Surrounding Rock in Entry Retaining along the Gob Side

School of Civil & Architecture Engineering, Xi’an Technological University, Xi’an 710021, China Shaanxi Key Laboratory of Loess Mechanics and Engineering, Xi’an University of Technology, Xi’an 710048, China State Key Laboratory of Coal Resources and Mine Safety, China University of Mining & Technology, Xuzhou 221116, China Jiangsu Vocational Institute of Architectural Technology, Xuzhou 221116, China


Introduction
Entry retaining along the gob side refers to that in order to recover the protective coal pillars reserved in the traditional mining method, the roadway in the upper section is resupported by certain technical means and left to the next section for use.
is is of practical significance to the technical transformation of production mine, relaxation of mining relationship, and extension of mine life. Luan mining area contains a total of 10 planned mines and 8 production mines, namely, Shigejie mine, Wuyang mine, Zhangcun mine, Changcun mine, Wangzhuang mine, Tunliu mine, Sima mine, and Gaohe mine. No 3# coal seam is the main mining area. Locating in the eastern part of Qinshui coalfield, the mining area is about 44-77 km long from north to south and 63.1 km wide from east to west. e total area of the mining area is about 2052.8 km 2 . Ordovician limestone is the basement of the coal measures in the mining area, and the coal-bearing strata are mainly Carboniferous and Permian strata, with Shanxi Formation of the Lower Permian and Taiyuan Formation of the Upper Carboniferous as the main bodies. No 3# coal seam locates in the middle and lower parts of Shanxi Formation as the main mineable coal seam in this area. In addition, Nos. 6,9,14, and 15-2# coal seams are only partially minable. Under the event that no pillar is utilized in the mining process of No.3 coal, a higher coal recovery rate can be achieved by eliminating the pillar and additional constructions [1][2][3].
erefore, the study of the mining pressure law of 3# coal seam mining can provide insightful guidance to the coal-free pillar mining in the whole Luan mining area. During the study, a typical mine called Gaohe was selected as the main study subject [4][5][6].

Engineering Geological Overview of E1316 Intake Entry.
e current production capacity of Gaohe mine is 8.0 mt/a, which belongs to high-yield and high-efficiency mine. E1316 intake entry is surrounded by the E1316 working face at the south side, the goaf of E1315 working face on the north side, and five main roadways on the west side, which serve as the auxiliary transportation roadway of the south wing at +450 m level. e E1316 intake entry is located in the 3# coal seam with a total length of 1087 m and drivage along the roadway floor. e specific roadway location is shown in Figure 1. Gaohe mine is a typical high-gas mine. e absolute gas emission volume of the mine is as high as 253.92 m 3 /min. Considering that it has the high gas content and emission, the ventilation mode of E1316 working face was changed from the "Y-type + High pumping roadway" ventilation mode to the "W-type + High pumping roadway" ventilation mode to optimize the ventilation system and recover coal resources as much as possible. In addition, the noncoal pillar mining was implemented.
No. 3 # coal seam in the E1316 working face is deposited in the Shanxi Formation of the Permian as lagoon facies. e working face locates in a syncline structural area with a stable coal seam thickness ranging from 6.2 m to 7.1 m and an average coal thickness of 6.5 m. e whole coal is sandwiched with a layer of carbonaceous mudstone, whose thickness ranges from 0.10 m to 0.35 m with an average thickness of 0.20 m. e coal seam has a dip angle of 1°to 7°a nd an average inclination of 5°. e depth of coal seam is 430.123 m-446.942 m, and the average distance from 9# coal seam is about 64.7 m. e test results of rock (coal) mechanical properties are summarized in Table 1.

Roadway Support Form.
e details of the roadway support section are shown in Figure 2(a) with roadway support parameters shown in Table 2. Due to the production needs, E1316 intake entry was reinforced by the adoption of a shelf structure as shown in Figure 2(a).

