Characteristics of in situ stress field in the Huainan mining area, China and its control factors

Due to the high in situ stresses, dynamic disasters occurred frequently in the Huainan mining area, China. Our understanding of the in situ stresses in this area is still insufficient. In this study, the in situ stresses of 18 sections in two boreholes in the Xinji No. 1 coalfield were measured using the hydraulic fracturing method, and the distribution of in situ stresses in the Huainan mining area were investigated. The relationship between in situ stress and geological structure in the Huainan mining area was summarized and the limitation of fault friction strength on in situ stress was discussed. The result showed that the maximum horizontal principal stress (σH) at Xinji No. 1 mine was 13.95–25.23 MPa, the minimum horizontal principal stress (σh) was 12.16–21.17 MPa. The average azimuth of the maximum horizontal principal stress was N83.61°E. The statistical results showed that the in situ stresses in Huainan mining area were characterized by a strike-slip faulting regime. Both the horizontal and vertical principal stresses increased approximately linearly with the increase of burial depth. The orientation of the maximum principal stress in the study area is closely related to the tectonic movement and the ratio of maximum principal stress to minimum principal stress was primarily limited by the friction strength of the faults. The outcomes of this research can provide some reliable engineering parameters and benefit the roadway layout and support design in the Huainan mining area.


Introduction
In situ stress is the stress that exists in the crustal rock mass and is the fundamental force that causes deformation and failure of surrounding rock in underground engineering excavations. Rock burst, coal and gas outburst events, and large deformation of surrounding rock are more likely to occur in the presence of high in situ stresses (Shan and Yan 2010;Xiao et al. 2016). Mastering the in situ stress state in the engineering area is a prerequisite for determining the mechanical properties of the engineering rock mass, analyzing the stability of surrounding rock, and realizing scientific design and decision-making of an excavation in geotechnical engineering (Zoback et al. 1985). Field measurement of in situ stress is the most reliable way to accurately understand the distribution of in situ stress in the engineering area (Finkbeiner et al. 1997;Martin and Lanyon 2003;Yaghoubi and Zeinali 2009;Funato and Ito 2017;Yang et al. 2017). In 1932, Liearace successfully performed the original rock stress measurement in the Hoover Dam discharge tunnel for the first time (Carder 1945). In the late 1960s, Haimson and Fairhurst (1969) proposed the theory of in situ stress measurement by hydraulic fracturing. Brown and Hoek (1978) summarized the measured in situ stress data in different regions of the world in 1978, proposed the law of vertical stress varying with depth in various countries, and drew a regression curve of the ratio of average horizontal in situ stress to vertical in situ stress varying with depth. Since the 1980s, dozens of countries in the world have carried out a variety of methods of in situ stress measurements, such as hydraulic fracturing, overcoring, borehole caving, and acoustic emission, which have been successfully applied in engineering application, earthquake prediction, oil and coal mining (Gay 1975;Greiner 1975;Cornet and Valette 1984;Bell 2006;Zhao et al. 2015;Oliver et al. 2018). Among them, the hydraulic fracturing method is a widely used one. In situ stress measurement and related research work in coal mines lag behind other industries. Mastering the distribution of in situ stress in coal mine areas is of great significance for safe and efficient coal and coal-bed methane extraction (Li et al. 2019a, b). Kang et al. (2010) gathered a total of 204 sets of stress measurement data by hydraulic fracturing in 49 coal mines within 13 Chinese coal mining areas to investigate the distribution characteristics of in situ stresses. Meng et al. (2011) analyzed the characteristics of in situ stress fields and the relationship between permeability of a coal reservoir and the in situ stress in the Southern Qinshui basin, China. Coggan et al. (2012) analyzed the effects of in situ stress on coal mine roadway stability by numerical modeling. Paul and Chatterjee (2011) discussed the cleat orientations and other structural features and their relationship with the in situ stress orientation pattern, based on the observed results in the outcrops of 14 major coal seams in the Jharia coalfield of India.
The Huainan mining area in China is characterized by a great burial depth and complex mining conditions (Guo et al. 2012). With the increase of mining depth, most of the mines enter deep mining at the first level and all of them enter deep mining at the second level, with an average mining depth of 875 m. There is a high in situ stress level at depth in this mining area. And disasters, such as impact ground pressure, gas outbursts, and large deformation of roadway surrounding rock, occur frequently, which seriously threaten the safety of mine production (Li et al. 2013(Li et al. , 2019aYang et al. 2012). Therefore, it is of great significance to investigate the distribution laws of in situ stress and its control mechanism in the Huainan mining area for safe and efficient coal mining. For a long time, many scholars have carried out in situ stress measurements in the Huainan mining area and achieved some fruitful results. For instance, Han and Zhang (2009) measured the in situ stress in the Panyi mine and Paner mine using a hollow inclusion stress-relief method and analyzed the influence of in situ stress on coal and gas outbursts. Liu and Liu (2012) used hydraulic fracturing and stress-relief methods to measure the in situ stress in Xieyi mine, Pansan mine, and Wangfenggang mine, and analyzed the characteristics of in situ stress fields at depth in the mining area.
Previously measured in situ stress data were rather scattered and are insufficient for the analysis of a regional stress distribution. In this research, the hydraulic fracturing in situ stress measurement method was used to investigate the in situ stress state in the Xinji No. 1 mine. In combination with previously measured data, the distribution characteristics of the in situ stress in the Huainan mining area are statistically studied, and the influence mechanism of the geological structure on the in situ stress field is also discussed. This study can provide a basis for the control of a surrounding rock deformation and failure and the optimization of a roadway support scheme in Huainan mining area.

