CRITICAL STATE SOIL MECHANICS REVISITED: FABRIC EFFECTS (SOMEF)
Soil structures or foundations of structures, such as dams, embankments, foundations of buildings, off-shore platforms, pipelines etc, constitute the largest constructions on earth human ingenuity has built, and possibly attract the largest investment in terms of funding and human toil. While in the past geotechnical engineering analysis of these structures was rather empirical, in modern times the analysis can proceed by solving appropriate boundary value problems, a task made possible because of the dramatic advance of computational capabilities. The full force of continuum mechanics in implementing equations of motion, compatibility and constitutive relations for soils in numerical codes, supplemented by the additional element of two or three phase continua (soil skeleton, water and air), are used in static and dynamic analysis of geotechnical structures. In this analysis the predominant element is soil and it is not an understatement that today, the frontiers of geomechanics research and applications are intimately related to the development and numerical implementation of soil constitutive modeling.
Soil Mechanics begun formally when the French engineer Coulomb proposed the fundamental soil friction failure law. Since Coulomb’s time the discipline of Soil Mechanics has evolved tremendously. If one wanted to place a mark of distinct progress, it would be related to the foundation of Critical State Soil Mechanics (CSSM) theory developed in Cambridge during the 50’s and ’60s (Roscoe et al., 1958; Schofield & Wroth, 1968), which has become a paradigm in the field. Critical state of particulate materials is defined as the state where under constant stress the material keeps deforming in shear at constant volume. In addition to being a failure theory, CSSM is a general theoretical framework within which soil constitutive modeling flourished in the last 40 years. No soil constitutive model can be accepted today unless it falls within the framework of CSSM, and if it does not, it is at the risk of the proposer.
Challenging a paradigm is a challenge in itself. The SOMEF project will constructively challenge the present CSSM paradigm by revisiting it from a missing fundamental perspective, namely the role of soil fabric. A soil state is defined as critical when continuing deviatoric strain develops at zero volume change under fixed stresses. The current CSSM theory postulates that critical state is reached when the stress and void ratios reach properly defined critical values. In doing so, CSSM is based on two scalar valued quantities while orientation aspects of the fabric, such as orientation distribution of particles’ long axes, vectors normal to particle contacts, void vectors etc, have been completely ignored in characterizing critical state. Yet, it has been shown (Yoshimine et al., 1998) that such fabric orientation related quantities drastically affect pre-failure and failure soil response characteristics. Thus, their absence from the statement of Critical State conditions on the one hand questions on physical grounds the completeness of the existing CSSM theory, and on the other it is the reason why soil response under several loading conditions cannot be interpreted within the current theory.
In this project, the missing fabric orientation element from the CSSM theory will be represented by a properly defined evolving fabric tensor. The hypothesis that critical state cannot be reached until this fabric tensor acquires a critical value, in addition to the critical values of stress and void ratios, will be investigated by coordinated theoretical, numerical and experimental research actions. These will include continuum and discrete elements methods (DEM) of analysis, X-ray computed tomography studies as well as traditional triaxial, biaxial and hollow cylinder experiments. Uniqueness issues will be addressed by thermodynamic principles. The final objective of the present SOMEF (SOil MEchanics Fabric) project is the incorporation of the fabric tensor’s critical value into an enhanced set of necessary and sufficient Critical State conditions, the development of constitutive models within this new theoretical framework and their implementation into numerical codes for solving various boundary value geomechanics problems, examples of which will be presented emphasizing the important difference had fabric not been accounted for. It is important to emphasize that this project is not a search for a specific soil constitutive model, but rather is in search of a new fabric-enhanced Critical State framework in which various soil constitutive models can be developed.
If this new paradigm of CSSM is successful, it will drastically change and enrich the way CSSM theory is taught at Universities and applied in advanced geomechanics analysis problems. It will also offer a more practical engineering perspective to the neighboring fields of granular mechanics, physics and material sciences, where a continuum approach to fabric effects for particulate aggregates can be very useful. Last but not least, the safer analysis and performance-based design of geotechnical structures resulting from successful completion of the SOMEF project, is a field with increasing beneficial social impact in regards to hazard mitigation related to catastrophic events such as earthquakes and landslides.
SOMEF is an ERC Advanced Grant programme supported by the European Union under the Seventh Framework Programme, individual programme “IDEAS”. This project has received funding from the European Union’s Seventh Framework Programme for research, technological development and demonstration under grant agreement no 290963.