Details of the Flexible Formwork Support.
e strength of the flexible formwork support in E1316 intake entry was achieved through the application of the C30 concrete to the E1316 intake entry flexible formwork wall. e concrete parameters are provided in Table 3.

Law of Mine Pressure of the Gob-Side Entry
Retaining during the First Mining Period  Figure 1 and Table 4, respectively.

Law of Mine Pressure Appearance along the Gob-Side Entry Retaining in Front of E1315 Working Face
(1) Surface Deformation Law of Entry. After processing the deformation observation data obtained from No. 1# measuring point, an entry deformation curve was developed and is presented in Figure 3.
As demonstrated in Figure 3, the surface deformation of the gob-side entry retaining in front of E1315 working face can be divided into three stages: (1) No mining influence stage: beyond 72 meters in front of the working face, the entry in this section is basically not affected by the mining. e cumulative deformation of entry roof and floor is limited with the entry maintenance in good condition. (2) Low mining influence stage: with the advancing of the working face, in the range of 32-72 m ahead of the working face, the advance abutment pressure of the working face causes the increases of the subsidence rate of the roof, floor, and two sides of the entry gradually. e collected field data suggest that the average moving rate of the two sides is 8 Figure 4 shows the deformation and deformation rate curve of the 2# measuring point. e surface deformation of the gob-side entry retaining at the rear of the E1315 working face can be divided into two stages.
No. 2# measuring station is 330 m away from the cut hole, locating at 201 meters behind the current working face. e cumulative displacement of the roof and floor reached 1121 mm with a total of 1070 mm cumulative displacement at sides. e roof subsidence is the most severe one, followed by the floor subsidence and lateral substance. e deformation of the coal pillar side in advance entry was not as severe as the solid coal side. At the beginning of the mining of the working face, the deformation was observed at the measuring station, which locates at about 10 meters ahead of the working face. Gradually, the roadway deformation speed increased. e roof and floor moved at the maximum rate of 72 mm/d, with the maximum rate of two sides at 28.5 mm/d. A "step sinking" on the roof in front of the working face was identified, which led to the rapid increase of deformation rate.
After retaining the entry, the deformation speed of the entry firstly increased significantly and then decreased gradually. When the distance between the measuring station and the back of the working face reached 32 m, the roof and floor moved at a rate of 14 mm/d with intensified deformation speed. e roof and floor moved at the maximum rate of 45 mm/d, with a maximum rate of the two sides at 28.5 mm/d. When the distance reached 40-60 m behind the working face, the deformation of the entry became severe and continued for a whole week. It was presumed that the main roof of the working face collapses for the first time.
Meanwhile, the advancing distance of the working face was 80-100 m. e deformation of the retaining entry near the No. 2 measuring station grew stable.
(1) e increasing stage of deformation speed of the working face after mining: within a certain range after the working face was pushed, the immediate roof above the goaf failed and collapsed, accompanied by the main roof failure and rotation sinking. e entry deformation speed reached the maximum [7][8][9][10]. Simultaneously the maximum deformation rate of the two sides reached 27.5 mm/d and the maximum moving rate of the roof and floor was as high as 42.5 mm/d in the radius of 80 m behind the working face. In this whole stage, the cumulative deformation at both sides of the entry reached 655 mm with 752 mm cumulative deformation in the roof and floor.
(2) e decreasing stage of deformation speed of the working face after mining: as the bearing capacity of the filling wall increased, the key blocks at the end of the working face gradually stabilized under the support of the lower falling rock and filling body, resulting in the decreasing deformation speed of the surrounding rock of the entry. Within the range of 80-192 m behind the working face, the deformation speed of the entry gradually decreased, resulting in a maximum moving rate of 11.7 mm/d at two sides and a maximum moving rate of 12 mm/d in the roof and floor [11,12]. e cumulative deformation at both sides of the entry reached 302 mm, and the cumulative deformation of the roof and floor was as high as 270 mm.        Advances in Civil Engineering 5 30 meters after the working face. Eventually, a stable pressure was reached. After the collapse of the immediate roof, which filled the goaf, the prop pressure firstly decreased and grew stable after the main roof rotation, sinking, and contacting the gangue.

