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Tracked and wheeled vehicles used during military training compact and rut soils. Compaction reduces infiltration and hydraulic conductivity in soils, thereby increasing the volume and residence time of surface water runoff. Vehicle ruts channelize runoff and increase its sediment transport capacity. Ice that forms in soil voids during the freezing process pushes soil grains apart, reducing particle cohesion and soil strength, making a soil more erodible. We completed 18 experiments to quantify differences in soil erodibility and rill development in simulated ruts due to freezethaw (FT) cycling of frost-susceptible silt.

For each experiment, we prepared two identical soil bins, one as a control to remain unfrozen, the other to be frozen and thawed. We tested three soilwater ranges, 1518%, 2628%, and 3638% (saturated soil) by volume; three flow rates, 0.2, 0.6, and 1.2 gpm; and two slopes, 8 and 15. We measured sediment losses from and the cross-sectional geometry of the in-rut rills developed in both bins during the experiments. The average width of the in-rut rills in the control and FT bins was similar, but rill depth was 2 cm greater in the FT bin when the soil moisture was 1518%. The rills developed in the FT bins when the soil moisture was 3638% ranged from 2 to 10 times larger than in the control bin. Comparing the total sediment mass contained in runoff samples from the soil bins provides an integrated measure for each series of tests. At low soil moisture, the sediment mass from FT samples exceeded that of corresponding control samples by 39%. For mid-range soil moisture the mass contained in the FT samples exceeded that of the controls by a factor of 2.9, and at high soil moisture this factor increased to 6.2. The differences in rates and quantity of soil eroded increased dramatically with the water content as a result of the FT cycle. These are the first results that quantitatively define the differences in sediment loss and in-rut rill formation caused by FT cycling. We will complete analyses of the data from these 18 experiments and perform additional experiments to further define the effects of FT in the soil-erosion process and to establish the importance of accurately modeling FT in erosion models.