Freeze-thaw-induced variability of soil shear strength

More than 50 percent of the erosion in the Palouse region of eastern Washington and northern Idaho is related to frozen and thawing soil conditions. However, knowledge of soil physical properties such as soil strength under these conditions is extremely limited. Input of those time variant properties is urgently needed for physically based erosion models such as the USDA-ARS Water Erosion Prediction Project.; This study examined the feasibility of determining soil physical properties relevant to erosion as they change over the season under field conditions. Soil shear strength was measured with two Torvane shear devices, two pocket penetrometers, a modified Swedish fall-cone device and a Shearometer. Soil moisture content was determined gravimetrically and bulk density was determined by core sampling and compliant cavity techniques. Soil matric potential was determined using the filter paper method. Soil temperature was measured with thermocouples, frost depth was determined with a CRREL-Gandahl type frost gage and also by probing the soil with a knife.; The older metal Torvane device worked most satisfactorily under the conditions encountered, but methods to determine bulk density and soil matric potential did not perform well. Automated procedures are recommended to increase consistency and reduce labor requirements.; Shear strength ranged from 1.18 kPa during soil thawing to 14.33 kPa under dry soil conditions. Under sunny and windy conditions, thaw weakened soil regained most of its shear strength within hours after thaw. Shear strength was found to be linearly related to the moisture content of the top 10 mm of the soil profile. Rain and snowmelt did not have as drastic an effect on soil moisture content as did soil freezing. Soil moisture content during or immediately after rain or snowmelt did not exceed 25 percent by weight while the soil had moisture content of up to 58 percent while frozen and up to 44 percent while thawing.