In-situ stress measurements and stress accumulation level analysis of coal 1 mines in northern Ordos Basin

For 350m, k to the principal by KE have error 15% with of OC, ASR, HF dominant orientation basically consistent with of focal mechanism solution The fracture stress of mines lower than friction limit state, its level Abstract For non-directional drilling cores, the sample selection and test method for Kaiser effect (KE) 12 in-situ stress measurement were proposed, and the magnitude and direction of its principal 13 stresses were theoretically derived. Based on this method, the KE of 423 samples in Burtai and 14 Baode coal mines in northern Ordos Basin (NOB) were tested. The results show that σ H , σ h and 15 σ v vary with depth and location, and their values increase with increasing of depths. Generally, 16 horizontal stresses play a leading role. There are main stress regimes in NOB: σ H > σ h > σ v 17 (Burtai, <172m; Baode, <170m) and σ H > σ v > σ h (Burtai, 170-800 m; Baode, 170-400 m), and 18 the σ v > σ H > σ h stress regime is mainly distributed in moderately deep to deep coal mines. For 19 rock masses with a depth of 350m, k (( σ H + σ h ) / 2 σ v ) tends to 1, indicating that deep critical 20 state will gradually emerge. The test results were compared with those of overcoring method 21 (OC), elastic strain recovery (ASR) and micro-hydraulic fracturing (HF). The relative errors of 22 σ H , σ h and σ v are 14.90%, 19.67%, 15.47% (Burtai) and 10.74%, 22.76%, 19.97% (Baode), and 23 they are all within a mechanisms in this area. Based on Byerlee-Anderson theory, the stress accumulation level of 27 mine rock mass was discussed. Under dry rocks or hydrostatic pressure rocks, the friction 28 coefficient of faults is both low, which is less than the lower limit (0.6) of strike-slip faults slip, 29 indicating that the fracture stress with a low level around the study area is lower than the friction 30 limit stress. The stress accumulation level in Baode mine is slightly larger than that in Burtai 31 mine. 32

generally, six or nine small samples (Qin et al. 1993) are drilled in different directions on the 179 parent rock block, and the samples are cylindrical or square, with a width to height ratio of 1:2. 180 However, the core of ground drilling is different from that of the shallow layer, most of the 181 drilling holes are straight holes, and it is difficult to achieve core orientation. The diameter of 182 the parent rock core is mostly about 75 mm, so it is impossible to drill as required, so it is 183 necessary to reduce drilling number of small samples. KE for testing in-situ stress based on the 184 non-directional core assumes that the vertical stress is one of the principal stresses and the other 185 two are horizontal stresses, it is necessary to process four small cylindrical samples, one along 186 the core axis, three along the perpendicular to the core axis, and drill in 0°, 45° and 90° or 0°, 187 60° and 120° anticlockwise. The diameter and height are 25 and 50mm, which conforms to the 188 rock mechanics test standard. The on-site coring location and sample processing process are 189 shown in Fig. 2, and a total of six drilling holes (Red; Burtai, five; Baode, one) were selected 190 for AE testing. Generally, the influence of core sampling damage on KE can be ignored (Lavrov,191 2003).  Before the test, 2-4 sensors are used to collect data for small samples, which are placed 197 in the middle of the sample side, as far as possible away from the upper and lower end faces. 198 To ensure the coupling effect between sensor and end face, vaseline or butter is used as the 199 coupling agent, and the sensors are fixed with adhesive tape or plasticine. According to 200 previous research experience (Qin et al., 1993), in this paper, AE monitoring system is set up 201 with preamplifier gain of 40dB, valve value of 45dB and sampling rate of 1MHz. 202 When the load rate is 0.05-0.25 kN/s or the displacement rate is 0.05-0.6 mm/min in 203 uniaxial loading, the AE effect is better. With the increase of the sample moisture content will 204 lead to the decrease of AE count and energy, so the natural or dry sample should be used (Yang 205 et al., 2018). When triaxial loading test is adopted, the stratum stress where the sample is 206 located can be estimated in advance, to set the confining pressure and load the axial load at a 207 constant rate. When the rock is buried shallowly and confining pressure is low, the KE can be 208 used to testing in-situ stress directly by uniaxial loading. 209 For identification of KE points, most scholars choose AE count or count accumulation as 210 parameters, but the author finds that the comprehensive identification method, which is mainly 211 AE count or count accumulation, supplemented by AE energy rate or count rate increment (RI), 212 is easier to determine the KE point. To avoid the interference of friction AE at the initial stage 213 of loading, KE points are usually identified using uniaxial double-cycle loading. Since more 214 AE are produced by crack closure at the beginning of loading, generally, the surge point of the 215 first cycle AE should not be selected, but that of the second loading cycle AE should be used 216 as KE point (Fig. 3). Moreover, the first cyclic peak load σP is less than σHmax, σP is about 30% 217 of σC. Specific identification method of this paper: first, based on AE count or count cumulative 218 versus time, find the counting group, then identify the surge point according to AE energy rate 219 or RI versus time, mutual verification, and comprehensively determine KE point. If there are 220 multiple KE points under cyclic loading or they are not obvious, AE-DRA method can be used 221 to try. The loading mode is multiple cyclic loading controlled by load and displacement. The 222 first cycle σP is less than σHmax, and the next cycle σP is greater than σHmax and less than σe. It is 223 suggested that the reasonable σP in the first cycle is less than 27% of σC in siltstone, fine 224 sandstone and coarse sandstone, 40% of σC in sandy mudstone, 17% of σC in medium sandstone, 225 13% of σC in mudstone and limestone, and it does not exceed 63%-70% of σC in nest cycles 226 (Yang et al., 2018). If further KE point recognition is still not ideal, the method of calculating 227 the fractal dimension by G-P algorithm can be used to try. In this paper, three methods are used.
If the corresponding Mohr stress circle is established, σα and τα on α slope can also be 240 obtained from it. Then, two principal stress values can be determined as follows: x y x y xy x y x y xy (2) 242 If the angle between x-axis and direction of σH on the unit is β, and clockwise direction is To determine the angle between two principal stresses and x-axis, first, calculate β and β 246 ± 90° from Eq. (3), and then compare the magnitude of σx and σy. If σx > σy, the angle between 247 the maximum principal stress (σ1) and x-axis is β; if σx < σy, the angle between σ1 and the x-axis 248 is β ± 90°; if σx = σy, then β = -45°. That is to say, σ1 is always inclined to the larger of σx and With an increase of depth, the mechanical properties of the rock are different from those 259 of shallow, and the stress is more significantly affected by pore fluid, which can be