Geological setting
The Huainan mining area is in the north central part of Huainan city in the Anhui province, China. It is one of China's 14 large-scale coal bases with a length of 140 km from east to west, a width of 20-30 km from north to south, and an area of 3200 km 2 . In terms of regional geological structure, the Huainan mining area is located in the south edge of the North China plate. It starts from the Tanlu fault in the east, ends at the Fuyang fault in the west, connects with the Bengbu uplift in the north, and adjoins with the Hefei depression in the south (Jiang et al. 1992). The Huainan coal field is in the form of a compound syncline, with the main structure in the northwest direction. The two flanked low mountains include the Proterozoic Wuhe group, Fengyang group, Bagongshan group of the Qingbaikou system, and Paleozoic Cambrian and Ordovician units. The axis has a secondary wide and gentle fold, mainly composed of Carboniferous and Permian coal-bearing strata, and the overlying Cenozoic has a general thickness of 200-500 m. There are Fufeng and Shungengshan thrust nappes in the south and Minglongshan-Shangyao gravity slip structures in the north. The NNE-trending regional faults are superimposed on the NW trending main structural line (Fig. 1).
The north and south wings of the compound syncline are well developed, showing a thrust nappe from south to north, and forming the nappe structure pattern of the two wings. The Shungengshan fault and Fuyang-fengtai fault on the south wing constitute Shungengshan, Bagongshan, and Liuzhuang nappes from south to north. The Shangyao-Minglongshan-Shangtang fault on the north wing forms a north-south sliding nappe. In the compound syncline, reverse faults in the striking direction and NNE normal fault in the dipping direction are developed, the latter is close to an eastern and western structure belt, mainly including the Wudian fault, Xinchengkou-Changfeng fault, Chenqiao-Yingshang fault, Koziji (Xifanlou) fault, etc., forming a group of ladder structures which are generally parallel to the Tanlu fault and inclines to the west. Studies have shown that near EW-trending folds and fault structures in the Huainan coalfield were mainly formed in the Indosinian and Yanshan periods, while normal faults in the NNE direction were mostly new structures and formed in the Himalayan movement period (Song et al. 2005).
The Xinji No.1 mine is located in the middle section of the Xieqiao syncline flank of the Huainan complex syncline and the Yingfeng-Fufeng nappe structure. The overall structural form in this mine is that the Fufeng thrust fault pushes the foreign strata system from south to north over the in situ system (coal-bearing strata). Affected by a strong compressive stress from south to north, the overlying branch fault form of the Fufeng nappe structure has undergone a full nappe, forming a shingle fan structure combination. The main fold structures in this well field include the Xieqiao syncline, Liuka anticline, and Qiandaliujia syncline. The main faults include the reverse fault group formed by the North-South compression of Fufeng nappe and the F10 normal fault group formed by the nearly east-west extension (Fig. 2). The exposed strata at Xinji No.1 mine are composed of Proterozoic Qingbaikou group, Cambrian, Ordovician, Carboniferous, Permian, Triassic, Neogene, and Quaternary. The Shanxi Formation and upper and Lower Shihezi Formation of Permian are the main coal-bearing strata. The lithology of the strata is mainly alluvial clay, various kinds of sandstone, mudstone, coal rock, and limestone (Shi et al. 2014).

