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Supplementary Material 1

Roadways in thick coal seams are widely distributed in China. However, due to the relatively developed cracks and brittleness of coal, the support failure of thick-coal-seam roadways frequently occurs. Therefore, the study of bolt failure characteristics and new anchoring technology is very important for the safety control of thick-coal-seam roadways. Based on field observations, the failure mechanism of selected roadway failures under distinct conditions at three representative coal mines in eastern and western China was analyzed. Recommendations are provided for roadway safety control. The results show that the strength and dimension of the anchoring structure in the coal roof of thick-coal-seam roadways are the decisive factors for the resistance of the roadway convergence and stress disturbance. The thick anchoring structure in the roof constructed by flexible long bolts can effectively solve the problem of support failure caused by insufficient support length of traditional rebar bolts under the condition of extra-thick coal roof and thick coal roof with weak interlayers. The concepts and techniques presented in the paper provide a reference for the design of roadway support under similar geological conditions and dynamic load.

The mismatch of speed of mining and excavating is the main contradiction of high efficiency production in China, improving the excavating speed of coal roadway is an effective way to solve the problem. According to the engineering geological conditions of the main haulage roadway 21205 in Hulusu Coal Mine, the technical problems existing in the original support scheme were analyzed, increased the excavating speed of roadway through improve support efficiency of single bolt and reducing density of bolt, a rapid excavation technology based on anchoring method of critical high efficiency and low density on roof of coal roadway was proposed. The influence of different bolt length and different pre-tightening force on the stability of roadway roof was studied by FLAC3D simulation, the results showed: (1) increasing the length and pre-tightening force can control the development of the roof vertical strain with a certain range; (2) as the length of bolt increases, the compressive stress zone of the rock layer between the two bolts increases and the compressive stress in the rock layer area of the anchorage section tends to increase; (3) with the increase of pre-tightening force, the effective, compressive stress zones between the two bolts overlap with each other, forming a whole, and the compressive stress values in the near zero stress zone increase. Decided the scheme that, bolt length is 4 m, pre-tightening force is 200 kN, and the support density is 0.5 root/m2, industrial experiments were carried out. The mine pressure monitoring showed that the roof of the roadway has almost no ionosphere, and the deep ionosphere was well controlled. The monthly excavating progress of the roadway was increased from 520 to 825 m, with a rise of 58.7%, the technical and economical effects were significant. It was proved that the scheme is in line with the actual. It can provide reference for roadway rapid excavating support technology for similar conditions.

The higher strength of a hard roof leads to higher coal pressure during coal mining, especially under extra-thick coal seam conditions. This study addresses the hard roof control problem for extra-thick coal seams using the air return roadway 4106 (AR 4106) of the Wenjiapo Coal Mine as a case study. A new surrounding rock control strategy is proposed, which mainly includes 44 m deep-hole pre-splitting blasting for stress releasing and flexible 4-m-long bolt for roof supporting. Based on the new support scheme, field tests were performed. The results show that roadway support failure in traditional scenarios is caused by insufficient bolt length and extensive rotary subsidence of the long cantilever beam of the hard roof. In the new proposed scheme, flexible 4-m-long bolts are shown to effectively restrain the initial expansion deformation of the top coal. The deflection of the rock beam anchored by the roof foundation are improved. Deep-hole pre-splitting blasting effectively reduces the cantilever distance of the “block B” of the voussoir beam structure. The stress environment of the roadway surrounding rock is optimized and anchorage structure damage is inhibited. The results provide insights regarding the safe control of roadway roofs under extra-thick coal seam conditions.

As mining depth extends continuously, complex geological environment and strong mining stress pose serious challenges against excavation safety and higher demand for bolt support. To meet the challenges, a new type of flexible bolt is developed that is free from the limitation of excavation height and can be installed quickly. The flexible bolts have a tail structure which is squeezed together by the locking casing and the steel-strand rod. The tail structure forms the thread through the rolling process, and then can be quickly installed by the nut rotation type, which avoids the disadvantages of the cable tension installation. Through the flexible long bolts, the thick anchoring layer of the roof can be constructed, and the safety control of the roadway is realized. To obtain the optimal specifications, locking casings of three diameters and lengths were used to prepare bolt samples for static tensile test. Test results show that when the diameter of locking casing is fixed, the peak resistance increases as the length of locking casing increases, but the increased amplitude tends to reduce gradually. When the length of locking casing is fixed, too large a diameter makes the mechanical property unstable. Finally, the optimal length and diameter of locking casing are set at 120 and 30 mm, respectively. In addition, cyclic loading tests were conducted, the results of which indicate that cyclic loading does not change the stress state within the bolts and that flexible bolts display a stable mechanical property. Field results indicate that flexible bolts achieve low damage of the surrounding rock and control the maximum roof fracture depth within 2.1 m, thus guaranteeing the roadway safety.