Abstract

In the underground environment with large buried depth and high ground stress, man-made disturbance is very easy to cause the rapid expansion of primary fractures in the rock, and then developed into the macrodynamic performance of rock. Based on the propagation law of elastic waves in discontinuous media, the application of acoustic emission detection technology can quickly determine whether there are primary fractures in the rock and predict its approximate location. In this work, CT scanning experiments of intact sandstone specimens and fractured sandstone specimens were performed. The gray value distribution of intact sandstone specimens and fractured sandstone specimens was studied. The sandstone specimens were divided into 4 zones (I~IV) from top to bottom. The height of each zone is from 0 mm to 25 mm, and the upper end face of each zone is the starting face. Acoustic emission experiments of intact sandstone and fractured sandstone are carried out based on the equilateral triangle sensor array. The dispersion of AE wave velocity and amplitude in intact sandstone specimens and fractured sandstone specimens is studied. The results show that the crack evolution law of sandstone specimens before and after preloading is closely related to the density distribution. The regular trend is from low density to high density. And the decay law of AE eigenvalue before and after preloading of sandstone specimen is consistent with the change trend of gray value. This shows that it is feasible to explore the spatial location of primary fractures and the degree of development of primary fractures in the rock through the equilateral triangle sensor array. In the actual project, it can provide some guidance and suggestions for related projects.

1. Introduction

With the development of crustal movement, the rock is repeatedly crushed and bonded, and it mainly exists in a discontinuous state [1]. Therefore, there are numerous preexisting fractures in the rock mass. The underground excavation project is gradually entering into large buried depth and high in-situ stress. This makes the original rock stress field in the deep surrounding very sensitive. When there is artificial excavation and unloading, the redistribution of surrounding rock stress can easily lead to the rapid expansion of preexisting fracture and even dynamic disasters such as rock burst, piece, and collapse. And it is also easy to form fault rocks and increase water erosion [2, 3]. There are great potential risks for on-site personnel. Therefore, it is necessary to quickly determine the location and distribution of existing and derived cracks in rock mass. It can be applied to the safety assessment of deep underground engineering.

When rock is in cracking or fracture, expanding generates a large amount of sound waves, also called variable elastic waves. It can be detected by Acoustic Emission (AE) Instrument. Many scholars have studied the fracture evolution process of rock through the parameters of AE count [46], duration [710], energy [1116], amplitude [1720], rise time [2124], and average frequency [2528]. Wang et al. [29] used different sensor arrays to analyze the feasibility of acoustic emission source localization for slab rock and optimized the results of the planar position. Long-Jun and Xi-Bing [30] optimized the AE sensor array coordinates and established the AE/MS source localization equation through a multidimensional monitoring network of sensor locations. Moreover, many scholars have done a lot of research on rocks through CT scanning technology and nuclear magnetic resonance (NMR) technology. Sun et al. [31] carried out CT scanning uniaxial compression experiments on SRM samples with a rock mass ratio of 40%. An overall decrease in the average density of the samples was found during compression fracture. At the same time, the heterogeneity of the samples is greatly increased. Wang et al. [32] performed axial loading CT scanning experiments on SRM specimens with a rock mass ratio of 30% and found that the interaction between the soil and the rock mass has a greater effect on the crack geometry and probability density distribution impact. Li et al. [33] quantitatively studied the pore structure and hydraulic properties of broken rock mass by nuclear magnetic resonance (NMR) and put forward a Logistic regression model. At present, AE detection technology and CT scanning technology are the main means to study the evolution of internal fractures in rock. When using AE detection technology, many works focused on exploring the change law of fractures through the AE characteristic parameters by dynamic crack propagation. Few researchers show interest on the preexisting fracture of rocks in the static state. In many field tests of deep underground engineering projects, the preexisting fracture in the rock mass is one of the main factors causing the damage to the surrounding rock. Therefore, predicting the location and distribution of the preexisting fractures in the rock previously, which is important reference value for preventing the dynamic disasters such as rock burst, piece, and collapse. At present, CT scanning technology has high accuracy, but CT scanning technology is expensive and is mainly used in laboratory experiments. Therefore, it is possible to try to study the attenuation law of AE signals that caused preexisting fractures in rock and on how to find the location and distribution of cracks.