Core orientation using Paleomagnetic technology 266
For non-directional drilling core, the angle between σH and 0° mark line can be obtained   Among them, the data of BK209 is too discrete, the fitting coefficient is low, and others 318 are more than 0.7. The large discreteness of data is mainly caused by the heterogeneity of rocks 319 caused by different lithology, structure, bedding, and so on. It can be seen from Eq. (7) that σv 13 increases most rapidly with the increase of depth. In shallow mine, the horizontal stress is large.

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(5) The variation of k with depth is shown in Eq. (9). Compared with k envelope (Eq. (10)) different, but they are basically the same in the progressive value of the inner envelope. It can 350 be seen from Fig. 6 (a)    that. Fig. 7 shows stereoscopic projection and the orthogonal projection of the magnetization 384 directions of some samples. Table 2 shows the test results of the samples. Fig. 8 (a) shows the 385 σH azimuth half-rose of samples.   Among the 63 measuring points in Fig. 8 (a), there is 22 σH orientation between N30°E

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In the end, a total of 24 groups of in-situ stress data were obtained, including 13 groups of 420 measured σv and 11 groups of calculated σv by the fitting equation.  (Fig. 5 (g)). The fitting equations are as follows: (2) There are 23 with σh < 10 MPa, accounting for 96% of the total, and only one with 10 430 < σh < 18 Mpa (Table 1). Therefore, the in situ stress in Baode mine belongs to the low in situ 431 stress field as a whole.

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In-situ stress testing of three boreholes (Fig. 2, light green) by OC in the third panel of 531 Baode mine was carried out by Anhui University of science and technology. The depth is 485m.

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KE stresses of three measuring points close to their depth are selected and compared with that.

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Eq. (11) is used to calculate the 2 # and 6 # measuring points. According to Table 4, the seven 534 stresses of KE are greater than that of OC, accounting for 77.8% of the total. The relative error 535 of σH is 8.46%-12.41%, that of σh is 27.18%-35.65%, that of σv is 34.82%-45.59%, and that 536 of principal stress is 9.91%, 30.82% and 39.84% respectively, among which that between 537 fitting equation and OC is the largest.

