Gradual distribution of deteriorated Shirasu (dacitic pyroclastic flow deposits) composing steep slopes in southern Kyushu, Japan

九州南部第四纪火山碎屑流沉积的陡坡上发生了许多边坡破坏。该矿床当地称为“白苏”，为均质无焊接的英质凝灰岩，软质、渗透性强。根据这种类型的重复破坏，可以理解为由于岩石强度的时间降低而导致的发生。为了了解边坡内部强度降低的类型，作者试图测量几个边坡从边坡表面到内部的劣化分布。采用硬度计测透硬度作为劣化等级的指标。用x射线衍射法对粘土矿化进行了粒度分布筛分试验和干密度测定。结果表明，硬度分布呈现由表面向内逐渐变化的趋势。坡顶区域离坡面7 ~ 8米，而距坡脚仅1 ~ 2米。这可以理解为每个地点的风化和侵蚀的结合。这种矿床的变质过程不仅包括颗粒的崩解，还包括粘土矿化等化学变化。这些分布可以控制边坡破坏的体积大小和重现时间。从这些分布估计的退化的时间过程也可能支持其中一位作者提出的退化的数学模型。

Many slope failures have occurred on steep slopes composed of the Quaternary pyroclastic flow deposits in southern Kyushu. The deposits called as "Shirasu" in local name, are homogeneous non-welded dacitic tuff, and are soft and permeable. Based on the recurrence failure in this type, the occurrence can be understood to have been brought about due to temporal reducing in strength of rocks. To understand the style of reducing in strength within the slopes, the authors tried to measure the deterioration distribution from the slope surface to inward portion in several slopes in there. Penetrative hardness by hardness meter is used as the index of grade of deterioration. Sieving test for grain-size distribution and measuring of dry densities have also made with the X-ray diffraction for clay mineralization.As a result, distribution of hardness shows gradual changes from the surface to inward portion in the slopes. While deteriorated zone from the slope surface attains to 7 to 8 meters in top region, it is only 1 to 2 meter in foot of the slope. This can be understood as the combination between weathering and erosion in each locations. The process of deteriorating in this deposits may include not only disintegration of grains but also chemical changes such as clay mineralizarion. These distribution in slope surface may control the amount of volume for a slope failure and its recurrence time. The temporal process of deteriorating estimated from these distribution may also support a mathematical model on deterioration proposed by one of the authors.