Weathering distribution in a steep slope of soft pyroclastic rocks as an indicator of slope instability

Weathering of rocks on steep slopes influences slope instability and sometimes leads to slope failure during heavy rainfall. To determine the weathering intensity of hillslope rocks and its inward change from the slope surface, measurements of the penetrative hardness of rocks, which correlates with their strength, were made in some slopes composed of soft pyroclastic rocks on Kyushu Island, Japan. A gradual reduction in hardness from the interior to the slope surface was recognized within slopes. The hardness change was almost concordant with changes in other weathering indices, such as rock color, clay minerals and apparent grain size. athered zone indicated by the reduction in hardness was 1–2 m thick at the foot of steep slopes, which are frequently subjected to gullying and surface failures, whereas the zone was 7–8 m thick at slope summits, which are gentle and stable for a long period. Although the inward change in hardness was gradual on all parts of a slope, the change was marked on the foot-slope, where the strength contrast between weathered and non-weathered rock was greater. The thickness of the weathered zone on the foot-slope, therefore, controls the volume of failed materials when a failure occurs. The difference in weathered thickness between the foot and the summit of a slope much affects the style of failure in general, and consequently shapes the characteristic configuration of slopes in this region. timated weathering rate on a slope surface based on hardness distribution was on the order of 10−2 to 100 cm year−1, assuming a constant rate. This rate is far greater than that of other rocks. Therefore, the frequent occurrence of slope failures in this region was attributed to this rapid weathering.