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Liquefaction of sands under multi-axial loading

University of British Columbia

A fundamental study of the undrained behaviour of sands under multi-axial loading is presented. The study was performed by using the hollow cylinder torsional (HCT) device. The HCT is the only device that permits a soil specimen to be subjected to multi-axial loading with controlled variations in the magnitudes of the three principal stresses and the direction of the major principal stress with the vertical deposition direction. The main objective of the study was to assess the effects of principal stress magnitude, directions and their rotation on sand liquefaction. This is achieved by a systematic study of static and cyclic undrained behaviour of reconstituted loose sand. Shear loading is carried out under strain control. Only such loading permits the needed capture of post peak strain softening characteristics of loose sands. Undesirable runaway strains are inevitable in stress controlled loading modes. In addition to the investigations in the hollow cylinder torsional device, sand behaviour in simple shear as well as under the triaxial conditions was also assessed as reference for comparisons with that under multi-axial stresses. The investigations were carried out using two sands - Fraser River sand and Syncrude sand. Sand specimens were reconstituted by water pluviation, which is considered to duplicate the fabric ofin-situ fluvial and hydraulic fill deposits. Independence of the effective stress path and stress-strain characteristics from the total stress path under fixed principal stress directions and constant value of intermediate principal stress parameter is illustrated. The undrained response of loose sand is highly dependent on the loading direction, implying inherent anisotropy. The friction angle mobilized at phase transformation or steady state is a unique material property, independent of the mode of loading static or cyclic, direction of principal stresses, intermediate principal stress level, consolidation history and the stress and void ratio state prior to undrained shear. There is no unique relationship between steady state or phase transformation strength and void ratio that is independent of the stress path, implying that a unique steady state line does not exist for a sand. The influence of intermediate principal stress, on undrained response is small when the intermediate principal stress parameter, that reflects value of this stress relative to the major and the minor values, is less than about 0.5. At constant values of other parameters increasing confining stress and decreasing relative density under multi-axial loading promote a higher degree of contractive response. The history of principal stress directions during principal stress rotation does not seem to have any appreciable effect on the peak and steady state or phase transformation strength. These strengths are apparently controlled by the peak value of major principal stress inclination experienced during shearing with respect to vertical direction. Principal stresses undergo continuous rotation from 0 to about ±45° in simple shear deformation. A simultaneous change in intermediate principal stress occurs as the major principal stress rotates. The maximum shear stress and maximum shear strain in conventional simple shear deformation approximately equals the horizontal shear stress and shear strain respectively. For a given initial stress and void ratio state, the number of cycles to liquefaction is smaller under cyclic triaxial than under similar 90° jump rotation that do not invoke the weakest triaxial extension loading mode during shear. For a given direction of principal stresses, if the sand is contractive under static loading, it would also be contractive under cyclic loading, provided that the cyclic deviator stress amplitude is higher than the steady state or phase transformation strength in static loading.

Thesis/Dissertation

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2009-03-17

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http://hdl.handle.net/2429/6173

Civil Engineering

University of British Columbia

UBCV

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