Basic theory of hydraulic fracturing
Fracturing pressure during hydraulic fracturing is an important parameter and it can be expressed as follows (Haimson 1978): where P b is the strata breakdown pressure when the strata are hydraulically fractured; p p is the pore pressure; T 0 is the rock tensile strength; h and H are the minimum and maximum horizontal principal stresses acting on the cross section of borehole, respectively.
After the fracture of the borehole wall, if the injection continues to increase the pressure, the fracture will extend deep into the wall rock. If the injection stops and the fracturing circulation system keeps sealed, the fracture will stop extending immediately and tend to close. The equilibrium pressure that can keep the fracture open is called instantaneous shut-in pressure (P s ), which is equal to the minimum principal stress perpendicular to the fracture plane; i.e., If the sealing section is pressurized again to reopen the fracture, the fracture reopening pressure ( P r ) can be obtained. Since the rock has been broken, the tensile strength ( T 0 ) is equal to zero. P r can be obtained as follows: Combining Eq. (1) with Eq. (3), the tensile strength of rock ( T 0 ) can be obtained as follows: Combining Eq. (2) with Eq. (3), the maximum horizontal principal stress ( H ) can be estimated as follows: The vertical principal stress can be estimated according to the formula given by Hoek and Brown (Hoek and Brown 1980), i.e., where σ v is the vertical principal stress; H is the occurrence depth of the rock mass.

Measurement equipment of in situ stress
The Sy-2007 single-loop in situ stress measurement system was adopted for the in situ stress measurement, as shown in Fig. 3. The single circuit was to use an independent pressurization system to pressurize the packer and test section, which was characterized by only using a high-pressure pipe to pressurize the downhole during the measurement process, and the downhole was converted by a push-pull switch to seal the packer and fracture the well section, respectively.
The Sy-2007 hydraulic fracturing in situ stress measurement system is typically used for in situ stress measurements. The system is composed of an LSJ-4 × 400 highpressure oil pump, an ACP-4001 industrial control desktop computer, a control box, a push-pull switch, a directional device, packers, a moulage device, and high-pressure oil pipes. That is, it is a relatively complete measurement system. The control and data recording system are equipped with the following hardware: a field data processing computer, a multi-channel data acquisition card, and a recoding and processing code of hydraulic fracturing parameters.

Field test program of the hydraulic fracturing method
The specific field test program of hydraulic fracturing method is as follows: ① drill to the measuring position and isolate the test section with two packers; ② use a high-pressure pump to pressurize the sealed space (fracturing test section) through the high-pressure pipe and drill pipe. During the pressurization process, because the pressure sensor is installed in the high-pressure pipeline, the pressure value on the recording instrument will increase rapidly with the pumping of the high-pressure liquid. Due to the stress concentration around the borehole, the rock in the fracturing section will crack at the position of the minimum tangential stress under the action of enough hydraulic pressure, that is, in the direction perpendicular to the minimum horizontal principal stress. The critical pressure value P b recorded at this time is the fracture pressure of the rock. Once the rock has fractures, the pressure will drop sharply. If the displacement continues to be pressurized, the fractures will remain open and extend to some distance. ③When the fracture extends to 2-3 times the borehole diameter, close the high-pressure water system. At this time, the constant water pressure is called the shut-in pressure, which is recorded as P s . Finally, the pressure is released to close the fracture; ④ re-inject high-pressure water into the sealing section to reopen the fracture, and record the pressure when the fracture Fig. 3 Sketch of a single-loop hydraulic fracturing in situ stress measurement system (Cai et al. 2006) reopens, which is recorded as P r . This re-pressurization process is repeated 3 times; ⑤ take the packer out of the borehole after the packer is completely depressurized; ⑥ the moulage device, wrapped with a special rubber, is sent into the fracture section and pressurized to make the shape, size, orientation of the hydraulic fracture crack, and the original joint fractures in the hole wall be recorded by the rubber moulage. A typical pressure-time curve obtained during hydraulic fracturing is shown in Fig. 4.
The fractures orientation can be determined after the fracturing measurement in the isolation section to determine the direction of the maximum horizontal principal compressive stress. The method used in this test is the directional moulage method. In addition, it should be noted that the instantaneous shut-in pressure P s represents the minimum horizontal principal stress and is an important calculation parameter. In this test, a single tangent method is used to accurately identify the instantaneous shut-in pressure (Gronseth and Kry 1981).

Arrangement of in situ stress measuring points
Combined with the progress of the exploration project of Xinji No.1 mine, S5 borehole and S9 borehole for hydrogeological supplementary exploration of Shanxi formation were selected as the measurement boreholes for hydraulic fracturing. The wellhead elevation and depth of S5 borehole were 26.16 m and 656.70 m, respectively. The wellhead elevation and depth of S9 borehole were 23.27 m and 928.86 m, respectively. According to the specific geological and lithological conditions of the borehole, considering the actual needs of the project, ten sections were tested in S5 borehole, which were at depths of 440 m, 470 m, 485 m, 512 m, 540 m, 560 m, 580 m, 600 m, 620 m, and 640 m, respectively; eight sections were tested in S9 borehole, which were at depths of 500 m, 535 m, 580 m, 665 m, 715 m, 790 m, 820 m, and 845 m, respectively. To reveal the in situ stress distribution around the roadway, these test points were arranged as far as possible in the sandstone sections at the top and bottom of the coal mine roadway.

