Cause Analysis Of Lining Cracks In Long-dam Multi-arch Tunnel And Research On Construction Safety Countermeasures

During the construction of the multi-arch tunnel,due to the changing geological conditions around the tunnel,the complicated structure of the tunnel structure,the existence of multiple disturbances and the limitation of the construction site,the first hole excavated in the tunnel will produce multiple cracks of different sizes in the right arch waist to the right arch shoulder.The occurrence of cracks not only affects the bearing capacity of the tunnel structure,but also causes the phenomenon of water leakage.In serious cases,it may even lead to safety accidents such as partial collapse of the tunnel.This paper relies on the Changba multi-arch tunnel as the project.Based on the investigation of crack distribution patterns,theoretical analysis,numerical simulation and monitoring measurement are used to study the lining cracks of the Changba multi-arch tunnel.The main contents of the study are as follows:(1)The spatial distribution characteristics of the cracks appearing on the left lining of the Changba multi-arch tunnel are investigated.The cracks are mainly distributed in the right arch waist to the right arch shoulder,with longitudinal cracks and oblique cracks.The main causes of cracks are: localized bias in the tunnel,loose surrounding rock structure,poor bond strength,under the buried deep load and the right hole excavation,the partial sliding force of the rock mass and the self-weight of the rock mass are transferred to the left hole,which causes the left lining concrete to bear excessive load,so cracks appear.(2)Based on the actual stress and surrounding rock deformation of multi-arch tunnel lining,combined with the stipulation of the boundary height of deep and shallow burial in the code,the boundary burial depth of the Changba multi-arch tunnel is calculated.According to the theory of Pu's balanced arch,the formulas for calculating the surrounding rock loads of the multi-arch tunnels under deep buried,shallow buried and biased pressure are determined.According to the actual engineering geological conditions of the Changba multi-arch tunnel,the earth pressure load of surrounding rock is calculated.(3)Based on the MIDAS/GTS NX geotechnical simulation software,thenumerical analysis model of the multi-arch tunnel was established.The stress and displacement characteristics of the tunnel lining structure with the vertical depth and horizontal depth of the tunnel and the different overlapping heights of the initial steel support were studied.The results show that the sinking amount of the tunnel in the ZK6+390 section lining is 2.5mm,which is consistent with the actual crack location width.The lining sinking amount reaches 9.63 mm at the maximum buried depth.The maximum compressive stress of all sections of the tunnel does not exceed the design value of the compressive strength of the lining structure of 13.8 MPa.The tensile stress is 1.43 MPa in the ZK6+390 section,which exceeds the structural tensile strength design value of 1.39 MPa.The tensile stress concentration is the main factor leading to the cracking of the left right arch waist to the right shoulder lining.The upward support of the initial support steel lap height of 50 cm can optimize the overall stress of the tunnel lining structure,and effectively reduce the vertical displacement of the tunnel lining and the tensile stress in the maximum principal stress.(4)Grouting the surrounding rock behind the lining of the cracking section and pre-reinforcing the surrounding rock in front of the face with a small duct.Consolidation of broken surrounding rock forms effective bearing arch,which effectively controls the load on lining caused by subsidence and deformation of overlying surrounding rock.By monitoring the measurement points,the maximum vertical displacement of the effective section is 69 mm,and the maximum peripheral convergence displacement is 51.36 mm.The crack is repaired with epoxy resin and the width is increased by 0.06 mm at the maximum and is finally stable.After 20 days of reinforcing with steel mesh,the second lining tends to be stable on the 15 th day.The maximum interlayer pressure of the lining appears at 0.45 MPa at the vault top.The maximum strain of concrete and tensile stress of steel bar appear at the right shoulder of the arch,which are 91 ?? and 0.58 MPa,respectively.Compared with the simulation results,the reduction is larger,and the treatment measures of lining cracking are reasonable and effective.