Filling Body Pressure Monitoring.
In order to monitor the pressure change of the filling body [13][14][15][16], filling body pressure monitoring gauges were installed, respectively, at a distance of 0.3 m and 0.6 m from the edge of the filling body, as shown in Figure 6(a). e dimension of the upper load-bearing steel is 30 × 0.25 × 0.25 m (thickness, length, and width, respectively). e dimension of the lower protective steel plate was 0.25 × 0.3 (0.6) m, respectively. e pressure gauge observations are shown in Figure 6 As shown in Figure 6, within the range of 0-18.5 m behind the working face, the pressure on the filling body due to the flexible formwork concrete near roadway increased rapidly, from 141 t at 0.3 m to 275 t at 0.6 m. Consequently, the filling body pressure gauge failed to monitor at 0.3 m due to the pipeline damage. With the range of 18.5-32.2 m behind the working face, the main roof of the stope failed, rotated, and sunk resulting in the increasing pressure of the filling body consistently. According to the pressure gauge reading, the filling body pressure at 27.6 m behind the working face increased to 0  5  10  15  20  25  30  35  40  45  50  55  60  65  70  75  80  85    Advances in Civil Engineering 308 t at 0.6 m. e pressure of the filling body fluctuated, decreased, and rose after 32.2 m behind the working face, forming a microgrowth trend. e lowest pressure was spotted at 45 m behind the working face, which may be caused by the main roof fracture. e pressure of the filling body changed little after 50 m behind the working face, indicating that the movement of roof strata tends to be stable.

Law of Mine Pressure of the Gob-Side Entry
Retaining  Table 5 and Figure 7, respectively.

Anchor Cable Reinforcement Scheme of E1316 Intake
Entry.
e anchor cable reinforcement scheme of E1316 intake entry is shown in Table 6 and Figure 8.

Detection and Analysis of the Surrounding Rock Stability of
Entry before the Secondary Mining

Detection Scheme of the Surrounding Rock Structure.
Based on the specific conditions of E1316 intake entry and air return entry, the borehole detector was used to detect and analyze the internal deformation and failure of the surrounding rock of entry, which provides a basis for entry evaluation and reinforcement scheme.
(1) e layout of the borehole stations: as shown in Figure 9(a), a CHK7.2 (B) type rock borehole detector developed by Xuzhou Whitton Company was adopted [17][18][19]. During the observation, the list of the required instruments is provided in Table 7.     Figure 9: e detection scheme of the surrounding rock structure.

Advances in Civil Engineering
(3) Observation scheme: According to the specific production technical conditions of E1316 intake entry, the deformation and failure of surrounding rock were detected and analyzed by using the CHK7.2 (B) borehole detector in the early and middle working shifts on August 6, 2017. Boreholes were drilled along the roof of the entry upward and parallel to the floor along the pillar wall near E1316 intake entry, locating at 150 meters away from the cut hole. e boreholes' layout is shown in Figures 9(b) and 9(c). A total of seven detection holes were drilled, namely, 1# (E1316 air return entry coal pillar side), 2# (E1316 air return entry solid coal side), 3# and 3'# (E1316 air return entry roof ), 4# (E1316 intake entry coal pillar side), and 5# and 5'# (E1316 intake entry roof). e design parameters of each detection hole are shown in Table 8.
(4) e borehole detection and analysis: as shown in Figure 9(d), converting the borehole video into screenshot images can facilitate the analysis and processing of the borehole wall. According to the specific production technical conditions of the related roadways in E1315 and E1316 working faces of Gaohe coal mine, the borehole detection was conducted in the early and middle shifts on August 6, 2017. rough the detection and analysis of the separation and deformation of the surrounding rock, seven observation holes were drilled, and four out of seven were obtained. Some boreholes failed in the middle of the drilling, which were abandoned and excluded. Overall, a total of 22 valid video images were obtained. e law governing the internal fissures development in the surrounding rock was presented, providing the foundation for the reinforcement scheme and the selection of the supporting system.

