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

通过对干砂层中桩-结构模型的大型振动台试验，研究了土-桩-结构相互作用对桩身应力的影响。试验结果表明：（1）液化土中桩的水平地基反力增大，在孔压减小的一侧，桩受到土的拉力作用;这一机理与干砂中的机理完全不同，干砂中的水平地基反力总是由桩的受压侧土压力的增加引起的。(2)If上部结构的自振周期T_B小于地基的自振周期T_g，地基位移与上部结构的惯性力同相，增大了传递到桩上的剪力。反之，当T_B> T_g时，地面位移趋于与惯性力反相，抑制桩身应力的增加。(3)If当土体发生液化时，近地表土体可能会推挤基础，导致桩身应力增大。基础刚度越大，桩身应力越大。在这种情况下，轴向力的增加不能从倾覆基础力矩的平衡来估计，因为随着土壤液化的发展，力矩平衡中心变得更低。（4）在假定最大弯矩等于惯性力和运动力之和T_B T_g的情况下，无论是否发生液化，用拟静力法计算的桩身应力与实测值吻合较好。<T_g or the squire root of the sum of the squires of the two if T_b>

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.