On soil behaviour during field traffic

This thesis summarizes and discusses results of subsoil stress, strain and strength measurements during field traffic with several load intensities on five sites in Sweden during the growing season. Vertical subsoil stress and displacement were measured by installing sensors horizontally 1 m into the soil at 0.3, 0.5 and 0.7 m depth through holes drilled from a pit whereby the soil flanking the sensors was left undisturbed. The soil precompression stress was determined on each occasion by sequential loading of soil samples in the laboratory. The topsoil properties had a large impact on the vertical subsoil stress, and the concentration of stress below a loaded surface appears to be affected by topsoil characteristics quite different than normally anticipated. The stress was clearly transmitted more directly and undiminished in relatively dry soil than in relatively wet soil. The precompression stress as determined by classical, uniaxial compression tests was a poor measure of soil ability to sustain non-recoverable deformation. The study showed that plastic deformation occurred at stress levels far below that predicted by the precompression stress. A linear correlation between elastic (recoverable) and plastic (non-recoverable) soil deformation indicates that soil should be regarded as an assemblage of soil elements with a stochastic distribution of the strength of the contact points. The practical implication of the observed soil behaviour is that the subsoil appears to be vulnerable to plastic (non-recoverable) deformation at lower wheel loads than normally anticipated. Furthermore, it is important that the stress in the contact area is as evenly distributed as possible to avoid unnecessarily high peak stresses. This calls for relatively fast and reliable methods that enable the land-users to optimise the stress distribution below tyres or tracks for specific soil types, water content and field operation. Furthermore, the results suggests an urgent need for engineering developments in vehicles with small load intensities.