Mechanisms and processes of crust formation on artificial aggregates

Abstract

Simulated rainfall treatment (intensity^4.48 mm min−1; drop diameter, 2.17 mm; falling height, 1.5 m;. and duration, 2, 5, 8, 12 min) was applied to three artificially granulated aggregates, 1.0 to 2.0 mm in diameter, the initial moisture content of which was adjusted preliminarily to definite levels of matric suction, 0.98 ×10−3 to 9.81 MPa. Degree of crusting was measured in terms of negative logarithm of hydraulic conductivity, —log K value (Tanaka et al. 1995: Soil Sci. Plant Nutr., 41, 263-273). The effects of slaking and impact of rain drops on crust development were estimated by using the —log K values obtained after simulated rainfall treatment for 2 and 5 min under different initial moisture conditions. The mechanisms and processes of crust formation under different moisture conditions were discussed. The results are summarized as follows: 1) The action of rainfall involved “impact” and “slaking” as the major factors. The slaking action was further divided into “weak slaking” and “rapid slaking,” and the contribution of each factor to the degree of crusting was calculated for a 5-min rainfall treatment. 2) The impact action was observed regardless of the moisture conditions, whereas the effects of weak and rapid slaking appeared when the initial moisture conditions were in a drier range. And, 3) the susceptibility of each soil to the above actions was different.