Study On The Working Characteristics Of Screw Pile-net Composite Foundation In Loess Subgrade Of High Speed Railway Ballasttrack

The newly built Yin(chuan)Xi(an)railway Yin(chuan)Wu(zhong)section(referred to as the "Yinwu passenger dedicated railway")is the first high-speed railway built in China on the Loess Plateau.Due to the short construction period,the bridge construction and the passing of the girder truck during the construction,the original preloading measures cannot be adopted.In order to ensure the subgrade strength and control the post-construction settlement,it is decided to adopt the screw pile-net composite foundation to reinforce the Yinwu passenger dedicated railway subgrade,and to study the improvement of the subgrade performance.In this paper,through field investigation,field test,theoretical analysis and finite element analysis,the working characteristics of screw pile-net composite foundation are studied.Typical sections are selected to analyze the settlement control effect of foundation,and the following conclusions are drawn:(1)Yinwu passenger dedicated railway is located in the Yellow River terraces trailing piedmont diluvial plain,the average annual rainfall is less,but will drop to heavy rain in the summer,underground 35 m within the scope of the main stratum of silty clay,silt,silty and fine sand,fine round gravel soil and buried depth of ground water level,water depth is in commonly 0.4 ~3.5 m,the foundation for the soft ground prone to roadbed sinking,the screw pile-net composite foundation is adopted the form of ground treatment;It is recommended that a geocell with a width of 12 m should be added between the base layer and the lower part of the subgrade bed of Yinxi railway.The subgrade should be sealed with a composite geomembrane of two cloth and one membrane laid on the base layer.(2)Analysis of field load test result,screw pile bearing capacity of single pile and single pile composite foundation can satisfy the requirements of design,and making use of the finite element analysis software of screw pile load-bearing characteristics of the single pile composite foundation,the results show that the screw pile and single pile with the stress ratio of pile-soil composite foundation pile length,pile cap diameter of cushion modulus,increases,along with the increase of cushion thickness,pile diameter decreases;The vertical stress at the bottom of pile cap increases with the increase of cushion modulus,cushion thickness,pile length and pile cap diameter,and decreases with the increase of pile diameter.The pile axial force first increases and then decreases along the direction of depth.At the same depth,the pile axial force increases with the increase of cushion modulus,pile diameter and pile cap diameter.The negative friction resistance of pile increases with the increase of cushion thickness and pile length,and decreases with the increase of cushion modulus,pile diameter and pile cap diameter.(3)The finite element software is used to analyze the working characteristics of the pilenet composite foundation of Yinwu passenger dedicated railway.The results show that the pile-soil stress ratio of the screw pile-net composite foundation increases with the increase of embankment filling height and pile modulus,and the influence of cushion reinforcement and embankment reinforcement is small.The settlement value of the foundation surface decreases with the distance from the center line of the subgrade,and the settlement value at the center of the subgrade is the largest.The settlement curve is similar to the net shape,and increases with the increase of the embankment filling height and pile spacing,and decreases with the increase of the pile modulus.The pile axial force decreases gradually with the distance from the center line of the roadbed,and increases first and then decreases along the direction of pile depth.There is a peak,which increases with the increase of embankment filling height,pile modulus and pile spacing.Pile with friction,with a gradual increase in distance from the center line of the roadbed is friction with gradually reduce the distance from the center line of the subgrade,neutral position with gradually reduce the distance from the center line of the subgrade,screw pile in straight rod and screw paragraphs at the junction of pile side friction can produce severe attenuation,below the interface depth,pile side friction and gradually play,along with the increase of modulus of pile and pile spacing decreases,increases with the height of embankment filling,cushion layer reinforcement method and the embankment reinforcement method has little effect on it.(4)Four typical sections of Yinwu passenger dedicated railway were selected to analyze the foundation settlement rule,and the results showed that the settlement of the four typical sections mainly occurred during the subgrade filling period,during which the foundation settlement rate and settlement value increased rapidly,and the maximum settlement value was26.74 mm.The settlement value increased slightly within 3 months after the completion of the filling,with the maximum value of 9.6 mm and the minimum value of 0.95 mm.The settlement value increased mainly because of the beam transport vehicle and precast beam acting on the roadbed.Within 3 to 6 months after the completion of filling,the maximum settlement increase value is 1.68 mm,and the foundation settlement tends to be stable.The final settlement of each typical section predicted by the hyperbolic method and the exponential curve method all meet the requirements of the specification,which indicates that the screw pile-net composite foundation can effectively control the settlement of the soft ground in this area.