Study On Bearing Performance Of CFG Pile And Half-screw Pile Composite Foundation In Silt(Sand) Foundation

Half-screw pile is a variable cross-section pile with an upper part of cylindrical pile and a lower part of the screw pile. Compared with traditional screw piles, half-screw piles have advantages of high loading capacity, wide applicability, fast construction rate, flexible pile diameter, no noise, no vibration and so on. However, theoretical research of half-screw pile can't meet the requirements of engineering practice. In order to improve the application performance of screw pile in the engineering practice, it's necessary to research the load bearing performance of half-screw pile.Based on the existing conclusions about vertical load bearing performance, load transfer mechanism, destroy model of half-screw pile, a deep research on bearing performance of half-screw pile has been carried out with the method of indoor test and numerical simulation. In the following are the main research:(1)The current research status of half-screw pile on the pile foundation engineering field was summarized. Advantages of improving bearing capacity of half screw pile were introduced. Theoretical analysis on calculation method of pile side resistance, pile tip resistance, and ultimate bearing capacity of single pile was carried on respectively, and influencing factors of the screw pile single pile bearing force were analyzed.(2)Combined with the field of half-screw pile composite foundation in Hohhot to Zhungeer new railway engineering construction, the indoor model test was designed based on the similarity theory, the geometric similarity ratio of which was 1:10.Aspect ratioofthe test pile is 16:1, which was simulated with PTFE nylon rod. The length of CFG pile was 800 mm, and the diameter of pile was 50 mm. Half-screw pile was processed by lathe, the parameters of which are introduced in the following: straight rod section is 300 mm in length, and screw length is 500 mm, and screw blade diameter is 50 mm, and inner diameter is 30 mm,and the bottom width is 10 mm, and the top width is 5mm, and pitch width is 35 mm.(3)This test is carried out in two groups. The first group was carried on in sand with the lateral pile spacing of 6 times of the diameter of the pile, and the vertical pile spacing of 8 times of the diameter of the pile respectively. And 3 CFG piles and 3 screw piles were embedfor single pile bearing test. The second group of model tests were arranged in 5 rows and 4 columns in a square, and the lateral pile spacing is 4 times of the diameter of the pile, and the vertical pile spacing is 3 times of the diameter of the pile, which included2 rows of single pile composite foundation of CFG pile and 2 rows of CFG piles. In the process of the test, the inner 8 piles were chosen to be loaded. Two groups of CFG pile single pile composite foundation, two groups of half-screw pile single pile composite foundation bearing, two groups of straight bar pile single pile bearing and two groups of half-screw pile single pile bearing capacity test were carried on respectively. In order to compare and analyze the bearing capacity of CFG pile and screw pile, another group of natural silt foundation bearing test was carried on.(4)Analysis on the model test results was carried on. Results of single pile composite foundation bearing were analyzed by drawing curves such as load settlement curve, axial force distribution curve, pile-soil stress ratio distribution curve and pile side friction distribution curve. Results of the single pile bearing test were analyzed by drawing curves such as load settlement curve, and the curve of the axial force distribution of the pile. Analysis shows that the ultimate bearing capacity of CFG pile in sandy soil and silty soil is 4kN and 3kN respectively, and the ultimate bearing capacity of screw pile in sandy soil and silty soil is 5kN. The ultimate bearing capacity of natural foundation, single pile composite foundation of CFG pile, single pile composite foundation of half-screw pile is 167 kPa, 233 kPa, 333 kPa separately. Compared with the natural foundation, ultimate bearing capacity of single pile composite foundation of CFG pile increases by 40%. And compared to natural foundation and single pile composite foundation of CFG pile, the ultimate bearing capacity of single pile composite foundation of screw pile in CFG pile increases by 30% and 100% separately.(5)The single pile bearing performance of the CFG pile and half-screw pile was simulated by finite element analysis software ANSYS. Based on the results, relation curve between load and settlement, distribution curve of axial force and lateral frictional resistance were drawn. Results of indoor model test and numerical simulation were compared to verify the validity of the model test.Based on the study above, the difference between the CFG pile and half-screw pile on bearing behavior were compared. The advantage of half-screw pile on bearing performance was clearly recognized, which provides reference for future engineering practice.