Evaluation of Ultimate Strength and Deformation Capacity of Pile Foundations Considering Soil-Structure Interaction during Strong Earthquakes

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Effects of soil-pile-structure interaction on pile stresses are studied based on large shaking table tests on pile-structure models with and without embedded foundations that are constructed in dry and liquefiable sand deposits. The test results have shown the following :(1)The horizontal subgrade reaction of piles in liquefied soils increases in such a way that the piles are pulled by the soil on the extension side where the reduction of pore pressure occurs. Such mechanism is quite different from that occurring in dry sand where horizontal subgrade reaction is always induced by the increase in earth pressure on the compression side of the pile.(2)If the natural period of the superstructure, T_b, is less than that of the ground, T_g, the ground displacement tend to be in phase with the inertial force from the superstructure, increasing the shear force transmitted to the pile. In contrast, if T_b>T_g, the ground displacement tends to be out of phase with the inertial force, restraining the pile stress from increasing.(3)If soil liquefaction occurs, near surface soil may push the foundation, resulting in an increase in pile stresses. The higher the foundation rigidity, the larger become the pile stresses. The increase in axial force in this case cannot be estimated from the equilibrium of the overturning base moment, as the center of moment equilibrium becomes lower with the development of soil liquefaction.(4)The pile stresses estimated by the pseudo-static analysis, assuming that the maximum moment is equal to the sum of the two stresses caused by the inertial and kinematic effects if T_b<T_g or the squire root of the sum of the squires of the two if T_b>T_g, are in good agreement with the observed ones regardless of the occurrence of soil liquefaction.