Study On Seismic Response Law Of Shallow Buried Bias Double-arch Tunnel With Foam Concrete Damping Layer

Multi-arch tunnels are being used more and more because of their smooth plane alignment,less floor space,and ease of operation and management.Earthquake is a natural disaster with great destructive power and difficulty in defense.Once a tunnel is damaged by an earthquake disaster,it is difficult to repair.And multi-arch tunnel will have different seismic dynamic responses under earthquake disasters because of its different structural form from ordinary tunnels.At present,there are relatively few studies on the seismic response of shallow-buried and unsymmetrical double-arch tunnels,so the research on its seismic response and the research on seismic reduction measures are of great significance.Under the support of the National Natural Science Foundation of Hunan Province"Seismic response characteristics and damping mechanism of foam concrete damping layer in shallow-buried unsymmetrical tunnels",the shallow-buried and unsymmetrical double-arch tunnel with or without a foam concrete damping layer was designed and carried out.Numerical simulation is used to further study the shock absorption effect of the double-arch tunnel under different elastic modulus and thickness foam concrete shock-absorbing layers under different directions of seismic waves.The main research work and conclusions are as follows。1.Based on the similarity theory,according to the performance parameters of the shaking table and the actual situation,a physical test model of a shallow buried and unsymmetrical double-arch tunnel with a geometric similarity ratio of 1:20 was designed and produced.Select Kobe wave and EI wave as the input seismic wave of the table to carry out a large-scale shaking table test.The acceleration and dynamic strain response laws of shallow-buried and unsymmetrical double-arch tunnels with or without damping layers,different types of seismic waves and different seismic intensities are studied.Studies have shown that the average vertical acceleration of the double-arch tunnel is larger than the horizontal acceleration,and the vertical seismic wave effect of the double-arch tunnel is stronger than the horizontal seismic wave.After the shock-absorbing layer is installed in the shallow-buried and biased double-arch tunnel,the horizontal acceleration,vertical acceleration and strain of different measuring points have different shock-absorbing effects.The seismic waves have different degrees of coupling due to the reflection and refraction of the foamed concrete porous structure.And in the case of the greater the earthquake intensity,the better the shock absorption and energy absorption effect of the shock absorption layer.2.The numerical model of the shallow-buried and unsymmetrical double-arch tunnel is established by the MIDAS-NX.The lining and surrounding rock in the numerical model adopt solid elements.In the constitutive relationship used,the lining is elastic and the surrounding rock is Mohr Cullen construction model.Free field is used as the boundary condition,and 5%is selected as the critical damping ratio in this paper.The selection of surrounding rock,lining,and shock-absorbing layer parameters is consistent with the shaking table test.Established five models of no damping layer,and different elastic modulus for the damping layer for analysis.Studies have shown that although the damping layer cannot essentially change the dynamic response of the lining,it can have an excellent damping effect on the tunnel lining due to the principle of damping,absorbing and buffering of its own material;As the elastic modulus of the damping layer increases,the horizontal displacement of the lining slightly increases,and the vertical displacement slightly decreases,which has little effect on the displacement;The shear force is significantly reduced,the bending moment and the axial force change little,and the internal force is mainly affected by the partition wall;The first principal stress and the third principal stress increase significantly.Through comparison,it can be concluded that the shock absorption effect is the best when the shock absorption layer is 30Mpa elastic modulus.Appropriately reducing the elastic modulus of the shock-absorbing layer can reduce the stress but slightly increase the lining shear.3.Five numerical models with no damping layer and different thick damping layer were established to analyze the results.Studies have shown that Under the action of seismic waves,as the thickness of the damping layer increases,the horizontal and vertical displacements of the lining slightly increase,and the shear force decreases significantly.The bending moment and axial force,the top end of the partition wall increases,and the bottom end decreases.The first principal stress increases locally,but the stress distribution is more uniform,which has little effect on the third principal stress.However,the greater the thickness of the damping layer,the less obvious the damping effect.Therefore,appropriately increasing the thickness of the damping layer can effectively reduce and improve the internal force and stress of the lining.After the damping layer is installed in the tunnel,not only the shear force,bending moment and axial force of the lining can be effectively reduced,but also the internal force distribution of the lining can be more balanced and reasonable,and the effect of weakening the stress is also obvious.Through comparison,it can be concluded that the thickness of 16cm has the best shock absorption effect,and the addition of the thickness of the shock absorption layer in some places also has a good shock absorption effect,which can be used according to the actual project.