Experimental study on effects of lateral soil flow on group pile and its application to design procedure

在1-G振动台上进行了不同群桩的模型试验，该群桩埋设在液化的斜坡地基中。与前人采用4根或9根成桩的群桩试验不同，本研究采用36根或121根成正方形的桩，对模型进行不同强度和频率的振动，记录几根桩沿桩长方向的弯矩。此外，还利用埋置位移传感器的数据以及地表土标的变形来研究地表的侧向位移。还记录了加速度和孔压。结果表明，液化后土体沿下坡方向流动，桩周变形较大。这种大变形使得常规有限元分析桩土相互作用变得非常困难，甚至不合适。对弯矩数据的解释表明，弯矩的大小(以及由此产生的侧向土压力)在桩基础的上下游两端出现最大值。这意味着上游方向桩的存在降低了下游方向的土压力，这被称为阴影效应，而在下游端，由于桩阻止土壤流动而产生的地面高程差在端桩上产生了显著的侧向土压力。阴影效应随桩间距的不同而不同，间距越小，阴影效应越明显。最后，提出了既有群桩基础的加固措施。这就是在基础的上下游两侧安装额外的桩。液化土流产生的土压力由增加的桩承担，减轻了既有桩和重要桩的荷载。这些发现也在一个数值模型中进行了检验，在该模型中，液化沙子被建模为粘性液体。

Model tests were conducted on a 1-G shaking table with a variety of group pile which was embedded in a liquefied sloping ground. Different from foregoing group pile tests with 4 or 9 constituent piles, the present study employed 36 or 121 piles in a square configuration.Upon shaking the model with different intensities and frequencies, the bending moment was recorded along the length of piles at several piles. Moreover, lateral ground displacement was studied by using data of embedded displacement transducers as well as distortion of soil markers in the ground surface. Records were taken of acceleration and pore water pressure as well. It was revealed that soil flows in the downslope direction upon liquefaction and large deformation occurs around piles. This large deformation makes it very difficult or even inappropriate to analyze the pile-soil interaction by conventional FEM analysis.The interpretation of bending moment data revealed that the magnitude of bending moment (and consequently the lateral earth pressure) takes the maximum value at the upstream and downstream ends of the pile foundation. This implies that the existence of piles in the upstream direction reduces the earth pressure in the downstream direction, which is called the Shadow Effects, and that, in the downstream end, the ground elevation difference, which is produced by the prevented soil flow by piles, creates significant lateral earth pressure on end piles. The shadow effects vary with the pile spacing ; the narrower spacing makes the shadow effects more remarkable.Finally, a retrofitting measure for existing group foundation was proposed. That is installation of additional piles on upstream and downstream sides of a foundation. The earth pressure due to flow of liquefied soil is borne by those additional piles and the load on the existing and important piles is mitigated. These findings were examined in a numerical model as well in which liquefied sand is modeled as a viscous liquid.