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The stress values measured by KE is compared with that measured by ASR (Xu, 2018), 539 and the cores of the same depth in the same borehole is selected. The samples of 51-53 and 540 62-64 groups in Table 4 are selected by KE, and that of BD-2 (275.5m) and BD-6 (346.0m) 541 are selected by ASR, among which the average values of σH, σh and σv of 51-53 groups are 542 8.28, 6.16 and 7.31 MPa, respectively, and that of σH, σh and σv of BD-2 are 6.8, 4.7 and 7.3    Based on the Byerlee Anderson theory and in-situ stress test results, the stress state of 555 boreholes in two mines is drawn, as shown in Fig. 9. None of the six boreholes has a measured 556 point with μ > 0.6, and σH is all on the left of the lower limit of Byerlee range, and μ < 0.3.

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Below 350m, a small amount of data in Burtai mine is between 0.6 > μ > 0.3, which indicates 558 that with the increase of depth, the stress accumulation level is increasing. Therefore, it can be 559 concluded that the friction strength of the faults around the boreholes in two mines is weak, the 560 stress level is low, and they are in a relatively calm period, and the faults and surrounding areas 561 are in a relatively stable state.  The sliding failure caused by the increase of shear stress is a common fracture 565 phenomenon in the earth's crust. The main controlling factors are the maximum shear stress 566 and the normal stress of vertical shear plane. Therefore, the μmd and μmh calculated based on in-567 situ stress are also indicators to characterize the regional stress accumulation level (Jamison 568 and Cook, 1980). The relationship between μm and μ is as follows: The expressions of indexes μmd and μmh are as follows: Where σ1, σ3 and P0 represent the maximum principal stress, the minimum principal stress 573 and the pore pressure respectively. In the shallow low permeability rocks of the crust, P0 is 574 roughly equal to the static pressure of the water column (Townend and Zoback, 2000). To some 575 extent, μmd and μmh only represent the stress accumulation level, independent of the stress 576 direction. The larger the value is, the greater the shear stress around the fault is, the higher the 577 stress accumulation level is, the greater the possibility of fault activity is, and vice versa 578 (Jamison and Cook, 1980). The shallow μm is mostly 0.7, the deep μm is 0.5 (Tuncay and Ulusay, 579 2008), so when μm is 0.5-0.7, the crustal stress accumulation is in its friction limit state; when 580 μm is close to 0.5-0.7, the stress accumulation level is higher; when μm is less than 0.3, that is 581 lower. 582 Figure 10 gives the distribution of μmd and μmh with depth in two mines. In dry rock, the 583 μmd in Boertai mine is 0.04-0.76, with an average of 0.24 and a mean square deviation of 0.14, 584 among which, μmd > 0.1, accounting for 87.6% of the total, and only two points with μmd > 0.6,  0.32-0.50, which are less than the lower limit (0.6) of strike-slip faults slip. It shows that the 606 fracture stress state around the study area is lower than the friction limit state, which is at a low 607 level as a whole, and the risk of seismic activity is low. Historically, the number of earthquakes 608 in Ordos City and its vicinity was not many, and the intensity was not high, and the regional Baode mine, and the test results are compared with that of OC, ASR and adjacent mine HF.

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Based on Byerlee-Anderson theory, the stress accumulation level of the rock mass is discussed.

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The main conclusions are as follows:

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(1) For the non-directional drilling core, based on the elastic mechanics, the magnitude and direction of the principal stress measured by KE are derived. The angle between the 622 direction of σH and the marker line is β0 or β0 + 90°. The azimuth of σH relative to the geographic 623 north pole is calculated as D0-Da-β. 624 (2) The σH, σh and σv in two mines increase with increasing of depths. The ratios of σH, σh 625 to σv are 1.20, 0.83 (Burtai), 1.76 and 0.96 (Baode), respectively. k is 1.02 (Burtai) and 1.36 626 (Baode). Generally, horizontal stresses play a leading role. The ratio of σH to σh is 1.53 (Burtai) 627 and 2.04 (Baode), and (σ1-σ3)/2 is 2.05 MPa (Burtai) and 1.46 MPa (Baode). There are main 628 stress regimes in NOB: σH > σh > σv (Burtai, <172m; Baode, <170m) and σH > σv > σh (Burtai, indicating that the fracture stress with a low level around the study area is lower than the friction 646 limit stress. The stress accumulation level in Baode mine is slightly larger than that in Burtai