Localization in Dutch dune sand and organic clay. New insight into localization mechanisms

One of the challenging research puzzles in soil mechanics is the subject of strain localization. The investigation of the physico-chemical mechanisms and the conditions under which the strain will localize has been going on for more than a century. The focus of these studies was continuously adapted as new engineering problems occurred or new technologies became available. The objective of the present study is to collect new laboratory evidence of strain localization in Dutch dune sand and organic clay with the emphasis on the application of novel laboratory techniques. In addition to mathematical aspects at the macro level the interdisciplinary knowledge of micro-geology and bio-geochemistry was incorporated in the micro-mechanical approach. The first part of this thesis describes specific microscopic laboratory findings and their macroscopic effects of a typical Dutch dune sand. Chapter 2 reports on triaxial elemental tests, which were performed to revisit the interpretation of dilatancy of sand and its implication on failure mechanisms. Specific attention was given to pre-peak volume change characteristics. The corresponding characteristic stress state (CSS) was measured with respect to the evolution of deformations. A special mathematical approach in line with Desai yield surfaces was developed to model the observed volume change of sand by incorporating characteristic stress states with an additional state parameter. A hitherto unrecognised intermixing deformation mode, i.e. cone-radial bands of varying density, has been identified. The characteristic stress state is found to be a precursor of the formation of such a deformation mode. Once such an intermixing deformation mode occurs the measured strength, either peak or post-peak strength, is difficult to establish. Chapter 3 formulates the intermixing deformation mode using the technique of limit analysis. Two failure mechanisms were involved, i.e. a single shear rupture plane and double cones-radial loosening bands. Both associated and non-associated kinematical fields were considered. The Mohr-Coulomb yield criterion was applied. The classical mechanism of energy dissipation in a shear rupture has been adopted. Upper bound solutions were obtained and the orientation of shear-rupture surfaces (cones)-radial loosening bands in a cylindrical sample was formulated in terms of apparent friction angle, apparent dilatancy angle, confining pressure, slenderness of the sample and tensile strength. This finding is important for the interpretation of triaxial compression strength when measured in the laboratory. Dutch organic clay is the second important subject in this thesis, in recognition of its importance to geotechnical engineering practice in the Netherlands. This material has a distinct mechanical behaviour different from non-organic clays. The reason for its high effective strength index and low coefficient of lateral stress remains as yet unknown. Fundamental aspects of the behaviour of Dutch organic clays, including the effects of strain localization (Chapter 4), the fabric and micro-deformation of organic and clay mineral contents (Chapter 5 and Chapter 6) are reported in this thesis. Based on observed laboratory behaviour the strain rate dependent characteristics of soft clays was demonstrated and examined in terms of the instability of the material (Chapter 4). The results suggest that the observed phenomena of rate-dependent deformation mechanisms, creep rupture and pre-failure softening are related to this rate-dependent instability. This viscosity-induced instability can lead to strain localization of saturated soft clay, which is different from the critical state. The combined effect of viscosity and drainage was also addressed by demonstrating its correlation with the type of failure of soft clay. The commonly applied undrained conditions adopted for the usual theoretical analysis and the standard engineering interpretation for the strength of soft clay do not occur in the laboratory or practice. Local drainage has to be taken into account and awareness is growing that the introduction of localization into the study of soft clays is relevant, both scientifically and practically. A high quality electron microscope system (ESEM and EDAX) combined with a mini-loading module was made available. Chapter 5 includes the laboratory investigation of fabric and related micro-deformation of Dutch organic clay with this modern sophisticated experimental technology. Four types of clay states were studied: natural, remoulded, artificial inorganic and fractured. The results suggest that Dutch organic clay has an unusual fabric where microstructures of organics and microfossils play a central role. Emphasis was put on the identification of the complicated microstructures and on live measurement of related micro-deformations in a controlled environment (temperature and humidity). Eventually, at a scale of tens of micrometers, the clay fabric and its micro-deformation characteristics were analysed. Applying the technique of distinct element modelling (DEM) a sound relation between fabric and common geotechnical properties of clay could be formulated on basis of micro-scale observations (Chapter 6), including an explanation accounting for the fabric matrix suction with regard to the unusual mechanical properties of Dutch organic clay. Micro-scale approach is promising and inevitable for the enhancement of the fundamental understanding of the fabric of organic clays and its geotechnical implications. Moreover, the study of mechanical behaviour of modified or contaminated clay could benefit from it as well