Study On The Characteristics Of Static And Seismic Dynamic Responses At The Entrance Section Of Loess Tunnel

With the steady progress of the national "The Belt and Road" strategy,the infrastructure construction in the western region of China is also in full swing.At present,China has built a large number of railway and highway projects in the loess region of the west,which greatly promotes the local economic development.In order to ensure the high smoothness of high-speed railway,a large number of Bridges and tunnels will inevitably be built in the western mountainous area.However,the tunnel entry elevation is always a neglected problem when the route is designed in longitudinal section.How many times the slope of the tunnel into the hole can be the greatest degree to resist the tunnel entrance section of the earthquake damage is an urgent problem to be solved.Based on 300 of loess slope as the research object,firstly established by using finite element software Midas GTS/NX tunnel in the slope toe,0.3H elevation and 0.6H elevation into the hole and the numerical model,completed the static tunnel excavation analysis and seismic dynamic response analysis.At the same time,the shaking table model test under typical working conditions is completed by relying on the shaking table of lanzhou seismological bureau.The research results show that:(1)In the aspect of static calculation:the influence of the change of tunnel entry elevation on the displacement value of the secondary lining is not obvious.Under the condition of a certain tunnel entry elevation,the large principal stress value of the secondary lining has little difference in different tunnel monitoring sections.(2)The different elevation of the tunnel has a great influence on the vertical settlement displacement of surrounding rock caused by excavation.However,the maximum displacement of the secondary lining is less affected by the change of the height of the tunnel.(3)In terms of dynamic calculation,the elevation of the slope measurement point is the most critical factor affecting the numerical value of slope displacement and slope acceleration,and the overall change trend is that the higher the elevation is,the larger the corresponding numerical value of displacement and acceleration will be.However,the growth rate of slope displacement and slope acceleration near the tunnel will decrease to a certain extent.The existence of the tunnel has a certain "inhibitory effect" on the growth rate of the two in a local scope.(4)The maximum value of the secondary lining displacement and the maximum value of the large principal stress both appear in the monitoring section close to the tunnel entrance,with a significant downward trend within a range of about 80m from the tunnel entrance,and the value changes are relatively stable after 100m.Considering that the lining structure bears the maximum force within a range of about 80m from the tunnel entrance,it is suggested that the seismic fortification length of the tunnel can be 6 to 7 times the tunnel diameter.(5)Compared to dynamic and static calculation results concluded that the earthquake under the action of tunnel portal of second lining the maximal displacement of about 7.0～9.0 cm,the maximum principal stress is about 4.5～5.8 MPa,between two lining under the static state of maximum displacement and maximum principal stress of 11～15 times larger and 3～3.9 times,the earthquake damage to the tunnel lining structure has obvious effect.(6)In terms of model test,the peak acceleration(PGA)measured by accelerometers at the roof,arch and invert in the model test all increased with the increase of loading conditions,and the sudden increase of PGA amplification coefficient of the accelerometer at the top of the slope could be regarded as one of the signs of serious damage to the model.(7)The dynamic earth pressure in the model soil will be at least more than 1 times higher than the initial earth pressure,which will increase the surrounding rock pressure on the tunnel structure to a large extent,and is likely to cause the crack and dislocation of the tunnel lining.At the same time,both the maximum major stress and the maximum shear stress of tunnel lining reach the maximum within the range of 20cm from the tunnel entrance.The model test also shows that the tunnel entrance lining is in the most unfavorable position under the action of earthquake.