Study On Bearing Mechanism Of DX Piles Cast-in-Situ And Its Application In Osterberg Loading Test Method

DX pile is a new type of squeezed cast-in-situ pile with multi-branch. Comparedwith conventional cast-in-situ pile, DX pile can raise the bearing capacitysubstantially according to a lot of experiences of projects. Therefore, a remarkableeconomic benefit and technical effect can be acquired by reasonably using DX pile.However, because of the complexity of the interaction between DX pile and soilaround, the research on bearing mechanism of DX pile is quite insufficient, and itsapplication is limited. In this thesis, the mechanism of bearing capacity of DX piles subjected totop-upward tensile loading and down-upward compressive loading is studiedsystematically by FEM analysis, elastic theory and limit equilibrium method. Basedon bearing behavior of DX pile and characteristic of the Osterberg Pile Loading TestMethod, a new approach is developed for the application of DX pile to the Osterbergmethod. Through FEM analysis, the bearing behavior and load-transferring mechanismof DX pile under three loading conditions: compressive load and tensile load at thetop of pile, uplifting load at the tip of pile are discussed. The main conclusions aredescribed as follows: ⑴The application of DX pile to the Osterberg method canovercome the limitation of Osterberg method in which the shaft frictional resistancescould be insufficient, thus the applied range of Osterberg method is widen. ⑵Thebearing mechanism of DX pile under the three loading conditions are different. Themain potential mechanisms responsible for the differences lie in the differences instress paths in soil, rotation of the principal stress directions, Poisson domino effect ofpile shaft, the effects of gravity and the enhancing response for the soil underneaththe pile. ⑶ Behavior of enlarged disks has obvious time effect. The enlargeddisks exert its end bearing from top to bottom under compressive and tensileloading at the top of pile while from bottom to top under uplifting load at the tip ofpile. - II -Based on the elastic theory and the overlapped-form strain mode of the soilsurrounding a pile, an analytical solution for the load-displacement relationship ofDX pile in different ways is presented in this paper. The proposed solution can reflectthe influence of the deformation of pile and non-homogeneity of soils' modulus alongthe depth in load-displacement relationship. The comparison with FEM analysis andfull-scale test results show that the analytical solution can well predict thedisplacement of pullout DX pile. The solution is simple and convenient, and can beused as a practical reference to the design of pullout DX pile and DX-Osterbergmethod. Based on the limit equilibrium theory, the calculation formula of uplift capacityof DX pile is established. The various failure mode of DX pile under the ultimatebearing state are considered. By comparing with former experiential formula andfull-scale test results, formula obtained in this paper can reflect the bearing behaviorof pullout DX pile preferable, which is convenient to be used in the design of pulloutDX pile and DX-Osterberg method.