Explanation of Seepage Failure Mechanism at Steep Slope of Shirasu Soil

为了弄清Shrasu火山碎屑流存款陡坡的渗透破坏机理，需要进一步对原状样品进行研究。然而，从扰动样品的管涌试验中可以得出以下结论：土壤颗粒可以在一个多孔结构的间隙内迁移，如果受到足够的水力梯度，可以从边坡表面冲刷出来。这是渗透破坏的第一阶段。一种土如果不损失颗粒，则被描述为具有稳定的级配，而如果颗粒容易损失，则具有不稳定的级配。为了确定白须土是否处于内部稳定性，根据均匀度系数Uc = 20和稳定性指数H/F ≤ 1.3和D ≤ 2.4的颗粒容易不稳定，其中D是由粒度分布曲线的形状得出的。根据这个新的限制，白洲的三分之一是不稳定的国家。在此条件下，在约为经典管涌理论临界坡度的1/2 ~ 2/3的坡度处，发生了强烈的细颗粒管涌，因此，崖体的破坏过程可归纳为：细颗粒被冲刷，管涌形成小渗孔。之后，孔扩大，孔底部的侵蚀，如在孔周围的云母薄片脱落。然后上面的部分由于自身重量突然下滑。

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.