In this work, the AE sensor is arranged in a spatial equilateral triangle array to carry out a stratified analysis method to analyze the AE characteristic parameters of sandstone samples, which is similar to the scanning of CT layer by layer. By comparing and analyzing the CT scanning results and AE characteristic parameters of sandstone with fissures, its feasibility is verified.

2. Experiment Apparatus

AE detection technology and CT scanning technology are both methods of nondestructive testing. AE signal is an elastic wave with penetrating properties like X-ray. When it propagates in the rock, it is mainly divided into longitudinal wave, transverse wave, surface wave, and plate wave according to the vibration direction and propagation direction of the particles. Because the elastic wave is a kind of transient wave, the application of acoustic emission is mainly to detect the dynamic development of cracks. By using the attenuation properties of AE signals. It is possible to explore the location and distribution of the preexisting fractures in the rock.

In this work, the uniaxial compression experiment of rock is carried out by SAM-2000 electro-hydraulic servo rock triaxial loading system. PXDAQ24260B AE Instrument is used for acoustic emission detection. The CT test was carried out with GE phoenix v|tome|x s240 for comparative analysis and verification, as shown in Figure 1.

3. Analysis of Results

3.1. Experimental Schedule

In this work, tight sandstone samples () are used to reduce the adverse effect of preexisting fractures on the initial reference value as much as possible. And in order to ensure the comparability of the test results, the same sandstone specimen is used in AE detection and CT scanning.

First, CT scanning experiments were performed on the intact sandstone samples to explore whether there were preexisting fractures in them.

Secondly the sandstone samples were divided into 4 zones (I~IV) from top to bottom. The height of each zone is 25 mm, and the upper face of each zone is the initial face, as shown in Figure 2. The sensors in each area are arranged with positive triangular array, and AE experiments are carried out (Figure 3). This experiment adopts the two-dimensional polar coordinate system and takes the center of the circular section as the origin. Set 4 lead the break points (the point where the pencil lead breaks and emits sound waves) P1 to P4 correspond to I, II, III, and IV Zones. For example, the coordinates of the lead break point of I, sensor 1, sensor 2 and sensor 3 are P1 (25, 0°), (250, 60°), (25, 180°), and (25, 300°), respectively, use the same arrangement. The lead break point coordinates and sensor coordinates of I, II, III, and IV are the same. Both the lead break point and the sensor are located axially on the center plane of each zone. The first step is to break the lead ten times at the P1, P2, P3, and P4 lead break points, and then collect the average value of the wave speed and the average value of the amplitude of the acoustic emission signal.

Then uniaxial compression experiments of intact sandstone specimens were carried out by SAM-2000 electro-hydraulic servo rock triaxial loading system. Pressurization was stopped after the intact sandstone specimen reached the peak strength. In this experiment, the pressure method of displacement control is adopted, and its speed is 0.05 mm/min.

Finally, the previous acoustic emission experiment was repeated for the fractured sandstone specimen and the CT scanning experiment was performed. The experimental models are shown in Figures 2 and 3.

3.2. Physical and Mechanical Properties of Sandstone Materials

Some basic physical and mechanical properties of sandstone materials are obtained through uniaxial compression experiments. The density of sandstone is 2.75 g/cm3, the peak strength is 64.1766 MPa, and Young’s modulus is 18.1931 GPa. The main mineral composition of sandstone includes quartz, feldspar, and mica [34]. The specific experimental results are shown in Figure 4.

3.3. Analysis of Experimental Results of Intact Sandstone

The scanning step spacing of CT experiment was 0.06 mm. More than 3200 CT images of intact sandstone and fractured sandstone were obtained. Due to the limited computing resources, at the beginning, select 1 image of each zone. The calculation method of the CT scan is shown below.

In a homogeneous material, the incident X-ray is assumed to be a beam of monoenergetic photons with a sufficiently small cross-section. Based on its attenuation properties, the relationship between the incident and outgoing X-ray intensities can be obtained as [35] where is the incident X-ray intensity; is the intensity of the outgoing X-rays; is the mass thickness; is the mass attenuation coefficient.

