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物构成的陡坡发生了许多边坡失稳。当地称之为“Shirasu”的矿床是均质的非焊接英安质凝灰岩，柔软且可渗透。根据这种类型的复发性破坏，可以理解这种情况是由于岩石强度的暂时降低而引起的。为了了解边坡内部强度衰减的形式，作者尝试测量了该地区几个边坡从坡面到内部的劣化分布。使用硬度计的渗透硬度作为劣化等级的指标。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，硬度分布从坡面表层向内部呈渐变趋势。坡顶地区的退化带距坡面可达7 ~ 8 m，而坡脚仅1 ~ 2 m。这可以理解为每个地点的风化和侵蚀之间的组合。这种矿床的变质过程不仅包括颗粒的崩解，而且还包括粘土矿化等化学变化。坡面上的这些分布可以控制边坡破坏的体积量和复发时间。由这些分布估计的劣化的时间过程也可以支持由作者之一提出的劣化的数学模型。

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.