Theoretical And Application Research On Load Transfer Of Vertically Loading Large Diameter Piles

Along with the construction development in speedways, express railways, long-span structures and tall buildings in China, the requirements of bearing capacity of the foundations which support the upper structures have turned out more and more strict. The large diameter piles (LDPs) not only have stronger bearing capacity with less deformation, but also have some other advantages: flexible in application, simple in computation, convenient for construction, and economical in materials consumption, etc., thus LDPs are widely used as the first priority in deep foundations.It is unsuitable to use the theory of common small-medium piles to analyze LDPs, because there are distinguished differences in the mechanism of load transfer and deformation features between them. It is because LDPs have higher bearing capacity that the load test in-situ for them is confined for many reasons, therefore the tested results and experiential information are seldom reported, thus the theoretical research on it lays behind. Based on the present domestic and international research on LDPs and the integrated analysis, this dissertation combines the above-mentioned research with the research projects supported by the National and Hunan Provincial Natural Science Fund. By means of combining the research with the theoretical analysis, the model tests, load tests in-situ and numerical simulation, the research work has been accomplished with the following achievements:1) Based on the hyperbolic model of load transfer, this dissertation proposes an analytical method to analyze the relation between load and settlement of LDPs. Furthermore, it puts forward an iterative analytical method referring to the influence of pile-end sediment. Compared with tested results, the proposed method proved its reliability.2) Based on shear-deformation method and one-dimension consolidation theory, the present dissertation makes a systematical study on the law of load transfer of LDPs with the effect of consolidation and overload, and proposes the relative calculation method for LDPs' loadtransfer analysis for the first time, which, compared with results of model tests and load tests in-situ, proves to be reliable.3) According to the load test characteristics of LDPs, the dissertation improves GM(1, 1) model for the prediction of load-settlement relation, and provides a reasonable approach to calculate the bearing capacity and settlement of LDPs by applying limited load test data to predict the whole relation of the load-settlement of LDPs.4) Applying the principle of shear-deformation approach and with the influence of nonlinear deformation of soil considered, the dissertation puts forward a matrix method to analyze the load transfer of LDPs in layered soils. Based on this method, a study on the whole course of load transfer of LDPs in layered soils is carried out with the application of elasticity-fully-plasticity model and tri-linear softening model, and at the same time a corresponding calculating method is also proposed. According to the above theory, a study on load transfer of LDPs socketed into rock, and its corresponding analytic methods and theories are put forth in the dissertation. These studies show that the calculated results are in good agreement with the tested data.5) Through shear-friction test, deep-hole load test and Self-balance load test, the parameter of load transfer of LDPs is measured systematically. With a further study on mechanism of load transfer of LDPs, the following conclusions are found:CD In the process of load transfer, some factors have great influence on the development of pile side friction, such as the nature of pile side soil, pile-soil relative displacement, pile-soil connection conditions and pile diameter, as well as the supporting conditions of pile-end soil.?Along with the development of pile side friction and pile-end resistance, the depth effect appears in the process of load transfer. For LDPs, the critical depth of pile side friction and pile-end resistance is about 10 times of the pile diameter.(3) In the course of load transfer, pile side friction and pile-end resistance are strengthened mutually, which is important for the design and construction of engineering.(4) For LDPs, its size effect (pile shaft diameter and pile-end diameter) on the influence of pile-side friction and pile-end resistance cannot be neglected, which is the basic difference in load transfer of LDPs from ordinary piles.(5) Influence of soft underlayer on the bearing capacity of piles is great, while it is exists. Therefore, it is necessary to ensure the thickness of bearing stratum. For LDPs, the critical thickness of bearing stratum is about 10 times of the pile-end diameter.6) The relative computer programs are designed for the above proposed analytic methods for practical use. Furthermore, the finite element method (FEM) program is also designed to simulate the load transfer of LDPs. The conclusions and results from the simulation are in good agreement with the results obtained through theoretical analysis and experimental research.