Impact Tendency Characteristics of Borehole Coal Samples under Real-Time and Uniaxial Loading Conditions: Insights from Physical Experiments

Abstract

Real-time drilling depressurization technology is widely used in the prevention and control of dynamic disasters, such as deep-seated rock burst. However, current coal- and rock-loading tests under drilling conditions seldom account for real-time issues associated with drilling, thus failing to fully reflect the actual stress state of the surrounding rock during the implementation of drilling depressurization technology. Therefore, this study designed and implemented a uniaxial loading scheme for coal samples incorporating real-time-drilling characteristics. The results indicate a significant reduction in the uniaxial compressive strength ($R_{\mathrm{C}}$), elastic energy index ($W_{\mathrm{ET}}$), and impact energy index ($K_{\mathrm{E}}$) of the samples post-drilling. These parameters show a clear decreasing trend with increasing axial stress during real-time drilling. The weakening effect of impact tendency following real-time drilling depressurization is significant, and the depressurization effect is pronounced. The $R_{\mathrm{C}}$, $W_{\mathrm{ET}}$, and $K_{\mathrm{E}}$ of each real-time-drilled sample exhibit a notable decrease with increasing drilling stress, with the reduction rate significantly diminishing after the drilling stress reaches 20% of the peak strength.

1. Introduction

Currently, global energy demand is rising, while the extraction of conventional energy sources is gradually depleting. The exploration and development of unconventional energy sources, such as shale gas and deep coal, have become a primary approach to addressing the energy crisis [1,2]. During deep coal mining, the frequency and severity of dynamic disasters such as rock bursts are significantly higher due to the sudden release of accumulated stress [3,4,5]. The fundamental objective in addressing these deep-seated dynamic hazards is to mitigate the stress behavior within the surrounding rock or to eliminate the accumulated stress [6,7,8]. At present, the prevention and control of mine-rock bursts often adopts drilling measures to induce the failure of surrounding rock, thereby weakening its capacity to store deformation energy and dissipating the accumulated stress [9,10,11]. However, inappropriate drilling-design parameters may limit the effectiveness of the pressure-relief technique and even severely compromise the stability of the surrounding rock [12]. Therefore, understanding the mechanical properties of the drilled coal–rock mass and the transformation of stress characteristics before and after drilling is crucial for assessing the effectiveness of the pressure-relief technique and the stability of the surrounding rock.

Currently, extensive research has been conducted on the mechanical properties and failure behaviors of coal rock samples containing voids. Uniaxial compression tests are commonly employed as the primary method for studying rock mechanics and are widely used to investigate the decompression effects of various drilled specimens [13,14,15,16]. Li et al. [17] conducted a detailed analysis using a two-dimensional finite element model to examine the effects of key parameters such as drill diameter on model failure progression and stress field evolution, identifying the predominant failure types associated with drilling. Zhang et al. [18] performed uniaxial loading tests on coal samples with various drilled configurations to analyze the evolution of energy dissipation indicators in decompressed samples. Yin et al. [19] carried out extensive uniaxial loading tests on drilled coal samples, revealing that increases in drill size and quantity significantly reduced the elastic strain energy and energy release rate of the samples. Given that surrounding rock in geological pressures is typically in a confined-stress state, researchers have increasingly employed uniaxial lateral confinement and triaxial loading methods to study drilled specimens, making the experiments more reflective of actual engineering conditions. Chen et al. [20] conducted confined-pressure tests on specimens with varying hole diameters, depths, and spacings, demonstrating that increases in diameter and depth, as well as reduced spacing, significantly enhance dissipated-strain energy, achieving better decompression effects. Lu et al. [21] investigated the mechanical and failure behaviors of sandstone samples with drilled holes under true triaxial stress, finding that intermediate principal stress significantly influences the damage distribution around the drill holes. Ren et al. [22] performed true triaxial unloading tests on rock samples with different drilled configurations, analyzing the impact of drilling and bolting on micro-crack development and identifying the impact decompression mechanisms. Additionally, researchers have delved into specific geological or drilling conditions [23,24]. For instance, Fan et al. [25] used discrete element methods to study the mechanical behavior of rock samples, with voids as a function of joint parameters, revealing the damage mechanisms under the combined effects of drilling and joint interactions. However, a review of existing studies indicates a scarcity of research on the real-time characteristics of drilling. Most studies involve prefabricated drilled samples, which do not accurately represent the stress conditions of surrounding rock during actual drilling decompression operations.

