QUANTIFYING THE SOIL FREEZING CHARACTERISTIC CURVE IN LABORATORY AND FIELD SOILS

Abstract

The soil freezing characteristic curve (SFC) controls the hydraulic properties of soils and is especially crucial in understanding snowmelt infiltration and runoff, frost heave formation and thawing settlement in frozen soils. The SFC can be modelled by combining information from the soil moisture characteristic curve of unfrozen soils (SMC) with the Generalized Clapeyron Equation (GCE). While such an approach is straightforward and involves no additional free parameters, the resulting SFC is not always consistent with those observed in the laboratory and field. This study was therefore designed to obtain both laboratory and field data that quantifies the SMC and SFC for different soil textures and salinities and to compare the results with those obtained from the GCE and other alternative relationships.

In the laboratory, the SMC of a silica sand was measured using a Hydraulic Property Analyzer (HYPROP). The SFC of the same sand was measured using a series of column experiments with controlled total water content and pore-water salinity. In the field, data were collected from the St Denis National Wildlife Area (SDN), a mixed grassland cropped site in the Canadian prairies in Saskatchewan and the Boreal Ecosystem Research and Monitoring Sites (BERMS) Old Jack Pine (OJP) site in Saskatchewan, Canada. Three alternative models for the SFC were developed (capillary, salt exclusion, and the combined capillary salt models), and compared with observed data from the laboratory and field experiments.

The results show that the column experiments were successful in producing well-defined SFCs that matched expectations, where the form of the decrease in liquid water content with temperature was similar to the form of the SMC. Increasing the salinity resulted in enhanced freezing point depression as was expected. The field SFCs followed the same trend as those measured in the laboratory. The modelling results suggest that salinity is a dominant control on the SFC in real soils and that the combined capillary salt model is the most realistic of the three models considered in predicting liquid water content in frozen soils.