In situ stress measurement results
Through hydraulic fracturing in situ stress measurements, fracturing curves of in situ stress measurements in the Xinji No.1 mine were obtained. The fracturing curves of the first and fifth sections of S5 borehole and the fourth and sixth sections of S9 borehole are shown in Figs. 5, 6. According to the obtained data, the data quality of each section was satisfactory. The pressure record curves were quite standard with obvious peak values of fracture pressure and the regularity of each cycle was very strong. The fracture parameters measured by each cycle had good consistency. Therefore, the in situ stress datum of each measuring point was reliable.
According to the collation and calculation analysis of the test data, the breakdown pressure ( P b ), fracture reopening pressure ( P r ), instantaneous shut-in pressure of the hydraulic fracture plane ( P s ), rock pore pressure ( P p ) and tensile strength T 0 of rock were determined for each test section.
According to the measured pressure parameters and Eqs. (1-6), the maximum and minimum horizontal principal stress values and vertical principal stress values were obtained, as shown in Table 1.
To determine the orientation of the principal stresses, according to a comprehensive analysis of the fracturing test curves of S5 borehole, four measurement sections at depth of 469.5-470.5 m, 539.5-540.5 m, 599.5-600.5 m, and 639.5-640.5 m were selected for moulage and orientation. The method of moulage and fracture orientation following hydraulic fracturing is as follows: lower the moulage rubber cylinder with the compass orientation device to the hole section where the cracks have occurred, pressurize the impression device for a period of time, then take it out of the hole, and trace the crack marks on the moulage rubber cylinder on the special film, and record the corresponding parameters. Take out the photo paper from the compass in the dark box and put it in the developer and fixer to wash the photo to determine the angle of the baseline and the compass needle. Using a transparent film, trace the baseline and crack traces on the moulage rubber cylinder on the film to measure the angle between the crack and the baseline, and mark the upper and lower ends in time. According to the angle between the compass needle and the baseline and the angle between the baseline and the crack, the azimuth angle of fractures can be determined.
The moulage and orientation measurement results are shown in the circle development diagram (Fig. 7). The rupture morphology of the four measurement sections are all upright fractures. According to a set of radially opposite longitudinal fractures, the fracture surface directions of the four Environmental Earth Sciences (2021)  Through a comprehensive analysis of the fracturing test curves of the S9 hole, three measurement sections at depths of 534.5-535.5 m, 714.5-715.5 m, and 844.5-845.5 m were selected for moulage and orientation. The fracture direction of each section of the S9 hole was determined using the same method as above. The fracture surface directions of the three measurement sections are N66.1°E, N89.2°E, and To conclude, the maximum horizontal principal stress of Xinji No.1 mine was 13.95-25.23 MPa, and the minimum horizontal principal stress was 12.16-21.17 MPa. The average azimuth of the maximum horizontal principal stress in Xinji No. 1 mine was N83.61°E, near EW direction.

Characteristics of in situ stress field in the Huainan mining area
Based on the in situ stress measurements in the Xinji No.1 mine, a total of 76 sets of in situ stress data from the Pan No.1 mine, Pan No.2 mine, Xinzhuang mine, Guqiao mine, and Wangfenggang mine in the Huainan mining area were collected to systematically analyze the characteristics of their in situ stress fields, as shown in Table 2.

Type and magnitude of in situ stress field
Among the 76 measuring points in Huainan mining area, 48 measuring points belong to the type of σ H > σ v > σ h , accounting for 63.2% of the total measuring points; 16 measuring points belong to the type of σ H > σ h > σ v , accounting for 21.0% of the total measuring points; 12 measuring points belong to the type of σ v > σ H > σ h , accounting for 15.8% of the total measuring points. Sixty-four measuring points have a maximum horizontal principal stress greater than the vertical principal stress, which accounts for 84.2% of the total measuring points. Therefore, the in situ stress field in Huainan mining area is dominated by horizontal stress, that is, horizontal tectonic stress predominates (Kang et al. 2009). The overall stress field is characterized by σ H > σ v > σ h .
According to the stress magnitude, the stress can be classified into four levels (Kang et al. 2010): a low stress level with a stress magnitude of 0-10 MPa, a medium stress level of 10-18 MPa, a high stress level of 18-30 MPa, and an ultra-high stress level of greater than 30 MPa. Among the 76 measure points, 48 measuring points have a maximum horizontal principal stress of 18 MPa ≤ σ H ≤ 30 MPa, and 6 points have a maximum horizontal principal stress of greater than 30 MPa and most of these points have a burial depth of greater than 550 m. Twenty-two measurement points with a burial depth of less than 550 m have a maximum horizontal principal stress of less than 18 MPa. Therefore, the deep in situ stress state of Huainan mining area generally belongs to a high stress level.

