Measurement of soil aggregate settling velocities .II. Sensitivity to sample moisture content and implications for studies of structural stability

Abstract

The importance to settling velocity of initial sample moisture content of non-immersed sediment is presented for a well-aggregated, cracking clay soil (a Vertisol). Eroded sediment was analysed immediately following deposition and after drying to various moisture contents. Settling velocity distributions were measured using the modified bottom withdrawal tube method. Initial aggregate settling velocities are not retained if sediment is allowed to dry to pore water suctions greater than 1.5 MPa prior to analysis, which involves immersion. Coarse aggregate velocities are reduced, a result attributed to slaking, while fine aggregate velocities are increased, a result attributed to binding of dispersed clay during drying with reduced re-dispersion upon immersion. Exposure of air-dry sediment to a saturated atmosphere for varying lengths of time can improve agreement with initial levels of aggregation, but the improvement varies between different samples of this Vertisol. The settling velocity distribution of a sediment is shown to be an indicator of aggregate stability to wetting. Combination of a useful pretreatment of air-dry sediment, which affords improved reproducibility, with the recognized low technique variability given by settling tubes, is demonstrated to be a useful method for comparative studies of structural stability.