Analysis On The Settlement Reasons Of High-speed Railway Subgrade With Pile-net Structure

Due to the soft soil areas are widely distributed in China,with the rapid development of high-speed railway network,the settlement of soft foundation will occur when high-speed railway is built on soft soil.As a reinforcement measure for effectively dealing with soft foundation,pile-net composite foundation have a complex mechanism of action,there are many factors that lead to the settlement of pile-net subgrade accordingly.This paper is based on the settlement deformation of High-speed Railway subgrade with pile-net foundation which is located in east China.The causes of the settlement of the high-speed railway pile-net subgrade were studied by means of a combination of geological verification,non-destructive testing,in-site settlement monitoring,theoretical and finite element analysis.The main research contents and results are as follows:(1)Through the analysis and comparison of the geological verification results,the parameters of physical structure and soil physical and mechanical parameters revealed by the design,construction process and operation are basically the same.The results of the static cone penetration tests show that the lateral resistance of muddy-silty clay layer,silty clay layer and silty clay with brecciated layer is smaller than the standard empirical value.(2)According to the different detection characteristics of the geological radar and the transient surface wave,they are used for a comprehensive non-destructive testing of the the subgrade.Through detection it was found that subgrade subsidence,the uncompacted subgrade filling,and the excessive moisture content in local area.At the time,there are voids under the closed layer of shoulder,and the loss of soil between piles around the pile cap.(3)Analysis of manual leveling data show that since the high-speed railway operated,the subgrade settlement has been occurring,up to now,the settlement rate is relatively large,and the settlement trend is not convergence.The layered settlement monitoring data of main line shows no compression deformation occurred in the subgrade body.The layered settlement monitoring data of right connecting line show that the settlement mainly comes from the compressive deformation of the foundation below the pile cap to the silty clay layer.(4)The bearing capacity of the single pile calculated by the standard method meets the capacity requirements.The calculated settlement is smaller than the actual settlement measured by manual leveling.It can be seen that when the pile tip is deep into the strong-weathered mudstone layer 1m,and the pile length is 26 m,the pile-net structure can work normally and the large subgrade settlement can be effectively controlled.(5)Based on analysis of different pile length conditions with PLAXIS finite element analysis software,it is concluded that different pile lengths correspond to different settlement law.As the length of the pile is increased,the amount of settlement and the settlement rate are reduced accordingly.When the pile length is 20 m,the settlement law calculated is basically the same as the manual leveling data during the corresponding period.It is showed that the insufficient of pile length is one of the main factors affecting the settlement.In view of the settlement characteristics of high-speed railway pile-net foundations,the reinforcement measures of ballastless track lifting,lateral restraint of cement mixing piles,subgrade load shedding and grouting reinforcement in deep foundation were proposed.