Geological Core Ground Reorientation Technology Application on In Situ Stress Measurement of an Over-Kilometer-Deep Shaft

As mining progresses to depth, engineering activities face the extreme challenge of high in situ stress. To eﬃciently measure the deep in situ stress before engineering excavation, an innovative deep in situ stress measurement method capable of the geological core ground reorientation technology and acoustic emission (AE) technology was proposed. With this method, nonorientation geological cores collected from the thousand-meter-deep borehole were reoriented based on the spatial spherical geometry model and borehole bending measurement principle. The distribution of deep in situ stress of an over-kilometer-deep shaft in the Xiangxi gold mine was investigated with real-time synchronized MTS 815 material testing machine and PCI-II AE instrument. The results show that the in situ stress changes from being dominated by horizontal stress to being dominated by vertical stress with depth. The horizontal maximum principal stress and vertical stress gradually increase with depth and reach a high-stress level (greater than 25MPa) at a depth of 1000m. The direction of the maximum principal stress is near the north. Meanwhile, to analyze the accuracy of the measured in situ stress comparatively, the stress relief measurements were performed at a depth of 655–958m in the mine, using the Swedish LUTrock triaxial in situ stress measurement system. The distribution of deep in situ stress obtained by the stress relief method agrees well with that by the AE method, which proves the reliability of the AE in situ stress testing method based on the geological core ground reorientation technology.


Introduction
In situ stress is the internal stress in rock mass in the crust. It is the fundamental force that causes the deformation and failure of underground excavation structures such as mines, tunnels, and chambers [1][2][3]. In mining engineering, the influence of in situ stress increases with the increase in burial depth of the mine year by year [4][5][6][7]. e high in situ stress in depth causes an increased risk of rockburst and seismicity and brings significant challenges to the excavation and support of engineering structures [8][9][10][11]. erefore, it is a necessary prerequisite to conduct measurement and understand the distribution of in situ stress in the crust for the scientific design and excavation of deep engineering structures [12][13][14][15].
At present, the investigation and development of deep in situ stress measurement technology have attracted increasing attention of many scholars and engineers [16][17][18][19]. Compared with the shallow subsurface, the complex geological conditions and limited measurement space in depth make it challenging to measure in situ stress accurately. Correspondingly, there are only a few theories and methods available for deep (over one kilometer or up to a few kilometers' depth) in situ stress measurement [1,12,[20][21][22]. e theories and methods that are easy to operate, cost-effective, and readily available for largescale application are almost a blank.
For the in situ stress measurement of rock mass with considerable depth, the existing mainstream methods, such as stress relief, hydraulic fracturing, and acoustic emission (AE), have been applied in the measurement engineering, but there are some limitations in the practical applications. e stress relief method is a proven quantitative in situ stress measurement technique. However, the measurement process requires the measurement operator and equipment to reach the deep measurement position [1,21], which significantly limits the operability of this method in deep narrow space. Besides, the uncertainty in the constitutive behavior of deep rock affects the accuracy of the stress relief method. e hydraulic fracturing method is an effective method to measure deep in situ stress but is still essentially a plane stress measurement technique. To obtain the actual three-dimensional in situ stress distribution in depth, it is necessary to perform hydraulic fracturing measurement in more than three intersection boreholes [23,24]. Although a single borehole hydraulic fracturing method has been developed in recent years [25], the operation is complex, and many environmental conditions limit the success rate and accuracy. Compared with the former two methods, the acoustic emission (AE) method can be carried out in the laboratory with more stable test conditions and lower test costs [19,26]. However, this method requires special oriented coring tools to drill the orientation core in the borehole before testing. e high-cost, low-efficiency, and time-consuming oriented coring processes prevent the AE method from being effectively promoted [27]. Currently, some core orientation methods have been developed, such as paleomagnetic orientation, borehole TV, and stratigraphic scanning orientation, whereas they all have some limitations in actual engineering applications [28,29]. erefore, oriented coring or core orientation in the large-depth rock mass is still a very laborious task.
In this paper, a new method for the ground reorientation of the geological core is proposed. e geological core ground reorientation technology replaces the cumbersome operation of core orientation in the large-depth borehole, making the AE method an ideal technology for large-depth in situ stress measurement.

