Multi-Scale Analysis Of Characteristics Of Deep Foundation During Constructing And Loading

With the development of computer technology, many scholars begin to analyze the geotechnical engineering problems with numerical analysis method and make lots of progress. The finite difference method and the discrete element method are widely used in geotechnical engineering. While with the existing disadvantages of these two methods, domestic and foreign scholars put forward coupling analysis to take advantage of both. Discrete-Continue coupling analysis method has become a research hotspot these years.This thesis presents a three-dimensional Discrete-Finite difference coupling analysis method and the coupling calculation program with PFC3D and FLAC3D. Caisson-sinking, pile-driving and caisson-cushion-pile-loading process are analyzed from macro-mesoscopic scales using the coupled calculation program. The main research work and conclusions are as follows:(1) Discrete-finite difference coupling algorithm is proposed. The simulations of laboratory tests prove that the coupling calculation method can capture and transmit the normal force and tangential force transmit accurately at the coupling interface. It also proves the compatibility of both the constitutive relation and the deformation at the coupling interface when the macro-mesoscopic parameters are matching.(2)To explore the mechanism of caisson-sinking process, the process is simulated numerically. It shows that there is an obvious turning point of the pressure distribution curve. The pressure near the caisson edge decreases gradually, tending to the active earth pressure 7 which is a consequence of the occurring of a stress relaxation zone, using the theory of soil arching effect, the calculated earth pressure is found to be more closer to the measured values.(3)To investigate the mechanism of pile driving process, numerical simulations are performed. The results show that the pile diemnsion has a great influence on soil compaction effect. The soil displacement increases with the increase of pile’s diameter, and it decreases quickly along the radial direction. Vibrosinking can reduce the pile friction and pile end resistance effectively in pile-driving process. The vibration frequency should be adjusted according to the properties of soil to achieve the most appropriate vibration frequency.(4)The caisson-cushion-pile composite foundation is simulated numerically to investigate the influence of the thickness of the cushion. It is found that the thicker cushion is, the large the foundation settlement, the pile top penetration, and load sharing ratio of soil are. During loading, with the increase of load, the load sharing ratio of piles (load taken by the pile) declines gradually and the load sharing ratio of the soil increases. If the other conditions are same, the pile top penetration is related to the pile location, smaller for the internal piles and larger for the external pile.Based on the coupling numerical analysis, the characteristics of the three foundation types in the process of constructing and loading are investigated from viewpoint of multi-scales. And the validity of the Discrete-Finite difference coupling algorithm is verified.