Study On Filler Improvement And Settlement Control Technology Of High-speed Railway Subgrade In Mu Us Desert

The Yulin-Ordos high-speed railway is located in the northern part of Shaanxi Province and the southeastern margin of the Mu Us Desert in the southwest of the Inner Mongolia Autonomous Region.It is a ballastless track railway with a design speed of 350km/h.The section through the desert area accounts for more than 80%of the whole line.It is the first high-speed railway in the desert area that China will start construction.Due to the engineering characteristics of poor gradation,loose particles and low cohesion of local aeolian sand fillers,untreated aeolian sand can not meet the needs of high-speed railway subgrade and embankment fillers,and the bearing capacity of deep fine sand foundation is low.In order to meet the requirements of high-speed railway ballastless track post-construction settlement of not less than 15 mm,this paper improves the natural aeolian sand filler with cement and CG-2 curing agent,proposes an economical and reliable improved mixing ratio,and explores the influence of freeze-thaw cycle on the strength performance of improved soil.A three-dimensional finite element model was established by FLAC^3D finite difference method software.The settlement of aeolian sand foundation reinforced by shallow pile-board structure and CFG pile-net structure is studied,and the best settlement control measures are put forward.The main research contents and related results of this paper are summarized as follows:（1）The physical and mechanical indexes of aeolian sand in three soil sampling sites were tested.The results show that the non-uniformity coefficient Cu of aeolian sand is between2.70～3.25,and the curvature coefficient Cc is between 0.89～1.01,which is uniform graded fine sand and belongs to C3 group filler.Compared with the standard compaction test,the optimum moisture content and maximum dry density of aeolian sand obtained by vibration compaction are larger.The optimum moisture content is 15.95%and the maximum dry density is 1.79g/cm3.The cohesion of dry sand and natural aeolian sand at the optimum moisture content is 0.02k Pa and 5.31k Pa respectively,and the internal friction angle is 30.52°and 25.48°respectively.（2）The optimum vibration time of cement modified aeolian sand was studied.It was found that with the increase of vibration time,the phenomenon of compaction-vibration-compaction was shown,and the optimum vibration time was 330s.The vibration compaction test of cement and curing agent modified aeolian sand shows that the optimum moisture content and maximum dry density are greater than the results of standard test.The unconfined compressive strength of eolian sand improved by cement and CG-2 curing agent increases steadily with the increase of cement content.The curing agent needs to play a role when the cement mixing ratio is large,and the excessive amount of curing agent will weaken the strength.The optimum dosage is0.02%.The compaction coefficient has a considerable influence on the growth of unconfined compressive strength of cement and curing agent modified aeolian sand.The compressive strength increases with the increase of compaction coefficient,and the curing age has a great influence on the strength.With the increase of cement content,the cohesion and internal friction angle of cement improved aeolian sand without curing agent are improved to varying degrees.Compared with the mixed curing agent and the unmixed curing agent,the greater the cement content,the more obvious the cohesive effect of the curing agent,but the friction force decreases.（3）The higher the cement content,the better the resistance to freeze-thaw deformation of cement modified aeolian sand,and the slower the attenuation of unconfined compressive strength.The strength of the sample with 8%cement content has no obvious attenuation after freeze-thaw cycles.The CG-2 curing agent has stronger resistance to freeze-thaw deformation in the aeolian sand samples with more cement content.The cement content of 6%and 8%can meet the requirements of chemical improved soil filler for the bottom layer of subgrade bed and embankment below subgrade bed in severe cold and cold areas.（4）The stress and deformation characteristics of aeolian sand foundation reinforced by shallow-buried pile-board structure are studied.The results show that the pile length is 40～45m,the pile diameter is 1.0～1.2m,the longitudinal span is not more than 7m,and the bearing thickness is set to 0.9～1.1m,which can more effectively bear the track and train load to achieve the best settlement control effect.（5）The settlement and stress characteristics of aeolian sand foundation reinforced by CFG pile-net structure are analyzed.It is found that with the increase of pile length,the post-construction settlement decreases,and the pile-soil stress ratio increases.The increase of pile spacing will lead to the increase of post-construction settlement and pile-soil stress ratio.The geogrid plays an obvious role in the gravel cushion.The settlement deformation of the geogrid presents a wave fluctuation type,and the settlement gradually decreases from the center of the subgrade to both sides,showing a’mesh pocket’shape,which makes the pile body bear more load and is conducive to controlling the uneven settlement of the subgrade.