Variation of horizontal and vertical principal stress with depth
The relationship between in situ stress and depth of each measuring point within 350.7-1150 m is shown in Fig. 8.
To analyze the linear correlations between the vertical principal stress, the horizontal principal stress, and the burial depth, a least square method is used for a regression analysis, and the R test is used to test the significance of the regression curve.
Regression analysis shows that the relationship between in situ stresses and burial depth in Huainan mining area can be expressed as follows: (1) Vertical principal stress σ v : The significance test of the regression curve is performed: the number of samples is n = 76, and the fitting goodness R = 0.9428. It shows that there is a good linear  correlation between the vertical principal stress and the depth. Hoek and Brown (1980) obtained the Eq. (6) by studying the in situ stress measurement data of 116 sites around the world. By comparing Eq. (6) with Eq. (7), it can be seen that the gradient of regression curve of vertical stress with depth in the Huainan mining area is smaller than that of Hoek-Brown curve.
(2) Maximum horizontal principal stress σ H : The fitting goodness R = 0.6957 > R α = 0.290, α = 0.01 (significance level). It shows that there is a roughly linear correlation between the maximum horizontal principal stress and the depth.
It can be seen from Fig. 8 that the maximum and minimum horizontal principal stresses and vertical principal stresses generally increase with the increase in depth. However, due to the great difference in geological structure, stratigraphic configuration, ground temperature and groundwater, there is a certain discreteness in the measurement data of in situ stress, especially the discreteness of the minimum horizontal principal stress.

Relationship between lateral pressure ratio (λ) and depth
The lateral pressure ratio (λ) is the ratio of the maximum horizontal principal stress to the vertical principal stress, which is an index reflecting the level of tectonic stress. The relationship between λ and depth is shown in Fig. 9. It can be seen that λ is rather discrete. At burial depths of less than 550 m, λ has the largest discretization range, but with the increase of burial depth, the discretization range of λ tends to decrease gradually. In Huainan mining area, λ values of 76 measuring points range from 0.505 to 1.822, among which 12 measuring points with λ ≤ 1.0, account for 15.8% of the total number of measuring points, and 64 measuring points with λ > 1.0, accounting for 84.2% of the total number. It shows that the state of in situ stress in the mining area is dominated by horizontal stress, and the in situ stress field in the mining area is dominated by tectonic stress field. With the increase of depth, the lateral pressure coefficient is close to 1.0. λ is relatively greater in shallow strata and tends to decrease toward a critical value (9) h = 0.0145H + 5.2348 with depth increasing. The variation curve of λ versus H is similar to that from Brown and Hoek (1978) in general trend. However, there are considerable differences in magnitudes due to the distinct disparity in rock properties, depth of test sites and the range of data collected. According to classification of the macro-type of the original rock stress field (Peng and Yu 1998), it can be found that the in situ stress field in the depth of − 550 m to − 1200 m in the Huainan mining area is of the geodynamic stress field type, but with the increase of the depth, it transforms into a quasi-hydrostatic pressure field type, gradually.

Relationship between lateral pressure coefficient (k) and depth
The lateral pressure coefficient (k) is the ratio of the average horizontal principal stress to the vertical principal stress, which can be described below: For the 76 measured data, the variation range of k is 0.485-1.535, and the distribution is relatively discrete. The variation range of k is high at depths of less than 550 m and then decreases gradually with increase of burial depth with k approaching to 1.0 (Fig. 10).
Set k = a H + b , 1 H =x , where a, b are undetermined constants. k has a linear relationship with the reciprocal of H, i.e., k = ax + b.
a, b can be obtained from the linear regression analysis of the 76 groups of in situ stress data in the Huainan mining area. a = 196.9, b = 0.7, then the following equation can be written as: Fig. 9 Ratio of maximum horizontal principal stresses to vertical principal stresses with depth Fig.10 Ratio of average horizontal principal stress to vertical stress vs depth Brown and Hoek (1978) summarized in situ stress measurement results from around the world and found that the ratio of average horizontal to vertical stress, k, generally lies within limits defined by: The average value of k can be expressed as follows: Comparing Eq. (11) with Eqs. (12) and (13), within the buried depth of 460-1000 m in Huainan mining area, the variation curve of k with depth is within the Hoek-Brown inner and outer envelopes, that is, it is included in the Hoek-Brown lateral pressure coefficient range expressed by Eq. (12). And the change trend is similar to Hoek-Brown lateral pressure coefficient outer envelope (left part of Eq. (12)), even though there are significant differences in magnitude values. The reasons may be due to the following: ① in underground coal mines, the rock types are mainly sedimentary. However, Brown and Hoek's (1978) results were based on stress data records not only from sedimentary rocks but also from magmatic and metamorphic rocks; ② the maximum depth of the test sites in this study is 1150 m, while that in Brown and Hoek's data is 2806 m. Therefore, the difference between the regression curves may be due to the great disparity in depth; ③ the other possible reason for the large disparity between the data derived from this study and Brown and Hoek's data is that the range for data collection in this study is quite small compared with that by Brown and Hoek, which includes various data from many countries worldwide.

