Research On Lining Structure Response Of Metro Shield Tunnel Under The Influence Of Ground Cavity

The shield tunnel is influenced by the over excavation of shield machine,poor driving attitude,dense grouting,slurry shrinkage,groundwater erosion and other factors.There are often cavities behind the shield segments,which leads to the disappearance of stratum resistance,the eccentric pressure of the lining,which causes stress redistribution of segments,and stress concentration in lining at the edge of the tunnel,which may lead to segment cracking and breaking the damage of block and leakage water greatly reduces the safety and durability of the segments.Therefore,this paper takes the tunnel with holes behind the segment as the research object,and studies the deformation and stress characteristics of shield tunnel segments under the effect of single hole and multi hole combination effect of different positions,angles,depth and longitudinal length by means of model test and numerical simulation.The main research contents and achievements are as follows:(1)The similarity ratio of model test is determined according to similarity theory,and the size of test bench is determined based on this,the physical and mechanical parameters of test materials are measured and the lining segments are calibrated,and detailed test scheme is developed.(2)The displacement and deformation,contact pressure distribution and internal force characteristics of the segment under four working conditions of no cavity behind the segment,vault,arch waist and arch bottom cavity are monitored and analyzed by means of model test.The results are as follows:(1)When there is no cavity behind the segment,the segment has good contact with the surrounding stratum.The whole section of the segment is subject to uniform formation pressure.The surrounding stratum provides sufficient stratum resistance for the tunnel segment,and the axial force and bending moment of the segment are distributed symmetrically.(2)The appearance of cavity leads to deformation of the corresponding position of the segment pointing to the hole,the contact pressure at the cavity decreases sharply,the formation resistance disappears,the stress of the segment appears redistribution,the overall stress state of the segment worsens,and the internal force is asymmetrical.(3)The influence of different cavities behind the segment on the safety of tunnel structure is as follows: arch waist > arch top ≥ arch bottom.When there is cavity behind the arch waist,the bending moment of this section increases by 2.78 times than that without cavity,and the tensile stress on the outer side of the segment is larger;when there is cavity behind the arch roof and arch bottom,the bending moment value of the section is significantly reduced,while the bending moment value of the adjacent section on both sides increases greatly,and the original load is borne by the adjacent section on both sides;when the cavity size increases to a certain value,the bending moment at the arch top occurs in reverse direction,there are reverse bending points on both sides of the arch crown,which changes from the original positive bending to negative bending(it is specified that the inner side of the lining is under tension and the outside is positive bending moment when the lining is under pressure).The bending moment change rate of the corresponding section of the cavity is large,and the segment is easy to be subjected to concentrated load.(3)The two-dimensional plane and three-dimensional spatial models are established by using beam spring method to further explore the influence of different sizes of cavities on the lining structure of shield tunnel.Based on the results of the study,the load and deformation modes of shield tunnel under the influence of cavities are summarized.The results are as follows: the increase of the angle,depth and longitudinal length of the cavity will lead to the further decrease of the safety of the segment structure.The order of the influence of the different cavity size parameters behind the segment on the safety of the tunnel structure is: the longitudinal length value ≥ the angle value > the depth value;the segment is sensitive to the longitudinal length of the cavity.With the increase of the longitudinal length of the cavity,the safety coefficient of the segment is presented the "parabola" is reduced now;the increase of void angle will further lead to the increase of the affected section of the segment,and the distribution of internal force of the segment will change greatly;the influence of the cavity depth on the safety of the segment is limited,and when the depth reaches a certain value,the stress state of the segment will gradually stabilize.(4)Based on the study of single cavity,the influence of multi cavity combination on shield tunnel is further studied by numerical simulation.The results show that under the effect of multiple cavities combination,the shield tunnel is no longer subjected to uniform load,and the segment is more seriously affected by bias pressure,and the internal force distribution mode of segment is more complex than that under single cavity condition,and the overall safety of the structure is lower than that without cavity compared with single cavity,multi cavity condition is not only a simple superposition between single cavities,but also the coexistence of multiple cavities aggravates the influence of each hole on the segment of tunnel.The influence of each cavity on segments is mutually beneficial,and the existence of multiple cavities combination has great harm to the segment structure of shield tunnel.