Study On The Engineering Properties Of Super-long Piles

As high-rise buildings and the wide-span bridges in deep soft clay increase recently, super-long bored piles are used frequently due to its superiority of high bearing capacity. And with the development of the construction equipment and the technological level, the application of the super-long bored piles become possible.Though a large number of super-long piles have been used, its load transfer mechanism is not distinct. By now, its design method still adopts that of the ordinary piles, which disagrees with practical situation. The design method of super-long pile in the applicable codes and standards is not based on the mechanism of load-bearing and deformation. Therefore, the research on the load-bearing properties and the reasonable design method of the super-long piles is important to meet the requirements of the theory and the reality.Based on the research status at home and abroad, the engineering properties and the pile settlement theory of the super-long piles are analyzed in the paper.The nonlinear character of the single super-long pile is analyzed according to its specific load transfer mechanism. The lateral friction resistance and the concrete are simulated respectively using hyperbolic curve model and Kent-Park model. The influence of settled material on the relation of force and deformation of pile tip is considered. It is analyzed that the settled material affects the super-long nonlinear character.The super-long pile load-bearing properties in isotropic soil and stratification soil are studied by the axisymmetric finite element method. In the FEM program, the elasto-plasticity of concrete is considered, and the Mohr-Coulomb model and the contact surface model are adopted to simulate the characters of soil and the interaction of pile and soil respectively. The influence of the modulus ratio of pile to soil, of upper soil layer to the bearing layer, and ratio of pile-length to pile-diameter on the load-bearing properties, including the relation of settlement and force, the characters lateral friction resistance, axial force, and plastic deformation, are analyzed. According to the analysis results, the concept of the optimum ratio of pile-length to pile-diameter is put forward, and the super-long pile is defined theoretically from thepoint of view of pile stiffness.The influence of loading method of piling-up and anchor pile on the testing load-bearing properties is analyzed. The influencing factors of piling-up loading method include: datum point, negative skin friction, additive skin friction, and compaction. At the same time, it is analyzed in detail that the spaces of anchor pile to testing pile affect the load-bearing properties of the testing pile.Based on analysis of the pile design method in the applicable codes and standards, some problems about the super-long piles design are pointed out according to the load transfer characters. The design method submitting to the principle of deformation compatibility is put forward for the super-long piles. The method is more rational for considering the intercoordination of the force and deformation of piles.Finally, it is concluded that it is feasible to analyzing the pile foundation by FEM, according to comparing the computing results with measuring results in an engineering example. Some useful conclusions are obtained after analyzing the measuring results about the relation of force and settlement, the distribution of axial force and lateral friction resistance.