Experimental Analysis and Control Technology of Deformation and Failure Mechanism of Inclined Coal Seam Roadway using Non-contact DIC Technique: A Case Study

13 In order to study the deformation and failure mechanism of surrounding rock of roadway in inclined coal 14 seam, the physical similarity model of right-angle trapezoidal roadway in inclined coal seam, in which the non- 15 contact digital image correlation (DIC) technology and the stress sensor is employed to provide full-field 16 displacement and stress measurements. The deformation control technology of the roadway surrounding rock 17 was proposed and applied to engineering practice. The research results show that the stress and deformation 18 failure of surrounding rock in low sidewall of roadway are greater than those in high sidewall, showing 19 asymmetric characteristics, and the maximum stress concentration coefficients of roadway sidewall, roof and 20 floor are 4.1, 3.4 and 2.8, respectively. A concept of roadway "cyclic failure" mechanism is proposed that is, the 21 cyclic interaction of the two sidewalls, the sharp angles and roof aggravated the failure of roadway, resulting in 22 the overall instability of roadway. The roadway sidewall is serious rib spalling, the roof is asymmetric "Beret" 23 type caving arch failure, and the floor is slightly bulging. On this basis, the principle of roadway deformation 24 control is revealed and asymmetric support design is adopted, and the deformation of roadway is controlled, 25 which support scheme is effective.

to study the deformation and failure mechanism of roadway in inclined coal seam to effectively support roadway 37 to ensure its stability. 38 At present, many scholars have studied the deformation control of roadway surrounding rock in inclined coal 39 seam by using the physical model test, theoretical analysis and numerical simulation. Manchao He et al. studied 40 technique is employed to provide stress and full-field displacement measurements, respectively. For damage 71 characteristics, an asymmetric support plan is proposed to strengthen key parts' support, which is applied to 72 engineering practice.

Test setup 91
In order to control the deformation of asymmetric roadway surrounding rock in inclined coal seam and mine 92 inclined coal seam safely and efficiently, a large-scale variable angle model test (length × width × height = 1.2m 93 × 0.12m × 1.1m) apparatus is assembled to reveal the deformation failure mechanism of roadway surrounding 94 rock without support (see Fig. 3). In this study, in order to simulate the effect of in-situ stress, uniform load was 95  The real profile of right angle trapezoidal roadway in inclined coal seam is adopted in the model test. Based 106 on the laws of similitude 21-25 , the similarity constants of the geometry, bulk density, and stress of the calculated 107 model are determined to be 30, 1.6, and 48 respectively by Equation (1), then the width of the roadway and the 108 height of the low sidewall of roadway are 150 mm and 100 mm respectively. Meet the requirements of the roadway 109 to boundary distance/roadway radius ≥ 3 26,27 . The model's prototype dimensions are 33 m in height, 36 m in length 110 and 3.6 m in width. 111 Where ασ is the strength and stress similarity constant; αr is the similarity constant of bulk density; αL is the 113 geometric size similarity constant; rP is the average bulk density of the original rock, taking 2.5 g/cm 3 ; The rM is 114 the bulk density of the model material, generally rM in 1.5-2.5 g/cm3 is suitable, too large molding compaction is 115 difficult, too small makes the model material loose and difficult to form. 116 Simulated materials with a mixture of sand, gypsum and CaCO3 are used to mimic the physical and 118 mechanical properties of coal and rock strata, and mica flake is used to simulate the joint layer between coal and 119 rock. In this study, the ratio of similar materials after optimization from strength test trials is shown in Table 2. 120 The meaning of proportioning number: the first digit represents the ratio of sand to binder, and the second and 121 third numbers represent the proportion of gypsum and CaCO3 in the two cements, such as 737 in the table, which 122 means the sand binder ratio is 7:1, and the ratio of gypsum and CaCO3 in a cement is 3:7.