Analysis of the Measurements
(1) Due to the change of stress field in tunnel excavation, the shallow deformation and destruction of coal pillar in E1316 air return entry were severe, including the failures of the boreholes. e stress concentration of solid coal pillar was high with fully developed cracks in the coal body. e overall condition was featured with fractures and cracks in the coal roof. e mudstone and the deep rock mass remained intact.
(2) e coal body of the pillar side of E1316 intake entry was mostly loose from shallow to deep with limited cracks. e coal roof was severely fractured. e separation phenomenon was observed at the joint of coal seam with the mudstone, and the deep rock seam remained intact.
(3) e surrounding rock fragmentation of the two roadways gradually diffused from shallow to deep, and the degree of fragmentation gradually decreased from shallow to deep.  Table 9.

Investigation on Premining
According to the field investigation of E1316 intake entry, the main failure modes of entry include the bolt and cable failure, roof subsidence, coal pillar side moving towards the entry, severe coal body outburst at the shoulder corner of the entry, and bulging of the flexible formwork wall. e deformation mainly occurred in the roof and the coal pillar side. e estimated deformations reached about 450 mm and 300 mm, respectively. e coal body outburst at the shoulder corner can reach 550 mm. e analysis demonstrates that the deformation and destruction of the roof and the coal pillar side are severe. Meanwhile due to the high pressure, the coal pillar side and the flexible formwork wall failed. e existing supporting system faces the risk of uncontrollable entry deformations. ③ e development of the surrounding rock outside the bolt anchorage zone (3.8-10 m) is in the basic integrity state. In this range, no obvious cracks and fractured zones were found in the surrounding rock with the good integrity. Cracking of entry floor. 151∼154

Borehole Detection and Analysis
Two bolts in the middle of coal pillar side are pulled out by the mining stress. 163 Anchoring cable for pillar reinforcement has been removed in later period. 175∼180 Extrusion and crushing at shoulder corner of the roof and coal pillar side is serious. 188 Unanchoring of the third bolt from the bottom of the coal pillar side.

