5

The stress–strain curve of samples under static load, a C, b R, c RC-1, d RC-2, e RC-3, f RCR

Full size image

The peak stress of C, R, RC-1, RC-2, RC-3 and RCR samples were in average 22.14 MPa, 35.80 MPa, 15.87 MPa, 15.39 MPa, 26.00 MPa and 15.09 MPa, respectively. The crack initiation stress (CIS) point refers the energy threshold when the combined samples begin to damage, accompanied by the generation of AE energy. After the CIS point appears, the stress–strain curve of the rock shows the nonlinear stage. The CIS of different samples is shown in Fig. 6. The CIS of the combination decreases with the rock proportion increases, less than the single rock sample. The degree of fracture development is higher than that of the rock part. The strain is larger and the accumulated energy is more in the coal part. The CIS of coal part is less than that of rock part. Therefore, the crack firstly expands in the coal part.

Fig. 6
figure 6

The CIS relationship of different samples

Full size image

Figure 7 shows the changes of peak stress, peak strain and elastic modulus of combined samples under static loading. The peak strain of combined sample is \(\varepsilon_{RCR} > \varepsilon_{{RC{ - 1}}} > \varepsilon_{{RC{ - 2}}} > \varepsilon_{{RC{ - 3}}}\). With the increase of rock proportion, the peak strain of the sample decreases gradually. The coal part continues to deform before overall failure result in peak strain of RCR is larger. The larger the proportion of rock, the greater the elastic modulus. The peak stress is similar to the trend of its elastic modulus. Liu et al. (2004) and Zhang et al. (2018) got a similar result.

Fig. 7
figure 7

Elastic modulus, peak stress and peak strain curves of samples

Full size image

The relationship between the elastic modulus, stress and strain of combination with the change of dynamic load level is shown in Fig. 8. The mechanical parameters of the combination under the coupled static and dynamic load were analyzed. The dynamic load level of 20%UCS has little effect on the mechanical parameters, which is basically close to the mechanical parameters under static loading. The elastic modulus decreases with the dynamic load. When the sample is damaged at the same strain level, the decrease of elastic modulus leads to the decrease of bearing capacity. The worse the integrity of the rock mass, the smaller the corresponding elastic modulus.

Fig. 8
figure 8

The parameters of combinations under different dynamic loading levels, a elastic modulus, b peak stress, c peak strain

Full size image

The stress of single rock sample and single coal sample increase first and then decrease with the increase of dynamic load level. The strain and stress of combinations are significantly affected by the dynamic load level and the rock proportion. The strain is positively correlated with the dynamic load level. The same is true for stress to rock proportion. That is obvious strain-rate effect (Yin et al. 2012; Liang et al. 2023), which may cause strain rock burst.

The result of combinations under different dynamic load levels was analyzed in Fig. 9. The dynamic load level has a significant influence on the strength of combination. The peak stress of the sample under dynamic disturbance is less than that under static load. It indicates that disturbance makes the combination more prone to damage and instability. When the dynamic load exceeds 60%UCS, the instability of the most samples appear with the unevenly changed hysteresis loop area. The hysteresis loop area can be used to characterize the amount of energy released that promotes the further development of microcracks in the sample. The sudden increase of the hysteresis loop area indicates that the energy release increases, and the damage of the sample increases rapidly. The hysteresis loop area in the instability stage of the combined sample increases with the increase of the number of cycles, and the strain increment is also greater than other dynamic disturbance stages. The rapid increase of strain and hysteresis loop area during the disturbance loading is the precursor for the instability of the combination.

Fig. 9
figure 9

Stress–strain curves of combination under different dynamic load levels, a C, b R, c RC-1, d RC-2, e RC-3, f RCR

Full size image

3.2 AE characteristics and rock damage evolution

AE and DIC technology were used to monitor the damage and crack development of samples. Figure 10 shows the AE response and damage evolution of the samples under static loading. The post-peak failure mode of the combined sample presents a stepped reduction in the stress–strain curve. It shows yield process instead of brittle failure. The coal part absorbs energy in the pre-peak stage and plays a buffering role. The maximum energy rate corresponds to the value of UCS. When the stress decreases by step, the AE energy rate increases sharply.

