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An experimental study of the small strain response of sand

University of British Columbia

Fundamental behaviour of Ottawa sand, in the strain range of 1 x 10⁻² to 1 x 10⁻⁵, is investigated by direct measurement of deformations in a load controlled conventional triaxial system. Experiments are aimed at examining common concepts and previous experimental justifications for incremental elastic, elasto-plastic and particulate frameworks for characterizing sand behaviour. From fundamental interpretation of test data, an alternative stress-strain relationship is proposed for proportional loading with relative density represented as a separate parameter. Maximum Young's moduli evaluated from resonant column tests are found to be approximately equal to initial unloading moduli from conventional triaxial tests and initial moduli from virgin loading and subsequent reloadings are much less. Initial unloading moduli are relatively unaffected by the cycle of loading and deviator stress level from which unloading is initiated. The value of Young's modulus at a stress state Is not unique but depends on the stress path and strain history. Nonrecovered strain directions, at small strain, depend on stress direction as opposed to the generally accepted dependence on stress state at large strain. Proportional loading paths are uniquely related to linear strain increment directions and maintain parallel mean normal stress equipotentials in strain space. Energy density increments in two proportional loading paths having identical mean normal stress histories remain proportional. Parallel nonproportional loadings result in a unique strain increment direction, relatively independent of hydrostatic stress level, with linear stress ratio equipotentials in strain space. In small strain response, shear strains result mainly from shear stress increments and not from changes in stress ratio. Shear volume response is contractant for both an increasing and decreasing shear stress increment, whereas the sense of shear strain increment alternates with the sense of shear stress increment. When the sense of strain state is opposite to the sense of an applied stress increment, the resulting stress-strain response is softer than when both are of the same sense. Strain paths for compression side shear loading are identical to paths of extension side shear unloading and vice versa. More shear and volumetric strains develop on extension side shear loadings and the ratio of volumetric to shear strain is also higher as opposed to comparable compression side shear loadings. At higher stress ratio states, decreasing mean normal stress at constant shear and increasing stress ratio conditions; extension side volumetric strain responses are associated with contraction following initial swelling and prior to dilation. This contraction phase is not present on the compression side.

Text

2010-06-23

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http://hdl.handle.net/2429/25946

Civil Engineering

University of British Columbia

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