Ratio of maximum to minimum horizontal principal stresses
The ratio of the maximum horizontal principal stress to the minimum horizontal principal stress (m) in Huainan mining area ranges from 1.004 to 2.275 (Fig. 11). According to the rock mechanics theory, the maximum shear stress is half of the difference between the maximum and the minimum principal stress, and the failure of rock mass is usually caused by shear failure. Therefore, the ratio of maximum to minor horizontal principal stress, to a certain extent, reflects that the effect of tectonic stress in the mining area is relatively constant. According to statistics of the ratio of the two horizontal principal stresses in some countries and regions in the world (Oliver et al. 2018;Brown and Hoek 1978), the ratio of the maximum horizontal principal stress to the minimum

Direction of maximum horizontal principal stress
Among the 76 sets of in situ stress data collected, 36 sets have measured the orientation of the maximum horizontal principal stress. Influenced by the geological structure, the maximum horizontal principal stresses are in the NE and NW directions, with 66.6% in the direction of NEE. Therefore, the dominant direction of the maximum horizontal principal stress in Huainan mining area is in the NEE direction, which is close to the EW direction. The coal-bearing strata in the Huainan mining area are mainly affected by compressive stress in the EW direction.

Influence of tectonic evolution on the current in situ stress field
Tectonic movement is the main cause of in situ stress field, especially in the horizontal direction. The current state of in situ stress is mainly controlled by the latest tectonic movement, but also is related to the tectonic movements that occurred in the geological history.
In the geological history, the strata in Huainan mining area mainly experienced three major tectonic movements, including the Indosinian movement in the late Triassic, the Yanshan movement in the Mesozoic, and the Himalayan movement in the Cenozoic. In the first stage, a series of EW striking compressional and torsional thrust faults and imbricated structures were formed in the south of the mining area under the action of compressive stress in the SN Fig. 11 The ratio of maximum to minor horizontal principal stress varying with depth Page 13 of 18 682 direction, and "X" type joints in NW and NE directions were generated in the syncline. The maximum principal stress was in the SN direction. In the second period, the strata were compressed laterally in the NW-SE direction and twisted to the left by NE-NNE spinning. Except for the further development of faults and folds in the near EW direction, the structural differentiation was rather obvious. The structures in the sub-EW direction were transformed into ones in the NWW-SEE direction, and a new fracture system was formed in this direction. The maximum principal stress was in the NWW-SEE direction. In the third period, the tectonic stress field was transformed from compression to extension. Under the action of tensile stress in the NWW-SEE direction, a large number of normal faults in the NWW-SEE direction were formed. At the same time, the stress field under the right-handed couple action in the sub-NE direction was formed, and the maximum principal stress was in the NEE-EW direction.
According to a large number of in situ stress measurement data, the maximum horizontal principal stress in the Huainan mining area is primarily in the NEE-EW direction. This shows that the current in situ stress field in the mining area basically inherits the third stage tectonic stress field. The maximum principal stress of the tectonic stress field is in the NEE-EW direction. For example, the maximum horizontal principal stress of seven survey sections at the Xinji No.1 mine is in the direction of NE83.61°, which is the in sub-EW direction. This result is consistent with the direction of the tectonic stress field analyzed above. The results of in situ stress measurement at the Xinji No.1 mine further confirmed the mechanical mechanism of the tensile-torsional fractural structures in the Huainan mining area. The good agreement between the two shows the accuracy of the measurement results of the in situ stress field at this mine. Han and Zhang (2009) used hydraulic fracturing methods to measure in situ stresses of 1202# and 201# boreholes at the Xinji No.1 mine. It was found that the average directions of the maximum horizontal principal stresses in 1202# and 201# boreholes were 55 ° and 51 °, respectively, which was in the near NE direction. The positions of S5, S9, 1202# and 201# boreholes are all at the Xinji No.1 mine, but their maximum horizontal principal stress directions are quite different. The preliminary analysis shows that geological structure is the main factor causing the difference of the direction of maximum horizontal principal stress.