For nonuniform materials, the mass attenuation coefficient can be used for calculation, and the specific expression is where represents the mass attenuation coefficient of the first component; represents the weight percentage of the first component; represents the number of cascaded units.

The first scan results are shown in Figure 5.

The grayscale images of different sections can be obtained through the complete sandstone scanning experiment. According to the principle, the grayscale value is proportional to the object density, and the average grayscale value of CT cross-section image is statistically analyzed. Here, the grayscale image is a 16-bit image. According to Figure 5, it can be seen that almost no cracks are generated inside the intact sandstone specimen. Its average gray value is between 25800 and 27600. This shows that the density of the intact sandstone specimen is high, and it has a dense structure. And Figure 5 also shows that the internal density distribution of the intact sandstone specimen is not uniform. The density of I is the smallest, the density of II to III is increasing continuously, and the density of IV first increases and then decreases. Therefore, when the intact sandstone specimen is tested under uniaxial compression, cracks may appear first in the intact sandstone specimen I and IV. With the continuous increase of axial pressure, the cracks gradually develop to II and III and form through cracks.

AE detection was carried out on intact sandstone specimens. The AE eigenvalues extracted in this experiment are the mean wave velocity and mean amplitude, respectively. Both are calculated as follows:

Based on the principle of the time difference method, the calculation formula of the wave speed is where and are the signal propagation distance from the acoustic emission source to the acoustic emission sensor when the pencil lead is broken; and are the arrival time of the signal from the acoustic emission source to the acoustic emission sensor when the pencil lead is broken; is the difference of signal propagation distance between the simulated acoustic emission source and different acoustic emission sensors when the pencil lead is broken; is the signal propagation time difference from the simulated acoustic emission source to different acoustic emission sensors when the pencil lead breaks; is the propagation speed of the acoustic emission signal; is the average speed of acoustic emission signal propagation.

The amplitude of the acoustic emission signal is usually expressed in . The sensor output is defined as 0 dB. The specific formula is as follows: where is the amplitude value. The parameter definitions are shown in Figure 6.

It can be seen from Table 1 and Figure 7 that the discrete coefficient of wave velocity is 0.0482. The discrete coefficient of the amplitude is 0.0016. Both of them show that the integrity of the intact sandstone specimen is better. There are no primary cracks in it that greatly attenuate the acoustic emission signal. The average amplitude of III is the highest, so its compactness is the best. This is consistent with the CT scan results.

3.4. Analysis of Experimental Results of Fractured Sandstone

In this paper, sandstone is used as the experimental object. Due to the high strength of sandstone, the plastic strain of sandstone is very small and difficult to control. Therefore, in order to generate cracks inside the intact sandstone specimen, in this experiment, uniaxial compression experiments of intact sandstone specimens were carried out by using SAM-2000 electro-hydraulic servo rock triaxial loading system. When the intact sandstone specimen reached the peak intensity, the acoustic emission experiment of the previous step was repeated, and the CT scanning experiment was performed again. The specific results are shown in Table 2 and Figure 8.

From the above analysis, it can be seen that the dispersion coefficients of the wave velocity and amplitude of the sandstone have changed greatly after reaching the peak intensity. The dispersion coefficient of the wave velocity is increased by a factor of nearly 6. The discrete coefficient of magnitude is increased by a factor of 66. This shows that the homogeneity of the sandstone specimens has been greatly reduced. It can be seen from Figures 9(a) and 9(b) that the wave speed and amplitude of the acoustic emission signal are severely attenuated at the same time. This shows that the acoustic emission signal produces more reflection and refraction in the process of propagation. This phenomenon mainly occurs when the acoustic emission signal propagates between different media. Therefore, the interior of the sandstone specimen may have been completely penetrated and large cracks have been generated. And from Figure 9(c), it can be seen that the attenuation degree of AE signal from I to III increases rapidly, and III reaches the maximum value of attenuation. After that, the degree of attenuation began to decrease, and IV reached the minimum value. This shows that the upper end of the intact sandstone specimen may first reach the peak strength and form cracks in the process of uniaxial compression experiment. Then the fissure gradually develops to the lower end to form typical shear failure. The development degree of fissures from I to III deepened gradually. The fissure development of III is the most serious. On the other hand, the degree of fracture development of IV gradually slowed down and did not form through cracks.