The monitoring system consists of stress sensor (L-YB-150 (φ28 mm × 10 mm), AD-64 data acquisition 125 instrument, DIC, displacement meter and computer. The monitoring content includes the stress, surface 126 displacement and deformation damage characteristics of the roadway surrounding rock. 127 (1) Stress monitoring of roadway surrounding rock 128 Throughout the backfilling process, a total of 19 stress force sensors are embedded in the rock mass around 129 the roadway (the roof, two sidewalls, floor and four sharp corners of the roadway) as schematically presented in 130 In this experiment, the photographing format is 1.2 × 1.1m, the magnification is 3.383 pixel / mm, and the accuracy 147 of DIC displacement measurement is expected to be 0.0296mm, as shown in Fig. 5

151
The mixture of sand, calcium carbonate and plaster powder was mixed evenly with water, and then it was 152 layered and filled into the model test apparatus to make the physical similarity model, in which the layered 153 thickness was controlled at 1-3 cm. The stress sensor is embedded in the corresponding position. After the model 154 was placed for one month, round measuring points were pasted around the roadway and speckle patterns were 155 prefabricated. According to the size of the roadway, the sawing device is carefully used to expand the excavation 156 range from the center of the roadway to the opening until the roadway contour is reached. Then the load is applied 157 to the top of the model step by step until the roadway is completely destroyed. During the loading process, the 158  surrounding rock of the two sidewalls of roadway was bulging into the roadway. Combined with the displacement 239 nephogram Fig.12 of all the measured points on the two sidewalls of roadway, it can be seen that the deformation 240 of the shallow surrounding rocks of the two sidewalls of roadway is large, and the upper side of the two sidewalls 241 of the surrounding rocks severely bugles. With the increase of the distance from the surrounding rock in the 242 roadway to the sidewall, the deformation is smaller, indicating that the deep surrounding rock of roadway is in the 243 compaction stage and is relatively stable. With the continuous increase of load, the roadway's two sidewalls 244 seriously bulge, and the maximum displacement of the high sidewall and the low sidewall is 1.934 mm and 2.98 245 mm, respectively. There is no data for the last loading of the low sidewall measuring point of the roadway, 246 indicating that the surrounding rock of the roadway's low sidewall surface is spalling and the measuring point is 247 damaged. As shown in Fig. 18 (d), the surface damage of the roadway's low sidewall is more significant than that 248 of the high sidewall surface, which shows asymmetric characteristics. 249  The correlations between internal displacement of roadway and loading are plotted in Fig. 17. It can be seen 285 that when the loading is at a low level (i.e. less than 0.063MPa), both the internal displacement of roadway are 286 insignificant. When the load increases beyond 0.063MPa, the displacement at roof and two sidewalls increase 287 rapidly, whereas the deformation of the two sidewalls of the roadway is more serious than that of the roof and the 288 displacement of low sidewall is larger than that of high sidewall. The asymmetric displacement could be caused 289 by dip angle of coal seam and section type of roadway. 290

Deformation and failure characteristics of roadway surrounding rock 291
The deformation and failure characteristics of the roadway surrounding rock are the most direct embodiment 292 of its failure. With the increasing external load, the roadway's surrounding rock begins to deform slightly, and then 293 cracks, crack propagation, local fracture, and overall failure occur, as shown in Fig. 18. 294 (1) When the load reaches 0.049 MPa, the stress concentration of the roof angle of roadway's high sidewall 295 reaches the maximum value, and the stress concentration coefficient is 3.1. The first fine cracks appear on the 296 surface of the roof angle of the high sidewall of roadway. The fine cracks' length is 20-40 mm, and the roof angle 297 of the high sidewall occurs local collapse, as shown in Fig. 18 (a). The roadway's low sidewall slightly bulged, and a crack appeared on the surface of the roadway's low sidewall. 