CHANGES IN MICROSTRUCTURE, VOIDS AND B-FABRIC OF SURFACE SAMPLES OF AVERTISOL CAUSED BY WET DRY CYCLES/

A finely structured (self-mulching) Vertisol from Zimbabwe was sampled to 150 mm (bulk samples) or 30 mm (undisturbed samples) in order to p rovide material for an investigation of the effects of wet/dry cycles on microstructure. Bulk samples were worked in the plastic state and t hen taken through a number of wet/dry cycles using flood, slow and fas t capillary and simulated rainfall wetting. Thin sections were produce d from the resulting clods/aggregates at different stages during the c ycles and changes in microstructure, voids and b-fabric were examined and compared to undisturbed samples of the field soil. The initial wor ked soil had a dense, massive structure but developed complex, blocky, crumb and platy structures during wetting and drying. Fast capillary wetting produced the finest structure in comparison to the other three wetting methods. Observable porosity (pore widths > 5 mu m), at 100 x magnification in horizontal thin sections, increased from 2.5% in the moist worked soil to 23.9, 13.7, 12.8 and 12.3% in the fast capillary , rainfall, slow capillary and flood wetted soils, respectively, after one wet/dry cycle. Tensile strengths of dry soils followed the conver se ranking, decreasing as observable porosity increased. The shape of voids did not have a significant effect on strength. The worked soil s howed predominantly planar voids but compound voids became dominant af ter wetting by all four methods. Changes in b-fabric during wetting an d drying showed no clear trends apart from a general decrease in the p roportion of oriented micromass. The b-fabric was commonly mosaic to s tipple speckled with granostriations. Porostriations were rarely obser ved. The field soil showed ultrafine to fine crumb structure with mosa ic to stipple speckled b-fabric and an observable void space at 100 X magnification of 53%. This structure was approached most closely by th e fast capillary wetted soil, indicating that a fine porous structure may be rapidly regenerated from an unfavourable dense structure throug h this method of wetting. (C) 1998 Elsevier Science B.V. All rights re served.