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

摘要

Weathering of rocks on steep slopes influencesslope instability and sometimes leads o slope failure during heavyrainfall. To determine the weathering intensity of hillslope rocksand its inward change from the slope surface, measurements of thepenetrative hardness of rocks, which correlates with their strength,were made in some slopes composed of soft pyroclastic rocks onKyushu Island, Japan. A gradual reduction in hardness from theinterior to the slope surface was recognized within slopes. Thehardness change was almost concordant with changes in otherweathering indices, such as rock color, clay minerals and apparent grain size.The weathered zone indicated by the reduction in hardness was1-2m thick at the foot of steep slopes, which are frequentlysubjected to gullying and surface failures, whereas the zone was 7-8m thick at slope summits, which are gentle and stable for a longperiod, Although the inward change in hardness was gradual on allparts of a slope, the change was marked on the foot-slope, wherethe strength contrast between weathered and non-weathered rockwas greater. The thickness of the weathered zone on the foot-slope,therefore, controls the volume of failed materials when a failureoccurs. The difference in weathered thickness between the foot andthe summit of a slope much affects the style of failure in general,and consequently shapes the characteristics configuration of slopesin this region. The estimated weathering rate on a slope surface based onhardness 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 otherrocks. Therefore, the frequent occurrence of slope failures in thisregion was attributed to this rapid weathering.