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 ungesattigten Boden sind sehr komplex, wozu in besonderem Mase auch Einflusse der Bodenbewirtschaftung und Pflanzen/Boden-Interaktionen beitragen. Aufbauend auf der Beschreibung von Spannungs- und Deformationstensoren werden in dem Ubersichtsartikel unterschiedliche Modellansatze zur Quantifizierung von mechanischen und gekoppelten (mechanisch/hydraulischen) Prozessen beschrieben und anhand eines Beispiels validiert. Mechanische Kenngrosen z.B. Spannungen und Partikelbewegungen (= displacement) sind Tensorfunktionen und mussen daher im dreidimensionalen Raum mittels Stress-State-Sensoren (Transducern) (SST) bzw. mehreren Displacement-Transducer-Systemen (DTS) erfast werden. Das Spannungs-/Deformationsverhalten von Boden selbst ist nichtlinear, irreversibel und hysteretisch, wobei Bodendeformationen je nach den hydraulischen Leitfahigkeiten unterschiedlich schnell erfolgen. Folglich mussen die entsprechenden Wechselwirkungen zwischen mechanischen und hydraulischen Prozessen berucksichtigt werden. Je nach Bodenentwicklung bzw. Bodenbearbeitungs- oder Nutzungssystemen variieren die Tensorgrosen: Spannung, Verformung, Wasserleitfahigkeit in starkem Mase. Die Quantifizierung derartiger nichtlinearer und hysteretischer Prozesse kann mit Hilfe moderner numerischer Methoden wie z. B. der Finite Elemente Modelltechnik (FEM) erfolgen. In dem vorliegenden Ubersichtsartikel wird eine kurze Beschreibung von Modellstrukturen gegeben. Empirische Modelle zur Prognose der mechanischen Belastbarkeit werden im Hinblick auf die Anderung physikalischer Eigenschaften im Erstverdichtungsbereich aufgrund von Belastung und/oder Lockerung diskutiert; allerdings konnen diese Ergebnisse nur als grober Hinweis auf die tatsachlich im Boden ablaufenden Vorgange gewertet werden. Als Beispiel fur die Einsatzbereiche von FEM-Ansatzen werden Berechnungen und Messungen zur Bedeutung ehemaliger Pflugsohlen dargestellt und hiermit auch die Moglichkeiten einer eindeutigeren Vorhersage der Wirkung von visuell noch sichtbaren (Platten-) Strukturen auf die Spannungsverteilung gegeben.

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