A Study on the Deformation Characteristics of Foundation Ground reinforced with Micropiles.

研究目的在于阐明砂土地基上微型桩基础的承载机理。进行了一系列的模型荷载试验，其中的基础与一组微型桩加固受到垂直和倾斜荷载。对微型桩基础的性状进行了观测和对比分析。重点分析了砂土地基相对密度、微型桩长度、微型桩刚度、微型桩数量、微型桩倾角等因素对微型桩承载力的影响。试验结果表明，微型群桩对地基承载力的提高具有显著的作用。这种效果在稠密、湿润的地面上得到了强调。由于基础与群桩之间的相互作用，基础下的地基材料受到约束，围压和抗剪强度提高。为了充分发挥相互作用的约束效应，需要考虑地基材料的粘性、足够数量的微型桩和微型桩的抗弯刚度。在竖向荷载作用下，微型群桩对地基承载力的提高是在基础沉降量一定的情况下出现的。在倾斜荷载的情况下，由于缺乏基础的基底压力，在普通条件下不能认识到承载力的提高。试验结果表明，初始基底压力是稳定提高地基承载力的必要条件。因此，对微型桩施加预拉力，并使桩底压力相应升高，是微型桩基础抗震施工的有效措施。

The aim of this study is to clarify the mechanism of bearing capacity of Micropile Foundation on sand ground. A series of model loading tests were conducted, where the footing reinforced with a group of micropiles were subjected to vertical and inclined loads. The behavior of the Micropile Foundation was observed and comparatively examined. The effects of the relative density of sand ground, the length and stiffness of micropile, the number and inclination of micropiles on the bearing capacity were focused. The conclusion concerned with the interaction between footing and a group micropiles derived from the test results is summarized as follows ;- The significant effect of group micropile was recognized on the improvement of bearing capacity by the reinforcement of foundation ground with micropiles. The effect was emphasized in dense, dilatant ground.- Due to the interaction between footing and group micropiles, the ground material beneath the footing is confined, and the confining pressure and shear strength raised.- To mobilize the confining effect of the interaction, dilatancy of ground material, sufficient number of micropiles and bending stiffness of micropiles are dispensable.- In the case of vertical load, the improvement of bearing capacity by group micropiles appeared under the certain amount settlement of footing. In the case of inclined load, the improvement of bearing capacity was not recognized under the ordinary condition due to the lack of base pressure of footing.- It was found that initial base pressure is indispensable for the stable improvement of bearing capacity. Then the application of pre-tension on the micropiles and a complimentary rise in base pressure is effective measure for the construction of earthquake resistant micropile foundation.