Study On Effects Of Vibration-isolation Trenches On Dynamic Response Of Shallow Tunnel

A large number of construction projects in the city will inevitably lead to the interaction between adjacent projects,especially the dynamic problems caused by dynamic compaction.At the construction site,when there is not enough construction safety distance reserved for the existing structures,dynamic compaction operation will lead to the vibration of surrounding underground structures.Vibration-isolation trench is an effective measure to isolate vibration,but the effect is greatly influenced by its parameters.Taking appropriate parameters of the vibration-isolation trench,can not only obtain better vibration-isolation effect,but also save the cost of the project.At the beginning,the dynamic analysis method and technique of finite difference software FLAC3D are discussed.And through the program and data of a previous test,the correctness and feasibility of dynamic analysis method for dynamic compaction are verified.Next,the vibration acceleration and the amplitude attenuation coefficient at the arch of shallow tunnel(near the vibration source)are taken as the indexes to evaluate the vibration-isolation effect of vibration-isolation trench.By changing the horizontal distance from vibration source,the depth,and the filling material of vibration-isolation trench,1reference model“dynamic compaction and tunnel”and 75“dynamic compaction,vibration-isolation trench and tunnel”analysis conditions are set up.Finally,Quasi-Newton method is used to obtain the relationships between the amplitude attenuation coefficient and the vibration source distance and the depth of vibration-isolation trenches filled with foam and steel.Then the distributions of applicable scopes under two filling materials(foam and steel)are drawn.And influences caused by changes of these vibration-isolation parameters are analyzed.The main conclusions are as follows:(1)Under dynamic compaction,value of vertical peak acceleration is larger than horizontal at the arch of shallow tunnel,while it is smal er than latter at the top of the dome.(2)Under dynamic compaction,it is necessary to set up vibration-isolation trench in shallow tunnel when its surrounding rocks are II grade.And the vibration source distance,the depth of vibration-isolation trenches,and filling materials all have influences on the vibration-isolation effects at different levels.(3)The maximum value(3.728cm/s~2)of the vibration acceleration at the arch of shallow tunnel(near the vibration source)is got in the reference model“dynamic compaction and tunnel”,and it can be used as the main indicator to reflect the dynamic response of shallow tunnel.Among 75“dynamic compaction,vibration-isolation trench and tunnel”analysis models,the best isolation-vibration effect is obtained from the condition3-20,in which the trench is filled with foam,and the value of its amplitude attenuation coefficient is 0.439.While the worst is under 3-16 isolation-vibration trench filled with concrete,with the value 1.051 of its amplitude attenuation coefficient.(4)The larger the difference of the impedance values between filling materials and site material,the later the peak acceleration at the arch of shallow tunnel(near the vibration source)appears,the smaller the value of the peak acceleration is,and the better isolation-vibration effect can be obtained.(5)Values of the amplitude attenuation coefficient obtained from isolation-vibration trenches filled with foam or steel reduce with the increase of depth of trench.Taking into account the cost of the project,steel filled isolation-vibration trench should be set at 4.5meters away from the vibration source,and foam filled isolation-vibration trench should be set nearly to the vibration source.