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Modelling of the Behaviour of Soft Swiss Lacustrine Clay - Theoretical and Practical Approaches

Abstract:

Soft lacustrine deposits can be found in appropriate geological and morphological conditions all around the world, and Switzerland is no exception. A number of factors combine to focus construction increasingly on such sites, which offer potentially troublesome foundation conditions and which were considered unsuitable just a couple of decades ago. Lacustrine soils are deposited in peri- or post-glacial lakes under a variable sedimentation environment, leading typically to the formation of varved clays. Thin layers of silts sediment out under water in summer between clay layers that are deposited in winter. This leads to significant structural anisotropy. By definition, these soils are normally consolidated following deposition, although a desiccated surface crust may form too. Within this project, Swiss lacustrine clay from several regions are investigated in terms of mineralogy and mechanical properties. In particular, the deformation behaviour is examined within a series of triaxial stress path tests. A triaxial stress path test device is equipped with local axial and radial strain measurement devices to investigate small strain behaviour. Axial local displacement transducers are mounted on the sample and the radial strains are measured by scanning the surface with lasers in continuous time steps. The test programme has begun with testing remoulded samples in undrained triaxial compression and extension, to reduce sample influences as much as possible and to compare data obtained from the new measurement devices with past results. Test series 1 involves multi-stage stress path tests on remoulded clay samples, trimmed from a 400 mm diameter cylinder. These are being reconsolidated along a constant stress ratio (q/p`), unloaded along the same q/p` and then reloaded along a different q/p` in compression or extension to create an appropriate stress path history and to obtain information about non-linear elastic behaviour. The load - unload - reload excursions will be drained, and will be followed by undrained shearing to failure. Subsequently, natural clay samples will be taken from the field from block samples and tested in Series 2 to investigate structural effects. The aim of the project is to develop advanced constitutive models that consider small strain as well as anisotropic soil behaviour for modelling Swiss lacustrine clay. This research project will focus on excavation applications, for which the small strain behaviour and stress path within the yield locus becomes important. The anisotropic elasto-plastic constitutive models, will be evaluated for use on Swiss lacustrine clay by simulating laboratory stress path tests from this series of data as well as those from past diploma theses and research work at IGT on a representative range of tests. Subsequently, a real construction project (an excavation) will be simulated using advanced constitutive models. The calculation results will be compared with the field measurements.

Authors:

Messerklinger, Sophie and Springman, Sarah M.

Index Terms:

SoilGroup; anisotropy; nonlinear elasticity; Swiss lacustrine clays; Triaxial stress path tests; constitutive modelling; deformation measurements; soft clay; numerical modelling; Messerklinger, Sophie; Springman, Sarah M.

Further Information:

Date published: 2004