315 The stress concentration at the roadway floor's low sidewall reached the maximum value, with the stress (4) When the load was 0.112 MPa, it is that the roadway's plastic deformation occurs at this time and the 318 stress concentration of the surrounding rock of the low sidewall of roadway transfers to the deep surrounding rock 319 of the low sidewall. Continuous loading until the roadway is completely destroyed, the two angles of roadway roof 320 collapse and the failure of roof angle of the low sidewall of roadway is greater than that of the high sidewall. The 321 roof of roadway is separated and collapsed, showing an asymmetric ''Beret'' type arch failure, and the cracks in the 322 roadway floor deepened, and slight floor heave occurred, but the stress at the two corners of the roadway floor 323 changed little and was relatively stable. Finally, only one crack appeared at the low sidewall angle of floor surface. 324 The two sidewalls of roadway are seriously fragmented. The upper side of the roadway's high sidewall is partially 325 collapsed, and the low sidewall of roadway is bulging, all of which are collapsed. The surrounding rock of roadway 326 surface is exfoliated, and the damage of the roadway's low sidewall is greater than that of the high sidewall, 327 showing asymmetric characteristics, as shown in Fig.18 (d). 328 (1) (2) (3) Under the action of load, the right angle trapezoidal roadway in inclined coal seam is subject to cyclic failure 331 and the failure of roadway presents asymmetric characteristics. Since the section type of inclined coal seam 332 roadway is right-angle trapezoid, the stress concentration is easy to occur at the sharp angle of the roadway roof, 333 and the stress concentration at the sharp angle of the high sidewall of roof first reaches the maximum value, 334 resulting in cracks. With the increase of load, the roof's stress concentration, two sidewalls, and floor of roadway 335 also reached the maximum. At this time, cracks appeared in the roof's internal sidewall, the high sidewall of floor 336 and the two sidewalls, and then the roof was slightly separated, the two sidewalls bulged, and the floor of roadway 337 was slightly heaved. As the load continues to increase, the stress concentration of the roadway's low sidewall 338 transfers to the deep, and the failure of the surrounding rock of low sidewall of roadway extends from the shallow 339 to the deep. At this time, the two sidewalls of roadway appear spalling and slight collapse, and the failure of the 340 low sidewall of roadway is greater than that of the high sidewall, which leads to the decrease of the support capacity of roadway, increases the span of roadway, and aggravates the separation of roof. The deformation of roof 342 increases the pressure of the two sidewalls and intensifies the two sides' failure. As the connection part, the corner 343 continues to deteriorate the stress state. Finally, the two sidewalls' failure extends to the deep part, which 344 deteriorates the roof's stress conditions, and the roof rapidly collapses from the floor, showing an asymmetric 345 "Beret" type of caving arch failure. That is, the roadway two sidewalls, two sharp angles of the roof, and the roof 346 fall into a vicious cycle that increases each other's stress, weakens the material, and intensifies the failure. This 347 cycle continues until the roadway is completely unstable. 348 It can be seen from the above that the two sidewalls, roof and roof sharp angle of roadway are the key positions 349 of roadway deformation and failure, and show asymmetric characteristics. Therefore, the key to breaking the 350 vicious circle and controlling the roadway's stability is to improve the asymmetric stress state of the roadway 351 surrounding rock, adopt the supportive measures of asymmetric and key parts strengthening and improve the 352 strength of roadway surrounding rock. 353 5 Support design based on asymmetric failure mechanism of roadway 354

Principle of Support Design 355
According to the principle of cyclic failure and asymmetric failure of right angle trapezoidal roadway in 356 inclined coal seam, the control principle of surrounding rock deformation is proposed: 357 (1) In order to increase the support strength of roadway surrounding rock and improve the asymmetric stress 358 state of roadway surrounding rock. The roof of roadway is supported by anchor net, anchor bolt and anchor cable 359 combined with asymmetric support. Because the roof of roadway is destroyed by asymmetric ''Beret'' type caving 360 arch, the roof anchor bolt and anchor cable are arranged by inclined installation and inclined to the low sidewall. 