Abstract
Based on the phenomena of core damage and core discing in deep strata of metal mines in Laizhou area, the physical and mechanical mechanism behind these phenomena and characteristics is discussed. The analysis shows that the damage effect of deep high stress rock is obvious after in-situ unloading, the core discing occurs when the damage reaches a certain degree, and the time effect of damage increases with the increase of buried depth. The characteristics of core discing are closely related to the stress environment and the mesostructure and mineral composition of the rock. Monzogranite with a large particle size and high content of potassium feldspar is easier to occur core discing than granodiorite in this area. The mechanism of core discing in Sanshandao gold mine is as follows: tensile failure occurs in the middle of the core, and composite failure of shear and tension occurs at the edge. The crack extends from the middle of the core to the outer edge, and finally, the tensile crack and shear crack intersect and connect to form a cake crack. Considering the stress condition and strength conditions of core discing, the energy release rate of rock mass in the core discing area is large, which has the conditions for strong rock burst. In addition, the ratio of maximum stress to tensile strength at the starting position of core discing is about 6.5 in this area, which is consistent with the previous analysis, so as to provide a strong basis for calculating in-situ stress based on the core discing phenomenon.
1. Introduction
Deep resource exploitation is an urgent task to solve the bottleneck of resource shortage in China. The deep metal mine shaft construction project is an important foundation and premise to realize the large-scale development of deep mineral resources. At present, the construction of some metal mine shafts in China is stepping into the stage of ultradeep (more than 2000 m depth), such as Sanshandao gold mine in Shandong Province. The field test shows that the in-situ stress at 2000 m depth is as high as 70 MPa, and the deep rock mass is in a strong compression in-situ state. During the excavation of underground works, the rock is disturbed by cyclic loading and unloading, resulting in a series of changes in its deformation and failure mode.
It is well known that extracting core from deep boreholes can lead to a significant increase in microcrack porosity commonly referred to as sample disturbance or stress-induced damage [1]. With the increase of stratum depth, the in-situ stress of rock mass is higher and higher, and the microcrack damage propagation finally leads to the core fracture into cake, that is, the core discing phenomenon. Hast [2] discovered the core discing for the first time in the process of drilling and coring in 1958. Then, with the rock mass projects of civil engineering, water conservancy, and mining marching into deep strata, the in-situ stress of rock mass is higher and higher, and the core discing phenomenon is more frequent and common. As a special phenomenon in the deep stratum, the research on the characteristics and mechanical mechanism of core damage and core discing plays an important role in revealing the variation of physical and mechanical properties of deep strata and the original rock stress environment.
The field investigation found that core discing occurred during deep drilling and coring in Xincheng gold mine and Sanshandao gold mine in Laizhou, Shandong, China. According to the on-site core discing characteristics, we found the following interesting questions: (1) The surface of some cores was intact when they were just taken out in Sanshandao gold mine, why did the core discing occur after they were placed for a few days to a few weeks? (2) The core discing is very strong when drilling at 1100 m–1200 m depth in Xincheng gold mine (Figure 1), and the pancake thickness is mostly more than 10 mm. While in Sanshandao gold mine, the core discing is more common when drilling at 1800 m depth, and the pancake thickness is mostly less than 10 mm (Figure 2). The distance between the two mines is no more than 10 km, why are there great differences in the initial depth of core discing and pancake thickness? (3) The phenomenon of core discing in Sanshandao gold mine is concentrated in the hanging wall of the vein but less in the footwall of the vein. Why does the core discing appear in the hanging wall of the ore vein with shallow burial depth rather than the footwall of the ore vein with greater burial depth? This study attempts to answer the above questions through research.
