Load Transfer Mechanism Of Concrete-cored DCM Pile

Concrete-cored DCM (Deep Cement Mixing) pile is a new kind of composite pile that is developed for the ground treatment in soft clay ground. The composite pile is composed of an external DCM pile socket and an inner precast concrete pile that is inserted right after the completion of the DCM pile. Based on a great deal production about studying pile foundation and the characteristic of practical material, this paper studies the mechanism of load transfer for concrete-cored pile using in-situ test and Finite Element Method (FEM). As the literature concerned, the research is original. This study will establish the foundation for applying this kind of pile and is also valuable for research on mechanism of other composite piles. In this paper, we think that Concrete-cored DCM pile has high bearing capacity, where the high strength concrete pile is designed to bear the structural load, and the large surface DCM pile socket, which takes the advantage of reinforcement, acts to transfer axial force into the surrounding soils by skin friction. With the divide the work of inner core pile and external pile socket, concrete-cored DCM pile cap use the strength of the material in pile body adequately. It's designed not only much higher working load than other ground treatment methods, but also is cheaper. Another advantage of the pile is that it can avoid the environmental problems of noise, vibration and extruding soil that are common for driven pile, also, slurry pollution for cast-in-situ pile. Our study indicates that inner concrete pile bore a majority of structural load, and share more and more the load of building as the applied load increases, which inevitably to leads to the concentration of stress on concrete core. We find that the characteristic of load capacity about concrete-cored DCM pile is same as that of a rigidity pile, and is different with that of a flexible pile. The structural load, which can transfer down to the bottom of DCM pile and the surrounding soil, is very limited. So the end resistance of composite pile is negligible and this kind of pile works as a friction pile in this situation. We find there are two failure modes involved in concrete-cored DCM pile: One is pile body failure type, the structural load exceeds the material strength of inner concrete pile and causes the concrete pile body to fail. And the other failure type can be named ground failure type, it refers to the shear failure of pile-soil interface or plastice failure of pile end soil layer. The ultimate capacity of concrete-cored DCM pile is controlled by both the strength of inner concrete core and pile-soil skin friction, and could be larger comparing with DCM pile and cast-in-situ pilefairly. It could be most economical, If inner pile body and ground soil reach their ultimately bearing capacity simultaneously. Now we can suggest a two-step model of load transfer: load transfer from inner pile to external pile, and from external pile to surrounding soil. This two-step load transfer system can make the load spread into a larger range. The study indicates that this composite pile used in soft ground has an acceptable settlement, which is relatively small and not exceed 1% of pile diameter under working load. Pile-cap settlement is significantly influenced by modulus of surrounding soil, also length and diameter of inner concrete pile. There is domino effect of group piles within Concrete-cored DCM piles. The efficiency modulus of group piles is larger as pile distance increases, it is 0.9 or more as the distance equals to 4D, and after 6D and more distance, it is close to 1. Finally this paper discusses the design about the length and diameter of concrete core pile.