| The shield method is often used in the construction of tunnels in urban rail transit.Due to the terrain characteristics and the backfill transformation in the early stage of construction,there are a large number of backfill land layers in Chongqing.The backfill soil is a mixture of soil and rock with poor grain grading,and it is easy to have tunnel collapse,water gusher,uneven settlement of the surface and tunnel bottom and other hazards during shield construction,which will affect the construction progress and safety.Therefore,it is of great significance to study the deformation response and control measures of shield construction in backfill soil layer.In this paper,the shield construction in the backfill section of the north extension of Chongqing Rail Transit Line 5 is taken as the engineering background,and the existing research methods and achievements are referred to.By means of theoretical analysis,numerical simulation analysis,field test and monitoring,the deformation laws of strata and segments in different construction stages are analyzed.This paper studies the control effect of palm surface stability control,formation pre-grouting reinforcement and wall backfill grouting on formation deformation.The main research contents and conclusions are as follows:(1)This paper introduces the general situation and geological conditions of the northern extension section of Chongqing Rail Transit Line 5 supported by the project,summarizes the difficulties in shield construction of backfill soil layer based on theoretical analysis and existing engineering experience,discusses the mechanism of formation deformation caused by shield construction,and analyzes the control mechanism of excavation face stability control,formation pre-grouting reinforcement and wall backfill grouting on formation deformation.(2)Finite element software ABAQUS was used to establish a three-dimensional numerical model to simulate the shield construction process under different cutter head opening rate and different soil tank pressure.Through displacement response analysis,it was found that before the arrival of the shield,a small amount of uplift occurred on the front surface,and the surface began to settle after the arrival of the shield.When the soil tank pressure was higher than the excavation surface pressure,When the earth pressure of the excavation face is lower than the earth pressure of the excavation face,the excavation face is extruded to the earth tank as a whole.The greater the earth tank pressure and the smaller the cutter head opening rate,the more favorable the surface settlement control.(3)The stratum pre-grouting reinforcement technology is introduced and the grouting effect is evaluated by means of unconfined compressive strength test,core sampling observation and ultrasonic test,etc.At the same time,a two-dimensional heterogeneous layer model is established based on the principle of Monte Carlo method for numerical simulation analysis of shield excavation.It is found that,The deformation of surrounding rock caused by shield construction decreases with the increase of stone content,and the more uneven the particle size distribution of block stone,the greater the influence of the change of stone content on the construction deformation of backfill soil layer.The physical and mechanical parameters of backfill soil can be improved by pregrouting reinforcement measures,and the integrity and compactness of the original stratum can be improved to effectively reduce the surrounding rock deformation caused by shield construction.And the lower the stone content of backfill soil layer,the better the control effect of pre-grouting reinforcement measures on formation deformation.(4)Internal force monitoring of segments is carried out on the construction site to draw the distribution diagram of internal force of segments at key stages in the shield construction process,and the stress and deformation state of segments after grouting of the rear wall are studied combined with numerical simulation analysis.The research results show that in the shield tunneling process,the axial pressure of the arch bottom of the segment is the largest,the bending moment of the side wall is the largest,and the deformation of segments after grouting of the rear wall is the largest.As the shield continues to drive forward,the differences in axial force and bending moment values of each part of the segment tend to decrease gradually,which indicates that the segment gradually changes from being subjected to high concentrated force locally to uniform compression. |
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