Explanation of Seepage Failure Mechanism at Steep Slope of Shirasu Soil

对于沙拉苏(火山碎屑流堆积)陡坡的渗流破坏机理，还需要对原状样品进行进一步的研究。然而，通过扰动试件的管道试验可以得出以下结论：土壤颗粒可以在骨架结构的空间内迁移，如果受到足够的水力坡度，土壤颗粒可以从坡面上被冲走。这是渗透破坏的第一阶段。为了判断石拉苏土是否具有内部稳定性，本研究提出了分形维D≦为2.4的石拉苏土是一种稳定状态，对应于均匀系数UC≧2 0和稳定性指数H/F≦1.3，而D≧2.4的稳定状态是不稳定的，其中D来源于颗粒分布曲线的形状。根据这一新的限制，Shirasu的五分之三是不稳定的国家。在这种情况下，细粒的强烈管涌发生在经典管涌理论所给出的临界坡度的1/2~2/3的坡度处，因此，悬崖的破坏过程可以分为以下几个过程：细粒被冲刷掉，管涌形成小的渗漏孔。之后，随着孔底的侵蚀，孔洞扩大，孔洞周围呈云母状的薄片脱落。然后，它上方的部分突然因自身重量而下滑。

To clarily the mechanism of seepage failure at the steep slope of Shrasu (pyroclastic flow deposit), further works are required to study on undisturbed samples. However, the following conclusion can be drawn from piping tests of disturbed samples.Soil particles can migrate within the interstics of a skelton structure, and can be washed out from the surface of slope if subjected to a sufficient hydraulic gradient. This is the first stage of seepage failure. A soil will be described as having stable gradation if it does not lose the particles, whereas it has an unstable gradation if particles can casily be lost.To determine whether Shirasu soil was on internal stability or not, the useful limit was proposed in this research that Shirasu soil having a fractal dimension D≦2.4 was a stable state coresponding to unifomity coefficient Uc≧20 and stability index H/F≦1.3 and these having D≧2.4 was liable to be unstable, where D was derived from the shape of the grain-size distribution curve. According to this new limit, third fifth of Shirasu were unstable states. In this condition, the strong piping of fines took place at gradients of almost one half to two third of the critical gradient given by the clasical piping theory.Therefore the process of failure at the cliff can be smmurized as follows ; the fines are washed out and the small seepage hole is formed by piping. After that, the hole is enlarged with the erosion at the bottom of the holes as coming off in thin section like mica around the hole. And then the part above it slides suddenly down by its own weight.