In this research, the compression experiment of real-time borehole coal samples was designed and carried out. Firstly, the impact tendency transformation of real-time drilling samples and complete samples was compared and analyzed. Then, the evolution of the impact tendency index of the sample under the influence of real-time borehole stress changes was studied in detail, and the optimal borehole stress was determined. Finally, the stress state of the deep surrounding rock and the optimization direction of a drilling-pressure relief indoor experiment were discussed in depth.

2. Experimental Scheme

2.1. Sample Preparation

The coal samples required for the experiment were sourced from the 21 # coal seam at the Hulusu Mine in the Dongsheng Coalfield, which exhibits significant rock burst issues. The 21 # seam averages 2.6 m in thickness, with a basic roof consisting of thick sandstone layers ranging from 13.8 to 23.3 m. Coal blocks from the 21 # seam were collected and processed into 50 mm × 50 mm × 100 mm cuboidal samples, as shown in Figure 1. Due to the coal’ s pronounced heterogeneity, anisotropy, and discontinuity, samples with a smooth surface and no defects or voids were selected based on the macroscopic integrity of the processed samples. The size parameters of each test sample are shown in

Table 1. Coal sample size parameters.

Sample Number	Length (mm)	Width (mm)	Height (mm)
i = null–1	49.97	49.75	101.41
i = null–2	50.81	50.38	100.12
i = 0.1–1	51.99	51.30	100.76
i = 0.1–2	51.12	50.28	100.19
i = 0.15–1	51,35	50.82	99.75
i = 0.15–2	51.51	49.06	99.75
i = 0.2–1	50.82	50.80	99.07
i = 0.2–2	50.08	51.37	100.21
i = 0.25–1	50.96	49.81	99.51
i = 0.25–2	51.59	49.86	100.36
i = 0.3–1	51.21	51.11	99.89
i = 0.3–2	50.94	49.28	100.24

.

2.2. Experimental Procedure

The impact tendency test of the coal samples was conducted using the RMT-150 rock mechanics testing system, as shown in Figure 2. This equipment, developed by the Wuhan Institute of Rock and Soil Mechanics under the Chinese Academy of Sciences, is a digitally controlled electro-hydraulic servo testing machine. It features an axial maximum load capacity of 1000 kN, a horizontal maximum load of 500 kN, a load accuracy of 1.0 × 10⁻³ kN, a maximum confining pressure of 50 MPa, an axial maximum stroke of 50 mm, a lateral displacement of 2.5 mm, and a deformation accuracy of 1.0 × 10⁻³ mm.

To address the requirements of real-time-drilling and pressure-relief research, the stress on the sample is maintained at a predetermined drilling and pressure-relief stress level. A handheld drill is used to create a hole with a diameter of 10 mm and a depth of 50 mm at the center of the sample’s surface. After completing the drilling and pressure-relief operation, loading is continued, as shown in Figure 3. In this study, five drilling and pressure-relief stress levels were set based on the average peak strength of intact coal samples in uniaxial strength tests: ${\sigma }_r$ = $i{\sigma }_c$ = 0.1, 0.15, 0.2, 0.25, 0.3.

2.3. Calculation Method and Standard of Impact Tendency Index

The Chinese national standard GB/T 25217.2-2010 [26] specifies four key indicators for the assessment of coal’s impact tendency [27]. The main impact tendency indices for coal samples include dynamic failure time ($DT$), uniaxial compressive strength ($R_{\mathrm{C}}$), elastic energy index ($W_{\mathrm{ET}}$), and impact energy index ($K_{\mathrm{E}}$). According to

Table 2. Classification of impact tendency of coal.

Index				Burst Tendency	Category
Dynamic Failure Time/ms	Elastic Energy Index	Impact Energy Index	Uniaxial Compressive Strength/MPa
DT > 500	WET < 2	KE < 1.5	RC < 7	No	I
50 < DT ≤ 500	2 ≤ WET < 5	1.5 ≤ KE < 5	7 ≤ RC < 14	Feebleness	II
DT ≤ 50	WET ≥ 5	KE ≥ 5	RC ≥ 14	Strong	III

, the impact tendency of coal is classified into strong impact tendency, weak impact tendency, and no impact tendency based on the magnitude of these indices.

