Army maneuvers compact and rut soils on training lands thereby increasing the likelihood of hillslope runoff, concentrated flows over the land surface and soil erosion. Research results conflict regarding the efficacy of soil freeze-thaw (FT) in reducing soil compaction and are absent regarding FT effects on vehicular ruts. My objective is to determine the effectiveness of FT in changing infiltration into compacted soil, the cross-sectional shape and roughness of vehicular ruts and the water-carrying capacity of ruts which often reduces the tractive force of flows in the ruts.
Based on initial experiments last year, I built four soil bins to conduct controlled FT experiments in the CRREL's Frost Effects Research Facility (FERF). The same Hanover silt at a water content of 22-23% (by volume) was placed in each bin to achieve similar soil density conditions. Thermocouples, resistivity gauges and Vitel probes were installed in each bin to measure soil temperature, frost depth and moisture every three hours. We rutted the soil in the bins by making multiple passes with a pickup truck. Soil density, penetrometer resistance, infiltration and soil shear strength in and out of the ruts were measured to determine differences between the disturbed and undisturbed soil. We then froze the soil to about 13" from 29 February to 19 March 1996 when we started the first thaw cycle. Soil surface profiles across the ruts were measured before and after the freeze cycle to determine soil surface microrelief, rut cross-sectional shape and the amount of frost heave.
The instruments in the bins are operating properly and results after one FT cycle show that: 1) rut soil starts to freeze and thaw later than adjacent, uncompacted soil; 2) once the rutted soil starts to freeze, it freezes faster than the unrutted soil; 3) the rutted soil heaved more than the unrutted; 4) the infiltration rate in that ruts increased after one FT cycle; and, 5) the shape and roughness of the ruts and penetration resistance and shear strength of the rut soil remain virtually unchanged after one FT cycle. Additional FT cycles are underway.
Future experiments will investigate the response of rutted and compacted soils in different soil types at various soil water contents exposed to different FT rates. Our results will be used to modify the winter processes, hydrology, soils and hydraulics of overland flow components of the WEPP soil erosion model for application on training lands.
Lawrence W. Gatto U.S. Army Cold Regions Research and Engineering Laboratory 72 Lyme Rd. Hanover, NH 03755-1290 Phone: (603) 646-4273 Fax: (603) 646-4785 E-mail: lgatto@hanover-crrel.army.mil