The behavior of pile foundation during earthquake is affected by soil-pile-building interaction and its nonlinearity. When we calculate the pile stress, the nonlinearity of soil material should be considered. For failure criterions which can express nonlinearity of soil, Mohr-Coulomb failure criterion and Drucker-Prager failure criterion are used. However, the influences on soil resistance around piles caused by the difference of these failure criterions are not clear yet. On the other hand, horizontal soil resistances in pile groups are affected by the interaction between piles, which is called the effect of pile groups. In our past researches, we have analyzed about soil resistances around each pile in pile groups in sandy soil, but we have not discussed the case of cohesive soil. The failure criterion of cohesive soil is different from sandy soil. This difference may affect the behavior of pile groups. Therefore, it is necessary to discuss soil resistance of a pile group in cohesive soil.
This paper discusses the influences on soil resistance of a pile group caused by different failure criterions. Nonlinear static analyses on the 25-pile groups were performed using 3-dimensional finite element method. The piles are steel pipe piles with a length of 11m and a diameter of 600mm. The pile group is arranged in a 5x5 pattern. Each pile is located with center-to-center spacing of 1.5m. Soil parameters of internal frictional angle and cohesion are mainly discussed.
Both springs at the pile head and around pile shaft are calculated. When calculating the spring at the pile head, the piles are modeled by the elastic shell elements and forced displacement is given to the pile head. When calculating the soil spring around the pile shaft, the piles are modeled as the excavated ground and uniform forced displacement is given to the whole of the piles.
The major conclusions of this paper can be summarized as follows:
1) Under the Mohr-Coulomb failure criterion, the strength of sand differs between triaxial compression and triaxial tensile. The difference determines the subgrade reaction of piles. Near ground surface, the subgrade reaction is determined by the strength under triaxial compression, while in deeper parts of the ground, the subgrade reaction is determined by the strength under triaxial tensile.
2) In the case of sandy soil, subgrade reaction concentrates on front piles in a pile group. While in the case of cohesive soil, the concentration on front piles is softened by side piles.
3) Concentration ratio of the subgrade reaction is discussed. In sandy soil, concentration ratio on front piles goes higher with the growth of displacement, while the concentration ratio of side piles gets lower. But in cohesive soil, concentration ratio of front piles and side piles hardly change with displacement.
4) Both in sandy soil and cohesive soil, the subgrade reaction concentration ratios on front piles and side piles get lower near ground surface. While in deeper parts of the ground, they are almost constant values.