The $DT$ of coal is determined from the stress–time curve, where the failure time is defined as the time difference between the peak strength and the residual strength. The average $DT$ for each set of specimens (rounded to the nearest integer) is calculated using Equation (1) [28]:

$$D{T}_S={\displaystyle \frac{1}{n}}{\displaystyle \sum _{i=1}^{n}D{T}_i}$$

(1)

where $D{T}_S$ represents the average $DT$, ms. $D{T}_i$ denotes the $DT$ of the i-th specimen, ms. And n is the number of specimens in each set.

The unloading value of the sample can be estimated based on the uniaxial compressive strength, specifically at the point where the curvature of the stress–strain curve decreases or the torsion declines. By unloading the force to zero at the same rate, and then integrating the resulting load–displacement curve, as illustrated in Figure 4, the total strain energy and elastic strain energy can be computed. The $W_{\mathrm{ET}}$ is defined as the ratio of elastic strain energy to plastic strain energy.

After the peak point, if the stress–strain curve to the left of the vertical line at the peak stress point is classified as Type I curve, there is no need to calculate the impact energy index, indicating a strong impact tendency. Conversely, if the curve is classified as Type II and is on the right side, the impact energy index needs to be calculated. The impact energy index is computed using Equation (2) [19]:

$$K_{\mathrm{E}}={\displaystyle \frac{A_{\mathrm{S}}}{A_{\mathrm{X}}}}$$

(2)

where $A_{\mathrm{S}}$ is the accumulated deformation energy before the peak. $A_{\mathrm{X}}$ is the dissipated deformation energy after the peak, and $K_{\mathrm{E}}$ is the impact energy index.

3. Results and Analysis

3.1. Uniaxial Compressive Strength

Figure 5 displays the uniaxial loading stress–strain curves of the intact samples and those subjected to real-time drilling and unloading under peak strengths of 10%, 15%, 20%, 25%, and 30%. It is evident that compared to the intact samples, the strength and deformation capacity of the real-time-drilling and -unloading samples are significantly reduced. The deformation capacity of the real-time-drilling and -unloading samples exhibits an initially decreasing and then increasing trend with the augment in drilling stress, with the sample at i = 0.2 showing the lowest deformation capacity.

Figure 6 shows the peak strengths of real-time drilling and unloading under various axial stress conditions during uniaxial loading. The peak strength of the coal samples is significantly reduced due to real-time drilling and unloading, decreasing by 42.16%, 46.69%, 51.01%, 46.71%, and 52.42% compared to the intact coal sample’ s 48.36 MPa. Additionally, there is a slight decline in strength with unloading stress, and after samples with i = 0.2, the strength of the samples shows no significant change.

3.2. Dynamic Failure Duration

Figure 7 presents the load–time curves for intact samples and those subjected to real-time drilling under axial stresses corresponding to peak strengths of 10%, 15%, 20%, 25%, and 30%. The loading force and failure time of the borehole specimen are significantly lower than those of the intact specimen. The load–time curves exhibit a consistent trend across the samples, with a sharp decline observed in the curves following sample failure.

Figure 8 illustrates the dynamic failure times of samples subjected to real-time drilling during uniaxial loading. This time represents the duration from the maximum load until the sample loses its load-bearing capacity completely. For samples with i = null, 0.1, 0.15, 0.2, 0.25, and 0.3, the failure times are 258.5 ms, 370 ms, 254 ms, 470 ms, 397 ms, and 269 ms, respectively. It is evident that the failure times for drilled and unloaded samples show a noticeable increase compared to the intact samples, with the sample at i = 0.2 reaching the maximum. However, the failure times for different unloading stress samples are within the range of 50 to 500 ms, indicating a weak impact tendency. The study by Wang et al. also indicates that drilled samples can exacerbate brittle failure to some extent, leading to more rapid and severe damage [29]. This suggests that drilling and unloading, along with variations in unloading stress, have a minor effect on the $DT$ of the samples.

3.3. Elastic Energy Index

Figure 9 shows the loading–unloading curves for intact samples and samples subjected to real-time drilling under axial stresses, corresponding to peak strengths of 10%, 15%, 20%, 25%, and 30%. It is evident that the loading–unloading curves for the drilled and unloaded samples differ significantly from those of the intact samples, and the axial stress and strain of the drilled specimen are significantly lower than those of the intact specimen. However, there are no noticeable differences between the curves of samples subjected to varying drilling and unloading stresses.

