Study On Mechanical Characteristics And Crack Propagation Law Of Shield Segment In Xuzhou Area

As the scale of urban subway development becomes larger,the stratum span during shield tunnel construction is gradually wider,and it is inevitable to cross the complex surrounding rock environment.Under the influence of various boundary conditions,the segment may produce crack damage,which will degrade the concrete stiffness near the crack,resulting in a decrease in the bearing capacity of the segment,and may even lead to a large area of crushing of the segment concrete.The prevention and control of segment cracking requires in-depth study of the formation mechanism and propagation law of cracks.Based on a shield section of Xuzhou Metro Line 6,this thesis uses field investigation and measurement,numerical simulation and comparative analysis to study the distribution law of cracks and the cracking factors that have great influence on the segment in the stratum environment,so as to analyze the stress,displacement and damage characteristics of the segment and internal components under the boundary conditions of various cracking factors.Then XFEM is used to analyze the propagation law of existing cracks in the segment,and reinforcement measures are proposed to optimize the segment design.The specific work and main conclusions are as follows:（1）The crack parameters of a line under construction in Xuzhou were measured on site,and the segment cracking data of a cross-river shield tunnel were collected.Combined with mathematical statistics,the distribution of segments at spatial points and the crack inducement under the influence of strata were studied.The results show that the cracks are mainly circumferential and longitudinal cracks,mainly concentrated on the standard blocks and adjacent blocks at the upper space points of the arch waist,and it is concluded that in the same type of stratum environment,the lateral pressure coefficient,groundwater level and upper soft and lower hard composite ratio are the main cracking factors of the segments along the line.（2）Based on the CDP damage plastic constitutive model of concrete,the stress,displacement and damage distribution of segments,steel bars and bolts under lateral pressure coefficient,groundwater level and upper-soft and lower-hard composite ratio are studied by numerical simulation.The results show that the stress at the vault and haunch decreases with the increase of lateral pressure coefficient,and the damage on the outer side of the haunch is the most serious when K₀=0.4.The change of groundwater level has little effect on the stress and displacement of the segment.Only when H_w=15.1 m,slight damage occurs at the haunch.When the soil-rock composite ratio m=1,a large area of tensile stress damage occurs on the outer side of the right hance and the inner side of the vault of the segment.The maximum principal stress and displacement of the segment,steel bar and bolt increase with the increase of the composite ratio.（3）Based on the XFEM extended finite element method,the numerical simulation method is used to study the existing crack propagation law of the segment when the damage is the most serious under the influence of various cracking factors.The results show that in the process of crack propagation,the geometric parameters show an increasing trend with the increase of the time history.When K₀=0.4,the initial cracks on the inner side of the vault and the outer side of the arch waist eventually expand into I-II composite penetrating cracks.When H_w=15.1 m,the cracks in the vault are not developed,and the cracks in the arch waist only expand in the shallow surface;when the stratum composite ratio m=1,the crack damage at the hance is more serious,and the bearing capacity of the segment is greatly weakened.（4）The numerical simulation method is used to study the crack propagation after the segment is reinforced by the steel plate ring.The results show that the steel plate ring has a good inhibitory effect on the growth of the crack depth direction.The research conclusions can provide theoretical support and method guidance for segment design optimization and operation maintenance.This thesis has 91 pictures,15 tables,and 86 references.