Research On Variable-section Tension Pile With Model Tests And Numerical Simulation

Variable cross-section piles as a new type of pile, is in the traditional cross-section pile on a certain position to expand diameter and form the cross-section pile. Compared with the traditional cross-section pile, the the variable cross-section increase the contact area with the soil around the pile, and the soil resistance is increased. As the deformation of the soil near the enlarged section from shear to under pressure, significantly increased the capacity of the pull-up capacity. Compared with the traditional equal section piles, the variable cross-section pile is quite complex, because of the pile shaft resistance and the support force at the variable cross section influence each other, which making the bearing capacity of the variable cross-section pultruded pile become a very complicated problem. The position, spacing, number and size of the variable cross section have a great influence on the failure mode and the bearing capacity. The literatures of the variable cross-section piles are mostly used as the research of compressive piles, but research on variable cross-section piles as tension piles are relatively few. Therefore, it is very important to study the variable cross-section pile under tension force.Based on the study and analysis of domestic and foreign research, this paper studies the influence of variable cross-section spacing on the uplift bearing capacity of piles with variable cross-section in sandy soil on the basis of model test and theoretical analysis and numerical simulation.Effects of spacing on bearing capacity of piles with variable cross-section are analysed. The study mainly includes:(1) The bearing mechanism of equal section piles and variable cross-section piles is analyzed and compared by means of the five-section test of the uplift piles in the sand foundation. The influence of the variable cross-section spacing on the bearing capacity of piles are analyzed. The results demostate that the ultimate tensile capacity of the piles with variable cross-section is larger than that of the piles, and the relative displacement of the pile and soil is also large. The ultimate bearing capacity increases with the increase of the variable cross-section spaing increase, but when the variable cross-section spacing greater than 6 times the cross-section pile diameter (18cm), the increase is smaller.(2) The numerical analysis of the uplift piles with different variable crossings is carried out, and the failure modes are studied. Studies have shown that when variable section pitch is less than 6 times the pile diameter, the plastic zone mainly in the vicinity of the variable cross-section, and is a twelve small large intermediate gourd-shaped.When the variable section spacing is greater than 6 times the pile diameter, the plastic zones are separated from each other and extend to the center as the center.(3) According to the numerical simulation of the plastic zone,according to the different variable spacing, two failure modes are proposed. When the variable cross-section spacing is greater than 6 times the pile diameter, it is assumed that the failure mode is a logarithmic helical type that is independent of the upper part of the two variable sections. When the variable cross-section spacing is less than 6 times the pile diameter, it is assumed that the failure mode is the logarithmic helical fracture surface of the upper part of the variable cross section, the elliptical fracture surface between the variable cross section and the upper variable cross section. According to the two failure modes, the calculation formula of the variable cross - section support force is put forward, and the calculation formula of pile - side resistance is obtained considering the change of earth pressure in the pile. Finally, the formula of calculating the ultimate bearing capacity is obtained.