Geological Core Ground
Reorientation Technology During the process of deep drilling and coring production, bending of the borehole will occur without exception due to geological and technological factors. Moreover, the axis of the core is always in line with that of the borehole under normal drilling conditions. erefore, it is possible to characterize the bending state of the core column using the borehole incline-measure data. e geological core ground reorientation is based on the above conditions and uses known incline-measure data to calculate the other additional incline-measure related data. Further, the continuously linked inclinometry is carried out in reverse with an inclinometer calibration stage to reorientate the in situ orientation of the core.

Spatial Spherical Geometry Model of Incline-Measure
Data. Figure 1 shows the spatial spherical coordinates of the upper (A) and lower (B) measuring points of single inclinemeasure data. PP′ is the upright diameter of the sphere, OP′AP and OP'BP are two large upright circles, and OAB is the large inclined circle. OA and OB are the adjacent upper and lower measuring points of the borehole, θ 1 and θ 2 are zenith angles, c is the total bending angle, and Δα is the change value of azimuth between two large upright circles. φ 1 and φ 2 , angles between the two upright large circles and the large inclined circle, are the special values of the end angle of the upper (A) and lower (B) measuring points. us, the three sides of the spherical triangle ABP are θ 1 , θ 2 , and c, and the three angles are π−φ 1 , φ 2 , and Δα. For the above parameters, if any three parameters are known, the other three parameters can be calculated according to the spherical triangle correlation.

Geological Core Ground Reorientation Based on an Inverse Process of Continuously Linked Inclinometry.
Continuously linked inclinometry is the use of measuring tools to obtain the zenith angle θ i , zenith angle θ i+1 , end angle φ i , and end angle φ i+1 (Δφ � φ i+1 − φ i ) at the upper and lower end of a measuring section of the borehole. After that, the change value of azimuth Δα can be calculated to orient the borehole. e inverse process is to calculate the total bending angle c, end angle φ 1 , and end angle φ 2 under the condition that the zenith angle θ 1 , zenith angle θ 2 , and azimuth change value Δα are known. Further, π−φ 1 , φ 2 , c, and the other three parameters (θ 1 , θ 2 , and Δα) constitute the unique spherical triangle, as shown in Figure 1. According to the above six parameters (φ 1 , φ 2 , c, θ 1 , θ 2 , and Δα), the in situ spatial posture of the core is restored using an inclinometer calibration stage.

Procedure of Geological Core Ground Reorientation
Technology. Ground reorientation method of the nonorientation core includes the following steps. (1) Sampling: the fresh geological cores are collected from the drilling site or geological core library. e geological cores need to ensure that the structure is complete, the length-diameter ratio is greater than 5, the upper end is marked, and the sampling depth is precise. e cores whose fractures coincide with each other should be collected as much as possible to help improve the measurement accuracy. (2) Collection and analysis of incline-measure data: the incline-measure data of the borehole are collected, including the borehole depth, zenith angle, and azimuth angle. Based on the borehole depth, the upper and lower ends of each core sample are matched with the corresponding zenith angles θ and azimuth angle α. If the sampling point is between the adjacent incline-measure points, the zenith angles and azimuth angles of the upper and lower ends of each core sample can be obtained by mathematical interpolation or stereographic projection-spherical triangle method. Further, φ 1 , φ 2 , and c of the upper and lower ends of each core sample can be calculated by spherical trigonometric formula as follows: cos c � cos θ 1 · cos θ 2 + sin θ 1 · sin θ 2 cos Δα.
(3) Reverse measurement and reorientation: the inclinometer calibration stage, inclinometer shell, core coaxial fixture, and small-diameter inclinometer or fixed-plate inclinometer are used together to restore the spatial posture of nonorientation cores. e small-diameter inclinometer or fixed-plate inclinometer is fixed in inclinometer calibration stage, and the upper section of the core is clamped into the core coaxial fixture first. e inclinometer calibration stage is then adjusted to calibrate the zenith angle and azimuth angle of the upper section of the core. After that, the core coaxial fixture is released, and the core and inclinometer are rotated axially until the end angles of the upper and lower section coincide with the calculated values. Further, the spatial posture of the core is locked, and a generatrix line is marked on the core in the vertical direction (the direction indicating the weight of the inclinometer). e generatrix line is the intersection line of the plane where the total bending angle is located and the core column surface. Finally, a horizontal ellipse is marked on the core column surface, and the north (N) direction is also marked on the horizontal ellipse according to the azimuth angle, as shown in Figure 2

