Study On Mechanical Characteristics And Construction Method Of Supporting Structure Of Super-large Section Urban Highway Tunnel

With the rapid development of Chinese economy,the investment of highway transportation and other infrastructure is also increasing.Highway tunnels have been more widely used and the construction scale is also getting laiger and larger.Due to the large excavation section,the extra large section tunnel bears more complex load.The mechanical properties are different from those of ordinary tunnels.Furthermore,the construction of large section tunnel has great influence on adjacent buildings.Therefore,the control of tunnel deformation and settlement in the construction process of large section urban highway tunnel is particularly important.Meanwhile,for the variable cross-section tunnel,the mechanical characteristics and deformation law of its supporting structure is more complex because of the influence of the change of cross-section.Based on the Huofengshan Tunnel project,with the combination method of literature research,field monitoring,model test and numerical simulation,this paper studied the mechanical characteristics of the supporting structure of the super large section urban highway tunnel.Furhtermore,the paper also explored the influence of tunnel construction on the building,and optimizes the construction method.The main research work and conclusions are as follows:(1)With the method of field monitoring,the deformation,surrounding rock pressure and internal force of steel arch under 17 m and 21 m sections were monitored.The results show that the relative deformation value of the crown of the super large section tunnel is larger than the clearance convergence value,with a difference of 3-5 times.The results show that the distribution of the surrounding rock pressure and the internal force of the steel arch frame is small in the upper part and the lower part,and the internal force of the steel arch frame at the arch shoulder and the arch waist is relatively large.(2)The mechanical characteristics of the tunnel supporting structure in the section of super large section are studied by model test.The three-dimensional numerical model of the super large section is established to simulate the excavation process of different section tunnels.The results show that the vertical relative deformation value of the super large section tunnel is larger than that in the horizontal direction.The difference increases with the increase of the tunnel section shape.For the super large section tunnel support structure,the minimum principal stress value at the arch waist of both sides is relatively large.The stress value increases with the increase of the tunnel section shape.The settlement value of the building pile foundation due to tunnel excavation is less than the allowable value of 20 mm.The uneven settlement value between the piles is about 50% of the maximum settlement value.(3)The tunnel with variable section is studied by model test and numerical simulation.The analysis results of the super large section are compared.By comparison,it is found that the stress concentration exists in the change of section of the supporting structure of each variable section,which is mainly distributed within 3m near the transition section.By comparing the displacement of surrounding rock in the super large section,it is found that the influence range of the section change on the displacement of surrounding rock is about 15 m near the transition section.The influence range under three kinds of the transition sections is the same.The uneven settlement value of pile foundation caused by tunnel excavation in variable section is larger than that of the large section.(4)The numerical simulation method is used to optimize the construction method of super large section and variable section tunnel.By comparing the construction methods,it is found that the both side heading method is obviously better than the CRD method and the CD method.The displacement value of surrounding rock and the stress value of supporting structure under the CRD method are relatively close with that of the CD method.By comparison,the optimized construction process can effectively reduce the stress value of temporary support in the construction process of super large section tunnel.The improvement effect of CD method is more obvious than that of double side heading method.When the tunnel with variable crosssection is excavated by sudden change,the influence range of the cross-section change on the stress of the supporting structure is mainly within 5m of the sudden change of the cross-section,which is larger than that of the original construction method.The stress value increases by13%-48%,and the plastic zone area also increases.