Synergetic Supporting Mechanism And Application Of Prestressed Anchorage System In Large-span Tunnelling

With the rapid development of China’s railway and highway transportation,largespan tunnels have become the key node of railway and highway projects.Large-span tunnels have unique advantages in the construction of underground stations,multi-track railways,or highway tunnels.However,the construction of a large-span tunnel is extremely difficult due to the large excavation span and adverse geological conditions.The supporting technology and theory of large-span tunnels have become the key points in the current tunnel engineering discipline.The prestressed rock bolt and anchor cable support system has become a significant method to solve this problem.This paper focuses on the prestressed bolt-cable(rock bolt and anchor cable)support system in large-span tunnelling.The data investigation,theoretical analysis,numerical simulation,and field monitoring test are used to study the synergetic support mechanism of prestressed anchorage system.This research is mainly divided into the following parts:mechanical characteristics of prestressed anchoring unit,analytical method of prestressed bolt-cable synergetic support system,mechanical behaviors of prestressed rock bolt and anchor cable,the regional synergetic effect of bot-cable support,the evaluation and design methods of prestressed anchorage system synergetic support,etc.The main work and conclusions are as follows:(1)The mechanical characteristics of prestressed anchoring units are analyzed,and the active and passive supporting effects of prestressed rock bolt and anchor cable are revealed.Based on the analysis of the prestressed rock bolt anchoring unit and prestressed bolt-cable composite anchoring unit,the active support effect and stress superposition characteristics caused by prestressed rock bolts and anchor cable were analyzed.The effect of sufficient prestress on the supporting capacity of rock bolt and anchor cable is illustrated.The passive support effect of rock bolt and anchor cable,which is caused by coordinated deformation between anchoring members and rock mass,has been analyzed.The results show that the pretension forces,as well as the level and distribution of load,have an important influence on the supporting characteristics and passive supporting effect of rock bolt and anchor cable.(2)The analytical method of prestressed bolt-cable synergetic support system,considering the interaction between rock bolt,anchor cable,and rock mass,is proposed.Based on the strain-softening characteristics of rock mass,the elastoplastic effect and delay effect of the bolt-cable support,and the stress release effect of tunnelling,an analytical model considering the interaction between the rock bolt,anchor cable,and rock mass is proposed.The following solutions can be obtained based on the analytical model: the stress,strain,and displacement of rock mass;as well as the supporting force of rock bolt and anchor cable.To study the synergetic support effect,parameter analysis of pre-tension forces,strength parameters,and stiffness parameters based on the analytical model was carried out.Then,the Ground Response Curve of tunnel reinforced by the bolt–cable support is established.On this basis,the significance of the whole process synergism time effect of the prestressed anchorage system is clarified.The supporting parameters should be determined based on the whole process synergism of the prestressed anchorage system.(3)The mechanical behaviors of prestressed rock bolt and anchor cable support in large-span tunnel are analyzed,which reveals the evolution rules of the anchoring forces.According to the field test research on the mechanical behaviors of rock bolts and anchor cables,the anchoring forces of the rock bolts and anchor cables are determined by the initial pretension forces,loss of pretension forces,and passive support forces.The evolution process of the anchoring forces can be divided into three stages: pretension force rapid loss stage,supporting force fluctuation stage,and supporting force stability stage.The dominant position of the initial pretension force in the supporting force of rock bolt and anchor cable is illustrated,and the effect of the initial pretension forces synergism on the whole process synergism of the bolt cable system is clarified.It is revealed that the evolution process of supporting forces is essentially the interaction process of pretension force loss and passive supporting force growth.(4)The regional load-bearing mechanism of prestressed bolt-cable support in largespan tunnelling is proposed.The regional synergetic effect of bot-cable support are clarified.According to the composite structural characteristic of surrounding rock mass,the distribution characteristics of the inner and outer layer of surrounding rock mass during large-span tunnelling are analyzed based on the results of the microseismic monitoring test.The bearing mechanism and coupling characteristics of prestressed rock bolt and anchor cable are analyzed.Based on the above analysis,the regional load-bearing mechanism of prestressed bolt-cable support in large-span tunnelling is proposed,and the regional synergetic effect is analyzed by numerical simulation later.It is clear that the regional synergetic support is the requirement of the final support state of the bolt-cable system.The stiffness and strength parameters should be suitable for the requirements of regional synergetic support.(5)The evaluation and design methods of prestressed anchorage system synergetic support are established.The evaluation indexes considering the synergetic characteristics of bolt-cable support are carried out.The evaluation standards of each index and engineering measures for supporting parameters are clarified in this method.Then,the evaluation method of prestressed anchorage system synergetic support is established.Based on the synergetic support mechanism of the prestressed anchorage system,combined with the evaluation method,the design method of prestressed anchorage system synergetic support is established.The research results were applied to the Beijing-Zhangjiakou high-speed railway large-span tunnel project,which verified the effectiveness of the evaluation and design methods.