Discussion
It can be seen from Fig. 2 that the relative positions of the four boreholes and the geological structure features nearby can be determined. The structural position of 1202# borehole is just located in the south wing of the Liuka anticline. The existence of Liuka anticline indicates that the original system strata in this position were obviously pushed in the NE direction. From the regional point of view, it should have the same formation age as the Huainan syncline. Because of the relatively high degree of stratigraphic folding here, in situ stress characteristics of the NE-thrusting process were well preserved, while the other parts of the Huainan syncline were relatively flat, which was covered by the later structure and not obvious. 201# borehole is close to the Fufengxiajiapian fault, which was also pushed and compressed in the NE direction. The Fufengxiajiapian fault is a series of NE-direction compressional faults formed by the compression and destruction of the old strata in the NE-direction thrusting process of the Fufeng nappe. Because these faults had a relatively unified lower boundary, that is, the bottom interface of the nappe, and their top was relatively continuous with the nappe and discontinuous with the newer strata, which was similar to the splint between the nappe and the in situ system, the faults in this part of the strata were called the splint fault layer under the nappe in the mining area. The existence of the lower splint fault showed that the NEtrending extrusion pressure of the nappe at this location was huge, which has affected the underlying in situ system strata, especially in the shallow part of the in situ system. Therefore, the maximum horizontal principal stress in 1202 and 201 boreholes was in the nearly NE direction.
According to the location of S5 and S9 boreholes, the Fufeng nappe structure is weak in this area. S9 borehole is located in the non-structural area and the location of the moulage point is deep. The influence of the nappe on the in situ system gradually decreases, and the in situ stress characteristics of the borehole are completely controlled by the regional characteristics in the deep. S5 borehole is located in the skylight position of nappe structure and the location of the moulage measuring point is also deep and it is not affected by nappe and is primarily affected by the East-West tensile movement represented by the F10 fault. Therefore, the test results of two holes represent the regional in situ stress characteristics and the orientation of maximum horizontal principal stress is in the near EW direction.
In addition, according to the direction of the fracture surface of each depth moulage section of S5 and S9 boreholes, it can be seen that vertically the orientation of maximum horizontal principal stress has a trend of deflecting from NE to EW with the increase of depth. The geological structure is the reason for the difference of principal stress orientation in situ system in the mining area. Due to the influence of the Fufeng nappe, the shallow strata are subjected to strong compressive stress in the north-south direction, and the stress field in the mining area is nearly NE direction. The Fufeng nappe has little influence on the stress field in the deep crust. The in situ stress of coal measures is mainly affected by the in situ system, and the orientation of the maximum horizontal principal stress is close to EW.

Constraint mechanism of fault friction strength on in situ stress
Tectonic movement has a clear controlling effect on the direction of the in situ principal stress, but the limiting conditions for the magnitude of the in situ stress are unclear, and it is a question worth considering. In essence, the magnitude of the in situ stress in the crustal rock mass is the result of the constraint of the frictional strength of discontinuities, such as faults and fractures. Therefore, it is necessary to clarify the frictional strength of the fault and its constraint mechanism on the in situ stress. On this basis, the limit of in situ stress by the F10 fault in Xinji No.1 mine is analyzed.

Fault friction strength
Friction strength is a basic mechanical property of faults. Byerlee (1978) summarized a large number of experimental data of different types of rocks and faults, and found that under the action of high normal stress ( n ≥ 10MPa ), the friction along a fault surface has nothing to do with surface roughness, normal stress, sliding speed, etc., and the friction coefficient ( ) fluctuates over a small range, i.e., 0.6 ≤ ≤ 1.0 . The frictional strength of a fault is one of the mechanical properties of the fault. It is the shear stress of the fault during frictional sliding under the condition of in situ stress.
The test results show that the materials in the faultaffected zone are mainly composed of fault breccia and mylonitic debris (Logan et al. 1992). The measured density of the samples is 1.5-2.3 g/cm 3 , the uniaxial compressive strength is 0.51-0.65 MPa, the elastic modulus is 66.4-101.4 MPa, and the Poisson ratio is 0.37-0.38. The materials in the fault zone are characterized by extremely low mechanical strength and rigidity, obvious Poisson effect, and easy deformation. Table 3 shows the tangent stiffness, normal stiffness, and the friction coefficient of some structural planes in the sedimentary rock mass (Li and Yang 1999). Cohesion C of different types of structural planes is very low, and tangential stiffness is less than normal stiffness. For sedimentary structural planes, the normal deformation and shear deformation are influenced by the lithology characteristics on both sides and the friction coefficient of fracture structural planes is about 0.6. Sedimentary rock mass is prone to tensile failure in the direction perpendicular to the structural plane or shear failure along the structural plane.

