Fracture propagation of cohesive soils under tensile loading and desiccation
thesisposted on 28.02.2017 by Shannon, Benjamin Michael
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Tensile fracture of clay soils either due to loading or due to desiccation is a common problem encountered in many geotechnical, geoenvironmental and resources engineering applications such as in compacted clay liners, dams, embankments, slopes, seabed trenching for pipeline placement and in mine tailings. However, the fundamental understanding of this process and its modelling capability has not yet advanced satisfactorily. This research intends to fill this gap, following on the past and concurrent research undertaken on this topic by Monash Geomechanics Group. The current research is to develop fundamental characteristics of fracture properties, develop relevant measuring and analysis techniques and provide the basis for theoretical modelling. The research undertaken comprised of three main laboratory testing stages, and analytical, numerical, theoretical and predictive modelling. Five main different soils were used throughout this thesis including: Werribee clay, Merri Creek clay, Altona North clay, Prestige NY kaolin clay and HR1F kaolin clay. The first three are naturally available in Victoria whereas the two kaolin clays are sourced from commercial dealers in NSW. A comprehensive soil properties database was compiled for all soils tested. Advanced image analysis techniques were extensively used throughout testing to capture strains caused by loading and/or desiccation and determine fracture propagation surfaces. Tensile crack surfaces of compacted soils with varying compaction pressure were analysed on a macro scale to identify voids and aggregate conglomeration. The tensile strength of soils was rigorously tested for mechanical loading and desiccation induced cracking. Mechanically loaded samples were examined for effects of preconsolidation pressure, compaction pressure, soil type and water content at failure. Tensile loading tests were completed using the indirect diametrical tensile (IDT) test. Results on tensile strength found from past literature were compiled and analysed using the MPK framework for volumetric behaviour of unsaturated soils. A line of optimum tensile strength was found from void ratio and moisture ratio for various soil types. An extensive restrained shrinkage desiccation test (Monash desiccation cracking test) was introduced to determine tensile strength, fracture toughness, shrinkage strains and suction from changing water content. Tests were modelled using analytical and numerical modelling. A theoretical and predictive model was determined using MIT and critical state methods based on the restrained shrinkage desiccation test. Fracture properties of clay soils were analysed under four-point bending notch beams and cylindrical ring tests. Linear elastic fracture mechanics, elastic-plastic fracture mechanics and plastic fracture mechanics were all used in calculating fracture energy and toughness. Numerical modelling was undertaken using FLAC3D and UDEC codes to model laboratory and analytical test results for restrained shrinkage tests. UDEC was used to model fracture properties from laboratory restrained tests. Finally, comparisons between different tensile strength tests and numerical models were examined.