The Dynamic Performance Evaluation Of Pile Supported And Geogrid Reinforced Embankment On Soft Soil During High-speed Train Operation

The thorough study of a structure results in a clear understanding of its behavior as a springboard that leads to an efficient design while meeting comfort and security criteria.Nowadays,the objective of better serving the population moving from one environment to another has led to the spreading of high-speed rail lines in all localities,including those at risk,i.e.,where soils are weak.New railway design methods suitable for these risk areas have been pioneered to ensure railway transport safety and passengers' comfort.However,the behavior of geogrid reinforced embankment supported by piles,one of the most widely used methods on weak soil,is still poorly understood.During the high-speed train operation,degradation and severe vibrations occur within the railroad structure,affecting railroad transport security.Embankment reinforced by geogrid and supported by pile system is a productive construction method adopted to enhance the railroad structure performance and ensure stability.Still,several studies on this structure type have focused on its behavior under the effect of a static load,leaving the research world unaware of how the system reacts under the impact of a moving load.This work treats the dynamic behaviour of an embankment reinforced with a geomembrane and supported by a cement fly-ash gravel pile,arranged in a triangular pattern,system during a high-speed train operation as well as various factors and means capable of optimizing the dynamic response of the railroad.A railway system's representative simulation model subjected to a moving HST load established a crucial first stage towards the consistent design of an embankment reinforced with geogrid and supported by pile system.Thus,a three-dimensional nonlinear finite element model has been implemented to simulate a railroad built over weak soil,referring to the HarbinDalian railway section.Each carriage of the train was modeled through a user-defined Dload subroutine as a transitory dynamic load.The established model was efficaciously validated by the dynamic response measured from the field test section.The evaluation of the railway dynamic response showed that the geogrids and piles contribute significantly to the reduction of vibrations in the system.The vibration attenuates quicker with the structure depth,even under overload conditions.Additionally,the resonance phenomenon that should occur when the train reaches a certain speed has been eliminated.The dynamic arch,due to the presence of reinforcements,forming in the embankment allows the transfer of the major part of the dynamic load towards the piles during the passage of the train.The closer the geogrid is to the soil,the greater the tensile stress developing in the geogrid at the piles' verge.Furthermore,geogrid and piles coupling influence with various strengths showed that high-strength pile and geogrid combination drastically diminishes the displacement gap due to the train speed change.Consequently,the rail track's vibrations were almost constant during the HST operation;thus,guaranteeing comfort to passengers and decreasing the derailment risk.In order to improve the optimal dynamic response of the railway,by increasing its bearing capacity and reducing vibrations induced by train passage,asphalt concrete(AC)has become a new material to be included in railroad design.To investigate the impact of AC material in a geogrid reinforced embankment and supported by Pile(GRSP)structure subjected to HST moving,the AC viscoelasticity behavior was incorporated into the 3D GRSP FE model through the Prony series for a more efficient characterization of its mechanical response.The use of the AC material in GRSP railway construction improves the dynamic system characteristics by regulating the structural vibrations and maintain the low vibration level despite the speed variation during the HST operation.The less the sub-layers are rigid,the more the effect of the asphalt layer in the structure is large.In winter,the AC layer deforms less and withstands more stress due to the dynamic HST load than in summer.Besides,the construction of a high embankment supported by rigid piles of large diameter and less spacing improves the rail system performance on low bearing capacity soil.That limits the distribution of dynamic pressure to the ground and decreases vibrations in the system.An analytical method capable of estimating the stress(static and dynamic)distribution in a GRSP system with piles arranged in an equilateral triangular pattern has been established based on the BS8006 and cone model method.First,a finite element model was developed and validated by comparison with field and analytical data.Then,the assessed results through the established analytical method were compared to the results obtained from the numerical model.