190-214
Cracking of entry floor. 198∼201 Four rows of ladder beams were broken in the middle and lower parts of coal pillar side. Roof breakage and subsidence are very serious, especially near the side of the coal pillar. 276 Unanchoring of the third and fourth bolts from the bottom of the coal pillar side. 296 Unanchoring of the third bolt from the bottom of the coal pillar side. 308 Unanchoring of the second bolt from the bottom of the coal pillar side. 311, 312 Unanchoring of the second bolt from the bottom of the coal pillar side. 320∼350 e upper part of flexible formwork wall badly bulge phenomenon. 334∼336 Flexible formwork wall with bulge phenomenon, bulging range is 2 meters. 352∼355 Cracking of the flexible formwork wall is serious. 358 e second repaired cable at the bottom of the coal pillar side has failed. 399∼406 Cracking of entry floor. 413∼422 ere are timbers to strengthen support in this area, and there are chambers nearby. 476∼479 Over-excavation of coal pillar side is serious, and the cross section is large and irregular. 537∼541 e roof is broken seriously, the displacement is large, and the flexible formwork wall has bulging problem. 581 Unanchoring of the third bolt from the bottom of the coal pillar side. 583 Unanchoring of the second bolt from the bottom of the coal pillar side.  No, 1 # boreholes failed in the middle of drilling, which were abandoned and excluded.
Advances in Civil Engineering 11 e hole with the depth of 7.2 m was located at the junction of the direct roof and the main roof, which was relatively fragmented. A large amount of smoke and dust in the hole was within the main roof, due to the friction between the drill bit and the sandstone of the main roof. e helix characteristics were clearly observed. e detection results are shown in Figures 12(a)-12(h).
(2) e solid coal side of E1316 return entry ① Good integrity of the surrounding rock in the bolt anchorage zone. In this range, no obvious cracks and fractured zones were found in the surrounding rock with better integrity. e preload of the bolt support was high at the initial stage. Better supporting effects were identified in the bolting range. is range of images was selected from the detection hole, and the results are shown in Figures 13(a)-13(d). In this range, no obvious cracks and fractured zones in the surrounding rock due to unmined working face were present. e overall integrity remained untouched. e coal powder accumulation in the pillar side hole was severe. is range of images was selected from the detection hole, and the results are shown in Figures 15(a)-15(d).
① Fractured zones found in the surrounding rock of bolt anchorage zone e integrity of the surrounding rock in 0.1-0.7 m range of the roof anchorage zone was good.
e detection results are shown in Figures 16(a) and 16(b). e surrounding rock in 0.7-2.3 m range of the roof anchorage zone was partially broken, and the cracks had expanded and developed. is range of images was selected In this range, no obvious cracks and fractured zones in the surrounding rock were found, but the coal body is loose and had no bearing capacity. is range of images was selected from the detection hole, and the results are shown in Figures 19(a)-19(d).
② e fracture zone in the surrounding rock outside bolt anchorage zone.
No obvious characteristics of crack development and fragmentation were found in the surrounding rock. Due to the high lateral stress caused by the mining in the upper working face, borehole fragmentation and deformation were severe with poor integrity. is range of images was selected from the detection hole, and the results are shown in Figures 20(a)-20(h). Combining with the statistical analysis of entry damage and field investigation results of borehole detection in the previous section, the overburden structure of E1316 intake entry changed due to the mining of E1315 working face. During the advancing process of E1315 working face, the periodic weighting phenomena occurred in the roof. After the first weighting, the continuous advancing of the working face had made the main roof of E1315 working face prone to form "O-X" fracture structure. Due to the top coal collapse and the 2.6meter thick immediate roof, the main roof was suffering from the fracture, rotation, and subsidence. In the mining process of E1315 working face, the flexible formwork support was used to ensure the safety of gob-side entry retaining. Before the mining of E1316 working face, under the overall structural environment of the goaf, flexible formwork wall, and E1316 intake entry, the main roof of the section direction of E1316 intake entry can temporarily form a masonry beam structure. e fracture structure was often caused by many factors including the thickness and mechanical properties of the main roof, the immediate roof, and the coal seam, the mining depth, the stress state of the original rock, and the mining height [20][21][22]. According to the features of the failures of the roof of E1316 intake entry, the main roof failed at the "masonry beam" and the breaking line was above E1316 working face, suggesting that most of the weight of rock block B was above E1316 intake entry. In addition, the weight was borne by the flexible formwork wall and the coal pillar, which made the support of entry more difficult.

Location Analysis of the Main Roof Fracture.
e location of the fracture in the main roof of the E1316 intake entry holds an important role in the stability of the surrounding rock. Under the lateral abutment pressure of the working face, plastic zone, elastic zone, and original rock stress zone tend to appear from the edge of the coal pillar side of the entry to the depth of the coal pillar. e fracture position of the main roof was located near the junction of the elastic zone and the plastic zone in the coal body, as shown in Figure 21.
In the plastic zone, the fracture of the coal pillar was fully developed and the damage was severe. On the contrary, the damage in the elastic zone and original rock stress zone was limited. e depth of plastic zone affects the selection of support parameters. erefore, the limit equilibrium theory was adopted to calculate the depth of the plastic zone of the surrounding rock for dynamic mining pressure entry such as E1316 intake entry. According to the calculation, the limit equilibrium zone depth of E1316 intake entry was 6.14 m. In other words, the junction point of the plastic zone and elastic zone on the coal pillar side of E1316 intake entry was 6.14 m away from the coal wall. e calculation suggests that the breaking line of key block B of the main roof was above the coal pillar, which was about 6.14 m away from the coal wall.