Fig. 10
figure 10

The result of AE and DIC under static loading, a relationship between AE response and stress–strain curve, b the strain distribution

Full size image

The AE energy events mainly occur in the post-peak stage, and only a relatively small amount of energy events are detected in the pre-peak stage (Fig. 10a). These are compaction stage I, elastic–plastic stage II, post-peak stage III and instability stage IV during the loading process. The AE energy rate is less than 1 × 105 in compaction stage I because the primary microcracks closed and energy dissipation. The cumulative energy increases significantly with the stress increases, the new cracks begin to develop and penetrate. The strain is obtained by DIC technology to characterize the development of cracks (Fig. 10b). The strain concentration begins to appear in the coal part of sample with AE energy mutation. With the increase of stress, the strain region becomes more concentrated and expanded. When the sample is destroyed, the coal part is doomed to failure firstly.

Figure 11 shows the failure and AE energy of the combination under the coupled static and dynamic load. The AE energy changed obviously into three stages, namely, energy growth stage, interval release stage and instantaneous release stage. It is different from the “four-stages” under static loading.

Fig. 11
figure 11

The relationship between stress–strain and AE energy under dynamic loading, a 20%UCS, b 40%UCS, c 60%UCS

Full size image

In the stage I*, no new cracks are developed inside the combination, the cumulative energy is at a low level. The release energy rate is less than 2 × 104, which is mostly frictional signal of primary crack closure. In the stage II*, the damage of the combinations under dynamic load level of 20%UCS is less, and the peak energy rate meets the peak stress. The dynamic load level of 40%UCS has a great influence on the damage of the combinations. The released energy increases significantly when the dynamic load is 60%UCS, and the combination is unstable during the third dynamic disturbance loading. When the peak stress occurs or the disturbance stage fails, the instantaneous release stage (stage III*) happens, then the sample breaks. The energy release is concentrated in the early stage of the disturbance loading process. It indicates the influence of dynamic disturbance to the combined sample is more significant in the early stage.

3.3 Crack propagation characteristics

The crack propagation and failure modes of combinations under static loading are showed in Fig. 12. The local high strain of the single coal sample is generated firstly at the end of the sample. During the loading process, axial splitting cracks are generated on both sides, and expand upward along the axial direction of the sample. Tensile cracks are developed at both ends of the sample, then extend to the middle, leading to splitting tensile failure (Fig. 12a). And one main crack gradually extends to both ends of the sample, and semi-X shear failure finally formed (Fig. 12b).

Fig. 12
figure 12

Failure mode and crack propagation of samples under static loading, a C, b R, c RC-1, d RC-2, e RC-3, f RCR

Full size image

The failure characteristic of combinations is different from that of single coal and rock samples. As shown in Fig. 12c–e, the length and number of cracks in the rock increase with the increase of rock proportion. The bearing capacity of the combinations is determined by the coal part with low strength. Stress concentration appears in the splice of the sample. The cracks are densely developed in vertical parallel. With the decrease of the bearing capacity of the coal part, the regional stress concentration in the rock part makes the rock occur tensile cracks. There are no penetrating cracks in the rock mass above and below the coal part in the RCR sample (Fig. 12f). Statistics on the failure modes of all samples found that of the combined sample are divided into three types: shear slip failure, splitting tensile failure and tension-shear combined failure.

Compared with shear failure in the single rock and single coal samples, tensile failure often occurs in the combined samples. The failure is mainly flake stripping. It corresponds to the falling of sidewalls in coal-rock roadway. The tensile failure can be reduced by bolt support on sidewalls to enhance the integrity of surrounding rock.

The failure mode changed from shear to splitting under the coupled dynamic-static loading and final impact damage occurs (Lyu et al. 2020). Figure 13 shows the crack characteristics and failure modes of combinations under the coupled static and dynamic load.

Fig. 13
figure 13

Crack characteristics of samples under the coupled static and dynamic load, a rock samples, b coal samples, c combinations

Full size image

The failure of single coal sample and single rock sample are tensile-shear failure mode under the dynamic load. When the dynamic load level is 20%UCS, the shear cracks are main with V-shaped type. When the dynamic load level is 40%UCS, the shear crack penetrates the sample with mainly tensile failure. The overall instability of the sample occurs during the loading process when the dynamic load level is 60%UCS. The spalling fragments number of coal is more than that of rock. And the average fragments size of coal is smaller than that of rock.

The failure characteristics of combinations are similar under the coupled static and dynamic load. However, the low strength coal part is more broken than the high strength rock part (Chen et al. 2021; Wen et al. 2022a). The crack initiation position is mostly in the coal part, and the crack is splitting tensile. With the increase of stress, the crack gradually develops the coal-rock interface. Based on Griffith theory, when the stress is greater than the strength of rock at the crack tip, the crack begins to develop inside the rock. Compared with the static loading, the failure mode of the combination is mostly tension-shear combined failure (Luo et al. 2020). The initial failure of the sample is dominated by splitting tensile, followed by shear crack in the later stage.

