Development of Method for Estimating Lateral Resistance of Pile Groups in Liquefied Soil during an Earthquake

自1964年新泻地震以来，饱和砂土中的液化和土在地震中的抗力机理逐渐被弄清楚。然而，近年来，由大地震引起的液化破坏也有报道，例如1993年的北海道-南海地震，1995年的Hyogoken-Nambu地震，2000年的Tottori-Seibu地震等等。在Hyogoken-Nambu地震中，从原海岸线到填海造地的大范围内发生了大规模液化，破坏发生在建筑物下的桩基。一些研究人员已经对桩基破坏的原因进行了调查。然而，建立支撑建筑物的水平荷载桩的设计方法仍然存在许多不确定性，特别是在考虑地震时孔压增加导致桩周土体有效应力降低的情况下。因此，我们设计了和…制作了相应的渗流降低试验砂层有效应力的试验装置，并进行了水平加载试验，研究了模型桩上的路基反力。研究发现：1.某一水平的路基反力与桩的横向位移近似为双曲线。然后，双曲线的初始模数和收敛速度分别与地基横向反力的初始系数和地基的极限横向反力有关。2.地基横向反力的初始系数和极限值均随深度呈线性增加(即三角形分布)，随砂层中有效应力的减小而线性减小。在此基础上，提出了基于次荷载反应分析的桩基设计新方法，该方法考虑了地震作用下地基有效应力的降低。3.提出了基于三维理论的群桩基础分析，考虑了桩周土体的弹塑性连续体和极限侧摩阻力。较少

The mechanism of the liquefaction in saturated sand and soil resistance during an earthquake has been gradually made clear since 1964 when the Niigata earthquake occurred. In recent years, however, the damages due to the liquefaction caused by the large earthquakes have been reported, for example the Hokkaido-nansei-oki earthquake in 1993, the Hyogoken-nambu earthquake in 1995, the Tottori-seibu earthquake in 2000, and so on. In the Hyogoken-nambu earthquake, the large-scale liquefaction occurred in the wide area from the former coastline to the reclaimed land, and the damages occurred in the pile foundation under buildings. The cause of damages of the pile foundations has investigated by some researchers. However, many uncertainties to establish the design method for laterally loaded piles supporting a building still remain, especially when considering effective stress reduction in soil around piles due to pore water pressure increasing during an earthquake. Therefore, we devised and … More made the appropriate test apparatus for decreasing effective stress in the test sand bed by using the seepage flow, and performed laterally loading tests in order to investigate the subgrade reaction acting on the model pile.The following points are found out from these studies :1.The lateral subgrade reaction on the pile at a given level is approximated by the hyperbola with respect to the lateral displacement of the pile. Then the initial modulus and the convergence of the hyperbola are related to the initial coefficient of the lateral subgrade reaction and the ultimate lateral subgrade reaction, respectively.2.Both of the initial coefficient of lateral subgrade reaction and the ultimate value increase linearly with depth (i.e. triangle distribution), and decrease linearly with decreasing the effective stress in the sand bed. From these results, we are able to present the new design method of pile foundations on the basis of the subgread reaction analysis which considers the reduction of effective stress in ground during an earthquake.3.Furthermore, the analysis for the pile group foundation based on the three dimensional theory, which considers an elasto-plastic continuum and the ultimate lateral resistance of the soil around the piles, is also proposed. Less