Theoretical principles for modelling soil strength and wheeling effects - a review

Physical and mechanical processes in structured unsaturated soils are very complex and are still more difficult to understand or to predict if tillage processes and plant growth effects are included. Based on the definitions of stresses, strain and combined processes, mathematical models will be defined in the literature review and some proofs will be given. The stress‐strain‐failure processes are non‐linear and irreversible or hysteretic. They should be coupled as the various processes are not independent of each other. If mechanical properties and processes in structured unsaturated soils are considered, the stress and strain tensors have to be quantified by the combined application of Stress State Transducers (SST) and Displacement Transducer Systems (DTS) in order to derive the stress and strain components in soils. Depending on soil development or tillage system both stress and strain components vary to a great extent and result in differing physical and ecological soil properties. Each soil deformation has to be coupled to hydraulic properties, because deformation and failure in soils depend on the advection and diffusion processes and their history and vice versa. These coupled, non‐linear and hysteretic processes can be modelled using modern numerical methods, such as the finite element method (FEM). A brief description of the modelling procedures is given in this paper. Empirical models are available to define soil strength, stress distribution and changes in physical properties due to loading or loosening, their results can be primarily used as a first rough estimation e.g. for mapping purposes. However, the theoretical modelling procedures have now advanced beyond our understanding of these processes and our ability to measure the material parameters and the field boundary conditions. This paper describes some of these problems such as the coupled nonlinear irreversible behaviour of unsaturated structured soils. In addition the FEM approach as the most applicable instrument can be used to predict changes in ecological properties like permeability, moisture characteristics and failure/yield behaviour of soils. Modellierung von Bodendeformationen —mechanische und modelltheoretische Grundlagen und Anwendungsbeispiele Physikalische und mechanische Prozesse in strukturierten ungesättigten Böden sind sehr komplex, wozu in besonderem Maße auch Einflüsse der Bodenbewirtschaftung und Pflanzen/Boden‐Interaktionen beitragen. Aufbauend auf der Beschreibung