361 The two sidewalls of the roadway are asymmetric supported by anchor net and anchor bolt. The stress 362 concentration and deformation failure of the roadway's low sidewall is greater than those of the high sidewall, so 363 the anchor bolt installation density should be appropriately increased in the same area. 364 (2) Strengthening the support of weak parts of roadway. The deformation of the two corners of the roadway 365 roof is serious, and the anchor bolt and anchor cable should be set up. Anchor bolt and anchor cables with sharp 366 angles of the high sidewall tend to the high sidewall, and anchor cables with sharp angles of the low sidewall tend 367 to the low sidewall, so as to strengthen the control of high-stress concentration and deformation and failure at the 368 corner. It is also necessary to strengthen the support of the local position of the bottom boundary. By reducing the 369 risk of damage at both ends of the floor, the instability of the whole floor is prevented, and the bearing capacity of 370 the roadway is improved. 371 (3) The supporting parameters of the anchor bolt and anchor cable are optimized to reinforce the surrounding 372 rock of roadway. The length of the anchor bolt and anchor cable is determined by calculating the range of loose 373 circle of roadway roof and two sidewalls, and the anchor cable should be anchored in the stable rock layer in the deep of roadway roof to ensure that the anchor cable can provide stable and long-term suspension force. 375 (4) In order to control the stability of the surrounding rock on the surface of the roadway, the metal mesh 376 should be used to support the two sidewalls of roadway. 377

Parameters of support design 378
According to the deformation control principle of asymmetric roadway in inclined coal seam, considering 379 the asymmetric characteristics of surrounding rock stress, deformation and failure and the range of loose circle, 380 combined with the support scheme of inclined coal seam roadway in No.2 mining area of shitanjing, the 381 asymmetric support design was carried out, and the combined support of anchor bolt, anchor cable and metal mesh 382 was adopted. The theoretical calculation was carried out according to Pu's theory and elastic mechanics. The 383 specific support parameters are as follows: 384 385 Fig. 19 Loose ring of roadway roof 386 Due to the influence of dip angle on the roof of inclined coal seam roadway, the stress distribution of the 387 two sidewalls shows asymmetric characteristics, which leads to the asymmetric "Beret" type caving arch of the 388 roof loose circle of the roadway (as shown in Fig. 19). According to Pu's theory, the range of roof loose circle H 389 is shown in Equation (2): 390 Where H is the range of roof loose circle; α is the dip angle of rock stratum, is 23°; a is the width of roadway, and 392 its value is 3 m; b1 the height of high sidewall of roadway, and its value is 4.91 m; b2 is the height of low side of 393 roadway, and its value is 3 m; f is the Pu's coefficient of roof rock, and its value is 2.3; φ is the internal friction 394 angle of the rock mass in the two sidewalls, and its value is 28°; c is cohesion, and its value is1.4; The calculation 395 results show that the range of roadway roof loose circle is 1.93 m. 396 Then the length LR of roof anchor bolt is shown in Equation (3): 397 Where LR is the length of roof anchor bolt; LR1 is the exposed length of roof anchor bolt, and its value is 0.15 m; 399 According to engineering analogy and experience, the length of roof anchor bolt is 2.7m, and the row spacing 402 of roof anchor bolt is 0.8m. 403 Due to the plastic zone of the two sidewalls of the right angle trapezoidal roadway in inclined coal seam is 404 asymmetric, the section form is simplified to different circular roadways (as shown in Fig. 20). According to the 405 elastic-plastic mechanics, the range of the loose circle of the high sidewall and the low sidewall of the roadway is 406 RH and RL, respectively, as shown in Equations (4) and (5) Where RH and RL are the range of loose circle of high and low sidewalls of roadway, respectively; P is vertical 411 original rock stress, and its value is 10 MPa; α is the dip angle of rock stratum, and its value is 23°; a is the width 412 of roadway, and its value is 4.5 m; b1 the height of high sidewall of roadway, and its value is 4.91 m; b2 is the 413 height of low side of roadway, and its value is 3 m; φ is the internal friction angle of the rock mass in the two 414 sidewalls, and its value is 28°; c is cohesion, and its value is 1.4. The calculation results show that the range of the 415 left side loose circle is 1.58 m, and the range of the right side loose circle is 1.1 m. 416 The length of the high and low sidewall anchor bolts of the roadway is shown in (6)  Where LLS is the length of low-sidewall anchor bolt; LS1 is the exposed length of two sidewalls anchor bolt, and its 420 value is 0.15 m; LS2 is the anchorage