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In this paper, particular attention was paid to the seismic enhancement effect of group-pile foundation with partial ground improvement method that is used for existing pile foundations in practical engineering. A model test on a full system with a superstructure, a nine-pile foundation and a sandy ground was conducted with the shaking table test device. The model pile is made from aluminum and the model ground is made from Toyoura Sand. The shaking table test device is 120 cm in width and 160 cm in length. The maximum acceleration is 1 g and the maximum displacement is 5 cm. The maximum payload is 16 kN and the highest frequency is 10 Hz. The model ground is carefully prepared to obtain a ground with controllable unified density. Before the shaking table test, the pattern of the partial ground improvement for an existed group-pile foundation is carefully selected using numerical tests with a 3D elastoplastic static finite element analysis. In the analysis, the nonlinear behavior of ground and piles are described by the cyclic mobility model ( Zhang et al., 2007 ) and the axial force dependent model (AFD model) proposed by Zhang and Kimura (2002) can take into consideration of axial-force dependency in the nonlinear moment–curvature relations. The applicability of the numerical analysis has been verified in previous works by comparing the numerical results with a real-scale field tests ( Kosa et al., 1998 ). Based on the results from the numerical tests on seismic enhancement effect of group-pile foundation with ground improvement, an optimum pattern of partial ground improvement of an existing pile foundations has been picked out for shaking table test. A numerical analysis using the program DBLEAVES ( Ye, 2007 ) is also conducted for the same optimum pattern for comparison purposes. The effectiveness of the partial ground improvement method has been proved by both the shaking table test and the numerical analysis.

In this paper, three-dimensional (3D) finite element analyses of a real-scale group-pile foundation subjected to horizontal cyclic loading are conducted using a program named DBLEAVES. In the simulations, nonlinear behaviors of ground and piles are described by subloading tij model and the axial-force dependent model (AFD model) which considered the axial-force dependency in the nonlinear moment-curvature relations. In order to consider the inuence of an effective stress path on the prediction of the group-pile foundation, the analyses are conducted within the framework of the soil-water coupling method with finite-difference and finite-element regime. The material parameters of soils are determined based on conventional triaxial drained compression tests on undisturbed and remolded specimens. The applicability of the proposed numerical method is encouraging, and therefore, it is quite confident to say that the method can be used to predict the mechanical behaviors of group-pile foundation to a satisfactory accuracy, particularly with the effective stress analysis.