Study On Installation Mechanism And Calculation Method For Bearing Capacity Of Concrete Pipe Pile

Prestressed concrete pipe piles are widely used in China owing to their advantages. However many engineers are not familiar with the mechanisms of the installation and bearing capacity of the prestressed concrete pipe piles. Thus there are a lot of problems existing in the design and construction of the prestressed concrete pipe piles, such as mischoosing pile types, unsuitable structures, and severe influences of the squeezing effect on the bearing capacity of adjacent piles and surrounding buildings during pile installation. This dissertation mainly deals with the installation mechanism and calculation method for bearing capacity of the prestressed concrete pipe piles consideraing their characteristics.Firstly, the squeezing effect of prestressed concrete pipe piles is studied with cavity expansion theory. Analyses of spherical and cylindrical cavity expansion in elasto-plastic soil are performed based on the elastic theory considering different moduli in tension and compression. The characteristics of dilation are taken into account by using the non-associated Mohr-Coulomb yield criterion. The analytical solutions for the stresses in elastic and plastic regions, maximum radii of the plastic regions and expansion pressures are obtained. Then the influences of different tension and compression moduli ratios, Poisson's ratios and dilation angles on the expansion pressures and radii of plastic regions are discussed by parametric study. The results show that different ratios of tension and compression moduli, Poisson's ratios and dilation angles have significant influences on the development of plastic regions and the stresses in the soil. Subsequently, the incremental filling ratio is introduced to quantify the degree of soil plugging in the open-ended pipe pile. The radii of plastic regions and the expansion pressures on the external wall are formulated including ratio of tension and compression moduli and incremental filling ratio. Moreover, the influences of incremental filling ratios on the squeezing areas induced by installation of open-ended pipe piles are investigated. The results of the parametric analyses show that the squeezing effects of the open-ended pipe piles with smaller incremental filling ratios are more severe than those with larger incremental filling ratios.Secondly, the dissipation of the excess pore water pressure after pile installation is studied. The dissipation of the excess pore pressure is analyzed by radial consolidation theory with a variable consolidation coefficient based on the linear responses of effective stress-void and permeability coefficient-void. The governing equation of the radial consolidation considering variable compressibility and permeability is solved by the variable separation method. Bessel functions are used to solve the differential equations for the time independent part. Then the accuracy of this method is validated by comparing calculated values with numerical results and field data observed from two project cases. And the influences of variable compressibility and permeability on the variation of consolidation coefficient and excess pore pressure dissipation are analyzed. Moreover, the dissipation of the excess pore water pressures in the soil plugs of open-ended pipe piles is analyzed by one-dimensional vertical consolidation theory. And the differential equation of vertical consolidation is solved with iterative method. The solutions obtained can effectively reflect the variation laws of permeability and compressibility of soils during excess pore pressure dissipation in soil plug.Thirdly, the calculation method of tip resistances of the prestressed concrete pipe piles is investigated. The results of static load tests of prestressed concrete pipe piles in Wuhan are accumulated and analyzed. These results are compared to the values obtained from local standard of Hubei province "Technical code for building foundation", which show that test data are larger than the calculation values. Then a method to evaluate the end bearing capacity of pile by twin shear slip-line theory is recommended. The difference between the tension strength and compression strength of soil is considered and the effect of intermediate principal stress is included in the formula. The stress transferring formula of the slip-line method is applied during the calculation process. Whereafter, the calculation methods of tip resistances of open-ended pipe piles in different specifications are summarized. The characteristics of the soil plugs are analyzed. And the tip resistances of soil plugs under fully drained and undrained conditions are obtained, which can reflect the characteristics of bearing capacities of open-ended pipe piles.Finally, the lateral bearing characteristics of prestressed concrete pipe piles are analyzed. The anti-crack requirements of prestressed concrete pipe piles are verified under the condition of lateral loads, which act on the top of the piles and takes the characteristic values of lateral bearing capacity. Then based on the assumption that the soil is elastic-perfectly plastic material, the theoretical solutions of deflection, rotation, moment and shear force at any depth of the pile under lateral force are obtained, which can be applied to analyze the lateral bearing capacity of the piles mainly exerted by lateral force and with large lateral deflection.The achievements in this dissertation can be valuable references for the design and construction of the prestressed concrete pipe piles.