Numerical Stability Analysis Of Rigid Pile Composite Foundation And Study Of Ground Beam Reinforcement

With the rapid development of modern transportation,a growing number of high-speed railway and expressway projects have been completed in the last decades.As the speeds of highspeed railways increase and the periods of construction accelerate,standards with respect to the post-construction settlement of subgrades are getting more and more accurate.The rigid piled embankment is commonly utilized in the fields of railway and expressway owing to its high bearing capacity,small post-construction settlement and short construction period.However,instability failure of rigid piled embankments designed according to the current specifications frequently occurred.This may indicate that the traditional stability method for piled embankment,which assumes that the shear failure is the dominant failure mode of rigid piles,fails to reflect the real failure mechanism of rigid piled embankment and overestimate its stability.To investigate the global stability and the failure mechanism of the piled embankment,a 3D numerical model of piled embankment is established.Besides,the strenghthening strategy of piled embankment,aimed at enhancing the overall stability,is also discussed.The main contents of the thesis are as follows:(1)By adopting the concrete model simulating the strain-softening behavior of piles after failure,a three-dimensional finite element analysis of the piled embankment is conducted to analyze the mechanical response and the three-phrase failure process of piles under embankment,and to reflect the progressive failure of the piled embankment.The results revealed that piles near the embankment toe were primarily subject to bending failure within the clay layer.And the fracture energy released from the cracks transferred to the adjacent piles,which resulted in successive bending failure of piles.Wih the propagation of cracks throughout the damaged section,a continuous slip surface was ultimately formed.As a result of that,this phenomenon caused the instability failure of embankment.Based on the progressive failure of the piled embankment discovered from the simulation,an efficiency factor of the bending capacity,is proposed via fitting analyses to describe the degrees of bending capacity mobilized by piles located in different areas and this can provide a guidance for the stability analysis method of piled embankment in engineering practice.(2)As a novel ground modification technique,orthogonally crisscrossing ground beams bridging across the top of individual piles have been gradually adopted in subgrade engineering.Based on the individual piled embankment numerical model,the piled beam-supported embankment(PBSE)model is developed to study the influence of ground beams on the mechanical characteristics and the deformation of piles,as well as the global stability of the piled embankment.Meanwhile,the geometries of the ground beam structure are also discussed to analyse their effects on the stability of piled embankments.The results showed that the ground beam could effectively improve the stress condition of piles avoiding from the occurrence of bending failure,reduce the additional stress of subsoil to mitigate the deflection of subsoil and markedly improve the stability of embankments.Compared to the increment of the height of beams,it is more effective to enhance the stability of piled beam-supported embankment by increasing the width of ground beams.Besides,the embedment depth of piles plays a more important role in improving the global stability of embankment as compared with the concrete grades.(3)Considering that piles at different locations have diverse impacts on the stability of embankment,the improved ground can be quantificationally classified into four zones,namely,extension-bending zone,bending zone,bending-compression zone and compression zone according to the difference regarding to mechanical characteristics of piles and the relative displacement between piles and soil.The zoning reinforcement strategy of ground beams is elucidated as well.The analysis manifested that the installation of ground beams in extensionbending zone or compression zone had little influence on the stability of embankment,while the stability of embankment was improved efficiently by introducing ground beams to bending zone and bending-compression zone in the meantime.An optimal and reasonable ground beam arrangement is proposed by taking the anti-sliding performance and the construction cost into account and this may promote the optimal engineering application of piled beam-supported embankment.