Study On Mechanism And Countermeasures Of Seismic Dynamic Instability Of Loess Tunnel Portal Sections

The Loess Area in Western China has a special geological environment,with large loess thickness,frequent seismic activities and high seismic fortification intensity.Under the background of continuous improvement of transportation infrastructure in Western China and rapid increase in the number of tunnels,a large number of tunnel projects are facing the threat of earthquake disasters at any time.Therefore,it is of great practical significance to study the seismic dynamic response law,instability mechanism and countermeasures of the portal sections of loess tunnel.In this dissertation,combined with the seismic damage data and relevant research literature at the tunnel portal sections,through numerical simulation and large-scale shaking table model test,considering the main influencing factors such as slope height,slope angle,tunnel entrance elevation and seismic wave spectrum characteristics,taking acceleration,acceleration amplification coefficient,displacement,wavelet packet transform and model failure mode are taken as the starting point,The seismic response law of the portal sections of loess tunnel is systematically studied,the seismic dynamic instability and failure characteristics of the portal sections are analyzed,the seismic dynamic instability mechanism and the discrimination basis of limit state are discussed,and the "estimation method of fortification range of the portal sections of loess tunnel under multi factor conditions" is proposed.The following research results are obtained:(1)The results of numerical simulation and model test show that the seismic response of the tunnel front slope along the elevation direction increases at first and then decreases,it is shows that when it is close to the top of the slope,the high-range amplification effect is no longer obvious under the filtering and attenuation effect of soil on seismic waves.(2)The seismic response of the tunnel is mainly driven by the deformation of the surrounding rock,which is manifested in the amplification effect of the tunnel portal.At the same time,the tunnel will also bring a certain restraint effect on the surrounding rock,which is expressed in that the slope seismic response decreases to a certain extent near the tunnel portal.(3)The height and angle of the front slope as well as the elevation of the tunnel entrance are important factors affecting the seismic response law of the tunnel portal sections,which will affect the seismic response strength and law,failure trend and range of the tunnel portal sections.The spectrum characteristics of different seismic waves will also lead to the difference of seismic response intensity.(4)By analyzing the deformation and failure modes of the model test,the instability failure process is divided into four stages: "elastic deformation stage","plastic deformation damage accumulation stage","local failure stage" and "overall failure".Different "stages" correspond to the variations of the overall dynamic characteristics of geotechnical structure of portal sections.(5)The change of acceleration peak value can reflect the deformation and failure state of geotechnical structure of portal sections in real time,and it also corresponds to the change of input seismic wave strength.The difference is that the change of acceleration amplification factor can reflect the change of internal state or characteristics of the model.When the acceleration amplification factor curve increases abruptly,it indicates that the rock and soil will enters the limit state before deformation and failure in the next stage,Generally,the measuring points with violent curve fluctuation will be destroyed firstly.(6)By analyzing the energy proportion of different frequency bands obtained by wavelet packet transform of acceleration response signal,the change trend of the difference between E1 and E2 in the energy proportion of the first and second frequency bands is regared as the index to judge the critical state of rock and soil deformation: when the difference between E1 and E2 reaches the maximum,it signs that the model is in the limit state before overall instability and failure,and when the difference between E1 and E2 begins to decrease from increasing,It indicates that the model is in the process of overall failure;when E1 is no longer dominant(less than 50%),it can be considered that the geotechnical structure has completely lost its strength and bearing capacity.(7)Through fitting,the quadratic functions of the relationship between the fortification range of tunnel portal sections and slope height,slope angle and tunnel entrance elevation are obtained.After solving them respectively,the average value is obtained which regared as the fortification range of loess tunnel portal sections under the condition of considering multiple factors.By means of the verification of an example,it is proved that this method is basically reasonable and feasible.