Sample Processing for AE Test.
All the geological cores of in situ stress measurement were collected from the ZK2 geological borehole (the maximum borehole depth of 1040 m) in the Xiangxi gold mine of Hunan Province, China, as shown in Figure 3. To grasp the distribution of in situ stress with depth and highlight the distribution of in situ stress in deep areas, geological cores with a diameter of 65 mm were taken from 200, 400, 600, 800, 900, and 1000 m depths of the borehole, respectively. After the cores were reoriented accurately, the cylindrical samples for AE test were drilled from four directions of the cores, as shown in Figure 4. ree to five samples were drilled in the vertical direction, and its role was to determine the vertical stress according to the Kaiser point. ree to five samples were drilled, respectively, from three directions at 0°( N), 45°, and 90°on the horizontal ellipse plane of the core, to determine the direction and value of the horizontal maximum principal stress and the horizontal minimum principal stress according to the Kaiser points. To ensure the reliability of test results, samples were drilled and cut to a diameter of 25 mm and a height of 50 mm. Both ends of each sample were carefully polished on the diamond grinding table.

Test Equipment.
e loading instrument used in the test is American MTS 815 servocontrolled rigid material testing machine with the 793 control system, as shown in Figure 5(a). e testing machine has a maximum load of 2600 kN and can precisely control load, displacement, and strain in a variety of modes. Besides, the test system is equipped with PCI-II multichannel AE instrument [30,31]. Before the test, the above two instruments are modified synchronously, and the load data of the MTS 815 testing machine are introduced into the AE instrument as external parameter signals, as shown in Figure 5(b). us the AE events and the corresponding stress values can be simultaneously collected by the AE instrument at a megahertzlevel rate, which improves the accuracy of judgment of the Kaiser point.

Test Methods and Parameter Settings.
e purpose of the AE test is to stimulate AE signals of rock samples by uniaxial compressive loading and then identify and  Advances in Civil Engineering determine the Kaiser points corresponding to in situ stress in AE signals. e displacement rate control mode of MTS testing machine was adopted in the test. e loading speed was set at 0.1 mm/min under the graded cyclic loading state.
e first-level stress of cyclic loading should be higher than the predicted in situ stress. Moreover, the dual-channel data acquisition mode of the AE instrument was adopted and the two AE probes (resonance frequency of 20-400 kHz) were placed on the opposite side of the sample. e preamplifier of the AE instrument was set to 40 dB, the noise threshold was 40 dB, and the AE sampling frequency is 1 MHz. In addition, the coupling agent was applied to the contact surface between the test sample and the AE probe, to ensure that the AE signal can be well received; a thin layer of petrolatum was applied at the interface between the press head and the test sample, to reduce the effect of end noise on the AE test results. Figure 5(c) shows that both the loading value and the AE signals are collected simultaneously on the same time axis during the loading process. e mutation point of AE signal is well matched to the external load for the identification of Kaiser point and the corresponding stress.

Calculation and Analysis of In Situ
Stress. According to the above AE test method, the initial Kaiser points of samples drilling from four directions are determined, and their corresponding loads are found, as shown in Figure 6. To ensure the validity of the test results, a minimum of 15 samples per depth were taken for AE test, and the average value of the results after removing astronomical deviation data is taken as the Kaiser stress, as shown in Table 1.
e Kaiser stress in the vertical direction in Table 1 is regarded as the vertical stress corresponding to the buried depth. e three Kaiser stresses in the horizontal direction (0°, 45°, and 90°) in Table 1 are taken into the plane stress calculation equation (2) to calculate the magnitude and direction of the horizontal maximum and horizontal minimum principal stress [29], as shown in Table 2.

Advances in Civil Engineering
where σ I , σ II , and σ III are measured stresses at 0°, 45°, and 90°f rom clockwise direction due north, respectively; σ H is the horizontal maximum principal stress, σ h is the horizontal minimum principal stress, and β is the angle between the direction of horizontal maximum principal stress and the north direction, which is positive when turned counterclockwise to the north direction. Figure 7(a) and Table 2 show that the in situ stress in the drilling area is dominated by horizontal tectonic stress, especially at the borehole depth of less than 600 m. With the further increase in the depth, the dominant effect of the horizontal tectonic stress decreases, while the effect of vertical stress increases. In addition, the horizontal maximum principal stress increases with borehole depth. e horizontal minimum principal stress also increases with borehole depth, with a higher value near 400 m depth. e abnormal point of in situ stress may be related to the implicit geological structure in the local area. e azimuth of the horizontal maximum principal stress at different depths is consistent, all of which are near the north, distributing between N5°W-N18°E, as shown in Figure 7(b). Figure 7(c) shows that the vertical principal stress increases linearly with the borehole depth, which is consistent with the gravitational stress at the corresponding buried depth. e linear regression analysis is performed on the stress values in Figure 7(a), and the calculation formula of the in situ stress with depth in the drilling area can be obtained:

Distribution and Variation of In Situ Stress.
where σ v is the predicted vertical principal stress, MPa; σ H is the predicted horizontal maximum principal stress, MPa; σ h is the predicted horizontal minimum principal stress, MPa; and h is the depth, m.

