Study On Working Performance Of Large-Diameter Belled Pile

Large-diameter belled pile, which has been widely used in long-span bridge,high-rise structure, has the advantage of high bearing capacity, little settlement and good stability. So far, it has been developed for more than 30 years. A great deal of work about mechanical hole and settlement calculation of large diameter belled pile was performed. A large amount of literature on dynamic and static response property under horizontal load developed well. However, compared with the conventional straight piles,large diameter belled pile has great differences on the expansion pressure, settlement calculation and seismic response, due to the enlarged end of pile.Therefore, sphere expansion pressure, settlement under vertical load and dynamic response caused by earthquake load of large diameter belled pile was investigated to develop the design theory of large diameter belled pile. The main contents in the paper are as follows:(1) The expansion pressure of large diameter belled pile was presented. On the basis of the spatially mobilized plane theory, the theoretical solution of spherical cavity expansion pressure of belled piles was derived. First, the energy dissipation principles were applied to investigate to the whole process of spherical cavity expansion. Using stress invariant, the yield criterion of spherical cavity expansion was obtained. Then, the differential equations of the problem were simplified to achieve the expression of stress,displace and strain in elasto-plastic zone. Finally, the solutions of the expansion pressure were derived based on the volume conservation and energy conservation respectively.The elastic deformation of plastic zone was considered in this method and the relation among the expansion pressure p and the plastic radius R and expansion radius a was obtained. The examples demonstrated that the value of the expansion pressure in the present paper agreed well with the field test result. The results also showed that the plastic radius and expansion pressure will be increasing with the increase of the value of expansion radius, but the increase gradually decreased and tended to be stable. The dilatancy angle of soil has significant influence on the plastic radius and expansion pressure. With the increase of dilitancy angle the plastic radius and expansion pressure increases greatly.(2) A method of settlement calculation of large diameter belled pile was put forward. Considering depth effect of compression modulus, the existing settlement calculation formula of large-diameter belled cast-in-place pile is modified. The relationship between compression modulus and deformation modulus is established from the micro element stress analysis. The effect of depth on compression modulus is introduced into the relationship. The relationship is introduced into the existing settlement calculation formula of belled piles and the modified calculation formula is obtained. The settlement of the belled pile in an engineering practice is calculated using the existing calculation formula and the modified calculation formula respectively. The results showed that the settlement value calculated by the modified calculation formula showed better agreement with the measured results. Numerical results also show that the settlement value considering depth effect of compression modulus is more consistent with the measured results compared with that not considering depth effect of compression modulus.(3) Dynamic response of large diameter belled pile under earthquake load was analyzed. The Drucker-Prager elasto-plastic model is used to consider the nonlinear behavior of soil, and the interface element is modeled as nonlinear of contact between soil and pile. By using ABAQUS, the 3D finite element models of belled pile-soil-structure interaction system and conventional straight pile-soil-structure interaction system is established respectively. The dynamic response of the two type of piles in time domain was numerically modeling to analyze the differences on anti-seismic behavior. The influence of dynamic interaction on pile-soil modulus ratio,damping ratio, soft layer, construction quality was also discussed.