Suction characteristics and elastic-plastic modelling of unsaturated sand-bentonite mixture

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Tang, Gary Xiangmin
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This thesis examines two aspects of unsaturated sand-bentonite behavior. The first is the suction characteristics of sand-bentonite at different saturation, water content, ion concentration, and moisture change history. Suctions of sand-bentonite are significantly outside the range normally encountered in normal geotechnical testing. The high suctions necessitate their measurement by using filter paper, vapour equilibrium and psychrometer techniques. Results produce suction-water content relationships and soil-water characteristic curves using different procedures. The second aspect is the strength properties and elastic-plastic stress-strain behavior of the compacted unsaturated sand-bentonite. Quick undrained triaxial tests were conducted on specimens at saturations ranging from 50%-98% to examine how saturation, matric suction and osmotic suction influence strength. The specimens possessed initial suctions arising from unsaturation, some of them had even higher suctions that resulted from introduced chemical agents. Strength properties were also examined using a series of tensile tests. Specimens compacted to 85% saturation at dry density of 1.67 Mg/m3 and water content of 19.4% were used for this series of tests. Some specimens were also dried to lower water contents, that is, to higher suctions. The pivotal component of the testing program in this thesis is an extensive series of stress-controlled triaxial tests that involved monitoring suction changes and measuring deformations during the tests. The specimens were compacted at dry density of 1.67 Mg/m3 with initial water content of 19.40% (corresponding to 85% saturation) and their subsequent alteration through drying to suction of approximately 6 MPa by imposing controlled relative humidity. The specimens were sheared under several selected stress paths. The results have developed a database to examine elastic-plastic stress-strain behavior of unsaturated compacted sand-bentonite mixture. The equipment and test methodologies for measuring or controlling suctions have been developed for the first time in the geotechnical laboratories at the University of Manitoba. Data showed that suction changes under pressure correspond to ch nges in the mean stress component of the stress tensor and are independent of the shear stress component. In unsaturated buffer specimens, the changes in suction with mean stress are largely reversible. The stress-strain behavior and strengths are found to be significantly dependent of suctions. A new elastic-plastic conceptual model has been developed for highly plastic clays. The new model stems from two stress variables-stress ratio and equivalent pressure which are proposed as alternatives of commonly used matric suction and net mean stress. Based on the developed model, new concepts of yield envelope and stress state boundary are defined in p-q-S space. The model provides interpretation to all aspects of the elasticplastic behavior and failure mechanism of unsaturated sand-bentonite observed in the testing program. For the first time, experimental yielding data of unsaturated specimens and saturated specimens can be normalized to a unified yield envelope.