The Dynamic Response And Long-term Strength And Settlement Of Loess Railway Embankment Subsjucted To Train Load

The cumulative settlement and strength degradation of subgrade are key factors for the operation of railway transportation.There are two essential issues need to be figure out during evalusting the cumulative settlement and strength degradation of subgrade,i.e.the dynamic response of subgrade system and deformation and strength evolution rules of subgrade material under train traffic loading.The permanent deformation behaviors of subgrade soil have attracted significant interest among researchers.However,many studies conducted on this topic have focused on granular materials,marine soft soils,or frozen soils.Relative reports about compacted loess are rare.In recent years,more and more railway construction projects have been carried out in the loess region.The accumulative deformation and the dynamic parameters of compacted loess vary with cyclic train traffic loading are experimentally investigated and the microscopic mechanism of intensity degradation has also been addressed.Furthermore,the 2.5D finite element algorithm was adopted to analyze the environmental vibration and dynamic stress distribution of subgrade.Effects of speed,axle weight,embankment height,foundation treatment measures and irregularity on the dynamic responses of subgrade were discussed.Finally,the long-term settlement of roadbed induced by train load was predicted based on the empirical formula and some engineering suggestions for subgrade filling project were given.The main research contents and conclusions are as follows:?1?Effects of confining pressure,static deviator stress,cyclic number,and frequency on the stress-strain relations of compacted loess were studied through dynamic triaxial test.The influence mechanism of static deviator stress on dynamic constitutive of sample is analyzed emphatically and the calculation of dynamic modulus has been revised based on dynamic creep theory.?2?The development of permanent strain and dynamic parameters of compacted loess was addressed using a number of long-term cyclic triaxial tests.The effects of cycle number,cyclic amplitude,confining pressure,moisture content,and compaction degree were examined in detail.Moreover,a predictive formula was proposed in order to describe the permanent strain,dynamic modulus and damping ratio versus cycle number,cyclic amplitude,and confining pressure.The cyclic strength of compacted loess was also addressed.The results indicate that cumulative strain of stable type specimen increases linearly with the logarithmic cyclic number,that there is an obvious threshold stress in the dynamic stress-strain relationship of the sample when considering the static deviator stress and that the dynamic stress-strain point shows an approximate linear relationship if dynamic stress is less than the threshold stress.In addition,the dynamic modulus of samples increases first and then decreases gradually.The maximum dynamic modulus increases linearly with the increase of confining pressure.?3?The variation of microstructure and static strength of compacted loess,before and after long-term traffic load,is discussed.The test results indicate that the rapid increase of accumulation strain and gradually increase of dynamic modulus in the logarithmic drop stage result from macro-behaviors of the closure of“pit”and connected pore,while the continuous improvement of accumulation strain and gradually reduction of dynamic modulus in the stable-oscillation stage result from macro-behaviors of the breakage of the cement among particles and the broken and redistribution of particles.After cyclic loading,the strength of the samples increased and the failure strain decreased.Moreover,the stress-strain relationship of the sample has“sudden drop”in descending section.?4?Principles determining the critical state of compacted loess were preliminarily summarized through recalling the comprehensive mechanical performance of samples in the loading process.The deformation states of compacted loess under traffic load can be divided into three types:stable,metastable and unstable.The cumulative rate of strain and damping ratio belong to prior parameters and are suitable for predicting the stability state of the samples.Moreover,the final cumulative strain,as a posteriori parameter,reflects the deformation capacity of the samples,which can provide a reference for the prediction of subgrade deformation in the limit state.Meanwhile,it can also be used as the cumulative strain standard to determine the dynamic strength of soil.?5?A 2.5-dimensional finite element program was established to calculate the dynamic response of subgrade system under train load,using the MATLAB software.Additionally,the stresses at the wave front of plane and cylindrical waves were calculated using the wave motion theories and it is adopted to optimize the setting of artificial boundaries in frequency domain.The results indicate that the inside vertical dynamic stress decays rapidly along the depth,and the attenuation curve of which along the horizontal direction is similar to“S”type.The width of the core area can be taken as 4m,and the depth is roughly between 2.6m and3.6m,once using rectangular core area to define the influence range of traffic load.Moreover,the attenuation velocity of acceleration on the surface is much larger than that of displacement.The increase of vehicle speed will significantly increase the dynamic response indexes of subgrade,and the change of embankment height has very small influence on it.When the“Mach effect”occurs,the stress path of the measuring point at the middle section becomes complicated.Most closed loops no longer exhibit the typical“heart-shaped”stress path characteristics under single axle loads.The dynamic characteristics of foundation after consolidation by the CFG-pile change greatly than that consolidated by the cement-soil compaction pile.The changes,before and after the reinforcement,are reflected in that the peak area of frequency response function moving to the higher vehicle speed and the main stress difference??₁-?₃?at each point is reduced.However,the dynamic stress attenuation remains the same.About the unevenness of rail,the vibration intensity on the surface and dynamic stress are significantly greater than those unconsidering the unevenness.The vibration intensity attenuates slowly along the surface while the dynamic stress attenuates rapidly along the depth.?6?The initial strain contributes to a large proportion of overall permanent strain.The long-term settlement of compacted loess embankment induced by train load meets the relevant requirements on embankment settlement in operation period,once ensuring compaction degree.Moisture content and compactness are key factors affecting accumulated strain of soil.