Study On Construction Mechanical Characteristics And Optimization Of Construction Parameters Of Soft Rock And Variable Cross-section Tunnel

The surrounding rock of soft rock tunnel has poor stability and is easily deformed by construction disturbance.When the tunnel changes suddenly from a small section to a large section,a staggered position of variable cross-section is formed.The surrounding rock of this section is prone to stress concentration,the stress and deformation of surrounding rock and support structure are complicated,and local large collapse is likely to occur during the construction process.It is important to study the construction mechanics characteristics of surrounding rock and supporting structure of soft rock variable cross-section tunnel to guide the safe construction of tunnel.Based on the Cave Laboratory tunnel engineering in this thesis,the theoretical analysis,field tests and numerical simulation methods are used to syudy the changes rule of stress and displacement of the surrounding rock and the internal force of the supporting structure at the variable cross-section in the process of tunnel construction,and the excavation method and support structure parameters are optimized.The main research work and results are as follows:(1)Based on the analysis of the measured data of the five sections near the variable section of the tunnel,the laws of the displacement time-history curve of the vault settlement and horizontal convergence are studied.Three kinds of function models are used to perform regression analysis on the measured data.The results show that the hyperbolic function has the highest fitting accuracy and can better fit the change rule of the measured data,Therefore,the function model is used to predict the final deformation of the surrounding rock and the best construction time of the second lining.(2)The finite element software Midas/GTS is used to establish numerical model of the variable section of soft rock tunnel,analyze the bearing capacity of surrounding rock and the internal force of the initial support structure,summarize the change rule of the stress and displacement of surrounding rock at the variable section,compare the numerical calculation results with the measured data,and verify the reliability of the calculation model.(3)Through the optimization analysis of excavation method and support structure parameters,the results that the displacement of surrounding rock increases with the increase of excavation footage,and the larger the excavation footage,the larger the increase,the defoementiont of surrounding rock can be controlled by reducing the excavation footage;with the increase of bench length,the displacement of surrounding rock increases,but the increase is not large;with the increase of shotcrete thickness,the displacement of surrounding rock decreases,but the spray When the thickness of shotcrete increases to a certain range,then increases the thickness of shotcrete,the deformation of surrounding rock decreases very little;with the increase of the geometric cross-sectional area of steel arch,the deformation of surrounding rock decreases very little.