Mechanism And Safety Evaluation Of Leakage Accident In Shield Tail

This study aims at introducing main causes,failure mechanism and risk assessment of leakage accidents at the tail of shield tunnel in urban subway construction.By analyzing the working mechanism of sealing system at the tail of shield tunnel,the main types and causes of leakage accddents at the tail of shield tunnel are summarized.Through the analysis of the structure and working mechanism of the multi-layer seal system at the tail of the shield consisting of several seal brushes,the common failure mode and the influence mechanism of main risk sources of leakage accddents at the tail of shield tunnel are summarized and analyzed.Combined with fuzzy comprehensive evaluation method,failure mode and effect analysis method,a method is proposed to comprehensively consider the influence of various main risk factors on the main failure failure mode,which is called failure mode and risk factor analysis method.In this paper,this method is proposed to evaluate the probability level of leakage accidents at the tail of tunnel shield by using the actual monitoring data and environmental conditions of shield tunnel engineering.The finite element numerical simulation model is established to verify the probability level evaluating method and to explore the influence of different conditions including different leakage accident probability levels and different leakage point positions on the deformation and internal force of shield tunnel lining structure.The rationality and feasibility of the evaluation method for the possibility grade of leakage accident at the tail of shield tunnel are verified based on the leakage and water permeation accident at the tail of shield tunnel in actual engineering,and the modification opinions of the method are proposed according to the comparison results.The main conclusions of this study are as follows:(1)There are two main failure modes of leakage accddents at the tail of shield tunnel,which are affected by many risk factors related to the safety of shield tail structure.The leakage phenomenon at the tail of shield tunnel is mainly related to the structural quality and safety performance of lining sheets,synchronous grouting,multi-layer seal system at the tail of the shield,working position of shield machine,and surrounding soil and rock layers environmental conditions.The influence mechanism of different factors on leakage phenomenon at the tail of shield tunnel is not the same,but there is a correlation between the main factors.In engineering field,according to positions of leakage point,main influence mechanism and damaged components,main failure modes of leakage accidents at the tail of shield tunnel are classified into two kinds: leakage point at the outermost shield tail seal brush and leakage point at lining sheets joint.The main risk factors of leakage accddents at the tail of shield tunnel include: the external resultant force acting on the outer seal shield tail brush which equals to the sum of synchronous grouting pressure,soil and water pressure from the surrounding soil and rock layers;sealing grease injection effect(mainly including injection and pressure of sealing grease);consruction quality of lining sheets;working position of shield machine.(2)When the locations of possible leakage points at the tail of shield tunnel are the same,the influence of different probability levels of leakage accidents at the tail of shield tunnel on the internal forces of shield tail lining structure is also different.When the probability level of leakage accidents at the tail of shield tunnel is higher than R1,the possible leakage accidents will cause the redistribution of the annular axial force,longitudinal shear force and transverse bending moment of the lining ring around the possible leakage point,and the deformation and displacement of the lining sheets around the possible leakage point will occur.When the position of possible leakage points at the tail of shield tunnel are the same,the higher the probability level of leakage accidents at the tail of shield tunnel is,the greater influence range and fluctuation range of annular axial force,longitudinal shear force and transverse bending moment redistribution are and the larger the displacement and deformation of shield tail lining ring sheets are.(3)When the probability levels of leakage accidents at the tail of shield tunnel are the same,different locations of possible leakage points will have different effects on the leakage of shield tailings.The larger burial depth of the lining sheets is,the greater the soil and water pressure from surrounding soil and rock layers is.The closer the location of possible leakage point to the bottom of the lining ring,the greater the influence range and fluctuation range of the circumferential axial force and transverse bending moment are,and the larger the maximum value of the axial force and bending moment on the lining ring is.However,the closer the location of leakage point to the horizontal axis of the lining ring,the larger the influence range and fluctuation range of the longitudinal shear force will be,and the larger the maximum shear force on the lining ring will be.(4)The s leakage accidents at the tail of shield tunnel is a kind of accelerated damage form of shield tail structure controlled by various risk factors which can be superimposed on each other.The actual engineering performance of leakage accidents at the tail of shield tunnel is a kind pf development and accelerated failure.When the possibility level of leakage accidents reaches a certain degree,various risk factors will influence each other and have obvious superposition effect,which leads to the rapid development speed of shield tail slurry leakage accident in the later stage and makes it difficult to take emergency remedial measures.Therefore,emergency treatment of shield tail slurry leakage should be completed before the risk level of shield tail slurry leakage of shield tunnel reaches R4...