2. Research Status of Core Discing
2.1. Fracture Tensile Failure Mechanism of Core Discing
Through the indoor triaxial test, Jaeger and Cook [3] found that the higher the lateral stress of the original rock, the greater the possibility of the existence of core discing, and the thinner the rock cake. The cake fracture generally starts from the center of the core, and the section is neat. They considered that the cake fracture was the result of tension failure. Sugawara K [4] proposed the tensile stress criterion for core discing. Maury et al. [5] pointed out that the pie fracture started from the inside of the core through the scanning analysis of the meso characteristics of the deep hole core. Matsuki [6] put forward the tensile stress failure criterion of a long cylindrical core. Li Y [7], Song I [8], Hakala M [9], and Kaga [10] all believe that cake fracture is dominated by tensile failure. S. S. Kang [11] studied the horizontal stress state at the bottom of the core on-site by using the integral cone end drilling and casing method CCBO and considered that the core fracture was mainly caused by tensile stress. R. Corthe´sy [12] simulated the core discing process and pointed out that the cake failure perpendicular to the core axis is dominated by tensile stress. Li Zhanhai et al. [13] pointed out that, due to the axial relaxation and lateral expansion of the core, the cracks around the core crack at the small defects and expand to the middle of the core and finally tensile fracture. Based on the tensile stress condition and energy theory, Zhang Hongwei et al. [14] gave the criteria for the occurrence of the core discing phenomenon in the coal mine.
2.2. Fracture Shear Failure Mechanism of Core Cake
Obert et al. [15] pointed out through the indoor triaxial compression core drilling test that the core discing failure starts at the outer edge of the disc, established the linear relationship between the radial stress and axial stress of core failure, and speculated that the cake is caused by shear stress. Durelli [16] found that the maximum shear stress is distributed at the outer edge of the core notch through 3D photoelastic experiment, so it is considered that the pie starts at the outer edge of the disc. Hou Faliang [17] and Yao Baokui et al. [18] put forward the fracture failure mechanism of core discing and considered that the core discing was formed by the initial fracture expansion of the circular arc crack near the core surface, rather than the central crack of the core. Shang Yuequan et al. [19] put forward the genetic view that the formation of core discing is “shear first and then pull, and mainly tensile fracture”. Ma Tianhui et al. [20] pointed out that high ground stress can cause core discing, radial stress is the main factor affecting core discing, and axial stress only causes local damage to the core surface. The formation process of core discing is mainly a shear failure, accompanied by a small amount of tensile failure.
To sum up, there are still some differences in the mechanical mechanism of core discing. One is that the cake is caused by the tensile stress concentration at the root of the core; the other is the shear fracture mechanism; the third is the tensile shear composite failure mechanism, and that is, core discing can be dominated by tensile failure or shear failure. At the same time, there are also differences in understanding whether the initiation of cake fracture starts from the middle to the edge or from the edge to the center.
3. In Situ Unloading Damage Characteristics of Deep Stratum Core
3.1. Experimental Design
The test samples are taken from granite buried in the range of 1800 m–2000 m in Sanshandao gold mine, Shandong Province. The length of the rock sample is 20–30 mm, and the diameter is consistent with the diameter of the drilling rig casing, which is 90 mm. Figure 3 shows the test pieces. In order to reduce the exposure time of the drill core in the air, the surface of the sample shall be cleaned immediately after taking back the sample, and the strain gauge and acoustic emission probe shall be quickly pasted and waterproofed, then placed in a constant temperature water bath system to maintain a constant temperature, and finally tested.
The original unloading strain recovery value in the test is monitored by the strain gauge connected with it. The resolution can reach 0.1με, and the strain detector has six compensation plates, which can compensate for each measuring point (Figure 4).
The constant temperature water bath system can keep the temperature around the sample constant at 24° with an error of ±0.1°C, so as to reduce the influence of temperature change.
The wave velocity measuring instrument can measure the transverse and longitudinal wave velocities of the sample. The probe is closely combined with the smooth surface of the sample through vaseline.
PCI acoustic emission acquisition instrument can realize multichannel data acquisition and storage. Four-channel data acquisition is adopted in the experimental process.
