Train-Induced Vibration Response And Subsidence Prediction Of Permafrost Subgrade Along Qinghai-Tibet Railway

Until recently little work has done on the vibration response and stability of subgrade induced by passing trains in permafrost regions at home and abroad, and specially much less on the effect of frozen layer on vibration response and long-term vibration subsidence of subgrade from trains. The frozen regions are widespread in China, and over half of the trunk railway lines are distributed in those regions. The phenomena of foundation settlement and cracking occurred in the partial sections in two months after the track release of the Qinghai-Tibet railway, which caused the high regard of railway interests. Afterwards, the specialists indicated that, due to less attention before, the permafrost engineering problem of Qinghai-Tibet railway from the repeated load of trains had been gradually becoming a prime important issue. In view of above-mentioned problems, this paper carried out the fundamental research on the dynamic performance of frozen soil, subsidence prediction, and the vibration response of frozen subgrade and its main influencing factors under traffic load in the Qinghai-Tibet railway. The pursuit is to maintain the normal operation of the Qinghai-Tibet railway and perfect the technical detail of anti-vibration design of railway subgrade in cold regions. The specific contents, methods, and results are as fellows.Firstly, the paper systematically investigated, in view of the characteristics of rail transit repetitive loading and the low temperature dynamic triaxial test, the dynamic nonlinear constitutive relationship and dynamic parameters of the frozen clay from the subgrade of the Beiluhe test section along the Qinghai-Tibet railway, and in addition their main influencing factors. Moreover, the paper also revealed the relation curves and corresponding empirical expressions of these dynamic parameters versus frozen temperature, confining pressure, moisture content, frequency, dynamic shearing strain amplitude, and dynamic stress amplitude.Secondly, the properties of dynamic residual strain increasing rate of above-mentioned frozen clay and its major influencing factors were explored based on the low temperature dynamic triaxial test. Adopting the power function fitting the dynamic and static stress ratio, temperature, moisture content, frequency versus axial dynamic residual strain increasing rate was put forward, and then the corresponding fitting expressions were given. Simultaneously, the laws of dynamic accumulated residual deformation of frozen soil mentioned above and its major influencing factors were studied, and the two empirical models of vibration subsidence prediction, i.e. dynamic accumulated residual strain of frozen soil, were brought forward according to the low temperature dynamic triaxial test. Especially the model two, put forward by combining dynamic residual strain increasing rate and long-term dynamic loading test, considers comprehensively the effects of element stress condition, vibration number, and temperature, moisture content, frequency, confining pressure etc. of frozen soil.Thirdly, based on the field monitoring on vibration response of rail, sleeper, embankment, and site, induced by train traffic, both on the Qinghai-Tibet permafrost site and Daqing seasonally frozen site, the characteristics of the vibration responses of those factors and the effect of the frozen layer on embankment vibration response were investigated. Then the two cognitions were acquired as follows: (1) the embankment attenuation laws and corresponding fitting expressions in different seasons in the Qinghai-Tibet railway and Daqing railway, (2) the amplifying effect of frozen layer, during freezing period, on the vertical and longitudinal vibration responses of embankment, and in the lateral direction to the contrary, relative reduction in the vertical and longitudinal directions, during spring thawing period, and, in contrast, amplifying effect in the lateral direction.Fourthly, the vertical dynamic coupling model of the train-track system was built, and at the same time the accompanying simulation procedure ZL-TNTLM was programmed based on the unified analytical concept of large-scale system of train, rail, sleeper, ballast and subgrade. Furthermore, the validation of the model reliability was carried out against the field monitoring data of the Beiluhe test section along the Qinghai-Tibet railway induced by train traffic. Subsequently, the effects of subgrade freeze-thaw condition, running speed and width of rail gap on the vibratory load from trains were investigated. Then the analytical model of subgrade vibration response, namely nonlinear viscoelastic model, was built, and, at the same time, the finite element simulation procedure ZL-RNTLM was compiled by the dynamic coupling concept of the sleeper-ballast-subgrade-field system. Thus, the quantitative assessment of permafrost subgrade vibration response from trains, monitored in the Beiluhe test section, was made. Consequently, the effects of seasonal variations, vehicle type, running speed, as well as embankment height on the permafrost subgrade vibration response were analyzed comparatively.Fifthly, the paper explored the settlement deformation characteristics of the different layers of frozen subgrade and their main influencing factors via the long-term subsidence monitoring on the road bed, embankment base, and permafrost table in the Beiluhe test section. The results indicates that the compression deformation of permafrost is prominent, about 57.4~69.6% of the accumulated settlement of road bed, and increasing with the rising of embankment height and ice-rich permafrost layer thickness. Until now, some sections are in the stage of undamped deformation rate, which should be highly emphasized as the deformation may rise to a considerable magnitude in the future.Lastly, based on the research findings mentioned above, the paper primarily investigated the train-induced subsidence prediction approach and its influencing factors of permafrost table. Furthermore, the settlement of the subgrade within the future fifty years, induced by train traffic, was forecasted for the Beiluhe test section. The results are favorable for further understanding the vibration response and train-induced diseases of the subgrade both on permafrost site within thawing interlayers and seasonally frozen site. Especially, the date, results, and recognition, obtained by the substantive dynamic triaxial tests of low temperature, the long-term subsidence monitoring of permafrost subgrade in the Qinghai-Tibet railway, along with the field experiments on the vibration response of the subgrade on the permafrost site along the Qinghai-Tibet railway and Daqing seasonally frozen site, have an important significance for further studying both on dynamics of frozen soil and dynamics of track and vehicle, and simultaneously provide valuable basic data for gradually developing the anti-vibratory technicality of subgrade in cold regions.