Restriction of fault friction strength on in situ stress
Discontinuity surfaces, such as faults, folds, and joints of different scales and in different directions, are widely distributed in the coal mining area, and the magnitude of in situ stress is obviously constrained by the friction strength of these discontinuities (Zoback 2007). The in situ stress values measured in many areas are balanced with the friction strength of discontinuities and therefore the in situ stress values can be inferred. However, it cannot be simply considered that this is always the case. In fact, the in situ stress values of the mining area cannot exceed the friction strength of the primary fault. Figure 12 gives an understanding of this concept. Figure 12a shows a two-dimensional fault. Ignoring the effect of the intermediate principal stress, and setting the angle between the fault normal and the maximum principal stress ( S 1 ) as , the shear and normal stresses acting on the fault plane can be expressed by the following equations: where 1 and 3 are the maximum and minimum effective principal stresses, respectively, 1 = S 1 − P p , 3 = S 3 − P p ;S 1 and S 3 are the total maximum and minimum principal stresses, respectively;P p is the pore pressure.
The faults (No.1) indicated by red lines in Fig. 12b are in the dominant direction. The faults (No.2) represented by the black lines are nearly perpendicular to S 1 , with larger normal stress and smaller shear stress. The faults (No.3) represented by the blue lines are nearly parallel to S 1 , and its (14) = 0.5 1 − 3 sin 2 n = 0.5 1 + 3 + 0.5 1 − 3 cos 2 normal stress and shear stress are smaller than those in the dominant direction. For any given 3 , there is a maximum value of 1 determined by the friction strength of the fault. The Mohr circle cannot exceed the upper limit of the friction strength, as shown in Fig. 12c. The critical friction angle in which the fault is the easiest to slide can be expressed as: According to Amonton's friction theorem (Archie 1950), Eq. (14) can be written as: Jaeger (1971) found that when the fault in the critical friction direction is in a limit friction equilibrium state, the relationship between the ratio of the maximum to the minimum effective principal stress and the fault friction coefficient can be expressed as follows: The above concept is explained by shear stress and normal stress on fault planes in three different directions, as shown in Fig. 12c. Figure 12c can be regarded as a case of strike-slip fault with 2 = v . In this case, the ratio of maximum to minimum effective principal stress, namely 1 ∕ 3 , is limited by the frictional strength of the primary fault. For the faults represented by the number 1, if S 1 increases with respect to S 3 , sliding occurs when the frictional strength of the faults in the critical direction is reached. Once faults begin to slide, S 1 will not increase. These types of faults become critical stress faults that are in a critical state of sliding, while other faults are not. The faults almost perpendicular to S 1 have relatively large normal stress but have not enough shear stress to cause sliding; while for the faults almost parallel to S 1 , both the normal stress and shear stress on the fault planes are relatively small.
Fault friction strength is the maximum static friction force before fault sliding under the action of in situ stress.  In situ stress controls the type and nature of the fault and affects the friction strength of fault (Kim and Hosseini 2017). The in situ stress in the rock mass is a three-dimensional stress field with unequal pressure. The magnitude and direction of the three principal stresses vary with space and time. According to the Anderson's fault theory (Anderson 1951), there are three types of stress states including normal, reverse, and strike-slip faulting stress regimes. The upper limit value of the ratio of the maximum to minimum effective principal stresses is estimated as follows: According to Eq. (18) and the three states of the original rock stress of the Anderson's fault theory, the lower limit value of the minimum horizontal principal stress of the normal fault and the upper limit value of the maximum horizontal principal stress of the reverse fault and the strike-slip fault are obtained.
where q f =[( 2 + 1) 1∕2 + ] 2 , LB h and UB H are the lower limit of the minimum horizontal principal stress and the upper limit of the maximum horizontal principal stress, respectively.
In the stress mechanism that forms a normal fault, the minimum horizontal principal stress is located between the lower limit of the minimum horizontal stress and the overlying rock load. F10 is the largest normal fault in the mining area. It traverses east-west and has a strike length of nearly 10 km. There are two normal faults F10 and F10-1 in the shallow strata, and these two faults are combined into one in the deep. The F10 fault strikes N70°W, trending SW with a dip angle of 70° and a fault throw of 0-230 m. S9 borehole is located in the east side of 1# exploration line and north of 114# borehole (Fig. 13). It can be seen from the geological structure plan in Fig. 2 that S9 borehole is located in the north side of the F10 normal fault. The in situ stresses at S9 borehole in Xinji No.1 mine are controlled by the F10 normal fault, and the minimum horizontal principal stress should be between the lower limit of the minimum horizontal stress and overburden stress. Assuming the friction coefficient of the F10 fault is μ = 0.6 which was measured in laboratory (Ben-David et al. 2010), the lower limit of the minimum horizontal stress can be calculated from Eq. (19), as shown in Fig. 14.