Research On Crack Evolution And Bolting Mechanism Of Intermittently Jointed Rocks

Research on the fracture evolution and bolting of intermittently jointed rocks is one of thehotspots and difficulties in the field of rock mechanics and rock engineering. Although someprogresses were made by scholars in this field, there are still many problems that need to besolved properly. Funded by the National Natural Science Foundation of China (51074162,51179189) and using physical simulation, numerical simulation, theoretical analysis and otherresearch methods, this paper made an in-depth study on the fracture evolution characteristics andbolting mechanism of intermittently jointed rocks. The main research findings and conclusionsare as follows:(1) A large-scale (500mm×500mm×480mm) three-dimensional testing system wasdeveloped for the simulation of rock bolting. This system has functions of loading, confining andmeasurement, with which the whole process of loading, deformation and failure of jointed rockscan be truly represented. To monitor the real-time anchoring force of bolt, the high-precisionminiature ergometer was developed to measure the anchor end force and the dynamometric boltbased on fiber Bragg grating was made to get the multipoint anchoring force along bolt. A newmethod of making multiple joints was designed and the corresponding molds were fabricated.An ideal similar material of intermittently jointed rocks was developed, which has advantages offew components, stable mechanical properties, adjustable parameters, pollution-free, low priceand so on.(2) Fracture evolvement rules of intermittently jointed rocks without bolt were obtained byuniaxial compression tests. The peak strength, ultimate strain and the center bearing core widthof rocks at post-peak stress of0.7σcdecrease first and then increase with the rise of joint angles.The initiation position of secondary cracks is mainly at the periphery of specimen, and most ofthe cracks are tensile or tensile-shear cracks. The crack initiation stress increases linearly withjoint angles. The energy consumed by the destruction of unit volume rocks is the minimum whenthe joint angle is60°; the energy increases with the decrease of joint angle when it is less than60°, and with the increase of joint angle when it is greater than60°. The failure of rock bridges ismainly due to tension and tensile-shear cracks when the joint angle is less than45°. The ratio ofshear cracks increases with the rise of joint angle. The main fracture types of jointed rocks are oftension and tensile-shear fractures when the joint angle is less than45°, and the rocks tend toshear along the joint surfaces when the joint angle is larger than45°.(3) Model experiments were made on the intermittently jointed rocks with different jointangles under various bolting parameters by using the self-developed testing system. It was foundthat the strength of bolted rocks comprises the strength of rocks, the additional strength caused by both initial equivalent constraining stress (σ3i) which is aroused by pre-tension force of bolts and the equivalent constraining stress (σ3b) which is generated by deformation of bolts. The functional relations among peak strength and residual strength of bolted rocks, joint angle and bolting density were established and successfully used in the design of bolting parameters for a tunnel, through which good effects were achieved.σp=4.543+1.412/ρ-2.146/ρ2+1.564/ρ3-0.02146a-8.0×0610α2+1.307×105α3σR=3.624+2.039ρ-2.263ρ2+1.499ρ3-0.005564a-1.41x10-4a2+2.267×10-6α3(4) The relations between the elastic modulus, Poisson’s ratio, volumetric strain of bolted rocks and the bolting density, the joint angle were obtained. The elastic modulus grows nonlinearly with the bolting density. The generalized Poisson’s ratio and the horizontal strain at the peak stress grow with the bolting density for the same joint angle. In general, the position of peak strength of bolted rocks is not the same as the beginning point of volume expansion. In most instances, the peak strength lags behind the volume expansion point of the bolted rocks. Bolts can obviously inhibit shear fracture, but has less effect on tensile fracture.(5) The function mechanism of full-length anchoring bolts was revealed by analyzing the evolution and distribution characteristics of bolting force. For sparsely distributed bolts, the force distribution along each bolt is uneven and the force of its central and external sections is much greater than that of internal section in the whole lifetime of the bolt. Consequently, the constraining force of bolts on rocks is unevenly distributed and some weak parts are prone to occur, which is adverse to the stabilization of the rocks. For densely distributed bolts, the force distribution along each bolt is nearly uniform. It can fully mobilize the load-bearing capacity of each bolt and improve the stress state of bolts and rocks. Therefore, it is more conducive to the stability of bolted rocks.