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

Compared with conventional tunnels such as single-tunnel tunnels and small-clearance tunnels,double-arch tunnels have many advantages.Therefore,in recent years,the number and mileage of double-arch tunnels have increased throughout the country.China has a vast territory,complex terrain,and mountainous hills in the southeast.In the construction of a double-arch tunnel,it will inevitably encounter the terrain conditions of shallow buried bias.The double-arch tunnel has a complicated structure,which is equivalent to the ordinary tunnel being more easily damaged in the event of an earthquake.Therefore,the stability of shallow buried biased double-arch tunnels under earthquakes has become a guarantee of people’s lives and property safety.But the existing research results are far from meeting the needs of the project.Therefore,it is urgent to explore the seismic response law of shallow buried biased double-arch tunnels and its damping measures.This article relies on the National Natural Science Foundation of China"Earthquake Disaster Behavior and Seismic Performance Evaluation of Rock Slopes with Underground Caverns(Groups)"(No.51204215),Hunan Natural Science Foundation(2018JJ3882),and Hunan Emergency Management of science and technology projects(31971727).The large-scale shaking table test and MIDAS GTS/NX finite element software were used to study the seismic response law of shallow buried biased double-arch tunnels with damping layers.The research results have certain engineering significance for deep understanding of the seismic response law of shallow buried biased double-arch tunnels and the damping effect of foam concrete damping layer.The main research contents and results are as follows:(1)Based on the three theorems of similar theory,the test purpose of the shaking table test and the actual test situation.The geometric similarity ratio of the model is determined as 1:20,and the similarity relationship of the model is determined based on the geometric similarity ratio and the rest of the basic similarity ratios.Under the guidance of similar relations,through the proportion test and uniaxial compression test,the mix ratio of the foam concrete damping layer that meets the strength requirements and deformation requirements of the shaking table test was studied.The uniaxial compression test results of foam concrete show that:The foam densities of 250 kg/m3,300 kg/m3 and 350 kg/m3 all have good ductility.Among them,the 350kg/m3 test piece has suitable strength(0.5MPa)and good deformation ability.This was used as the damping layer material for the shaking table test of the double-arch tunnel,and the feasibility of foam concrete as a damping material in actual tunnel engineering was verified.At the same time,lining and surrounding rock materials meeting the requirements of shaking table tests were prepared.(2)According to the test characteristics of the shaking table test and the existing research methods,the rigid model box was selected as the test carrier,and the model box was treated as follows:The bottom of the model box is first covered with 4cm thick fine stone,and a layer of fine sand is laid on the fine stone to be processed into a friction boundary to reduce the bottom slip;The inner wall of the model box is pasted with a 10cm thick polystyrene foam board and processed into a flexible boundary to absorb the seismic waves reflected by the side wall of the steel plate and reduce its impact on the model;Based on the flexible boundary,a layer of polyvinyl chloride film is pasted on the surface of the inner wall of the long side of the model box to form a sliding boundary,thereby reducing the friction between the flexible boundary and the model.After the boundary treatment is completed,the surrounding rock is paved and buried in the model to complete the shaking table test.Based on the results of the shaking table test,the acceleration response,dynamic strain response of the shallow-buried biased double-arch tunnel with or without foam concrete damping layer and the influence of the damping layer on them are analyzed in detail.The research shows that the acceleration response,dynamic strain response of the shallow buried biased double-arch tunnel are significantly different from that of the horizontal direction,and the response intensity of the vertical direction is greater than the horizontal direction;The damping layer does not change the spectral characteristics of the acceleration time history curve,but only affects its peak value;The damping layer can not only reduce the peak strain response of the lining,but also make the original line of the vibration return to the starting position after reaching the peak strain when the lining is excited at 0.4g-WC-XZ,preventing the plastic deformation produce.It shows that the foam concrete damping layer can delay the lining from entering the plastic failure stage to a certain extent.(3)A numerical model matching the shaking table test was established using MIDAS GTS/NX.By comparing the acceleration response measured with the shaking table test,the error between the numerical calculation result and the shaking table test result is within an order of magnitude,which verifies the reliability of the numerical model.Based on a reliable numerical model,the displacement response and internal force response of a shallow buried biased double-arch tunnel with damping layers are studied.The research results show that in the model without the foam concrete damping layer,the displacement value of each measurement point increases with the increase of the excitation intensity.In the model with foam concrete damping layer,the response law of vertical displacement is similar to that of the model without damping layer,but the damping layer has a greater influence on the horizontal displacement.The internal force(axial force,shear and moment)of the foam concrete damping layer model is the largest in the middle wall,and the size increases with the increase of the excitation intensity,and the internal force values at the other measuring points are smaller.In the model with the foam concrete damping layer model,the internal force peaks at left and the top of the right sides of the middle wall are also larger,but the internal force values at the bottom of the middle wall of the right hole are smaller.The damping effect of the foamed concrete damping layer is not obvious when the low-intensity vibration is 0.1g and 0.2g.At 0.4g and 0.6g high-intensity vibration,the foam concrete damping layer can effectively reduce the displacement response value of the lining structure,and the greater the intensity of the vibration,the greater the reduction.The foam concrete damping layer has different damping effects on different internal forces,and should be considered separately when designing shock absorption.