Research On The Dynamic Stability And Long-term Settlement Of High-speed Railway Subgrade Crossing Ground Fissure Zone

High-speed railway is the name card of China’s infrastructure construction,in recent years,the constructing scale and speed of China’s high-speed railway is unprecedented!The construction of high-speed railway plays a positive and important role in promoting the implementation of"The Belt and Road"national strategy and the rapid development of national economy.However,China is a vast country with complex geological conditions.Adverse geological phenomena such as collapse,landslide,debris flow,land subsidence,ground fissures and karst are very common in China.A number of high speed railways,both built and proposed,will inevitably pass through the ground fissure zone.As a kind of geological weak zone,the ground fissures zone may lead to the dynamic instability and damage of subgrade due to the differential settlement of the stratum on both sides of the ground fissures zone under the long-term vibration load of the train,thus threatening the operation safety of the high-speed railway.Therefore,it is of great practical significance and practical engineering value to study the long-term dynamic stability and settlement of high railway foundation across the ground fissures zone.This paper takes the Daxi（Datong to Xi’an）passenger dedicated line crossing the ground fissures in Taiyuan basin of Shanxi Province as the engineering background,relying on the project of national natural science foundation of China"Study on dynamic response and long-term settlement of high-speed railway subgrade crossing ground fissures zone（41772274）".By combining field test,laboratory test,theoretical analysis and dynamic finite element numerical simulation,the long-term dynamic stability and long-term settlement of subgrade across ground fissures zone under the action of train vibration load were studied.The research results can provide a scientific basis for the design and disease control of high railway foundation in the cross-ground fissures zone.The main research results are as follows:（1）Through shear wave velocity and ground pulsation in-situ test,the dynamic characteristics of ground fissures zone were studied.The results show that the shear wave velocity of soil in the ground fissures site of Dongguan substation changes sharply along the direction of depth.The spectrum characteristics of ground pulsation signal are multi-peak.Both of the two test methods have good consistency in inversion of formation structure.Compared with the shear wave velocity test,the site pulsation test has a larger excellent period and a better site type.（2）Based on the cyclic triaxial test in the chamber of soil cumulative plastic deformation,the soil cumulative plastic deformation characteristics,deformation influencing factors and deformation description model under cyclic loading were studied.The results show that the cumulative plastic deformation of soil can be divided into three stages:rapid growth section,stable growth section and stable section.The cumulative plastic deformation increases with the increase of dynamic deviatoric stress,static deviatoric stress and water content,and decreases with the increase of confining pressure and depth.Model of Allometric1(?_p^（28）a N^b)has excellent accuracy and practicability in both describing the cumulative plastic deformation of soil and the influencing factors of deformation.（3）By means of three-dimensional dynamic finite element numerical simulation,the response characteristics of dynamic stress and vibration velocity along the subgrade longitudinal direction and depth direction are studied,and the dynamic response amplification coefficients within the affected zone of ground fissure and the affected zone are determined.The results show that after reinforced by the composite foundation,the dynamic stress in the embankment increases,the dynamic stress of foundation within the scope of CFG pile reinforcement decreases significantly,the vibration velocity of the foundation within the depth of the embankment and the train is reduced,the influence zones of dynamic stress and vibration velocity on ground fissures are unchanged in embankment and decreased in foundation,the dynamic stress amplification coefficient decreases in both embankment and foundation,while the vibration velocity amplification coefficient does not change in embankment,but increases in foundation（4）Based on the train vibration load and environmental excitation measurement method,the critical dynamic stress method and the effective vibration velocity method,the dynamic stability of high railway foundation across the ground fissures zone is studied.The results show that no resonance damage will occur when the train passes through the subgrade of the ground fissures zone.The minimum treatment depth of roadbed soil that meets the dynamic stability requirements of roadbed with cross-ground fissures zone is:Area inside the ground crack affects:H₀≥6.1m（from the top of the roadbed）,and the treatment range is hanging wall distance ground fissure ₀L=15m,footwall distance ground fissure ₀L=10m（the ground fissure is 90°orthogonal to the roadbed）;Area outside the ground crack affects:H₁≥5.5m（from the top of the roadbed）.（5）Based on the cumulative plastic deformation test of soil mass under multi-factor coupling effect,a long-term settlement prediction model considering multi-factor coupling effect was established,and then the temporal development characteristics and spatial distribution characteristics of the long-term settlement of high railway foundation across the ground fissures zone were studied.The results showed that the subgrade settlement developed rapidly in the early stage of the line operation.The area affected of hanging wall by the ground crack settlement was smaller than that of the footwall.After the composite foundation was adopted to reinforce the subgrade,the total settlement amount of hanging wall and footwall subgrade and the differential settlement amount in the affected area of ground crack both decreased,and the affected area of ground crack settlement also decreased.