Seismic Response Analysis Of Tunnel Structures Through Horizontal Soft Sandwich Layers

With the vigorous construction and development of domestic highway tunnels,railway tunnels and other infrastructures in recent years,a large number of underground projects with complex geological conditions have emerged,and the tunnel structure through the weak interlayer geological conditions is one of the representative projects.Combined with China’s national strategic deployment,it can be predicted that the development of urban underground space is one of the inevitable trends of urban development in the future,so with the passage of time will also face more and more complex underground works.There have been some studies on this issue by scholars in the past,but most of them are based on technical studies of the construction phase of tunnel structures through weak interlayers,and less on the seismic design phase of tunnel structures through weak interlayers.Studies have shown that the presence of a weak interlayer has a huge impact on both the site and the seismic response of the tunnel structure,and the design of the tunnel structure is often on the dangerous side if the influence of the interlayer is not considered in depth and comprehensively.Thus,it is clear that the current stage of research is difficult to meet today’s as well as future engineering needs,and that there exists practical engineering applications for the seismic design of tunnel structures crossing weak interlayers.Based on this,In this paper,we take a tunnel structure crossing a horizontal weak interlayer in an actual project as an example to investigate the influence of weak interlayer on the seismic characteristics of the tunnel structure,and parametrically analyze various influencing factors（presence/absence of weak interlayer,input ground vibration characteristics,thickness of weak interlayer,modulus of weak interlayer,etc.）.and investigates the seismic response law of such tunnel structure in both cross-sectional and longitudinal aspects.Finally,specific measures to reduce the effects of seismic damage of weak interlayer are explored by changing the external conditions to improve the understanding of the seismic mechanism of tunnel structures.And to propose reasonable suggestions for improving the seismic performance of such tunnel structures.The main results are as follows.（1）The presence of weak interlayer has little effect on the horizontal absolute deformation of the tunnel structure,but has a great impact on the top and bottom relative deformation of the tunnel structure.The presence of weak interlayer has the greatest impact on the tunnel structure From the structural internal force point of view,the presence of weak interlayer has the greatest impact on the bending moment of the tunnel structure,with a difference of more than 20%in the peak response,while the impact on shear and axial forces is limited.With the increase of the thickness of the weak interlayer,the relative deformation of the top and bottom of the tunnel structure and the peak internal force response of the structure will gradually increase.As the modulus of elasticity of the weak interlayer increases,its influence on the structural deformation and internal forces of the tunnel decreases.（2）There is a range of influence of the presence of the weak interlayer on the longitudinal seismic response of the tunnel,and its spatial effect on the seismic response of the tunnel structure is most pronounced at the demarcation location of the weak interlayer.When the distance exceeds this position（30m away from the tunnel）,the seismic response of the tunnel structure gradually approaches the results of the 2D plane strain analysis.（3）The presence of the weak interlayer has a small effect on the bending moment at the top and bottom of the tunnel arch,and the peak bending moment in each direction does not vary much with position.The weak interlayer has a greater effect on the internal force of the arch waist and foot of the tunnel,especially the bending moment（M_y）in the Y direction at the arch waist position of the tunnel(the moment reaches 400k N·m·m^-1 near the interlayer)and（M_z）in the Z direction at the foot of the arch(the influence of the interlayer exceeds500k N·m·m^-1).As a whole,the bending moment（M_y）in the Y direction of the tunnel structure is smaller than the bending moment（M_z）in the Z direction,but when there is a weak interlayer,the arched waist bending moment M_y at the demarcation position also reaches 400k N·m·m^-1.Therefore,in the seismic design of such structures,in addition to the bending moment M_zwithin the cross-section of the structure,the bending moment M_y generated by the longitudinal bending of the tunnel at the location of the weak interlayer demarcation should also be considered.（4）Applying a certain thickness of seismic isolation layer around the tunnel structure is an effective measure to reduce or even offset the effect of weak interlayer on tunnel seismic response.Take Case 12 in Chapter 2,where the weak interlayer has the greatest effect on the seismic response of the tunnel,as an example.a 0.5m seismic isolation layer around the tunnel can basically eliminate the influence of the weak interlayer on the deformation response and bending moment response of the tunnel.