Effect of the freeze/thaw process on the structural stability of soil aggregates
This thesis is an investigation of the action of the freeze/thaw process on the structural stability of natural soil aggregates from soils of different textures. Aggregates of 1-2 mm diameter from a sandy loam (0.11 kg kg -1 clay), a clay loam (0.33 kg kg-1 clay) and a clay (0.44 kg kg-1 clay) at water contents of 0.10 kg kg -1, 0.20 kg kg-1 and 0.30 kg kg-1 were subjected to a single freeze/thaw cycle. The cycle consisted of 12 hours of three dimensional, rapid, closed system freezing to -15°C and 12 hours of thawing to +15°C. Aggregates from all three soils exhibited decreased stabilities over initial unfrozen values as indicated by both lower wet aggregate stabilities (WAS) and higher values of dispersible clay (DC) following freeze thaw. Aggregate stability following freeze/thaw was dependent on both the clay content of the soil and the water content at the time of freezing. Higher clay content resulted in significantly greater WAS values and significantly lower DC values than for lower clay content. Freezing and thawing at higher water contents resulted in significantly lower stabilities as measured by decreased WAS and increased DC. This is in contrast with information from the literature which indicates that aggregate stability can improve when frozen at low water contents as a result of freezing induced desiccation of the aggregates as ice crystals grow in inter-aggregat epore spaces. Freeze drying, where the effects of the presence of liquid water following thawing are removed, resulted in improved aggregate stabilities at all water contents and for all soils as measured by increased WAS and decreased DC. The rapid freezing rate was surmised to be the cause of the difference by negating freezing-induced water migration. The effect of load pressure during freezing and thawing decreased aggregate stabilities as measured by decreased WAS and increased DC as compared to those resulting from freezing and thawing at zero load due to the reduction of the pore volume. A thermodynamic model is proposed to predict the effects of freezing and thawing under load.