In this study, three-dimensional finite element analysis was performed to investigate the behavior of piled gravity base foundation (GBF) subjected to the loading conditions of offshore wind turbine. The piled GBF was newly developed to support offshore wind turbine. The piled GBF consists of a gravity based foundation with five piles improving conventional gravity base foundation for the use in very soft clay soil seabeds. In this numerical analysis, 1) parametric studies were carried out to confirm the behavior of the entire system of piled GBF. The controlled parameters include loading direction, loading height, bearing condition of piles, rigidity of mat, elasticity of clay, undrained shear strength, field ground position and pile length. Second, 2) the p-y curves were calculated according to the foundation type (single pile, two group piles with 3x3 array and cross array, piled raft, and piled GBF) to analyze the interactions between gravity base foundation and supporting piles (pile-soil-mat). Finally, the P-multipliers expressed by the ratio of ultimate soil reaction for single pile to that for group pile were calculated. For the group piles, 3x3 array and cross array are spaced with the same spacing (=7.2 times of the pile diameter) with the target structure, that is, the piled GBF.
The results of the parameter study are summarized as follows. First of all, as the load height increases, the horizontal resistance of the piled GBF decreases and the maximum bending moment of the supporting piles significantly increase in the pile body because of the increase in overturning moment. Sencond, considering end-bearing and friction piles, the lateral resistance of the piled GBF supported by end-bearing piles is higher than that of friction pile and the maximum bending moments of the end-bearing piles is lower than those of the friction piles. Third, the horizotnal resistance increases and maximum bending moment of the pile decreases with the increment in undrained shear strength of clay because of the soil reaction force increased by undrained shear strength.
Regarding the p-y curves of the group pile with the 3x3 and cross array, ultimate soil reaction force of piles in the 3x3 array has similar tendency to previous studies, which is that ultimate soil reaction force of the first row is the highest, followed by the second row and the third row. The ultimate soil reaction force of the piles in the cross array is lower than that in the 3x3 array due to the less number of piles. For piled raft and piled GBF, the ultimate soil reaction force of the center pile increased higher than single pile and that of the leading pile decreased due to the interaction between mat and piles. The increase in horizontal resistance of center pile is due to soil compression by the weight of gravity base foundation. The decrease in the ultimate soil reaction force of the leading pile is caused by the shear force which significantly increased at the front corner of the mat. In conclusion, the weight of gravity base foundation significantly affects horizontal behavior of piled GBF. The P-multipliers calculated are 1.11, 0.90, 0.70, and 1.08 respectively for center pile, back pile, leading pile, side pile at the depth ratio of two times the pile diameter. To identify the factors affecting the behavior of piled GBF, additional analyses were performed by varying the vertical load and the rigidity of the mat for the piled raft. As the result, the effect of the rigidity on P-multiplier was negligible. As the vertical load increases, the P-multipliers of the leading pile in piled raft significantly decreases, which is similar with the results of the piled GBF. Therefore, the trend of P-multiplier on piled GBF were found to be caused by the vertical load induced by weight of the gravity base foundation.