Research On Numerical Simulation Of The Bearing Performance Of Large-diameter Bored Piles

With the development of highway, railway, large bridges and other large-scaleconstruction projects, in order to meet the requirements of large-span, high-rise, heavyloads and complex geological conditions, pile foundations become the first choice offoundation form for many buildings or structures. The design size and bearingcapacity of the pile foundations become larger, and the application of large-diameterpiles in projects has become more and more common.Comparied with small-diameter piles, large-diameter piles have some advantages,such as the high capacity, small deformation, convenience in construction etc. In themeanwhile, there are some differences, such as the load transfer mechanism,deformation characteristics, and load bearing characteristics. Due to the large bearingcapacity of large-diameter piles, there are some limitations on study of the bearingcharacteristics using field test. Especially in the actual projects, the bearing capacityof the pile is often determined by the static load test or Osterberg-cell test, andconsidering the project cost, or the location of the balance point in the Osterberg–Celltest and other reasons, the applied load is often not enough to make the test pile intodestruction state. The bearing capacity of test pile meets the requirement of the project,but the research value is reduced. So the theoretical analysis on bearing characteristicsof large-diameter piles is relatively lagging behind the engineering practices.Therefore, in this paper, the numerical simulation is using to analysis the deformationand the load bearing characteristics of large-diameter piles, the main findings andconclusions are as following:1. Study on evaluation methods of large-diameter pile bearing capacity.Systematic comparative study on the evaluation methods of the bearing capacity oflarge-diameter pile is made, including empirical parameter method, field test (PileStatic Load test, Osterberg-cell test, dynamic load test and Statnamic method),numerical simulation method and indoor model test. The advantages and limitationsof these methods in determining the pile bearing capacity of large-diameter pile areanalyzed. 2．Simulation is used to study the bearing characteristics under different test.Numerical simulation methods are used to simulate Pile Static Load test andOsterberg-Cell test to discuss the bearing behavior of large-diameter pile underdifferent test methods. Comparing with the measured data, it found that the simulationstudy of the pile bearing characteristics is a very important and useful.3. Through the simulation experiments, the influencing factors of bearingcapacity of large-diameter piles were analyzed, including the deformation modulus ofsoil around the pile, cohesion and internal friction angle of soil around the pile,deformation modulus of soil at the pile tip, pile length, pile diameter, the elasticmodulus of pile, the interface mechanics parameters between the pile and soil. In thestudy, obtaining methods of the parameters for simulation are analyzed and discussed.Through the study, it found that the bearing capacity of the large-diameter pile linearlyincreases with the pile diameter increases. In a certain range, the bearing capacity ofthe pile can be improved with the increasing of the deformation modulus of soilaround the pile, deformation modulus of soil at the pile tip, and the length of the pile.There is no signigicant effect to increase the pile elastic modulus for improving thebearing capacity of the pile. The settlement of the pile is impacted by the interfaceshear stiffness between the pile and soil. And the the bearing capacity is impacted bycohesion and internal friction angle of interface between the pile and soil.4. Back-analysis is used on the study of the mechanical parameters of interfacebetween pile and soil. Based on the simulation results, orthogonal design and BPneural network program, the displacement back-analysis model has been established,which was used to study the mechanical parameters of interface between pile and soil.5. Based on the load transfer theory of piles and the deformation characteristics ofpile in the Osterberg–Cell test, a simulation analysis program was applied to simulatethe relationship between the load and settlement in the test. The analytical methodspresented in this paper can be considered practically, since the results from thesimulation test and on-site measurement indicate that the theoretically predicted resultis consistent with the measured one.6. The simulation analysis program can be used to determine the location of the balance point in the Osterberg–Cell test. A reasonable selection of the location of thehydraulic jack-like device (O-cell) can maximize the bearing capacity of testing pilesto obtain a more accurate ultimate bearing capacity. So this study provides a referencefor the design of the Osterberg–Cell test as well as pile foundations.