4 Discussion

  1. (1)

    Stress characteristics and AE energy.

Weaker structure affects the mechanical properties of coal-rock mass (Han et al. 2020). Different from the rapid post-peak failure of a single coal or rock mass, that of the combined sample presents a stepped reduction until the sample is failure. With the increase of dynamic load, the stepped reduction of stress process gradually disappears. The strength of the sample under dynamic load is greater than that under quasi-static load. The reason is that the coupled static and dynamic load makes the sample strain strengthened, and the strength of the sample is improved. Singh (1989), Miao et al. (2022) and Xu et al. (2009) have reached the same conclusion based on a single coal or rock mass. The CIS of rock part is greater than that of coal part. There are differences in mechanical properties of different parts of the combination leads to its different failure modes and failure time. Microscopically, it shows a sudden increase in AE energy rate. Macroscopically, the sample appears splitting and shedding phenomena.

In Figs. 10 and 11, the stage of stress–strain curve corresponds to the energy dissipation process. The division of stress stages under static loading and the coupled static and dynamic load is different. The appearance of intermittent disturbance load makes the AE energy of the sample change intermittently. The interval release stage appears and the energy gradually accumulates.

  1. (2)

    Failure and crack evolution.

The failure mode of the combined sample under the coupled static and dynamic load is important for exploring the internal crack development and the failure mechanism (Hao et al. 2020). The distribution of rock fragments under different loading modes and failure modes is different. Therefore, the fragmentation index can be used to explain the failure mode (Yang et al. 2021). The size of fragments is closely related to the proportion of coal-rock, static load, dynamic load and internal original cracks, et al.

The fragments distribution of the sample is shown in Fig. 14. Under the coupled static and dynamic load, the scale span of the broken block of the combined sample is large, showing long strip block, irregular polygon block and powder debris, and accompanied by local burst, fragments ejection and other dynamic phenomena.

Fig. 14
figure 14

Sample failure fragments size screening classification

Full size image

The cumulative percentage and size of fragments under different dynamic load levels are shown in Fig. 15. The percentage of fragments with size greater than 10 mm is greater than 60%, and the fragments are mostly long strips. The minimum percentage of fragments with size less than 10 mm is only 8.4%, and the fragments are polygonal blocks and powder. The long strip-shaped fragments show that the stress is transmitted along the axial in the sample, and the failure is mainly tensile failure. The irregular polygonal block fragments and semi-powder fragments indicate that the stress is transferred along the non-axial direction in the sample. During the failure process, the fragments against each other results in the generation of powder debris, and the tensile-shear failure is presented. Therefore, the larger the cumulative percentage of fragments greater than 10 mm, the greater the possibility of tensile failure; on the contrary, it tends to tensile-shear failure.

Fig. 15
figure 15

The cumulation of fragments size of each combination under different dynamic loading levels, a RC-1, b RC-2, c RC-3

Full size image

5 Conclusions

The structure of coal rock mass affects the mechanical response of surrounding rock. The peak stress of coal-rock combination is determined by low-strength coal. The post-peak stage of the stress–strain curve of the combined sample under the coupled static and dynamic load presents a stepped reduction. The elastic modulus decreases under the coupled static and dynamic load. The peak stress of the combined sample is positively correlated with the dynamic load level within a certain range, and the peak strain is negatively correlated. When the stress by step decreases, the AE energy rate increases sharply. AE energy rate is positively correlated with peak stress.

Dynamic load aggravates the damage and instability of coal-rock combination. The AE energy under static load can be divided into four stages: slow growth stage, rapid growth stage, dissipation (relatively stable) stage and instantaneous release stage. The appearance of intermittent disturbance load makes “four-stages” develop into three stages: energy growth stage, interval release stage and instantaneous release stage. The sudden increase of AE energy rate, hysteresis loop area and strain can be the precursor information of instability of coal-rock combination under the coupled static and dynamic load.

The failure mode of coal-rock combination is affected by coal-rock ratio and dynamic load. The failure modes of samples under static load include shear failure, splitting tensile failure and tension-shear combined failure. The failure mode of the under dynamic load is mainly tensile-shear failure. The application of dynamic load leads to instantaneous energy release and stress concentration. The fragmentation is different under different failure modes. The fragmentation index can explain the failure mode and crack propagation characteristics of coal-rock combination under the coupled static and dynamic load.