Abstract
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.
Highlights
With the rapid development of China’s economy, shallow coal resources are increasingly in short supply
Through innovation to the tail structure of flexible bolts, the installation of flexible bolts is free from the limitation of excavation height
The locking casing and the spring are extruded onto the rod body of flexible bolts at first
Summary
With the rapid development of China’s economy, shallow coal resources are increasingly in short supply. It has become an inevitable trend to mine deep coal resources [1,2,3]. The high stress and complicated geological environment often cause serious deformation and damage in the roadway and a huge loss of personnel and equipment. In this case, greater importance is to be attached to bolt support, which serves as the active control over roadway deformation [4,5,6]. Evolution Process of Bolts under the terms and conditions of the
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