Thawing soil strength measurements for predicting vehicle performance

The CRREL Instrumented Vehicle (CIV), shear annulus, direct shear and triaxial compression devices were used to characterize the strength of thawed and thawing soil. Strength was evaluated in terms of the Mohr-Coulomb failure parameters c′ and φ′, which can be used in simple models to predict the tractive performance of vehicles. Use of an instrumented wheel (like those of the CIV) is proposed for terrain strength characterization for traction prediction because the conditions created by a tire slipping on a soil surface are exactly duplicated. The c′ and φ′ values from a portable shear annulus overpredict traction because of the curved nature of the soil failure envelope in the region of low normal stress applied by a portable annulus. Of all the tests, the direct shear test yielded the highest φ′ value, due to its slow deformation rate and drained conditions. The triaxial test produced results closest to those of the instrumented wheel. For all methods, φ′ increases with soil moisture but decreases rapidly beyond the liquid limit of the soil. The φ′ measured with the vehicle was also found to be strongly influenced by the freeze-thaw layering of the soil.