| Xi'an is located in the loess area,and its collapsible loess has a wide distribution and large thickness.Many subway lines have to cross the large thickness collapsible loess stratum.These strata have higher strength and carrying capacity when not immersed in water.However the soil's compressibility is increased and the carrying capacity is rapidly reduced when immersed in water,so there is a greater risk to the subway tunnels in it.When there are seepage passages in the soil mass,if the municipal pipeline in the soil layer around the tunnel leaks water,and the adjacent deep foundation pit is not well protected against water and drainage,in case of heavy rainfall,the water in the foundation pit flows through the seepage channel to the surrounding layer of the tunnel,so that the soil around the tunnel is soaked in water and collapsed,thus causing adverse effects on the tunnel.Based on the engineering background of Xi'an Metro Line 9 Zhiyang Five Road to Zhiyang Square interval tunnel,this paper uses model test and numerical simulation to study the stress and deformation of tunnel lining and stratum after local soil collapse,and the mechanical mechanism of the impact.The numerical simulation is used to study the influence of the change of the collapsibility range and the different water immersion modes on the tunnel lining.The main results are as follows:1.Through the research of the bending moment and axial force of the tunnel lining under the three different water-immersed conditions of the model test,and the vertical stress change of the stratum,it can be seen that the larger the collapsing area,the greater the increase of the bending moment and the axial force in the tunnel lining,and the larger the surface settlement.The bending moment and axial force of the lining near the collapsible area are larger than those on the non-collapsible side.The vertical stress of the stratum at the waistline is reduced in the collapsing zone,and the vertical stress is increased outside the collapsible zone.The size of collapsible area is the same,the shallower the buried depth,the smaller the surface subsidence.2.Three different immersion and collapsibility conditions corresponding to model test are simulated by numerical calculation software.The stress and displacement changes of tunnel lining and surrounding rock after soil collapsibility are analyzed and studied.It is found that the results of the model test are basically consistent with the results of the numerical simulation.After the local soil around the tunnel is immersed in water,the bearing capacity of the soil is reduced,and the principal stress is deflected.The pressure of the overburden soil originally carried by the collapsing soil will be transmitted to the surrounding soil and lining structure,which will increase the surrounding rock pressure of the tunnel.And the increase is greater at the upper and lower corners of contact with the collapsible area.Correspondingly,the bending moment and axial force of tunnel lining increase,and the deformation also increases,showing the law of overall subsidence and upper extrusion deformation.The vertical stress is reduced in collapsible soil,but increases in non-collapsible soil on both sides of the collapsible area due to the transmission of stress.3.When horizontal immersion occurs,the stress in the tunnel lining increases significantly only when the distance between the collapsible area and the middle line of the tunnel is less than 11m(1.8D).When the water is immersed in the direction away from the tunnel from the side of the tunnel,the stress in the tunnel lining will increase greatly at the beginning of the collapse.When vertical immersion,from top to bottom immersion and from bottom to top immersion,the increasing path of stress in lining is basically the same,and the increasing law is approximately linear.When the collapsible area is the same,the internal forces of linings with different immersion modes are slightly different,but the difference is not great. |
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