In order to verify the above conjecture, the distribution of internal fracture in fractured sandstone specimens was detected by CT scanning experiment. The results are shown in Figures 1012.

By analyzing the results of CT scanning, we can now know that, after the uniaxial compression experiment, the average gray value of sandstone specimens is about the same as that before compression. That means in the process of sandstone compression, the evolution law of the internal fractures mainly extends from the low-density zone to the high-density zone. And the law of fracture development is consistent with the law of internal density distribution of sandstone. The CT scanning results of intact sandstone specimens show that the low density zone is mainly distributed in the I and latter part of IV. The II, III, and anterior parts of IV are high-density zones, and the anterior part of IV has the highest density. Combining Figures 11 and 12, it can be seen that the increase of the average gray value difference of I is relatively large. This is mainly due to the low density and low intensity of I, so that I can reach the peak intensity quickly. When entering into II and III, the higher density of sandstone leads to a slower increase in the average gray value difference. When entering IV, the average gray value difference suddenly increases rapidly and then decreases rapidly. This shows that the intact sandstone specimen has reached the highest peak strength during the compression process and started to develop towards the low density region. This does not conflict with the law that sandstone fissures develop from low-density zones to high-density zones. Combined with Figure 5, it can be seen that the average density of IV is higher than that of I, and the anterior part of IV has the highest density in the whole sandstone specimen. When I cracks, IV does not generate fissures at the same time, and the development of fissures is easy to cause stress concentration at the tip of the sandstone with low plasticity, so that II and III reach the peak strength faster. It can be seen that the peak strength of the intact sandstone specimen is equivalent to the peak strength of the highest density zone of IV. This is due to the immediate unloading of intact sandstone specimens when peak strength is reached. Therefore, the cracks in the latter part of IV are mainly caused by the continued action of the internal dissipative energy of the intact sandstone specimen. Therefore, combined with Figure 10, it can be clearly seen that the number of cracks in IV is relatively large, but the width is small, and no penetrating fissures are formed. The continuity of the medium of IV is better than that of the medium of I, II, and III. By comparing the acoustic emission detection results of the sandstone specimens after the uniaxial compression experiment and combining with Figures 9 and 12, it can be seen that the overall development trend of the average gray value difference of the sandstone specimens is the same as the attenuation trend of the acoustic emission signal. The conclusions obtained from CT scanning experiments and acoustic emission experiments are consistent. This proves that it is feasible to explore the spatial location of primary fractures and the development degree of primary fractures in sandstone through acoustic emission detection technology.

4. Conclusion

(1)The gray value attenuation curve was obtained by comparing and analyzing the change of gray value of sandstone specimens before and after preloading. It is found that the evolution law of sandstone fissures is the development of low-density zones to high-density zones. This shows that in the process of rock failure, the degree of fracture development is negatively related to the rock density. In the same stress environment, the degree of fracture development in the low-density area must be higher than that in the high-density area, and the development direction of the crack must be from the low-density area to the high-density area. The compressive strength of the area with the highest rock density is equal to that of the whole rock(2)The average wave velocity and average amplitude of the sandstone specimens before and after preloading were compared and analyzed, and the attenuation curves of the two were obtained. It is found that after the sandstone reaches the peak strength, the dispersion coefficient of wave speed increases by 6 times, and the dispersion coefficient of amplitude increases by 66 times. The development law of wave velocity attenuation curve and amplitude attenuation curve is consistent with that of gray value attenuation curve. This shows that the results obtained by AE detection technology are the same as those obtained by CT scanning technology. Therefore, AE detection technology can be used to explore the spatial location and development degree of primary fractures in rocks

Data Availability

The data used to support the findings of this study are available from the corresponding author upon request. Correspondence should be directed to Xin Zhang at the following address: [email protected].

Conflicts of Interest

There are no conflicts to declare.

Acknowledgments

This work was supported by the Key Project of Xihua University (Z201036), the Graduate Innovation Fund Project of Xihua University of China (YCJJ2021075), and the Undergraduate Innovation and Entrepreneurship Training Project of the Sichuan Province in 2019 (S201910623016).