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Fabric, initial state and stress path effects on liquefaction susceptibility of sands

University of British Columbia

An experimental study aimed at improving the understanding of the mechanics of liquefaction is presented. Two aspects of the undrained behaviour; (i) the influence of initial state, characterized by void ratio, principal stresses and their directions, and fabric under a given undrained stress path, and (ii) the effect of undrained stress path at a given initial state, were studied in a systematic manner. Triaxial, simple shear and hollow cylinder torsional shear devices were used in the experimentation to enforce a range of stress paths that simulate the loading insitu. The effect of membrane penetration was duly addressed in order to confidently measure the truly undrained behaviour of sands. The influence of soil fabric was determined using triaxial and simple shear tests on specimens reconstituted by different techniques. In addition to specimens reconstituted in the laboratory, undisturbed, in-situ frozen sands from four different sites were tested to assess the relevance of the behaviour of reconstituted specimens to the sand in-situ. It is shown that in-situ sands and those water pluviated in the laboratory are inherently anisotropic, and their undrained behaviour is direction dependent. At a given initial state, they often strain soften to an essential steady state in triaxial extension, but invariably strain harden in compression. Moist tamped sands, on the other hand, strain soften both in compression and extension. The domain of states in void ratio effective stress space accessible to alluvial in-situ sands is shown to be similar to those that ensue on pluviation. The behaviour of water pluviated sands is similar to that of alluvial in-situ sands, at a given initial stress state and undrained stress path, both under static and cyclic loading conditions - an indication that they both possess very similar fabric. This opens up the possibility of using water pluviated sand specimens to characterize the behaviour of in-situ sands, an attractive economical alternative. It is shown that the initial effective stress state plays a dominant role on the subsequent undrained behaviour at a given void ratio. Hollow cylinder torsional shear tests on water pluviated fabric were used to assess the influence of the initial axisyrnmetric and non-axisymmetric stress states on undrained shear. The behaviour due to an increase in deviator stress alone, and that due to simultaneous changes in deviator stress and the direction of principal stresses is investigated. A larger inclination of major principal stress to vertical results in a softer behaviour at all levels of static shear. An increase in static shear at a given inclination of the major principal stress promotes more strain softening response. Even a small undrained perturbation may trigger flow failure in a sand that is otherwise stable if drained, in the event the initial stress state is highly anisotropic together with larger inclination of major principal stress to the vertical. It is demonstrated that flow failure may be triggered by a mere rotation of principal stress directions only. The sense of principal stress rotation with respect to the initial inclination of the major principal stress also plays a prominent role on the undrained behaviour, and hence on strain development. It is shown that the strain induced anisotropy evolves at an accelerated rate due to rotation of major principal stress direction, compared to an increase in effective stress ratio alone at fixed direction of principal stresses. It is shown that both steady state and quasi steady state can be treated within the same framework. The friction angle mobilised at these states is independent of fabric, void ratio, effective confining and static shear stress levels, direction of principal stresses, and undrained stress path. This angle appears to be a unique material property. Contrary to the commonly held notion, the minimum undrained strength is dependent on the stress path, initial confining and static shear stress levels, fabric and the directions of principal stresses, in addition to void ratio. This implies that the steady/quasi-steady state is unique in the effective stress space, but not so in the void ratio-stress space. However, the minimum undrained strength normalised by the major principal stress appears to be dependent only on void ratio and the direction of major principal stress (i.e loading mode). The friction angle mobilised at the trigger of strain softening is also dependent on the initial stress state and the loading mode.

Thesis/Dissertation

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2009-07-23

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http://hdl.handle.net/2429/11200

Civil Engineering

University of British Columbia

UBCV

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