Studies On The Dynamic Responsesof The High Slope Above The Tunnel Portal In Loess Of High Speed Railway

Since the 21 st century,the development of China's traffic has made remarkable achievements,which,At the same time,is accompanied by many safety issues.Because the mountainous west of China are dominated by loess mountains,many high-speed railways must go through tunnels toovercome theobstacles fromtopography and elevation,andthen shorten the mileage and straighten the line.The frequent earthquakes make it always a focus to studying on the seismic problems of tunnel,but there are few examples for studying the dynamicresponse of loess tunnel,whichmakes further research on the dynamics of loess slope-tunnel structure system significant.The loess slope itself has its own static field,and the dynamic amplification effect of it and the natural dynamic response of the tunnel entrance will influence each other to produce a more complex stress field,which means different slope angles,slope heights and entrance elevations will conduct different dynamic responses under earthquake effect.Therefore,it is of great significance to study the deformation laws,the damage forms and the dynamic responses of the entrance section of loess slope tunnel.In this paper,numerical simulations and large-scale shaking table testswere used to study the seismic dynamic response of high slope loess tunnels.By selecting different elevations for the entrance at the upward slope angles of 45° and 60°,the instability damage laws and stress characteristics were analyzed.The main research contents and conclusions are as follows:(1)The relevant domestic and foreign papers about the dynamic response of the entrance section of tunnel were consulted and classified.After summarizing the damage types,the damage mechanisms and the related research methods,we conformedthe research contents.(2)The tunnel has a certain inhibition on the peak acceleration working on the slope surface around it.When the upward slope is certain,the peak acceleration has a good positive correlation with the height of slope.The peak acceleration increases with the increase of the height,but it is not continuous.Whenapproaching to the top,the peak acceleration tends to be stable,and a saturated phenomenon of acceleration amplification effect begins to appear.(3)With the increase of the upward slope,the displacement of the lining also increases,and the maximum displacement generally occurs at 0-50 m away from the entrance and shows a tendency to move toward it with the increase of the height of the entrance.The range influenced by the surface emptying effect shall be fully taken into consideration when determining the seismic fortification length that shall be determined within 70 m away from the entrance by considering the change of the lining's displacement,namely 5.4 times the radius.(4)The displacement of slope increases with the increase of the height and the gradient,and reduces at inversion of the tunnel.The displacement changes slowly surrounding the tunnel,and the range affected by it is within 20 m from the center of tunnel,namely the 1.54 times the span of tunnel.The effect increases constantly with the increase of the height of entrance.(5)In the model tests,cracks along the axial direction of the tunnel first appeared on the top of the slope,when the basic earthquake intensity reached 9 degrees,and the shearing extrusion damage occurred due to stress concentration at the foot of the slope.With the increase of earthquake intensity,obvious external drum phenomenon on the top surface appeared and the destruction of entire slope surface happened,which means the top surface was the most obvious position on which the earthquake acceleration amplification effect acted.The outside drum appeared on both sides of the slope and 25-35 cm from it,caused the curved cracks,and then the cracks penetrated to form landslide face,finally,the entire destruction happened and the cracks and shocks were shaped on the top surface.(6)With the increase of the surface peak acceleration,the excellent frequency of the soil moves to the left as a whole,and the components of seismic wave become more and more abundant.It means that the increasing loading times from earthquake change the internal structure of soil,in other words,the shear stiffness is decreasing so as to the self-chatter frequency,and the damage is increasing.