Seismic Response Of Tunnel Entrance In Cold Region Considering Freeze-thaw Cycle

There are mainly high altitude cold regions and high latitude cold regions in China.The high altitude cold area is mainly characterized by high altitude and cold climate,mainly concentrated in the Qinghai-Tibet Plateau,Gannan,western Sichuan,northern Yunnan and other central and western seismic areas of the most dense.With the improvement of our transportation network,it has been concluded that the traffic arteries in these areas should be opened as soon as possible,among which the stability of the tunnel project is the decisive key.The freeze-thaw cycle itself will shorten the life of concrete projects in the seasonal frozen area.In recent years,high-frequency earthquakes in the high altitude cold area have made the tunnels in the cold area,which were originally subjected to different degrees of freeze-thaw damage,face more serious earthquake damage.At the same time,a number of new tunnels have been built in high-intensity earthquake zones in the high and cold regions of the plateau,such as the Muzhailing Tunnel,the Dangjinshan Tunnel and the Galongla Tunnel.Taking the entrance section of a shallow tunnel in a frozen area as the research object,the transient temperature field and moisture field of the entrance section of a tunnel in cold area were obtained by establishing the differential governing equation of transient temperature field of unsaturated soil and the differential governing equation of transient liquid flow in unsaturated soil.Then the relation equation between the temperature field and the water field was deduced by the principle of conservation of mass,and the law of unfrozen water content and volume ice content of surrounding rock behind the lining of tunnel entrance in cold area was obtained.Then the frozen soil is regarded as a kind of special thermal sensitive material.Based on the theory of global frost heave of broken freeze-thaw circle,the freeze-thaw load with time in the freeze-thaw circle behind the lining is obtained,and the initial ground stress at the entrance of the tunnel in cold area is obtained by superimposing with the dead weight stress.Finally,the seismic load was quantified and equivalent to the physical strength of each particle.Considering the influence of freeze-thaw cycle on the soil strength and lining mechanical parameters in the freeze-thaw circle of the tunnel entrance in cold area,the seismic response law of the tunnel entrance under the influence of freeze-thaw was investigated using COMSOL Multiphysics coupled finite element analysis software.The main research contents and achievements are as follows:(1)Seismic response of tunnel entrance section in cold area at typical moments of cold and warm seasons:No matter what analysis method is used,the seismic response of each measuring point of the tunnel in cold zone considering the broken freeze-thaw circle is weaker in cold season than in warm season.No matter in cold season or warm season,the tunnel in cold zone without considering the influence of freeze-thaw cycle is prone to plasticity when it is hit by earthquake,and the wall is damaged only when it is hit by earthquake in melting period.The invert exhibits a stress characteristic of internal tension and external pressure.The stress on the top arch ring and side curved wall is uniform and small.(2)Seismic response of tunnel entrance in cold region affected by freeze-thaw cycle in30 years:The influence of freeze-thaw cycle on seismic response of tunnel entrance in cold area shows that the possibility of failure increases with time,especially in the compressive stress zone inside the wall foot.Constrained by the statistical method,the seismic response of the lining in the first 10 years of freeze-thaw is small,and the seismic stability of the lining in the entrance section has deteriorated significantly since the 15 th year of freeze-thaw.Under the influence of freeze-thaw cycle,the lining structure at the entrance of tunnel in cold area may suffer from the following damage forms when it is subjected to ground motion: frost heave damage aggravated by lining cracking,local blocking,collapse,tunnel floor heave,etc.From the time axis of freeze-thaw cycle,these damages will be progressive from the previous to the later.(3)Seismic response of tunnel entrance section in cold region affected by freeze-thaw cycle under different ground motion peaks:Under the influence of freeze-thaw cycle,the possibility of failure is positively correlated with the peak ground motion acceleration a when the tunnel entrance is subjected to earthquakes of different intensities.Especially when the tunnel in the cold area is hit by high intensity earthquake in the warm season,the possibility of tunnel lining damage is significantly changed compared with the general seismic intensity.Regardless of the value of a,the maximum and minimum principal stresses of the tunnel entrance section in cold area affected by freeze-thaw cycles appear at the foot of the left and right walls under different seismic intensities,followed by the inside and outside of the invert.(4)According to the above conclusions,the seismic response of local water-rich water at the foot of the surrounding rock behind the lining of the tunnel entrance in the cold area is studied:The seismic response of the tunnel entrance section with locally water-rich surrounding rock is greater than that of the tunnel without locally water-rich surrounding rock under the same intensity.The left and right walls are still the high occurrence area of disasters,which still conforms to the rule that the earthquake response degree in cold season is weaker than that in warm season.The peak value of equivalent plastic strain appears in the plastic zone of the left and right walls,and the plastic zone of the left water-rich zone is obviously larger than that of the right water-free zone.In addition,two symmetrical plastic zones of almost equal size also appeared at the left and right wall feet of the lining.The stress of the lining structure may have exceeded the elastic range and produced plastic deformation.However,due to the constraints of surrounding rock,cracks appeared at the left and right arch feet.