Study On Large Deformation Mechanism Of Shallow Buried Underground Excavation Tunnel In Reclamation Area

With the rapid development of urban infrastructure in China,urban tunnel engineering has become an important part of the urban road traffic network.Urban tunnel engineering has become one of the important ways to relieve urban traffic jams and develop urban underground spaces.A series of engineering problems are often encountered in the process of urban shallow buried tunnel construction.When the shallow buried tunnel is built in the reclamation area,the large deformation of the ground appears.How to ensure the safety of the tunnel project and the surrounding environment is very important.Therefore,it is of great theoretical and practical significance to study the mechanism of large deformation during tunneling in the reclamation area.Based on the land section project of Xiamen Haicang undersea tunnel,this paper systematically studies the large deformation of tunnel excavation in reclamation area using geological survey,theoretical analysis,numerical simulation and model test.The mechanism of large deformation of tunnel surrounding rock in reclamation area is elucidated.The main research contents and conclusions are as follows.(1)Through field monitoring and measurement,the typical characteristics of stratum deformation during large section tunnel excavation in reclamation area were summarized.Influencing factors of large deformation after tunnel excavation were analyzed.Based on the stochastic medium theory,the calculation formula of surface subsidence caused by tunnel construction in the water-rich composite stratum of reclamation area was derived.A modified method of the main influence angle of the surface subsidence considering the influence of the composite stratum and the geometric characteristics of the tunnel section was proposed.Meanwhile,the calculation formula of surface settlement caused by consolidation settlement of composite stratum in reclamation area was derived.(2)Based on the modeling assistant software,the numerical simulation of tunnel excavation in the reclamation area in four cases were carried out.Through the comparative analysis of the numerical results,it can be found that the seepage of groundwater is the key factor for the formation of large deformation in the reclamation area.The maximum vertical displacement of surrounding rock with consideration of groundwater seepage is 234.8 mm,which is about 45.2%higher than that without consideration of groundwater seepage.The main stress release of the surrounding rock of the vault is concentrated in the excavation process of the upper steps of the middle guide tunnel.At the same time,the pore water pressure in the surrounding rock of the vault is almost completely dissipated.The release of stress and dissipation of pore water pressure lead to the intensification of formation deformation,which is transmitted upward to the surface and causes obvious surface subsidence.In addition,the influence of the surface traffic load on the surrounding rock of the tunnel in the reclamation area weakens gradually with the increase of the buried depth.Its main influence range is within 10 m below the surface.The surface deformation is affected by the traffic load,and the size and influence range of the surface subsidence are increased.(3)The study of similar materials for tunnel composite stratum in reclamation area was carried out.The aggregate,cementing material and regulator of three similar materials in reclamation area,were determined respectively.Orthogonal experimental studies of three similar materials in the stratum of reclamation area have been carried out.The sensitivity analysis of the physical and mechanical parameters of similar materials was investigated,and the main influencing factors of the parameters and their laws were qualitatively analyzed.The matching scheme of similar materials which can simulate miscellaneous fill,strongly weathered granite and slightly weathered granite in reclamation area was determined.(4)The geomechanical model test system of the tunnel in reclamation area was set up.The system restored the dynamic construction process of the tunnel in the reclamation area and revealed the evolution law of the stress field,displacement field and seepage field of surrounding rock.The excavation of the upper bench of the middle guide tunnel has the greatest influence on the displacement of tunnel arch and ground surface.The excavation of the lower step of the middle guide tunnel has the least effect on the surface.The surrounding rock stress of the vault can be divided into three stages:uniform release,sharp release and stable release.With the excavation of each section of the tunnel,the pore water pressure in the surrounding rock of the vault exhibits different degrees of stair-stepping dissipation at different construction stages.The pore water pressure around the middle guide tunnel is almost completely dissipated after the excavation of the upper bench.The change rate of the vertical deformation of the arch,the release rate of the surrounding rock stress and the dissipation rate of the seepage pressure are closely related.(5)The mechanism of large deformation of shallow buried tunnel in reclamation area is revealed.The special engineering geological conditions in reclamation area are the basic conditions for the formation of large deformation.The disturbance of blasting construction and section excavation are the inducing conditions of large deformation.Stress release and seepage action directly lead to the formation of large deformation.The disturbance of driving dynamic load for a long time is the secondary factor that causes the large deformation of surrounding rock.