Measurement of Stress Paths under Agricultural Vehicles and their Interpretation in Critical State Space

Abstract

Soil behaviour depends on the stress state of the soil and for the application of some soil compaction models, including the critical state model, knowledge of the stress path or the relationship between the generalized deviatoric stress q and the mean normal stress p is required. Stress paths under agricultural vehicles commonly used in the Australian cotton industry were measured using a soil stress state transducer, and found to be dependent on the horizontal and vertical position of the transducer relative to the tyre. The paths were interpreted in terms of critical state soil mechanics and were in agreement with expected and observed changes in void ratios under a tyre. Stress paths observed in a heavy clay field soil traced the Hvorslev surface, along a constant volume plane due to the near-saturated and incompressible nature of the soil. However, stress paths obtained in soil bins filled with unsaturated and compressible lighter textured soil appear to trace the Roscoe surface along a constant stress ratio plane. The stress path alone is not sufficient to predict soil behaviour; a knowledge of the initial conditions indicated by for example the preconsolidation stress is essential. The magnitude and direction of the major principal stress under a moving tyre were determined. It was shown that the horizontal and shear stress components of the stress state under a stationary tyre are much smaller than those under a moving tyre. An increase in speed had only a small effect on the magnitudes of the stresses and stress paths. Similar results were found for an increase in the draught force, generated by pulling a small cultivator.