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

自1964年新泻地震发生以来，饱和砂土的液化机理和土体在地震中的抗压能力逐渐得到明确。然而近年来，由于液化造成的大地震造成的破坏也有所报道，如1993年的北海道-南井地震、1995年的小古根-南武地震、2000年的鸟取县-西武地震等。在小古根南部地震中，从原海岸线到填海土地的广大地区都发生了大规模的液化，建筑物下的桩基发生了破坏。一些学者对桩基破坏的原因进行了研究。然而，横向荷载桩支撑建筑设计方法的建立仍存在许多不确定性，特别是考虑地震作用下孔隙水压力增大对桩周土体的有效应力减小。因此，我们设计并制作了相应的试验装置，利用渗流降低试验砂床的有效应力，并进行了横向加载试验，以研究作用在模型桩上的路基反力。从这些研究中发现了以下几点：1。在给定水平面上，桩上的横向路基反力近似为关于桩侧位移的双曲线。那么初始模量和双曲线的收敛性分别与路基侧向反力的初始系数和路基侧向反力的极限系数有关。路基侧向反力初始系数和极限值均随深度线性增大（即三角形分布），随砂层有效应力的减小而线性减小。从这些结果中，我们可以提出一种考虑地震时地面有效应力减小的基于地下反力分析的桩基设计新方法。在此基础上，提出了基于三维理论的群桩基础分析方法，考虑了弹塑性连续体和桩周土的极限侧阻力。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