Comprehensive Assessment of E1316 Gob-Side Entry
Retaining.
e measurement of rock mechanics properties, detection of the surrounding rock structure, and statistical study and theoretical calculation of premining failure of the surrounding rock and the flexible formwork filling body of E1316 gob-side entry retaining have suggested that the surrounding rock of E1316 intake entry had undergone great deformation and destruction. e serious damage of support structure had made the supporting system more vulnerable and fragile. In addition, due to the uneven occurrence of the layered surrounding rock, the uneven roadside support, and coal rib support, the deformation and failure of the support structure of gob-side entry retaining present nonuniform characteristics. e instability of the support structure can be jeopardized due to the significant deformation and failure occurred in the entry.

Advances in Civil Engineering
In view of the existing problems, the following solutions are proposed as follows: (1) To solve the problem of loose bolts, the twin-nut structure is suggested. In addition, the bolt supporting density and anchoring agent should be increased to enhance the length of grip. (2) To minimize the risk of ladder beam fracture, the ladder beam should be reinforced with the 16 mm round steel. e stress on the ladder beam should be verified. Regarding the continuous fracture area, the ladder beam can be replaced by using the steel strip for increased surface areas and strength. e comprehensive analysis indicates that E1316 intake entry is subject to a high superimposed stress. Due to the low strength of the coal and rock mass, the surrounding rock cracks developed and continued to expand. With the poor bearing capacity of the entry itself, the flexible formwork wall tends to crack easily. erefore, besides the proposed reinforcements and other preventative measures, monitoring the mine pressure constantly plays a vital role in ensuring the safety of the roadway and the E1316 working face.

Conclusion
(1) During the initial mining period, the roadway located at the stress increasing area was affected by mining in E1315 working face. e observation and analysis have suggested that the high pressure caused severe damage to the surrounding rock and the flexible formwork filling wall in the upper section of the entry. e roof and the flexible formwork wall experienced various levels of damage. (2) e flexible formwork filling wall provides great strength with poor contractibility. For the severe roof subsidence at the gob-side entry retaining, the hardness of the filling wall was not compatible with the coal roof, resulting in inconsistent deformation. e deformation eventually led to the failure of the coal roof and flexible formwork filling wall, jeopardizing the entry support.
(3) e mining and roof collapse of E1315 working face lead to the formation of long cantilever beams which are not easy to collapse in the goaf on the side of the flexible formwork wall of E1316 intake entry. Most of its own weight of the cantilever beams acts on the flexible formwork filling wall, support structure, and coal pillar side, and the overall structure of the entry experienced various levels of damages. (4) Due to the high stress, cracks developed in the top coal and the immediate roof rock layer (2.65 m above the entry), which continued to expand until the failure of the surrounding rock. Meanwhile, the coal pillar grew loose and fragmented beyond the range of 5 m. e deformation made the coal pillar become uneven and fragile, resulting in the moving out of the whole coal pillar. e deformation further jeopardized the bearing capacity of the surrounding rock. e coal pillar side experienced the most severe damage. (5) With the reinforcements in the roof and the pillar side of the entry, the entry damage and deformation can be contained. However, more maintenance was required. Multiple long cracks and heaves were found in the immediate floor where no reinforcements were added.
e study of the coal pillarless mining technology conducted can provide both theoretical and pragmatic references to the operation of No. 3# coal in the entire Luan mining area, as well as other mining locations sharing similar conditions in Qinshui coalfield. e wide adoption of the gob-side entry retaining technology in the Luan mining area or Qinshui coalfield can bring significant economic and social benefits.

Data Availability
e data used to support the findings of this study are included within the article. And all data are obtained through experiment and test by our research team in Gaohe mine and laboratory. All the data are true and effective. e right to using data belongs to the authors before the article being published, but after it was published, the data can be referenced.

Conflicts of Interest
e authors declare no conflicts of interest.