Weathering process of pyroclastic flow deposits from slope surface and occurrence of slope failures

In general, slope failures tend to occur repeatedly. Recurrent failures may be influenced by the proceeding of weathering within slopes. To obtain the weathering grade and its spatial extents, the distribution of penetrative hardness were measured in some slopes composed of Quaternary pyroclastic flow deposits in Japan. Distinct weathered zones expressed by reducing of hardness are recognized along the slopes, and the degree tends to decrease gradually from surface inward. While a weathered zone attains up to 7 to 8 meters at high elevation of slopes, it is only 1 to 2 meters at foot of the slope. This difference can be attributed to the balance between weathering and erosion at each position. The estimated rate of weathering of 10(-2) to 10(0) cm/year is far greater than those of other rocks. Based on these values, it may become possible to estimate the recurrence time and amount of failed materials on any slopes.