Research On Settlement Control Technology Of Composite Foundation With Half-screw Piles In Loess Roadbed With High-speed Railway

The Yinchuan-Wuzhong passenger dedicated line of the new Yinxi railway is the first high-speed railway built in the loess area in China.The base of the line is large,and there are unfavorable geological conditions such as loess.The composite foundation of the half-screw pile is used to reinforce the weak foundation.Although half-screw piles have gained more applications in the foundation treatment of high-rise buildings,there are few applications for soft ground treatment in high-speed railways,especially in the western loess area.There are few studies on the use of half-screw piles for soft foundation treatment in high-speed railway construction,and there is a lack of regional application experience.Therefore,the construction technology,bearing characteristics,settlement control effect of the half-screw piles and the load transfer law,bearing capacity and settlement characteristics of the composite foundation of the half-screw pile need further study.(1)Investigating the foundation project of the Yinchuan-Wuzhong passenger dedicated line,it was found that there are loose soil foundations such as silt and sand along the project.Precipitation and surface water may soften the soil through the subgrade surface,subgrade slope or foundation surface infiltration into the foundation soil.Therefore,the soft foundation is strengthened by half-screw piles composite foundation.(2)The test parameters of half-screw piles were obtained by test pile test,and the process flow of half-screw piles were summarized.The pile quality was tested by low strain detection and excavation of half-screw piles.The results show that half-screw piles in silty soil and sand layer have good pile quality and effectively form a composite foundation with the natural foundation.(3)The results of static load test of single pile and composite foundation of half-screw piles show that the bearing capacity of single pile and composite foundation of half-screw piles meets the design requirements of this project.Through the theoretical calculation of the ultimate bearing capacity of single pile of half-screw piles,the half-screw piles have higher bearing capacity.The calculation results of bearing capacity of single pile composite foundation are also consistent with the field test results.The composite foundation has a certain safety reserve.(4)During the embankment filling phase,the settlement of the composite foundation increases with the increase of the embankment load.After the completion of the filling,the settlement gradually slows down and tends to be stable.After the completion of the filling,the foundation has basically reached a steady state after 6 months of settlement deformation.the settlement has met the requirements of standard control,and the settlement has been effectively controlled.(5)The numerical model of the composite foundation of screw pile is established.The maximum settlement value obtained by simulation is 34.94 mm,which is slightly larger than the observation data of the site observation,but the basic trend is the same.Under the load of the embankment,the piles are subjected to most of the load,and stress concentration occurs at the top of the piles.At the same depth,the axial force and side resistance of the piles body increase with the increase of the load;in the depth direction,the axial force of the piles body gradually decreases,and the side resistance first increases and then decreases.(6)Through the simulation analysis of the influence of the form of the cushion layer and the height of the roadbed filling on the settlement,it is shown that the cushion is important for the homogenization of the embankment load and the reduction of uneven settlement,and the geocell can achieve certain effects on controlling settlement and restraining horizontal displacement.Within a certain range,the higher the embankment fill height,the greater the settlement of the foundation and the lower the stability of the subgrade.