| Pretensioned spun high strength concrete(PHC) piles are widely used in various soft foundation reinforcements due to its high pile quality and short period for construction, which are determined by industrial productions. For the highway of construction in the mountains, the existence of soft soils in the foundation is an inevitable problem in high fill embankments. Based on the characteristics of composite foundations which are different from often foundations under other buildings loadings, this thesis aimed to employ finite difference method(FDM) to analyze the stress and deformation behaviors of PHC pile composite foundations under the load of high fill embankment at Guangfozhao highway, and then complete the design optimization for the case in PHC the prestressed piles near K124+440. The main work is listed as follows.Firstly, applying ANSYS to set transition meshs to ensure the grid continuity, which aimed to solve the irrationality of previous finite element methods using inconsistent mesh density contact surfaces to deliver loading. As for the steep slope section reinforced by piles, the consideration of interface in numerical simulations will affect the solution to initial geostress field and the equilibrium problem. Therefore, the main study was carried out in the geostress balance, and then established the interface when the software of FLAC3 D was utilized to build the model.In order to determine whether the bearing capacity of foundation match the design requirements, the curve of p-s of single pile was drawn based on the data of plate- load tests in compound foundations which range from K124 +410 to K124 +800. In addition, a model of single PHC pile was built based on this test data, and the contact surface parameters of pile-soil interface was obtain by repeating calculation for the succedent model built.Meanwhile, revealing the deformation distribution, the pile bearing capacity and the load sharing characteristic of the PHC pipe piles was explored under the loading of high fill embankment by the numerical simulations. It can be shown that the horizontal deformation and bending moments play an important roles in the PHC piles due to the incline of the soil foundation and the trapezoid distribution of fill loading. The simulations results also show that the settlement distribution at the bottom of the embankment presents a trend of two small ends and large in the middle. When the eight rows of piles was spaced as 2.6m, the maximum displacement at the bottom of the embankment is mostly located in the area 1m far away from the pile reinforcement.Through numerical simulations, the following factors were seleted to analyze the deformation distribution of the subgrade, the pile stress and load sharing characteristics of piles, including the design parameters of geogrid modulus, pile modulus, pile spacing, pile length and pile cap size. The impact of geotechnical grille on deformation of subgrade can be neglect when the embankment filled soil is much higher than the soil arch. Subgrade settlement increases only by 15.4%, when the modulus of PHC piles decreased to 10% of the origin value. Comparing with pile axial force, bending moments are more sensitive to the change of modulus.From the simulations, it can be seen that pile length has little effect on the settlement and horizontal displacement when the pile runs through the soft soil layer. An optimal value was obtained for the size of the pile cap as 1.2 m. Below this optimal value, the overall embankment settlement is inversely proportional to the size of pile cap, while the cap size is larger than this critical value, the the deformation at the bottom of the pile changes slowly.The pile axial force and bending moment increase with the pile cap size, especially at the top of the piles. When the pile spacing becomes larger, the settlement at bottom of embankment in the reinforced area gets larger, while the area outside of the reinforcement area keeps almost constant. |
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