Direct shear behaviour of dry, granular soils for low normal stress with application to lightweight robotic vehicle modelling

Modelling of soil shearing behaviour under wheeled or tracked vehicles requires the knowledge of three soil properties: cohesion, angle of i nternal friction, and shear modulus. For lightweight robots it is necessary to characterize the soil for small normal stress (<15kPa) while most of the data collected in the literature regards higher stress testing conditions. Soil failure at l ow stress may diverge from Mohr-Coulomb envelope invalidating the fundamental assumptions behind classical terramechanics approach. Through the analysis of direct shearing performance of a dry, granular, soil, this paper address several is sues related to off-road traction mechanics (not necessarily limited to low stress c ases). We present an improved approach for shear modulus calculation that overcome the inaccuracies introduced by Wong method. We analyze the importance of density in modifying terrain response. Moreover, we show how erroneous estimation of soil cohesion and angle of internal friction may limit the applicability o f Bekker/Wong theory to lightweight tracked vehicles.