Post Freezing Age Hardening Effects On Water Stability of Soil Aggregates.

The freezing process is commonly implicated as a key factor in defining the state of soil structural stability following the winter months.  However, the conditions experienced throughout the over-winter period include an amalgam of processes including freezing, thawing, wetting and drying.  The specific combination of all these processes more likely dictates the net structural effect of the over-winter period than the dominance of any single process. Of particular note is the “cohesion recovery” noted during non-freezing periods.  The objective of the study was to assess the extent of age hardening as reflected in aggregate stability during thaw periods of increasing duration following freezing conditions. Naturally occurring 1-2 mm aggregates from soils of varying clay content (0.11 kg kg⁻¹, 0.33 kg kg⁻¹, 0.44 kg kg⁻¹), and an initial water content of 0.20 kg kg^-1, were subjected to 12 hr of freezing at -15°C followed by thawing at +15°C for periods of 12, 36, 60 and 84 hr. Post-treatment aggregate stability determination was via wet aggregate stability (WAS) and dispersible clay (DC).  While freezing had a negative effect on aggregate stability as measured by both WAS and DC, the length of incubation time in the thawed state was found to be significant in the case of both WAS and DC.  Positive slopes of these measures against time thawed were significantly different from zero thus indicating a slow age hardening with time.  Longer thawed incubation times may facilitate improved structural stability through processes including the redistribution of intra-aggregate pore water following freezing induced pore water migration to the aggregate surface, the reactivation of cementing materials and the re-establishment of particle to particle contacts disrupted by the processes associated with freezing.  Soil type was found to be a significant factor with increases in aggregate stability being more pronounced in the soils with higher clay contents.