Some observations on the cyclic shear response of natural silt with increasing coarse-grained fraction

Achala Soysa, Dharma Wijewickreme

In the proceedings of: GeoRegina 2014: 67th Canadian Geotechnical Conference

Session: Laboratory and Field Testing

ABSTRACT: The cyclic shear response of natural silt with increasing coarse-grained fraction was investigated using constant-volume direct simple shear tests. Natural silt, having sand contents that ranged from 20% to 35%, retrieved using thin-walled sharpened-edge tube sampling methods from the Fraser River Delta of the Lower Mainland of British Columbia, Canada were used for the study. During constant-volume simple shear cyclic loading, progressive degradation of stiffness and gradual increment of pore water pressure with increasing number of loading cycles were observed for all tested materials, despite the variation of coarse-grained fraction. The potential for excess pore water pressure and shear strain development with number of cycles increased with increasing sand fraction. In turn, the cyclic resistance (expressed as cyclic resistance ratio versus number of cycles to reach a shear strain of 3.75%) decreased with increasing sand fraction. RÉSUMÉ La réponse de cisaille cyclique en limon naturel avec une fraction à grains grossier progressive est examinée selon des essais de cisaillement simples direct à volume constant. Limon naturel, avec une fraction de sable entre 20% et 35%, retrouvé en utilisant des méthodes de prélèvement de tube à paroi mince et arêtes vives du delta du fleuve Fraser au Lower Mainland de la Colombie-Britannique (Canada), a été utilisé pour cette étude. Pendant charge cyclique simple direct à volume constante, une réduction progressive de la rigidité et une augmentation graduelle de la pression du eau interstitielle avec nombre haussier des cycles du charge a été observée pour tous les matériaux testés, malgré la variation de la fraction à grain grossier. Le potentiel pour surpression du eau interstitielle et déformation cisaille avec nombre haussier des cycles du charge a augmenté avec fraction à grain grossier haussière. Ainsi, la résistance cyclique (présentée comme proportion de résistance cyclique au nombre des cycles pour arrivé à une déformation cisaille de 3.75%) a diminuée avec fraction de sable haussière. 1 INTRODUCTION Cyclic shear response of clean sand and the factors that influence the liquefaction of clean sand are generally well-identified and understood by the field observations and laboratory experimental studies conducted during the past few decades. Nevertheless, the current knowledge on cyclic shear response or liquefaction behaviour of fine grained soil and sand-fine mixtures needs advancement; for example, at present, different opinions persist on the effect of the variation of fines content and the plasticity of soils on the cyclic resistance of fine-grained soils. Soil is typically considered fine-grained when the fines (grain size smaller than 75 m) content is greater than 50% (ASTM, 2011). Using results from cyclic triaxial shear testing of three medium sands with varying gradation, yet having same mean grain size, Vaid et al. (1990) implied that gradation of soil may control the occurrence of contractive deformation. Kuerbis (1989) showed that increasing silt content up to 20% in Brenda sand results in higher cyclic resistance against limited liquefaction of sand. Similarly, adding Kaolinite, crushed silica fines and silica sand up to 40%, Pitman et al. (1994) observed that the undrained shear strength of Ottawa sand would increase with increasing fines content. Taking the fine-grained portion of the Yatesville silty sand and then mixing with Monterey sand and coarser potion of Yatesville silty sand, Polito & Martin II (2001) prepared sand and silt specimens with silt content varying from 4% to 100% to perform cyclic triaxial tests; they reported that cyclic resistance of the sand with silt, initially decrease with increasing silt content until the silt content has reached about 37%~50%, and then cyclic resistance would increase as the silt content increases up to 100%. Law & Ling (1992) also observed similar response as cyclic resistance initially decrease as the silt content increases until some minimum resistance is reached, and then that would increase with increasing silt content. The possible reason for differences in the observations for the cyclic resistance may be the difference in the arrangement of coarse and fine grains in the specimen, as different methods of deposition have been used for specimen preparation. Kuerbis (1989) showed that void ratio of the sand skeleton might govern the cyclic shear response than the silt content. Essentially considering the fine fraction that constitutes the soil matrix or the soil skeleton, Mitchell & Soga (2005) suggested that when the fine fraction exceeds about 35%, the fine fraction would be the dominating load carrying matrix of the soil. This can be also indirectly observed from an another point of view, as the commonly used Youd et al. (2001) empirical correlation for cyclic resistance ratio (CRR) and penetration resistance for coarse-grained soils is not defined for soils having fines content exceeding 35%. Some studies have already been carried out to understand the cyclic shear response of fine-grained soils

RÉSUMÉ: observations on the cyclic shear response of

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Achala Soysa; Dharma Wijewickreme (2014) Some observations on the cyclic shear response of natural silt with increasing coarse-grained fraction in GEO2014. Ottawa, Ontario: Canadian Geotechnical Society.

@article{GeoRegina14Paper356,author = Achala Soysa; Dharma Wijewickreme,title = Some observations on the cyclic shear response of natural silt with increasing coarse-grained fraction ,year = 2014}