3.2. Result Analysis
It can be seen from Figure 5 that the recovery process of anelastic strain under constant temperature can be basically divided into two stages. The first stage is the deceleration deformation stage: the strain increases and the strain rate decreases with time. The second stage is the steady-state deformation stage: the strain increases slowly or no longer, indicating the end of the anelastic strain recovery process. With the different buried depths of rock specimens, the time required for the completion of anelastic strain recovery is also changing. The greater the buried depth, the longer the time required for the completion of rock recovery, indicating that the higher the original stress of the rock, the greater the compression degree of the rock mass, the more microfractures in the rock after unloading, and the greater the anelastic strain value.
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From the variation curve of wave speed and time in Figure 5, we can see that the rock longitudinal wave velocity decreases in varying degrees with time. The change of ultrasonic wave velocity reflects the damage evolution in the rock, and it is confirmed that there is deformation or damage fracture in and between the crystals in the rock during the strain recovery process. There is a positive correlation between the anelastic strain recovery rate and the change rate of damage degree. In the deceleration deformation stage, the deformation and interaction activity of crystals in the rock gradually decreases, the propagation speed of internal new cracks also gradually decreases, and the new cracks gradually decrease, so that the change rate of rock damage degree gradually decreases. After reaching the steady-state deformation stage, the anelastic strain recovery process of the rock specimen ends, the crystal in the rock is no longer deformed, the crack is no longer added and expanded, and the longitudinal wave velocity and damage degree remain basically unchanged.
From the change of strain-time-ring count rate curve in Figure 6, it can be seen that in the deceleration deformation stage, with the initiation and propagation of cracks, the acoustic emission ring count is relatively large, and the acoustic emission in the rock is relatively active. In the steady-state deformation stage, the axial strain of rock remains basically stable, and the acoustic emission ring count is less, indicating that the damage of rock has been maintained at a stable level after this stage. The cumulative acoustic emission ring count curve and the anelastic strain recovery curve maintain the same change process, and the results are also consistent with the test findings by Wolter and Berkhemer [21]. Therefore, the acoustic emission ring count curve can also be used to judge whether the anelastic strain recovery process of rock has ended.
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To sum up, it can be seen that there are significant damage and deterioration characteristics during core drilling in deep high stress and strong compression stratum, and the damage has an obvious time effect, which is the internal mechanical mechanism leading to core discing after core extraction for a few days or weeks in Sanshandao gold mine.
4. Characteristics and Mechanism Analysis of Core Discing
4.1. Influence Analysis of Rock Composition and Particle Size
Lim (2010) analyzed the characteristics of granodiorite and granite at the same location in the underground laboratory of Atomic Energy Corporation of Canada, including particle size, mineral composition, and strength (Figure 7). It was found that the uniaxial compressive strength and tensile strength of fine-grained granodiorite were 17% higher than that of coarse-grained granite. Under the same stress environment, the cake thickness of granodiorite was greater than that of granite, which shows that the core discing characteristics are very sensitive to the subtle differences between granodiorite and granite.
Through statistical analysis of rock mesostructure and ore rock composition, it is found that the particle size of granite at different depths is greatly affected by the occurrence environment in the Laizhou area. According to Table 1, the maximum equivalent particle size is 1.07–1.09 mm at the depth of 1100–1200 m in Xincheng gold mine, and the average value of other equivalent particle sizes is 700–950 μm. According to Figure 8, the composition of granite at different depths also has certain differences, and the content of potassium feldspar is relatively high at 1100 m–1200 m depth. According to the above analysis, the strength of granite in this section is relatively low due to coarse particle size and high content of potassium feldspar, which makes it easier to occur core discing under the same stress conditions. That’s why the core discing phenomenon is strong near 1100–1200 m depth in Xincheng gold mine.
According to Figure 9, the hanging wall of Sanshandao vein is mainly Linglong medium-grained monzogranite (including plagioclase). The footwall wall of Sanshandao vein is Guojialing granodiorite with medium grain and porphyritic structure. The rock composition of the hanging wall and footwall of the vein is shown in Table 2. Because the content of potassium feldspar in monzogranite is higher than that of granodiorite, its strength decreases. In addition, the content of quartz in monzogranite is higher, resulting in high brittleness and brittle fracture. Different rock compositions and particle sizes lead to different strengths, and the compressive strength and tensile strength of granodiorite are higher than those of monzogranite. Therefore, in the same stress environment, the monzogranite in the hanging wall of the vein is easier to occur core discing than that in the footwall, and the core discing thickness will be smaller. This explains why the core discing phenomenon is concentrated in the hanging wall of the vein in the Sanshandao gold mine.
