Research On Dynamic Response And Long-term Settlement Of Overlapping Tunnels Under Train Dynamic Load

With the rapid development of Chinese city and the constant improvement of subway network,the transfer and underpass between different lines are becoming more and more frequent.The spacing between subway tunnels and other buildings and subway tunnels is also becoming small,with a lot of overlapping up and down,or a small Angle overlapping tunnels.Stratum settlement during subway operation is a common problem affecting the safety of tunnel structure operation,and one of the important factors affecting the long-term settlement of tunnel is subway train load.Therefore,it is necessary to study the dynamic response and long-term settlement of soil around overlapping tunnel structure under train vibration load.Based on the complete overlap section between Yongshun Station and Beiguan Station of Beijing Rail transit Line 22,this paper establishes a three-dimensional dynamic finite element model using MIDAS GTS NX,and adopts linear time history analysis method to study and analyze the changes and rules of dynamic response components of overlapping tunnel structure and surrounding soil layer under different working conditions.By comparing the size,amplitude and peak value of dynamic response components of each measuring point under different working conditions,an optimal working condition is obtained,and the long-term settlement value between the optimal working condition and the actual working condition is calculated and compared.The main conclusions are as follows:(1)With the increase of tunnel depth and tunnel diameter,the dynamic response of train vibration load to overlapping tunnel structure decreases gradually.When the train speed increases from 80km/h to 100km/h and then to 120km/h,the effects of train vibration load on the vertical displacement response,vertical acceleration response and maximum principal stress response first decrease and then increase,and the effects on the minimum principal stress response gradually decrease.With the increase of net tunnel distance,the effects of train vibration load on vertical acceleration response and minimum principal stress response gradually increase,while the effects on vertical displacement response and maximum principal stress response gradually decrease.(2)The four influencing factors have the greatest influence on the vertical displacement response of the tunnel structure is the tunnel diameter,the greatest influence on the vertical displacement response of the surrounding soil is the tunnel clear distance,and the greatest influence on the vertical acceleration response is the train speed.Each factor has different influence on the tunnel structure and the surrounding soil.In the design process of the actual project,the diameter of the tunnel should be considered first and carefully and reasonably selected.Under the precondition of economic conditions and stratum conditions,it is recommended that the tunnel diameter should be controlled within 7～8m.In the actual operation of subway,it is recommended to control the maximum speed of subway trains at100km/h.(3)By comparing the magnitude,amplitude and peak value of vertical displacement,vertical acceleration and vertical stress of each measuring point under different working conditions,it is concluded that the third working condition is the optimal working condition,namely the buried depth of the upper tunnel is 11 m,the net distance of the tunnel is 5m,the tunnel diameter is 8m,and the train speed is 100km/h.Compared with the actual working condition,the tunnel buried depth in the optimal working condition is 4m more.In practical engineering,the influence of train vibration load on dynamic response of overlapping tunnel structure and surrounding soil can be reduced more effectively by changing the tunnel burial depth.(4)The settlement of the tunnel structure in the sandy soil stratum develops particularly rapidly within half a year after operation.The settlement of the sandy soil stratum is mainly characterized by rapid development and rapid stability.The settlement of the interlayer soil is the most significant,followed by the soil under the tunnel.With the increase of depth,the subsidence attenuation rate of sand stratum decreases gradually.It is suggested that the grouting strength of interlayer soil should be greater than that under the tunnel during tunnel construction.