Mechanisms and processes of crust formation on artificial aggregates

Abstract

Assuming that the degree of aggregate disintegration or stability is affected by the initial moisture conditions, and that the uppermost zone of the soil surface, where a crust is formed, is subjected to the fluctuations of the moisture content under field conditions, the relations among the initial soil moisture conditions, aggregate stability and crust formation should be analyzed in order to elucidate the mechanisms and processes of crusting. Since rainfall is the major factor involved in the disintegration of soil aggregates at the surface through the slaking and impact actions, we determined the degree of aggregate disintegration with slaking and of crusting formed by simulated rainfall treatment as follows; intensity, 4.48 mm min−1; water drop diameter, 2.17 mm; rainfall duration, 2 and 5 min, using artificially granulated aggregates, 1.0 to 2.0 mm in diameter, under moisture conditions preliminarily adjusted at definite levels of matric suction, 0.98 kPa to 9.81 MPa. The results are summarized as follows: 1) Degree of aggregate disintegration and crust development was affected by the initial moisture conditions. 2) Inflection point, where abrupt change of aggregate disintegration occurred, appeared at a certain level of initial matric suction and was designated as critical slaking point (CSP). Above the CSP, the degree of disintegration by slaking was considerable, e.g. the contribution of slaking to the disintegration to the fraction of <0.25 mm in diameter was estimated to be 80% or more for three soils studied. 3) Inflection point of initial matric suction, where the degree of crusting abruptly changed, was designated as critical crusting point (CCP), and it appeared in the wetter range relatively to the CSP. And, 4) initial moisture conditions were divided into three ranges based on the CCP and the CSP. In the wet range of the initial moisture conditions, corresponding to suctions lower than the CCP, no slaking and little crusting occurred, in the dry range, suctions greater than the CSP, slaking and crusting occurred conspicuously, and in the moist range between the CCP and the CSP, crust formation occurred but slaking was hardly observed.