4.2. Numerical Simulation Analysis of Cake Cracking Process
4.2.1. Numerical Simulation Scheme and Process
R Corthésy and Leite M H [12] simulated the drilling and coring process with strain-softening model and found that under the influence of radial stress level, the initiation of core discing can start from tensile failure or shear failure. When the radial stress is high, tensile and shear composite failure will occur near the core edge. The higher the radial stress, the more obvious the conical shear failure in the core, which hinders the outward penetration of the tensile failure in the middle of the core perpendicular to the mandrel plane.
Combined with the rock mechanical parameters and in-situ stress test data of Sanshandao gold mine, the drilling and coring process is simulated with the FLAC3D strain-softening model. The in-situ stress test data of Sanshandao gold mine are shown in Figure 10, and it can be seen that the maximum horizontal principal stress at the buried depth of 1200 m in this area is about 46 MPa, and the vertical principal stress is 32 MPa. The maximum horizontal principal stress at the buried depth of 1800 m is about 66 MPa, and the vertical principal stress is 45 MPa. According to the indoor mechanical test report, the tensile strength of granite rock in this area is about 7–10 MPa, the tensile strength of granodiorite exceeds 14 MPa. According to the above conditions, different simulation schemes are designed as shown in Table 3.
Numerical model establishment and drilling process simulation refer to reference [12]. Because the bit has a certain height, the core is in the state of radial displacement constraint before the bit is exposed during actual drilling. Therefore, in the simulation, before the core is exposed to the bit, the horizontal displacement of its radial interface is constrained, so that the contact surface between the core and the bit can only move freely in the axial direction. This is the difference between the simulation process in this study and reference [12].
4.2.2. Numerical Simulation Results
The simulation results of schemes 1 to 5 are shown in Figure 11 to Figure 15. According to scheme 1, because the applied radial stress is small, the tensile stress generated in the core does not reach the tensile strength of granite, so there is no plastic tensile failure and core discing during drilling. According to scheme 2, it shows that under the stress condition of 1200 m depth, core discing occurs when the tensile strength is 7 MPa. The comparison between schemes 3 and 4 shows that under the stress condition of 1800 m depth, when the tensile strength is 10 MPa and the cohesion is 30 MPa, the core discing is not obvious, while the rock cohesion increases to 50 MPa, and the core discing is obvious. The shear failure inside the core hinders the outward penetration of tensile failure. The comparison between schemes 4 and 5 shows that it is not easy to occur core discing when the tensile strength exceeds 15 MPa under the stress condition of 1800 m depth.
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It also can be seen from the distribution of plastic zone of discing core that tensile failure occurs in the middle of core, while tensile and shear combined failure occurs at the edge.
The above analysis shows that core discing is the comprehensive effect of stress conditions, tensile strength, shear strength, and strain-softening characteristics. Core discing needs to meet certain stress and strength conditions, and single conditions such as depth condition or strength condition cannot determine whether core discing occurs. For the core discing rock, its tensile strength should be lower than its stress level, so that tensile fracture is easy to occur, and the shear strength should be higher than its stress state; otherwise, a large number of shear failure will hinder the formation of cake. Because the rock stress and average strength change with the buried depth, the law of core discing with depth is also complex, which shows that some place occurs at the core discing and some not at the same depth.
At the same time, the high shear strength of the rock mass enables the rock mass to accumulate a large amount of elastic energy. The relatively low tensile strength makes the rock mass prone to brittle failure when the current stress state is suddenly disturbed. Similarly, the energy consumed by pull-down fracture in the stress state is much lower than that of shear fracture, which enhances the energy release capacity when the rock mass in this area is broken. Therefore, from the rock conditions and surrounding stress conditions, the rock mass in the core discing area often has a strong risk of rock burst.
