Study On Dynamic Response Characteristics Of Portal Zone Of Large-section Loess Tunnels With Considering Slope Effect

In recent years,the frequent occurrence of earthquake disaster,the loess region is one of the regions with frequent occurrence of earthquakes and loess slope disasters due to the special geological background and geographical environment in China.However,the implementation of the western development strategy will have a large number of infrastructure constructions in the Loess Plateau.Such as the under construction passenger dedicated line from Baoji to Lanzhou,which located in the loess area and is a total length of 400 km,there are 273 km is tunnel project with 78 loess tunnel.In addition,the Zheng Xi passenger line,it has been put into operation and there are 27 loess tunnels with the total length 50 km.Therefore,it is of great significance to carry out the research of dynamic response and anti-seimic technology of portal zone of loess tunnels.In this paper,contents are mainly carried out by numerical simulation method in the following aspects:(1)Based on the boundary condition of aseismic analysis of underground engineering and the selection problem of earthquake input types,conducts example trials of different boundary conditions and different earthquake input types on condition of a underground semi-infinite body model being vertically incident by a shear wave,and finds that the method in which the two sides adopt vertical displacement constrained boundary,the bottom adopts viscoelastic boundary,and the bottom of the model adopts equivalent nodal force to input seismic oscillation,has got the same result as the method in which all the three sides adopt viscoelastic boundary condition and equivalent nodal loads to input seismic oscillation.The former method can be realized through APDL programming in ANSYS more easily,and has provided a more convenient way for the usage of viscoelastic boundary in the aseismic analysis of underground engineering.(2)Conducted the modal analysis for loess slope of different height and angel,it shows that self vibration frequency of loess slope has a increase with the increase of the frequency order.Slope angel has a small influences for fundamental frequency at the same slope height and different slope angel.Slope height has a great influence for self vibration frequency.With the increase of slope height,the self vibration frequency of each order is obviously decreased and the change trend is consistent under the same slope.(3)Seismic dynamic response analysis has been conducted for loess slope with different height and angle.It’s shows that slope displacement increases with the increase of slope angel at each slope height,the slope toe displacement increases with slope high increases at the same slope angel and different slope height,in addition,slope height has greater influence than slope angel for slope displacement.The amplification of the slope displacement is more and more obvious with the increase of the slope height under the each slope angel.Although slope acceleration amplification factor has a great change with the slope height increaces,the overall trend is consistent and changing in the shape of ‘S’ with the slope height changes at the same slope height and different slope angels.(4)Researched on dynamic response of portal zone with different altitude slopes.It’s shows that the relationship of slope displacement magnification factor at different altitude slopes are natural slopes displacement amplification factor is minimum and distance 1.5D to tunnel structure is maximum and central surface with tunnel structure is middle.The acceleration amplification coefficient is basically in accordance with the law.Through analysis of displacement and internal force of tunnel structure under different tunnel altitude on slopes,designated seismic fortification length of loess tunnel portal zone as 60 meters when entering the tunnel in the height of 0,and 50 meters when entering the tunnel in the height of 1/6,and 40 meters when entering the tunnel in the height of 1/3.(5)Researched on dynamic response of portal zone with different slopes angel.It’s shows that slope displacement,displacement of each feature point of the tunnel structure and internal force of the tunnel structure have an increases with the increase of the slope angel of loess portal zone.The length and thickness of covering soil of the tunnel structure is embedded in the slope are different due to the different slope angels,lead to the internal force and displacement of the tunnel begin to stabilize distance portal zone are different.It is maximum at 30 angel and minimum at 45 angel,40 angel is in the middle level.