length of anchor bolts, and its value is 0.45 m; 421 According to the engineering analogy and experience, the length of the left side anchor bolt is 2.2 m, the 422 length of the right side anchor bolt is 2.0 m, and the row distance between the two sides of the roadway is 0.8 m. 423 According to the above theory's calculation results and the asymmetric deformation and failure of the roadway 424 and the existing support experience, and a new support scheme was developed for the roadway of No.2 Mining 425 Srea of Shitanjing, as shown in Fig. 21. The following support systems were applied: 426 (1) Support pattern: Combined support system with anchor bolt, anchor cable, beam, and metal mesh. It can be seen from Fig. 22-25, the maximum settlement of the roof is 75 mm, and basically stable after 13 448 days. The maximum relative convergence of the two sidewalls is 74 mm, which is basically stable after 15 days. 449 The maximum roof separation occurred in 1.2 m-1.6 m. After 12 days, the roof separation stabilized, and the 450 maximum separation was 10 mm, and its maximum separation occurred in the range of anchor bolt support. The 451 maximum axial force of the roof anchor bolt is 35 kN. The loose circle range of the roadway roof is 1.4-1.7 m, 452 and the loose circle range of two sidewalls is 1.2-1.5 m, which is basically consistent with the results of theoretical 453 calculation. The subsidence of the roof, the convergence of the two sidewalls, and the stress of the anchor bolt are 454 less than the design requirements, indicating that the asymmetric support scheme effectively controls the 455 deformation of the surrounding rock of the roadway, improves the stability of the roadway, and the support effect 456 is good. It can be seen that the asymmetric support technology of inclined coal seam roadway has good 457 applicability. 458 roadway in inclined coal seam are investigated systematically through a series of physical model tests with the 461 facilitation of non-contact full-field displacement measurements using the DIC technology. The asymmetric 462 support measures are proposed and applied to engineering practice. The following conclusions are drawn: 463 (1) It is found that the stress distribution of surrounding rock of right angle trapezoidal roadway in inclined 464 coal seam is asymmetric, and the maximum stress concentration coefficient of two sidewalls, roof and floor are 465 4.1, 3.4 and 2.8, respectively. The stress concentration of the low sidewall of the roadway is significantly greater 466 than that of the high sidewall, and the distance from the stress concentration position of the low sidewall to the 467 sidewall of the roadway is greater than that of the high sidewall. With the increase of load, the stress concentration 468 of the two sidewalls transfers to the deep. The stress concentration of the high sidewall of roadway roof is greater 469 than that of the low sidewall in the early stage of loading. With the increase of load, the stress concentration of 470 high sidewall transfers to low sidewall. The stress concentration of the low sidewall of the roadway floor is greater 471 than that of the high sidewall. The size of the four corner stress distribution value is: HSRA > LSRA > HSFA > 472 LHFA. 473 (2) The concept of "cyclic failure" of right angle trapezoidal roadway in inclined coal seam is put forward, 474 that is, the roadway's failure originates from the sharp angle of roof and sidewall of roadway, and the cyclic 475 interaction of the two sidewalls, the sharp angles and roof aggravated the failure of roadway, resulting in the overall 476 instability of roadway. Due to the influence of asymmetric stress concentration of roadway surrounding rock, its 477 deformation and failure also show asymmetric characteristics. The sidewall of the roadway is most seriously 478 damaged, and the low sidewall is greater than the high sidewall. Roof separate layer caving, showing asymmetric 479 "Beret" type caving arch failure. The corner deformation and failure of the low sidewall of the roof are greater 480 than that of the high sidewall. There is a large crack at the high sidewall of the bottom plate and a slight floor 481 heave. There is no large damage at the two corners of the bottom plate and only the crack at the corner of the low 482 sidewall. 483 (3) Based on the principle of cyclic failure and asymmetric failure of right angle trapezoidal roadway in 484 inclined coal seam, some asymmetric support principle of roadway surrounding rock in inclined coal seam was 485 proposed. In brief, the principles aimed to increase the support strength to the right angle trapezoidal roadway in 486