4.2.3. The Core Discing Criterion
Lim [22] pointed out that when the ratio of maximum principal stress to rock tensile strength is greater than 6.5, the core discing starts. It can be seen from Table 4 that the rock stress intensity ratio at 1800 m depth of Sanshandao gold mine and 1200 m depth of Xincheng gold mine is greater than 6.5, while the stress intensity ratio at 1200 m depth of Sanshandao gold mine is less than 6.5, which explains why the two mines are very close, and the initial depth of core discing is inconsistent. At the same time, it can be seen that at the same depth, due to the different lithology of footwall and hanging wall, the stress intensity ratio is different, and the results of core discing are also different, that's why the core discing phenomenon is concentrated in the hanging wall of the ore vein in Sanshandao gold mine.
The above analysis is consistent with the field situation, which confirmed that it is reasonable to take the stress intensity ratio as the core discing criterion. According to the core discing characteristics and rock strength characteristics, the maximum principal stress in the core discing area can be calculated, which provides a basis for calculating the original in-situ stress based on the pancake characteristics.
4.3. Micromechanical Mechanism of Core Discing
The analysis of the core discing-shaped section of Sanshandao gold mine shows that the fracture plane has a certain fluctuation, showing a saddle shape, and the section is rough and dry, and there are uplift ridges (Figure 16). Cut the half cake core from the middle along the axial direction, it can be seen that the crack perpendicular to the core axis has penetrated the whole plane inside, but no obvious crack is found outside the core (Figure 17). Some cracks develop in isolation inside, indicating that the core discing can be started from the middle.
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By scanning the cake fracture surface with an electron microscope (Figures 18, 19), it is found that there are step-like and fishbone-like patterns near the edge of the core, with shear slip stripes, regular micromorphology, both transgranular fracture and shear fracture, which is a shear tensile composite fracture mode. In the middle, the crystal surface is smooth, there are many hole defects, mainly transgranular fracture, and there are many tongues and flake patterns on the fracture surface, which indicates that the mica minerals in the rock are torn and opened to form a multi thin-layer structure, mainly tensile failure. These analysis results are consistent with the results of Bankwitz et al. [23].
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Based on the above analysis, the mechanism of core discing in Sanshandao gold mine is as follows: tensile failure occurs in the middle of the core, and composite failure of shear and tension occurs at the edge. The crack extends from the middle of the core to the outer edge, and finally, the tensile crack and shear crack intersect and connect to form a cake crack. The microscanning results are consistent with the numerical simulation results, which prove that the above analysis is reasonable.
5. Conclusions
(1)There is obvious damage after rock coring and unloading in deep high stress stratum. The damage process is consistent with the anelastic strain recovery process, which can be divided into deceleration stage and steady-state stage.(2)It is found that the core discing of the gold mine in the Laizhou area is closely related to the stress environment and the meso composition of the rock itself. Monzogranite with a large particle size and high content of potassium feldspar in the hanging wall is easier to pie crack than granodiorite in the footwall.(3)Numerical simulation and SEM analysis show that the mechanism of core discing in Sanshandao gold mine is as follows: tensile failure occurs in the middle of the core, and composite failure of shear and tension occurs at the edge. The crack extends from the middle of the core to the outer edge, and finally, the tensile crack and shear crack intersect and connect to form a cake crack.(4)Considering the mechanical conditions and self-strength conditions of core discing, the energy release rate of rock mass in the cake crack area is large, which has the conditions for strong rock burst.(5)The maximum horizontal principal stress calculated from the tensile strength of rock mass in the core discing area is consistent with the field measured value, which proves that the phenomenon of core discing in the deep stratum is of great significance for calculating the actual in-situ stress.Data Availability
Some or all data, models, or code generated or used during the study are available from the corresponding author by request.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
This paper was supported by the National Key Research and Development Plan (no. 2019YFC1509602), Major Scientific and Technological Innovation Project of Shandong Province (no. 2019SDZY05), and Open fund of Hubei Key Laboratory for Efficient Utilization and Mlock Muilding of Metallurgical Mineral Resources (no. 2021zy003).