Void ratio redistribution in saturated sand during undrained triaxial shearing by x-ray computerised tomography
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AbstractA general consensus has not been achieved within the geotechnical community regarding a number of fundamental aspects of undrained sand behaviour. Discrepancies in observed specimen responses have often been attributed microscopic fabric effects that are manifest at the macroscopic (global) level; however, assessments of void ratio redistribution at the intermediate, mesoscopic level within individual specimens during undrained shear have not been reported previously. X-ray computerised tomography (CT) methods and specialised triaxial equipment were developed in this research program to quantify void ratio redistribution within saturated sand specimens during undrained shearing. CT measurements were obtained at several points along the stress path of each specimen without significant removal of axial load. A relationship between CT measurements and bulk density is established and the accuracy of this method is determined. Undrained triaxial compression and extension tests were completed and the wet pluviation and moist tamping specimen reconstitution methods were used. Significant specimen nonuniformities at the end of isotropic consolidation are reported. It is established that significant void ratio redistribution occurred within each specimen during these tests. The term undrained is considered to apply to these tests in only the macroscopic, global sense. The influences of void ratio redistribution on macroscopically observed specimen responses are established. The appropriateness of using macroscopically observed specimen responses near the peaks of undrained effective stress paths to represent soil element behaviour is discussed. Fundamental differences between temporarily strain softening (TSS) behaviour and steady state deformation are established and an underlying mechanism is postulated to explain similarities between mobilised stress ratios during TSS response and steady state deformation. The appropriateness of specimen mean void ratios as estimates of the void ratios within failing zones during undrained triaxial compression and extension is discussed. The findings of this research program provide unique insight into the fundamental aspects of sand behaviour during undrained triaxial shearing. Opportunities for refined test procedures are discussed and recommendations for further investigation are provided.
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