High-speed Train Induced Ground Vibrations And Vibration Mitigation Analysis Of CFG Pile-supported Subgrade

The rapid uptake of high-speed railway has been in-part due to its superior economic,social,and environmental benefits in comparison to other kinds of transportations.One side effect of high-speed railway is the environmental vibrations,which can cause discomfort to inhabitants and interfere with the regular use of sensitive equipment in surrounding buildings.Therefor it becomes increasing significant to assess and mitigate the ground-borne vibrations induced by high-speed trains.In view of this,this paper conducted experimental and numerical investigations to research the ground vibrations generated by backfilled embankment and cutting,and the influence of transverse isotropy on vibrations is also discussed in detail.In addition,the vibration reduction mechanism of the composite subgrade reinforced by CFG piles has been revealed,and the effects of the design parameters of CFG pile-supported subgrade on the vibration mitigation performance are investigated.The main contents of the thesis are as follows:（1）The improved Hilbert-Huang transform is proposed through adopting the cubic B-spline and the improved Rilling terminal conditions.The results of the comparative study show that the time-frequency characteristics of ground vibrations can be described more clearly by the improved Hilbert-Huang transform,which can effectively suppress the end effects occurring in the conventional method.Based on the field measurements of ground vibrations performed on Paris-Brussels high-speed railway,the ground vibrations generated by backfilled embankment and cutting are analyzed in detail.The results show that the earthquake profile conditions do have a significant influence on the railway induced environmental vibrations.The attenuation process of ground vibrations generated by backfilled embankment can be divided into two stages.For the cutting,the vibration attenuation process can be divided into three stages,and rebound phenomena of ground vibrations occur at the top of cutting slope.Moreover,the first-order dominant frequencies of ground vibrations generated by embankment and cutting are determined by the fundamental frequency f₁ of high-speed train’s moving loads.（2）The 2.5D finite element governing equations of transversely isotropic ground is deduced.The track structure is simplified as a Euler beam resting on the transversely isotropic ground surface subjected to high-speed train’s moving loads which is simulated as successive axle loads.The 2.5D numerical model of the transversely isotropic subgrade reinforced by CFG piles is established,in which the CFG pile composite subgrade is modeled as pile walls with modified elastic modulus.The viscoelastic boundary is adopted herein to avoid the influence of reflected waves from boundary surfaces on the calculated results.Comparing with four different published results,the reliability of the proposed 2.5D finite element method has been verified.（3）Considering different train speeds,the influences of stiffness ratio n on the ground vibrations generated by transversely isotropic ground with different railway earthquake profiles.The vibration mitigation performances of embankments and cuttings are discussed.The influencing factors studied include embankment height,cutting depth,embankment elastic modulus,and the inclined angle of cutting slope.Transversely isotropic ground vibrations decrease obviously with the increasing stiffness ratio n,and this influence increases gradually with the distance from the track center.The ground vibration decreases with the increasing embankment height or cutting depth.The increasing elastic modulus of embankment could mitigate the ground vibrations.On the premise that stability of cutting slope can be ensured,a larger inclined angle has positive benefits in reducing ground vibration.（4）Comparing with the foundation without piles,ground vibrations generated by CFG pile-supported subgrade as well as the ground vibration attenuation with the distance away from the track centerline are investigated,and the vibration reduction mechanism of which has been studied.Results show that the CFG pile-reinforcement of subgrade could significantly contribute to the mitigation of ground vibrations,and this vibration reduction effect decreases gradually with the increasing train speed.Reinforced by the CFG piles,the critical speed for the near track zoon can be raised obviously.In the composite subgrade,the most of dynamic loads generated by train’s operation are undertook by CFG piles.The vibrational energy is more transmitted to the interior of the composite subgrade along the depth direction,so as to reduce the ground vibrations effectively.（5）The vibration mitigation performances of the CFG pile-supported subgrade are discussed.The influencing factors studied include pile diameter d,pile spacing s and pile elastic modulus E_P.From the perspective of environmental vibration control,some meaningful recommendations are proposed for optimization design.The ground vibrations generated by CFG pile-supported subgrade decrease with the increase of pile diameter or pile elastic modulus.The decrease of pile spacing could lead to consistent reduction of ground vibrations.When the area replacement rate m and the elastic modulus ratio between CFG pile and soil E_P E_S are not less than 0.06 and 140,the vibration mitigation performance of CFG pile-supported subgrade is effective and reliable.