Research On Mechanism And Treatment Technology Of Shield Tunnel Structures Affected By Submersion And Collapse Of Loess Layer

Collapsible loess is widely distributed in Northwest China,which has special engineering properties such as macroporous,water sensitive,structural and so on.However,the construction of subway in collapsible loess cities(such as Xi’an,Lanzhou,Taiyuan and so on)is just in the beginning,so the subway construction will inevitably face the collapsibility problem of loess.When the subway tunnel passes through the collapsible loess stratum,the potential collapsible deformation of its foundation will seriously threaten the safety and stability of the tunnel structure,which has become a key research topic to be solved in the current subway engineering construction in the collapsible loess area.Based on Xi’an Metro Line 8,with the help of field investigation,numerical analysis and centrifugal model test,this paper systematically studies the collapsible mechanism of loess stratum and the treatment technology of shield tunnel foundation,The main research results are as follows:1.Based on the theory of earth pressure increment coefficient,the collapsibility mechanism of loess stratum is studied.Based on the analysis of the stress and structural differences between the shield tunnel and the basement of the ground building,it is found that the pressure on the basement of the shield tunnel is generally less than the self weight stress of the upper soil layer.Combined with the triggering conditions of the loess collapsibility around the shield tunnel,the collapsibility mechanism of the loess stratum around the shield tunnel is analyzed,and the analysis model of the loess collapsibility process around the shield tunnel is proposed.2.The influence mechanism of different factors on the mechanical deformation of shield tunnel structure is studied.The results show that the greater the collapsible depth,the greater the influence on the stress and deformation of the structure;the greater the collapsible soil near the tunnel structure,the greater the influence on the stress and deformation of the structure;under the same collapsible conditions,the greater the buried depth of the tunnel,the smaller the influence on the stress and deformation of the structure.3.The effectiveness of the simplified air fall method for artificial preparation of collapsible loess to simulate natural loess is verified by laboratory experiments.The experimental results show that the compressibility index of artificial collapsible loess is similar to that of natural loess,and the variation law of P-δ S curve is basically the same.The ratio of collapsibility coefficient between artificial collapsible loess and natural loess is about 1.45,which can provide a certain basis for the experimental results to speculate the actual results.4.Centrifugal test was carried out to study the influence mechanism of shield tunnel base grouting treatment on the internal force of tunnel structure and surface settlement.The test shows that the grouting treatment can effectively improve the bearing capacity of the foundation,the internal force change of the tunnel structure is still within the safe range after the base soil is soaked and collapsed,and the surface settlement is effectively controlled to ensure the operation safety of the subway tunnel.5.The influence of different grouting parameters on the stress,deformation and surface settlement of tunnel structure is studied.The results show that the maximum variation of earth pressure and displacement at each point of tunnel structure is 87.7% and 74.7% respectively.The maximum changes of earth pressure at each point of tunnel structure caused by grouting angle and grouting strength are 21.5% and 32.7% respectively,and the maximum changes of displacement are 28% and 10.8% respectively.Therefore,when optimizing the grouting scheme,priority should be given to increasing the grouting range,rather than increasing the grouting angle or increasing the grouting strength.