Study On Failure Mehcanism And Optimatization Design Of Tunnel Prefabricated Arch Support System

With the continuous development of world resources and the rapid increase of infrastructure scale,more and more underground engineering cases,such as transportation tunnels,coal mine tunnels,water conservancy and hydropower tunnels are built in complex geological conditions,which show complex characteristics such as compression and rheology,leading to long-term dynamic coupling rock-support interaction.If the support structures are inappropriately designed,it might cause severe soft rock large-deformation and rock mass fragmentation,resulting in support failure,roof fall,collapse and other engineering disasters.Workers operate in dangerous engineering environment,the construction efficiency is low,and the safety cost is high.One of the most effective way to enhance construction safety in underground excavation is the prefabricated construction technology.In the fourteenth five-year plan of China,higher requirements for the prefabricated construction are also put forward.The tunnel fabricated support system is used to assemble the primary support arch in a mechanized way,which can avoid workers being exposed to dangerous environment and achieved good field application effect.However,in the practice of tunnel surrounding rock control and support design development,the engineering analogy method based on experience is still widely adopted,lacking scientific quantitative design.Although many scholars have carried out a lot of researches,how to reasonably consider the long-term deformation pressure of surrounding rock,how to accurately predict the failure mode of support structures under soft surrounding rock compression,and how to select different support forms to conduct tunnel design are still important subjects in the study of tunnel design.Therefore,it is of great significance for efficient tunnel construction and safe operation to clarify the safety control mechanism and design principles of prefabricated support system.Taking the tunnel construction and tunnel support under complex conditions as background,considering high geostress and extreme soft rock,taking fabricated arch support system and shotcretearch composite structures as research objects,aiming at the practical engineering problems including arch instability,local components failure and lining bond-slip failure,a series of researches of tunnel rock-support interaction mechanism are conducted by means of mechanical analysis,laboratory test,numerical simulation and field monitoring.The main research works are as follows:(1)Based on the interaction model between surrounding rock and support in tunnel,considering the influence of arch instability,lining bond-slip and cracking,long-term creep behaviour of rock mass,rock anchoring and grouting,the analytical solution of surrounding rock deformation and stress of non-circular tunnel considering support construction process and progressive failure condition is derived,and the control mechanism and design principle of prefabricated support system are clarified.(2)As the main bearing body in the early stage of support,arch is prone to instability failure.Therefore,based on the full-scale laboratory mechanical test of arch,the bearing mechanism and deformation failure mechanism of arch under different cross-section forms,different combination states and different spatial constraints are clarified,the analytical solution of critical strength of arch instability is derived,and the mechanical model of support characteristic curve considering the overall instability of arch is proposed.(3)The laboratory flexural and compression-flexural tests of the jointless components,casing joint,flange joint and fabricated joint components are carried out.The local component deformation failure mechanism and force-transmission mechanism of joint with different joint forms,different constructional parameters and different loading states are clarified.The component design theory is introduced to construct mechanical analytical model of casing joint,flange joint and fabricated joint.A complete package of strength-checking method of local members of arch is obtained.(4)Aiming at the common failure problems of initial support lining such as cracking and sliding in the process of viscoelastic plastic stress release of surrounding rock in the late stage of continuous advancing of tunnel face,based on the full-scale mechanical test of steel-concrete composite lining structure,the occurrence mechanism and failure mode of interface bond-slip clarified,and the influence of strength,stiffness,ductility and ultimate deformation capacity of each component of composite lining is obtained.The mechanical properties of lining structure are studied by contrasting different layout,different constructional parameters and different shotcrete strength.The support characteristic curve model of composite lining structure is constructed.(5)Taking the super large section transportation tunnel-Letuan tunnel as the engineering background,the mechanical tests of rock uniaxial compression,triaxial rheology and bolt grouting body are carried out systematically,and the key mechanical parameters are inversed.Combined with the analysis of surrounding rock-support interaction mechanism in the whole process of tunnel construction,the function of tunnel support design in each key stage is constructed.Based on the idea of reliability design,the original design scheme is optimized by using support vector machine fitting method and Monte Carlo calculation method.The deformation control effect of the optimized design scheme is verified based on field monitoring.The failure mechanism and safety control mechanism of surrounding rock support under different support schemes is obtained.The results show that the research content of this paper is scientific and effective.