Study On Mechanical Characteristics Of Shallow Buried Large Section Urban Tunnel Construction

With the accelerated pace of economic and social development and urbanisation,traffic congestion has restricted the development of cities and regions,and single twolane urban road tunnels are no longer able to meet the demand of urban traffic.In this paper,the current status and challenges of the construction process of large section urban tunnels are investigated by means of field monitoring,numerical simulation and other means,combined with the actual construction of large section urban tunnels with eight lanes in both directions and small clearances under class V surrounding rocks.The construction methods of large section urban tunnels have been studied and analysed,and some research results with engineering value and significance have been obtained,mainly including:(1)Using FLAC3 D finite-difference software,numerical calculations were carried out for the construction of the CD method for a small-clearance large-section urban tunnel using the Slurry Spring urban tunnel as the engineering background,and the vault settlement,horizontal displacement of the surrounding rock and stresses in the surrounding rock were studied and analysed.According to the deformation pattern of the target points of the surrounding rock,the excavation of the later cavern has the greatest influence on the surface settlement directly above the earlier cavern and the vertical displacement of the surrounding rock at various points of the earlier tunnel.(2)Numerical simulations of various construction methods were carried out for small headroom large section urban road tunnels,comparing the settlement of the tunnel vault,horizontal displacement of the intervening rock and intervening rock stresses under various methods.It is concluded that the use of the double sidewall guide trench method is the preferred method of construction for small clearances and large cross-section urban tunnels,as it provides better control of surface settlement,reduces the effects of tunnel proximity construction,and suppresses the extent of the plastic zone in the surrounding rock.In the construction of small clearances large cross-section tunnels,it is proposed that the half-step CD method is selected for construction,which can provide reference for the design and construction of similar projects and has important implications for the design and construction of urban tunnels.In addition the numerical calculation results and field monitoring data are compared and analysed,showing that the displacement monitoring data and engineering practice prove that it is reasonable and feasible to use the CD method for excavation of the Slurry Spring Tunnel under Class V envelope grade conditions.(3)A numerical model of a large cross-sectional continuous-arch tunnel with eight lanes in both directions was developed for the same surrounding rock conditions in the Slurry Spring Tunnel.When a large cross-sectional continuous-arch urban tunnel is constructed using the three-guide method,the displacement of the surrounding rock gradually increases during the construction of the rear tunnel due to the small tunnel spacing,and the displacement of the rear tunnel is smaller than that of the first tunnel at the end of the construction of the rear tunnel,and the displacement overlaps at the upper end of the intermediate diaphragm wall The situation As the continuous arch tunnel is constructed in two longitudinal sections,the surrounding rock is less affected by the stresses in the surrounding rock after the excavation of the preceding tunnel is completed,and is only affected by the interaction of the same excavated section.During the construction of the tunnel,the diaphragm wall has to bear both the loads from the roof of the cavern and the construction loads from different directions.(4)Two construction methods,the three-guide method and the CRD method,are compared and analysed for large cross-sectional continuous-arch urban tunnels.In terms of surrounding rock deformation control,both the three-guide method and the CRD method can be used to achieve safe construction,but the former is slightly superior.The maximum tensile stress of the CRD method is 1.7 times higher than the maximum tensile stress of the double sidewall method,depending on the stresses in the tunnel.The threeguide pit method,due to its high construction requirements,often results in the removal and replacement of temporary supports in tunneling projects;whereas the CRD uses symmetrical excavation in the left and right lines,with relatively few construction steps,which is more conducive to the safe and rapid construction of the tunnel.(5)A comparison is made between the two different construction forms of small clearance tunnels and continuous arch tunnels for large section urban tunnels.The differences between the small clearance tunnel and the continuous arch tunnel are analysed in terms of the displacement of the surrounding rock,surface settlement,the plastic zone of the surrounding rock and the initial support stress.In terms of the extent of the plastic zone in the surrounding rock,the plastic zone in the continuous arch tunnel is slightly larger,with the plastic zone in the arched shoulder of the continuous arch tunnel being larger and concentrated in the upper part of the diaphragm wall and at the base,but the overall yielding range is still manageable.In the small headroom tunnel,the higher initial compressive stresses are mainly located at the arch waist and arch shoulders,while the higher tensile stresses are mainly located at the arch base;in the continuous arch tunnel,the higher initial compressive stresses are mainly concentrated above the diaphragm wall,while the tensile stresses are mainly located at the arch top.In addition,corresponding construction recommendations and measures are given for large section small clearances and continuous arch tunnels,in order to better guide the construction of large section urban tunnels.