Research On The Interaction Between Pile And Soil Of Super-large Diameter Steel Pipe Piles For Offshore Wind Farm In Sand

The monopile foundation is the main type of offshore wind turbine foundation. The utilization rate of monopile foundation is more than 65% in the wind farm. In recent years, super large diameter steel pipe piles with diameter larger than 4 m have been widely used as monopile foundation with the increase of the unit capacity of wind turbine and the construction water. The increase of pile diameter leads the stress and deformation characteristics of the foundation soil change. Therefore, the interaction mechanism between the pile and soil for the super large and small diameter steel pipe pile is different. However, the research of interaction mechanism between the pile and the soil is not sufficient; there are no mature soil plug evaluation method and bearing capacity calculation method for super large diameter steel pipe piles. Based on the theoretical analysis, numerical simulation and model test, the interaction between the pile and soil is studied. The main research contents are as follow:1. The correct judgment of the soil plug effect during pile driving has an important influence on the evaluation of driving resistance and bearing capacity. Based on a numerical method, the failure mode at the pile tip in sand of super large steel pipe pile is analyzed. According to the numerical results of super large diameter steel pipe pile, Meyerhof deep foundation formula is determined to calculate the soil resistance at the pile tip; in addition, quasi static equilibrium method is proposed considering the inertia force of soil plug. Also model test is carried out to study the regularity of the soil plug effect.2. Because some restraints are made on the vertical bearing capacity in API code for security purpose, the value of vertical ultimate bearing capacity of pile foundation calculated by the code is conservative. For accurate calculation of the vertical bearing capacity, the soil stress characteristics at pile tip of super large diameter steel pipe pile under vertical load in sand is studied based on a numerical method. According to the numerical results, the influence of soil arching effect on the vertical bearing capacity is analyzed and the calculation method considering the soil arching effect is proposed. The applicability of this method is verified based on the model test. Also the influence of pile diameter, embedded pile length and wall thickness on vertical bearing capacity is studied, and the characteristic parameter for vertical bearing capacity is proposed.3. In offshore engineering, the P-Y curve method is applied to calculate the lateral displacement of pile foundation. The P-Y curve in API code is obtained from the test data of small diameter pile; and its applicability for large diameter piles remain to be proved. To get the calculation method for lateral displacement of super large diameter steel pipe piles, the stress and deformation characteristics of the pile body and the soil around the pile under lateral load in sand is analyzed based on a numerical method. According to the numerical results, the lateral displacement calculation method for pile top based on shell theory is proposed. The applicability of this method is verified based on the model test. Also the influence of pile diameter, embedded pile length and wall thickness on lateral bearing capacity is studied, and the characteristic parameter for lateral bearing capacity is proposed.