Research On Numerical Simulation Method For Large Deformation Evolution Of Loess Tunnel And Its Supporting Mechanical Behavior

The Northwest and north of China are covered by large areas of loess strata.With the increasing construction of roads and railways in recent years,numerous tunnels have been constructed in expansive loess strata.Significant deformation,failure of primary supports and collapse of tunnels are common occurrences in loess tunnels owing to the low strength,water softening or swelling of the loess.These disasters have seriously restricted the construction and operation in safety of transportation and water resources and hydropower projects of China.And it will cause huge economic losses and negative impact on society.Therefore,the numerical simulation of the whole process of deformation and collapse of loess tunnel excavation and the study of the mechanical properties of the supports are of great practical and practical significance for analyzing the causes of tunnel collapse,and guiding the construction on site,so that the stability of the tunnel can be ensured.In this paper,the evolution process of large deformation and supporting mechanics of tunnel was analyzed by numerical simulation and theoretical analysis,based on Shanxi Taixing railway loess tunnel project.The main research contents of this paper include:(1)Based on the manifold cover idea,the barycentric interpolation test function is established.A large deformation numerical simulation method is proposed by double discrete method with fixed Euler background mesh and moving material point,with discontinuous damage processes are implemented by continuous simulation.The material particles are also the integration points.This method is called manifold cover lagrangian integral point method based on barycentric interpolation.This method uses Euler mesh as the background integral mesh and describes the deformation behavior of macroscopic objects through the motion of particles between meshes.Therefore,this method can avoid the problem of computation termination caused by the distortion of the mesh in the calculation process.And this method can also make material particles keep moving without limits in the set region,so that it is suitable for simulating large deformation and collapse problems in geotechnical engineering.(2)Using the complex variable function method and the Mohr-Coulomb plasticity criterion,the elastoplastic two-section ground characteristic curves of non-circular tunnel after excavation are derived.Taking a horseshoe tunnel as an example,conformal mapping for tunnel section shape is implemented by complex shape method.And the elastoplastic ground characteristic curve expression of the horseshoe tunnel is obtained.Meanwhile,the FLAC numerical simulation method was used to verify the derivation results.(3)The analytical solution of ground characteristic curve of composite rock mass reinforced by bolts is derived by the homogenization method based on the elastoplastic analytical solution of tunnel excavation,with the bolts are regarded as a part of the common bearing system of the surrounding rock.The analytical solution considers the length of the bolt and the spacing of the support.Finally,the deduced analytical solution is compared with the numerical calculation results,and the results are in good agreement.(4)Aiming at the problem of landslide and support damage caused by rainfall in loess tunnel,the composite support method of "steel sets + steel grid + shotcrete",which is suitable for loess tunnel,is proposed.The mechanical analysis model is established by the theory of elastic thin shell and equivalent calculation method is introduced to obtain the force distribution mode,and the equivalent calculation method of support stiffness and maximum support force of composite support(5)Based on the aforementioned theoretical research and Shanxi Taixing railway loess tunnel project,the process of landslide and tunnel collapse are reappeared by the manifold cover lagrangian integral point method based on barycentric interpolation,which is proposed in this paper.From a theoretical point of view,the reason for the damage of the tunnel supported by original primary support is explained.The engineering applications indicates that the "steel sets + steel grid + shotcrete" composite support method proposed in this paper has advantages in the expansive loess tunnel,comparing with the original "steel sets + shotcrete" support method.The proposed composite support method fully utilizes the bearing characteristics of different materials and ensures the stability of the tunnel.The research contents in this paper can provide a new method for numerical simulation of large deformation of tunnels and the support measures and mechanical characteristics analysis of loess tunnels.It has important reference significance for the construction and support of loess tunnels.