Figure 10 illustrates the elastic energy index for real-time borehole pressure-relief samples under uniaxial loading with different axial stress states. Integration calculations reveal that the elastic energy indices for samples with i = null, 0.1, 0.15, 0.2, 0.25, and 0.3 are 8.950, 3.805, 3.662, 4.513, 2.351, and 1.911, respectively. Compared to the intact coal samples, these pressure-relief samples exhibit reductions of 57.49%, 59.08%, 49.57%, 73.73%, and 78.65%, indicating a significant improvement in borehole pressure-relief effectiveness. Following borehole pressure relief, the elastic energy index shifts from a tendency for strong impact to a tendency for weak impact. Furthermore, as the real-time borehole pressure-relief stress increases, the elastic energy index shows a decreasing trend, with higher stress conditions yielding better relief effects compared to lower stress conditions.

3.4. Impact Energy Index

Figure 11 shows the load–displacement curves for real-time borehole pressure-relief samples under axial stress states with peak intensity of 10%, 15%, 20%, 25%, and 30%, compared to the intact samples. For samples with i = null, 0.1, 0.15, 0.2, 0.25, and 0.3, the values are 18.026, 17.732, 9.952, 6.891, 5.379, and 5.854, respectively. These relief samples show reductions of 57.49%, 59.08%, 49.57%, 73.73%, and 78.65% compared to the intact coal samples, indicating a significant pressure-relief effect. However, the impact energy index for all borehole samples remains above 5, indicating a tendency for strong impact.

Figure 12 shows the impact energy index of real-time borehole pressure relief under uniaxial loading with different axial stress states. For the sample with i = 0.1, the reduction in impact energy index compared to the intact sample is not significant. However, as the borehole pressure-relief stress increases, there is a noticeable decline in the impact energy index. For samples with i ≥ 0.2, the variation in impact energy index becomes less pronounced, and it tends to stabilize.

4. Conclusions

Previous research indicates that borehole pressure relief technology is an effective method for preventing and mitigating dynamic hazards, such as impact ground pressure and deep-seated stress accumulation. However, practical engineering applications of borehole pressure relief face two key issues: the effectiveness of pressure relief and the stability of the surrounding rock after drilling. This highlights the significance of the current study. The following conclusions were reached:

(1) After borehole drilling, the $R_{\mathrm{C}}$, $W_{\mathrm{ET}}$, and $K_{\mathrm{E}}$ of the samples all exhibited significant reductions. The $R_{\mathrm{C}}$, $W_{\mathrm{ET}}$, and $K_{\mathrm{E}}$ exhibited a clear decreasing trend with increasing axial stress during real-time drilling. The $DT$ exhibited considerable fluctuations with no discernible pattern. As the axial stress state increased, the effect of reduced impact tendency following real-time borehole pressure relief became more pronounced, and the pressure-relief effect was significantly enhanced.

(2) Drilling and relieving pressure under high stress is more effective than under low stress. Compared to samples without drilling and pressure relief, which exhibit a strong impact tendency, samples with drilling and pressure relief at peak strengths of 10%, 15%, and 20% still show strong impact tendencies. However, samples with drilling and pressure relief at peak strengths of 25% and 30% exhibit weak impact tendencies. Specifically, the impact energy index of samples relieved at 25% of the peak strength (${\sigma }_c$) decreased by 70.16% compared to intact samples, while the impact energy index at 10% of the peak strength decreased by only 1.63%.

(3) Drilling and relieving pressure reduces the occurrence of rock burst disasters primarily by lowering the energy index. At peak strengths of 25% and 30%, the samples subjected to drilling and pressure relief show a change in impact tendency compared to samples without pressure relief. Specifically, at a peak strength of 25%, the dynamic failure time with drilling and pressure relief only increased by 4% compared to intact samples. For all stress states, coal samples with drilling and pressure relief exhibited a change from strong to weak impact tendencies, with an average reduction of 63.70% in the elastic energy index. The average reduction in the impact energy index was 49.06%, and at peak strengths of 25% and 30%, the impact energy index of samples approached a weak impact tendency, with an average reduction of 68.56%. The average reduction in uniaxial compressive strength was 47.80%, but the samples still generally exhibited a strong impact tendency.

However, the loading conditions in this study involve uniaxial loading, which does not fully represent the actual conditions where the coal body is subjected to confining pressure with a single face exposed. Thus, it does not completely reflect the mechanical characteristics of real-time borehole pressure relief. Therefore, it is necessary to conduct real-time borehole-loading tests under lateral confinement, which will be the focus of our future work.