Comparative Verification of Measurement Accuracy.
In order to analyze the accuracy of the measured in situ stress comparatively, Xiangxi gold mine carried out verification in situ stress measurement in the underground mine with the stress relief method. e measurements were performed at a depth of 655-958 m in the mine, using the Swedish LUTrock triaxial in situ stress measurement system, as shown in Figure 8. e magnitude and direction of the in situ stresses at the four measurement points were obtained from the LUT in situ stress calculation program, as shown in Table 3. Besides, based on the stress components obtained during stress relief, planar stress calculations were performed to solve σ H , σ h , and σ v at each measurement point, as shown in Table 4. Meanwhile, (3) fitted by the AE method was also used to calculate the AE in situ stresses at the four measurement points, which were listed in Table 4 for comparative analysis. Tables 3 and 4 show that the in situ stress measured by the stress relief method has the following distribution characteristics:  (1) e dip angle of the maximum principal stress (σ1) generally increases with depth, as shown in Table 3. e dip angle of σ1 at a depth of 655 m is 14.17°, indicating that the σ1 is a significant horizontal stress. When the depth increases to 756-765 m, the dip angle of σ1 increases to 24.47°-26.91°, indicating that the σ1 is near horizontal stress. Until the depth increases to 958 m, the dip angle of σ1 reaches 47.34°, indicating that the σ1 is inclined stress. e trend of dip angle of σ1 shows that the in situ stress changes from being dominated by horizontal stress to being dominated by vertical stress with depth, which is consistent with the resulting trend of the AE method. (2) e azimuth of σ1 is N41.16°W-N24°E, which is close to the azimuth measured by the AE method, pointing to the near north direction, as shown in Figure 9. (3) e vertical stress (σ V ) measured by the stress relief method increases with depth, as shown in Table 4, which is also consistent with the resulting trend of the AE method.
In summary, the measurement results of the reorientation core AE method and stress relief method are in good agreement. In particular, there is consistency in the change of maximum principal stress, the azimuth of maximum principal stress, and the change of vertical stress, which shows that the reorientation core AE method is reliable. Certainly, Table 4 shows that there is some deviation in the values of σ H , σ h , and σ v obtained by the two types of  methods, which is mainly due to the difference in geological structure (shown in Figure 3) of the sampling points and the fact that the shear stresses were not taken into account in the reorientation core AE method.

Conclusion
In this study, an innovative deep in situ stress measurement method capable of the geological core ground reorientation technology and the AE technology was developed. With this method, the distribution of deep in situ stress of an overkilometer-deep shaft in Xiangxi gold mine was investigated. Meanwhile, to analyze the accuracy of the measured in situ stress comparatively, the stress relief measurements were performed. e following are the conclusions drawn from this study: (1) e geological core ground reorientation technology combines the spatial spherical geometry model and borehole bending measurement principle to reorient the geological core on an indoor calibration stage. is technology allows geological core orientation to be separated from the drilling process without the need for in-borehole operation and has the advantages of simple operation, high accuracy, and low cost. (2) e AE in situ stress tests were carried out with reorientation geological cores to analyze the distribution of in situ stress in deep (1040 m) borehole in the Xiangxi Gold Mine. e results show that the in situ stress changes from being dominated by horizontal stress to being dominated by vertical stress with depth. e horizontal maximum principal stress and vertical stress gradually increase with depth and reach a high-stress level (greater than 25 MPa) at a depth of 1000 m. e direction of the maximum principal stress is near north.
(3) In order to comparatively analyze the accuracy of the measured in situ stress, Xiangxi gold mine carried out verification in situ stress measurement in the underground mine with the stress relief method. e distribution of deep in situ stress obtained by the stress relief method agrees well with that by the AE   method, which proves the reliability of AE in situ stress testing method based on the geological core ground reorientation technology.

Data Availability
e data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
e authors declare that they have no conflicts of interest.