Characteristics And Control Of Construction Mechanical Response Of Surrounding Rock Of Super Large Section Tunnel

Compared with the conventional scale tunnel engineering,the super-large cross-section tunnel are constructed with greater excavation disturbances,resulting in poorer rock stability,longer and larger load release,and higher demands on the support structure.Therefore,the proper construction method,reasonable pre-support,and anchoring system are the key points of tunnel construction safety control.The new Badaling Tunnel of Beijing-Zhangjiakou High-speed Railway is a supporting project for the 2022 Beijing Winter Olympic Games,featuring a large cross-sectional area(the largest single hole excavation area of 494.4m~2)and poor surrounding rock conditions.Based on this project,this paper adopts statistical theory,numerical simulation,theoretical analysis,and field measurement to study the mechanical response characteristics and deformation control of the super-large cross-section tunnel construction.The current research work mainly includes:the spatial and temporal distribution law of the surrounding rock pressure,the ground reinforcement mechanism of the pipe roof,the collaborative design method of the anchorage system,the optimization analysis of the construction parameters.The core innovations of this work are summarized as follows:(1)Calculation method of rock pressure in super-large cross-section tunnels and characterization of rock damage evolution:By analyzing the measured data of 242cross-sections in 130 super-large cross-section tunnels in China,the distribution law and evolution characteristics of the surrounding rock pressure are clarified.It can be concluded that the surrounding rock pressure of super-large cross-section tunnels conforms to the compound growth form in time,i.e."sharp growth-slow growth...sharp growth-slow growth-gradual stabilization".This property is not related to the physical and mechanical parameters of the surrounding rock but is caused by the superposition of excavation effects.Besides,the spatial distribution of surrounding rock pressure is"arch vault>arch shoulder>arch waist".Taking these statistical data as samples,a more accurate empirical formula of surrounding rock pressure is also proposed.In order to explore the evolution characteristics of surrounding rock failure of super-large cross-section tunnels,this paper systematically investigates the formation mechanism of the loose layer from the perspective of macro surrounding rock deformation and micro damage.The results show that the excavation of the upper part is the key stage of the loose layer formation,and the convergence around the tunnel has a significant impact on the development of the loose layer.Therefore,the expansion of the loose layer can be prevented by controlling the deformation of the surrounding rock.(2)Mechanical model of pipe roof in super-large cross-section tunnels and evaluation method of stratum reinforcement effect:In order to study the mechanism of the pipe roof,this paper develops a two-parameter Pasternak foundation beam model,considering the delay effect of the primary lining,the variable bed coefficient of the geotechnical material in front of the tunnel face,and the spatial distribution characteristics of the load.Using this model,the variation of deflection of pipe roof with its design parameters and tunnel construction parameters are studied.The results show that the reinforcement effect of?108mm and?159mm pipe roofs commonly used in practical projects is appropriate;the increase of excavation feed leads to a higher risk of collapse in unsupported intervals,and the increase of excavation height leads to a higher risk of collapse at the tunnel face.Through the field test,this paper further studies the stratum reinforcement effect of the pipe roof in the shallow buried loess super-large cross-section tunnel.It can be concluded that the control effect of the pipe roof on the vault settlement is stronger than the horizontal convergence.The pipe roof can prevent the deformation from transferring to the upper stratum and shorten the time for the stratum to reach stability.(3)Synergistic optimization design method of anchorage system for super-large cross-section tunnels:Considering the spatial effect of excavation face and the spatial state relationship between bolt and surrounding rock,an interaction analysis model of the bolt and surrounding rock is established.Using this model,the influence of bolt length,supporting time and other parameters on the deformation control effect of surrounding rock are analyzed.It can be obtained that the bolt needs to be installed before the plastic zone appears as far as possible.When the bolt is not effective enough to control the surrounding rock deformation,the anchor cable should be used to carry the load together.Considering the difference of supporting time and anchoring range of bolt and anchor cable,a synergetic mechanical model of the bolt and anchor cable is also proposed.This model is used to analyze the spatiotemporal evolution mechanism of interaction between anchorage system and surrounding rock,and to study the deformation control principle of tunnel anchorage system.The results show that the main function of the anchorage system is to improve the stress state of surrounding rock through equivalent support force and reinforcement circle effect,so as to control the rapid deformation of surrounding rock behind the excavation face.The deformation control effect is mainly determined by the anchor,so the anchor cable mainly plays the role of safety reserve.(4)Optimization of tunneling method for super-large cross-section tunnels and its application in the large span transition section of new Badaling Tunnel:Using numerical simulation software,this paper systematically studies the mechanical response of surrounding rock and supporting structure in the construction process of three benches-seven steps method,both side drift method,reserved core soil method,reserved middle rock pillar method,and half step CD method.Taking the convergence around the tunnel,the primary support load,and the plastic zone of surrounding rock as evaluation indexes,the comparison of construction methods and parameter optimization are carried out.The obtained optimal construction method was applied to the excavation of the large-span transition section of the new Badaling Tunnel.The final deformation of this section is controlled within 30mm,and the maximum range of the loose layer is only 8.1m.Therefore,the optimal construction method has an excellent control effect on the engineering response of surrounding rock.In addition,the density of the anchor cable can be reduced appropriately to coordinate the utilization of the bolts and anchor cables.