Numeric Analysis Of The Properties Of Vertical Bearing Capacities For Cast In Situ Concrete Tubular Piles

Thin-wall cast-in-situ concrete tubular pile is a new type of pile and created by Prof. XIE Qing-dao based on his years of research on Ocean Engineering. Compared to other piles this pile has many advantages, such as higher bearing capacity, less material consuming and easy for construction, so it has been gradually used in the soft ground and Ocean Engineering. Nowadays due to fewer studies on theoretical mechanism and load transfer system its wide use in practical engineering and further development are restricted. Further studies on this subject become more urgent and necessary.In this paper first the characteristics of this thin-wall cast-in-situ concrete tubular pile are summarized, and then its application in soft ground and the procedure of pile-driven are introduced. Other methods used for ground improvement are also briefly discussed and a review of present studies on this pile and other similar piles, such as steel tubular (or pipe) pile are depicted in the first chapter.In order to analyze the load transfer curve and study the main effects on the vertical bearing capacity numerical analysis is used. The program used in this research is ANSYS, which is commonly used in Geotechnical Engineering. Some details in finite element analysis are introduced, including element discretization, stiffness matrix for a single element, the global stiffness matrix assembling, boundary conditions, interfaces modeling and the constitutive model.By using the software ANSYS the mechanism of load transfer form is studied and the vertical bearing capacity as well. Different soil modulus along the pile and underneath the pile are considered, meanwhile different cohesion, friction angle, pilelength, diameter and the thickness of pile wall are also studied in order to grasp the main effects on pile bearing capacity and guide practical design.Numerical results show that higher compression modulus, longer length and larger outer diameter will cause a large increasing in pile bearing capacity. The reasonable outer diameter for practical use is around 10-15 m. Once fixing the outer diameter any changes in the wall thickness will have little influence on its bearing capacity. But it will increase with soil cohesion and friction angle. When the cohesion higher than 30 kPa no more increase in capacity will be induced. And the same conclusion can be got from increasing friction angle. As the friction angle larger than 15 degree the bearing capacity seems to stop improving.In the end of the paper a simplified method for estimating the limited inner friction force and the resistance of soil plug under the cover board is put forward. So the entire vertical bearing capacity consists of four parts, inner and outer friction force, pile resistance around the circular area and the resistance of soil plug under the cover board. This simplified method is verified in a case study and will be convenient for practical use.