MEASUREMENT OF AGGREGATE BREAKDOWN UNDER RAIN - COMPARISON WITH TESTSOF WATER STABILITY AND RELATIONSHIPS WITH FIELD-MEASUREMENTS OF INFILTRATION

This paper reports comparisons between aggregate breakdown on wetting by rainfall with breakdown measured by a range of alternative methods. It also reports correlations between measured breakdown and steady in filtration rates of simulated rain of high and low energy, and hydraul ic conductivities of surface seal layers formed under high energy rain . A wide range of soils in eastern Australia were studied. Highly sign ificant correlations were found between measurements of aggregate brea kdown to < 125 mu m caused by rainfall wetting and both steady infiltr ation rates and hydraulic conductivities. Significant, but poorer corr elations were found between steady infiltration rates and breakdown re sulting from immersion wetting. Deletion of swelling soils from the da ta set greatly improved correlations between steady infiltration rates of high energy rain and breakdown measured by both immersion and tens ion wetting, showing that these methods of wetting ace particularly in appropriate for swelling soils. No correlation was found between infil tration rates and measured clay dispersion. Different relationships be tween the proportion of particles (%) < 125 mu m at the soil surface ( P-125) and steady infiltration rates of low and high energy rain indic ated that compaction of the soil surface layer, rather than increased aggregate breakdown, is a major cause of surface sealing by raindrop i mpacts. Measurements of fall cone penetration confirmed that drop impa cts had compacted the surface layer. Suctions across the surface seal were related to P-125 in that layer, and the relationship obtained was used in calculating hydraulic conductivities. The results confirm tha t measurement of aggregate breakdown under rainfall wetting produces r esults of much greater relevance to soil behaviour under field conditi ons than do tests based on immersion and tension wetting.