Susan R. Bigl

Moisture migration during freeze and thaw of unsaturated soils: modeling and large scale experiments

Abstract A coupled heat flow and moisture flow model (FROSTB) was used to simulate large-scale freeze-thaw experiments to assess its ability to predict soil moisture conditions. The experimental data consist of temperature and soil moisture profiles measured during freeze-thaw cycles in a 1-m layer of frost-susceptible silty sand over roughly 2 m of gravelly sand. Two experimental conditions were modeled: (1) where the soil was fairly wet and the water table was shallow (1 m below surface), and (2) where the soil moisture was lower than specific retention and water table was deep. Overall, the model predicts the frost penetration and heave quite well; however, it tends to overpredict ice formation. The additional ice in the modeled frozen soil then causes a slower thaw. Matching the (total) moisture contents in the drier soils results in underpredicting the heave. The discrepancies stem from the model representation of the physical processes involved in freezing (and frost heave) of unsaturated soil. We propose improvements through using a “pseudo” three-phase flow potential and calculating volumetric segrated ice content starting at 90% of saturation. The effects of changing the constants related to hydrologic properties are also discussed.

Condition of in situ unexploded ordnance

Unexploded ordnance (UXO) become point contamination sources when their casings fail and their explosive fill dissolve. To determine the modes of failure, we documented the condition of UXO found on military training ranges and sampled soils for explosives beneath 42 in situ UXO. We found that oxidation caused the metal UXO casings to swell and fail catastrophically. Unlike previous work, pitting of the metal casings was not found to be an important release route for explosives. Of the 42 UXO sampled, eight were leaking explosives into the soil and of these, four had perforated or cracked casings, three were corroded and one was a partially detonated round. We estimated a surface density of 74 UXO per hectare for a subset of UXO sampled. We used the relative concentrations of explosives and their transformation products in the soil to determine if the explosives had recently dissolved or were from past military training.

Statistical Significance of Freeze–Thaw Related Factors on Cumulative Damage to Flexible Pavements

Although it is widely recognized that trafficking on thaw-weakened pavement structures significantly accelerates pavement failure in seasonal frost areas, contributions of specific freeze season characteristics, such as freeze season length, thaw depth, and depth to water table, are not as readily identified or quantified. To identify statistically significant contributing factors or combinations of factors, computer simulations were conducted with a seasonal mechanistic design and evaluation procedure and 21 years of environmental data from two original flexible pavement test cells at the Minnesota Road Research Project. Statistical analyses were conducted on various winter season characteristics to determine which characteristics or explanatory variables were statistically significant contributors to, or indicators of, cumulative pavement damage. Analyses suggest damage could generally be modeled as a function of one, two, or three explanatory variables. Thaw depth, or a variation thereof, was a significant explanatory variable for at least one regression equation corresponding to each damage model in both test cells evaluated. The combination of the surface freezing season length and corresponding average pavement surface temperature appears significant for fatigue cracking for the two test cells analyzed. Maximum thaw depth and the combination of surface freezing season length and air freezing index appear significant with regard to rutting, according to Asphalt Institute and U.S. Army Corps of Engineers criteria. Several additional characteristics of the winter season have been identified for inclusion in similar analyses to be conducted with the Enhanced Integrated Climatic Model and nine U.S. Department of Agriculture Forest Service–New Hampshire Department of Transportation pavement test sections. Such characteristics, once better defined, could provide useful information for design or evaluation purposes.