Analysis On Causes For The Cracks Of The Inverted Arch Of Highway Loess Tunnel

In recent years,with the implementation of China’s western development strategy,the construction of infrastructure projects in the western region has been launched on a large scale,especially the rapid development of High-speed railway and Highway construction.China is a mountainous area,the famous Loess Plateau is located in the famous West.As is known,in the process of loess tunnel construction,the main problems are the large settlement of the crown,the cracking of the earth surface,the uplift and cracks of the arch.A tunnel in the process of construction,surface cracking,the too large surface subsidence,surface subsidence,deformation of the lining and the cracks of tunnel invert cont continuously appear.Therefore,by closely combining the construction process of the tunnel,this paper will study the tunnel lining structure of the stress characteristics and deformation characteristics to explore the reasons for the cracks in this tunnel invert.By relying on Lanzhou south beltway on Lianyungang-Huoerguosi national highway,this paper mainly carries out the research work and obtains some conclusions of the following aspects.(1)By analysing of the engineering geological and hydrogeological charac-teristics of the project,and the field sampling for laboratory tests,the numerical calculation of the physical and mechanical parameters of a tunnel will be got.The result is that the surrounding rock of internal force test section is loose,the steel arch space is 1m,and the space is slightly larger.(2)By installing three monitoring sections at the construction site,the first section is installed in the standard section,the second section is installed in the test section(the test section is provided with a temporary inverted arch and the arch is provided with bars),the third section is installed in the temporary invert.By the analysis of the test data,it is found that the surrounding rock pressure is generally larger,the left and right ends of the packed layer are compressed,the middle of the packed layer is tensified,and the tensile stress of the middle of the packed layer exceeded the tensile strength.With the increasing of steel,the crack width of packed layer decreases obviously.However,the application of temporary invert failed to inhibit the development of cracks.(3)The crown settlement,clearance convergence,side wall settlement observation point and surface settlement observation points are installed in the settlement of three monitoring sections.Based on the analysis of the lining structure and the law of deformation of the ground surface,we can receive that the settlement of the vault is larger than the side wall,the maximum width of surface crack is 15 cm,the maximum fault is 10 cm,and there are many pits on the surface.(4)The finite element model of MIDAS/GTS is used to build three-dimensional model to simulate the construction process of the tunnel.By analyzing the stress and deformation data of lining structure,we find that the middle of the invert and packed layer is tensified,and the tensile stress of the middle of the packed layer exceeded the tensile strength.(5)Structural mechanics solver is used to calculate the force characteristics of invert arch and analyse the variation of safety factor of each part of invert.When the hight of inverted arch remains unchanged,with the increasing of uniformly distributed load,each part of the inverted arch safety coefficient decreased,and the left,right and bottom arches decrease more obvious;when the uniformly distributed load remains unchanged,with the increasing of hight,each part of the inverted arch safety coefficient increased.When the arch is high enough,each part of the inverted arch safety coefficient can meet requirements.By taking practical engineering problems as background this paper analyses the causes of cracks of invert arch by means of the indoor test,monitoring measurement and numerical simulation of structure monitoring and the analysis of the cause of inverted arch and surface cracking,puts forward the corresponding treatment measures and provides the reference for the similar engineering.