Dynamic Characteristics And Vibration Mitigation Research Of Twin Tunnels Based On PFC And Similar Experiment

The 21st century is the century of exploitation and utilization of underground space.The construction of the underground railway twin tunnels in large and medium-sized cities in China is increasingly extensive.Due to the increasingly crowded city,the spacing between the twin tunnels is often smaller,sometimes less than one times the tunnel’s diameter.The tunnel liner and surroundings of the adjacent tunnel will be significantly affected by the near-spaced tunnels during construction and operation.In addition,the construction of super-large diameter river-crossing twin tunnels is becoming more and more common,while the relevant research is still less.To systematically study the dynamic characteristics of twin tunnels during the construction and operation,this paper uses particle flow code(PFC)and similarity experiment to study the dynamic and static characteristics of adjacent tunnel’s segment,sleeper,and surroundings during construction and train vibration of one tunnel.The main research contents are as follows:(1)A PFC3D model was established to explore the dynamic characteristics of the surroundings of adjacent tunnels during tunnel excavation.The influence of the construction process on the surroundings of adjacent tunnels was analyzed,including the change of displacement,stress,porosity,and coordination of surroundings.The influence area is determined according to the analysis results,and the corresponding vibration isolation measures are put forward.The results show that the adjacent tunnel construction will have a significant impact on the surroundings of the existing tunnel,especially from the top of the tunnel to the area of the side of the other tunnel.The composite reinforcement method combining FRP sheet and grouting in a specific area can obtain a better protective effect.(2)To explore the dynamic characteristics of adjacent tunnels when trains run in tunnels with different buried depths,the dynamic and static characteristics of adjacent tunnel sleeper,segment,and surroundings are analyzed.Then,the distribution of earth pressure around adjacent tunnels and the acceleration and strain of adjacent tunnel segments are discussed using a similar experiment.The results show that the results of the similar experiment and PFC model are similar under different buried depths.The maximum variation of soil stress of twin tunnels occurs at the tunnel vault,which should be paid more attention to.In practical engineering,the buried depth of twin tunnels should be greater than one times the tunnel’s diameter.(3)The dynamic characteristics of the super-large diameter twin tunnels(D_L=15.2m)and the small diameter twin tunnels(d=6.2 m)under train vibration are compared.The displacement and velocity of smoke exhaust plates,vehicle lane plates,and left and right partition walls of adjacent large-diameter tunnels are discussed.Then,the displacements and velocities of sleepers and segments in super-large diameter and small-diameter tunnels are compared.The results show that the super-large diameter tunnel is more stable than the small-diameter tunnel because of the larger tunnel spacing,larger water pressure,and more stable cross-section.The dynamic characteristics of the super-large diameter tunnel are very different from those of the small-diameter tunnel.Therefore,the dynamic characteristics of the train vibration in the small-diameter twin tunnels cannot be directly applied to the super-large diameter twin tunnels.(4)The vibration reduction effect of steel plate,concrete zone,and the double liner on its tunnel and adjacent tunnel during the train vibration process was compared by PFC and laboratory test methods.The results show that the compactness of the concrete zone will affect the effect of vibration reduction.The concrete zone is recommended to be used first in engineering because it has a better vibration isolation effect on its tunnel and adjacent tunnel.When a steel plate is used,it is necessary to pay attention to the dynamic effect of the steel plate on the surroundings of its tunnel.In this dissertation,the dynamic characteristics of near-distance twin tunnels under construction and train operation are studied by using PFC and similar experiment.This study can be used as a reference for similar twin tunnels engineering projects.