Study On Failure Mechanism Of Rock Anchoring And Bearing Performances Of Prestress Anchored Surrounding Rock

In view of the influence of high ground stress and extra soft stratum to roadway excavation, the traditional supporting such as bolt and anchor cable, are easily to generate problems such as failure frequently, poor boltability, low strength, insufficient active supporting effect. They cannot give full play to supporting potential of components, reduce the strength of entire supporting system, thus further increase roadway supporting difficulties and insecurities, also increase roadway repairing rate and supporting cost. Considering the above mentioned problems, explaining the failure mechanism at the interface between anchorage body and soft surrounding rock, optimizing and proposing reasonable parameters of high strength bolt, and giving effective control measures are necessary. It is also of both theoretical and practical significance to ensure the roadway stability and coal resources safe-mining.In this paper, combined with theoretical analysis, numerical simulation, model test, field test and field measurement, the progresive failure mechanism at the interface between anchorage body and soft surrounding rock and the failure mechanism of prestress anchored surrounding rock were studied, strengthening mechanism and bearing performances of prestress anchored surrounding rock were revealed, and the design method and quantitative applying technology of high prestress, which are suitable for high-stress roadway excavation, were also developed and applied.The main work and results of the dissertation are described as follows:1. Mechanical characteristics analysis at the interface between anchorage body and soft surrounding rockFocused on the problems in soft surrounding rock, such as low bond capacity at the interface between anchoring agent and rock mass, poor boltability, etc, the theoretical analysis method of interfacial bond strength of anchorage body was established by considering factors including the design parameters and dilation effect of anchorage body, the strength and stress of surrounding rock. The bearing capacity at the interface between anchorage body and soft surrounding rock was revealed. The interfacial shear capacity is positive correlated with elasticity modulus, cohesion, internal friction angle, dilatancy angle and stress of surrounding rock. Improving the anchorage technology on the anchored section and employing grouting reinforcement technology was an effective way to improve the anchorage support boltability in soft surrounding rock.2. Progressive failure mechanism study at the interface between anchorage body and soft surrounding rockConsidering the dilatancy effect and softening effect at the interface between anchorage body and soft surrounding rock, a constitutive model used to describe the progress of elastic, plastic slide, plastic soften, debonding at the interface between anchorage body and soft surrounding rock was given, and the full-range analysis method of progressive failure mechanism was developed. Using the above mentioned method, the load transfer mechanism and the value of ultimate drawing force were analyzed. The progressive failure mechanism at the interface between anchorage body and soft surrounding rock was revealed.3. Failure mechanism study of prestress anchored surrounding rockFocused on the failure mode of prestress anchored surrounding rock, a mechanical model, considering the nonlinear failure characteristics of rock mass, was constructed. The failure mechanism of surrounding rock and ultimate pullout force were presented, based on equilibrium principle of energy-transfer. When anchored surrounding rock failure occurs, the magnitude of ultimate pullout force depend on rock mass quality mainly. With the improving of empirical parameter A, tensile strength, compressive strength and pressure from overlying rock mass, the ultimate pullout force of anchorage increases.4. Mechanical effect analysis of prestress anchoring in surrounding rockAccording to the superposition principle of prestressed bolt, mechanical model of anchored surrounding rock under different anchorage lengths was established, and the distribution of bolt stress and surrounding rock stress were deduced. Meanwhile, numerical test of 20 comparative schemes of anchored surrounding rock, was also conducted, with taking 4 categories of factors, such as bolt length, spacing, anchoring length and prestress, into consideration. The mechanical effects of prestress anchored surrounding rock was revealed by combining theoretical analysis results. The results show that:adopting high prestress and setting a certain length of free-segment or increasing bolt length, reducing bolt spacing can help prestress to spread in surrounding rock, form effective bearing structure in anchored rock mass; when bolt spacing increases, a better control effect of surrounding rock can be achieved through applying high prestress and reducing anchorage length appropriately,5. Strengthening mechanism study of prestress anchored surrounding rockThe support effects of non-wholly bonded prestressed bolt on surrounding rock mainly contains two aspects:passive reinforcement region at anchored-segment and active reinforcement region at free-segment. The theoretical method of calculation about active support strength and passive support strength of anchored surrounding rock were presented. The strengthening mechanism of prestress anchored surrounding rock was revealed. Meanwhile, the wholly bonded prestress anchoring (end anchored and wholly bonded) can be used as optimal way to reinforce surrounding rock, which can give full play to supporting potential of anchorage section and free section of bolt.6. Bearing performance model test study of anchored surrounding rockThe anchorage simulation testing apparatus was developed. Under the two cases (no bolting and bolting), model test of bearing performances of surrounding rock was conducted. The failure mechanism and bearing capacity of prestress anchored surrounding rock were presented and the theoretical analysis results were proved to be valid. The results indicate that the peak, post-peak bearing capacity and entire rigidity of surrounding rock were effectively improved, and the splitting and caving failure can be controlled with some ductile deformation behaviors of surrounding rock by using prestressed bolts.7. Simplified design method study of prestress anchoring support and its applicationIn view of the status quo of lacking effective design theory of bolt prestress, taking deep arched roadway for example, the roof failure mechanism, considering the nonlinear failure characteristics, stress of surrounding rock, and bolting support, was constructed. A simplified design method of prestress of roof bolts at the early stage after roadway excavation was proposed based on upper bound method, recommended engineering measures were also provided accordingly. Finally, the superiority of roof bolting support with high strength and high prestress was verified through filed application.8. Field test study of surrounding rock control effects under different anchoring modesConsidering the influences of anchoring length and prestress, serial comparative schemes of anchored surrounding rock were designed, field test of control effects of surrounding rock under different anchoring modes was conducted. The mechanical performances of bolts and control effects of surrounding rock were presented through field monitoring. The results indicate that:increasing prestress is an effective way to achieve better control effect of surrounding rock, and reducing anchorage length under a certain conditions will not weaken the control effect of surrounding rock significantly, thus also save support cost.9. Development and application of quantitative applying technology of high prestressIn view of the problems in prestress applying process for full thread bolt in mines, such as small torque transformation coefficient, easy deformability of thread thus difficult to retreat, low prestress and unable to quantify, difficult to secondary apply after rib spalling, complex construction technology and big labor strength, etc, a quantitative applying technology of high prestress was developed, and the effectiveness of this technology was verified through laboratory testing and field applications.