Dynamic Response Characteristics And Deformation And Failure Modes Of Cross Tunnels With Small Clear Space

With the rapid development of transportation construction,the above ground space has been basically used up,and the underground space has been developed and utilized reasonably and efficiently step by step.It is inevitable that there will be structural and spatial intersection between highway-railway,railway-railway,highway-highway and subway tunnels.In general,the structure of the three-dimensional crossing tunnel with small clear distance will be subjected to various seismic loadings at the same time under the action of seismic loading,resulting in larger seismic inertial force.Additionally,owing to the mutual influence between surrounding rocks,the crossing tunnel is likely to become the weakest section of the entire line,and severe damage will occur once an earthquake occurs.At present,the model test study on the crossing tunnel under the action of seismic loadings has not made a substantial breakthrough,and the spectrum characteristics in the dynamics of the crossing tunnels is almost blank.Therefore,to study the failure process and complex dynamic response characteristics of the three-dimensional crossing tunnels under the seismic loading,the shaking table tests were established by selecting typical spatial orthogonal and oblique crossing tunnels.First,the characteristics of seismic wave propagation and energy distribution are quantitatively analyzed by acceleration,strain response and spectrum characteristics.Then,the seismic response and deformation and failure modes of crossing tunnels with small clearance under seismic loadings are summarized.Finally,the effectiveness and rationality of the shaking table test results are proved by three-dimensional numerical simulation calculations.The research has the following conclusions:(1)The seismic responses of spatial orthogonal and oblique three-dimensional tunnels show different laws: For spatial orthogonal crossing tunnel,the seismic responses at the crowns all show linear distribution characteristics,while the acceleration response at the invert of the upper-span tunnel shows nonlinear and non-stationary increases.For spatial oblique crossing tunnel,the acceleration response at the crown of the tunnels is the largest,showing a "parabolic" distribution,while the acceleration response at the invert has an obvious surface-tending effect,which is basically an "arc" along the horizontal distributed.(2)Different from the magnification effect of acceleration along the elevation in a single tunnel slope,the seismic waves will be reflected and refracted between the two tunnels in the crossing tunnels engineering,resulting in the "superposition effect" of acceleration in space.The acceleration response in the surrounding rock of the crossing section increases abnormally,which easily becomes the weak link of the tunnel structure in earthquake resistance.(3)The dynamic strain response of the upper-span tunnel is basically smaller than that of the under-crossing tunnel in the spatial orthogonal crossing tunnel,which is the opposite in the spatial oblique crossing tunnel,that is,the peak strain of the upper-span tunnel is basically larger than that of the under-crossing tunnel.It shows that the type and angle of intersection will have a certain impact on the seismic response of the crossing tunnels.The smaller the clearance between the two tunnels,the smaller the angle of intersection,the greater the possible deformation and damage that may occur in the upper-span tunnel.(4)The failure process of the three-dimensional crossing tunnel is mainly determined by the low-frequency wavelet component.Furthermore,with the increase of the maximum input acceleration,the "frequency energy proportion migration phenomenon" appears,that is,the energy proportion of frequency component gradually migrates from the 1st frequency band to the 2nd frequency band that is,(0.1～6.26 Hz)to(6.26～12.51 Hz),but the 1st frequency band still plays a leading role.(5)The failure of the spatial three-dimensional crossing tunnel model has obvious deformation stages: elastic stage(0.1g～0.15 g),plastic stage(0.2g～0.3 g)and failure stage(0.4g～0.6 g).The failure deformation starts from the top of the slope,and cracks appear along the upper-span tunnel to the under-crossing tunnel.The damage degree of the upper-span tunnel is more serious than that of the under-crossing tunnel,and the cracks at the crown are more concentrated.Additionally,with the continuous loading,the through cracks appear from the top of the slope to the crown of the upper-span tunnel,the slope is basically unstable,and the model is basically damaged.