A Multilaminate Model for Overconsolidated Soils

Numerical methods such as the finite element method are established tools in geotechnical research and in practical geotechnical engineering but there is still a need for fundamental research on constitutive modelling in geotechnics. Because of the complexity of soil behaviour, resulting from its geological history, highly advanced constitutive models are required and so far no "universal" constitutive model for soils has emerged. In this proposal the development of a constitutive model for overconsolidated, structured soils is suggested. The model will be formulated in the context of classical plasticity theory but not, as more commonly employed, in terms of stress invariants but utilizing the multilaminate framework. In multilaminate models the overall deformation behaviour of a soil body is obtained by integration of the contributions of so-called integration planes and all constitutive relations are formulated on these planes, the orientations of which depend on the integration rule employed. In contrast to models formulated in stress invariants, multilaminate models take into account the effect of principal stress axes rotation and induced anisotropy inherently. The model developed at Graz University of Technology up to now is an elastic-plastic model which includes both deviatoric and volumetric hardening. It can account for destructuration, anisotropy in strength and strain softening employing a non-local formulation. However, the present formulation of the model is not adequate for modelling the behaviour of overconsolidated soil due to the fact that stress states on the "dry side" of the critical state line are not permissible. Therefore a bounding surface will be introduced and it will be investigated whether a continuous function or separate parts on the dry and wet side of the critical state line are more efficient. This will require detailed investigations because the concept of a Hvorslev surface in p-q-stress space has to be transferred to the multilaminate framework. The size of the bounding surface will be a function of bonding (structure) and destructuration will be a function of damage strain. When peak strength is reached strain softening will take place and as a first stage the applicability of the (available) non- local model to simulate strain softening behaviour of highly overconsolidated clay will be evaluated. If considered necessary alternative approaches, such as a strong discontinuity formulation, will be investigated. In order to take into account small strain stiffness behaviour two possibilities will be pursued: a) implementation of the small strain stiffness model already available, ii) implementation of kinematic hardening for the deviatoric yield curve. The main novelty of the research proposed is the introduction of a structure tensor which - in combination with the multilaminate framework - allows for considering anisotropy in strength, destructuration and stiffness in a mathematically convenient and straightforward manner. It is anticipated that the proposed model will be a significant step forward in modelling the mechanical behaviour of overconsolidated soils over the entire stress range from small strain stiffness to post peak behaviour.

In this research project a constitutive model for application in numerical analysis has been developed, in which typical characteristics of stiff, heavily compacted clay can be taken into account. Such soils are commonly the result of pre-loading due to glacial cover or the former presence (and subsequent erosion) of soil layers above the current ground level. Some of Europes most densely populated areas, e.g. South England, Belgium and the city of Vienna are built on stiff clays, which highlights the practical relevance of mechanically sound modelling of this type of soil. Typical for these soils is anisotropic, i.e. orientation dependent material stiffness at small deformations, which is a result of preferred spatial orientation of soil particles at microscopic level. After reaching the maximum shear strength, the shear strength of the material tends to reduce with proceeding deformation due to loosing of the material and destruction of inter-particle bonds. The constitutive model can account for these effects in a mechanically meaningful manner. Additional characteristics of soil behaviour such as stress dependent material stiffness and shear strength, orientation dependent shear strength and the occurrence of plastic, i.e. irrecoverable deformations have already been taken into account in the original model, and are hence also available in the current model. Anisotropic soil stiffness is in particular relevant for ensuring the serviceability of geotechnical structures during their life cycle. Detractions of serviceability in the context of geotechnical engineering are commonly the result of excessive ground deformations, which can cause damage to buildings or limit the usability of infrastructure. By using the advanced numerical model for the prediction of ground deformation in highly complex, stiff clays, it is possible to adjust the geotechnical structure or the construction method in advance and hence keep ground deformations within acceptable limits. Realistic modelling of shear strength reduction with deformation is rather relevant for assessing the safety level of geotechnical structures and for modelling soil behaviour at large deformations. Practical applications are safety calculations of natural and artificial slopes and the calculation of maximum bearing capacity of foundation structures.