Progressive deformation of glacial till due to viscoplastic straining and pore pressure variation

R. Harley, V. Sivakumar, D. Hughes, M.R. Karim, S.L. Barbour

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

Session: Laboratory and Field Testing

ABSTRACT: The stress regime in a cutting (slope) is complex, with different principle stresses acting in different directions along the potential failure plane. For example, stresses may be primarily in extension near the toe and in compression near the crest of a slope. Cuttings in heavily overconsolidated clays are known to be susceptible to progressive failure which usually starts at the toe of the slope. Softening and the development of rupture surfaces have been observed in the field and are well documented for London Clays. However, this failure mechanism is yet to be established for glacial tills. To better understand the progressive failure mechanism, this paper discusses a series of laboratory tests conducted on reconstituted glacial till samples from Northern Ireland. Initial observations indicate that, a soil with insitu stress states between 80-90% of peak strength may undergo significant viscoplastic straining as a result of the combination of pore-pressure cycling and elevated stress level. RÉSUMÉ Le régime de stress dans une coupe (pente) est complexe, avec des contraintes principales agissant dans des directions différentes le long du plan de rupture potentielle. Par exemple, les contraintes peuvent être principalement en extension près du pied et à la compression à proximité de la crête d'une pente. Boutures dans les argiles surconsolidés sont connus pour être sensibles à la défaillance progressive qui commence habituellement au pied de la pente. Ramollissement et le développement des surfaces de rupture ont été observés dans le domaine et est bien documenté pour Londres argiles. Cependant, ce mécanisme de rupture est encore à établir pour les tills glaciaires. Pour mieux comprendre le mécanisme de rupture progressive, ce document examine une série de tests en laboratoire sur des échantillons de till glaciaire reconstitué d'Irlande du Nord. Les premières observations indiquent que, un sol avec des états de contraintes in situ entre 80-90 % de la force maximale peut subir viscoplastique significative forcer comme un résultat de la combinaison de cyclisme pression interstitielle et le niveau élevé de stress. 1 INTRODUCTION The road network in Northern Ireland encompasses a substantial number of large cuttings in glacial tills. A potential mechanism for failures in these large cuttings in stiff overconsolidated clay is progressive failure, which has been attributed to the pore water pressure (pwp) dynamics (Hughes et al., 2007) and creep. Because of the predicted more acute climate scenarios in the future, as presented by UK Climate Projection 09 (Murphy et al. 2009), these effects are likely to be more severe. Creep is defined as the development of irrecoverable (plastic) deformations with time under an applied constant effective stress. This is distinguished from the term plasticity in soil mechanics, which implies that irrecoverable strains develop instantaneously subjected to change in effective stresses. It has long been recognised that, for many soils, this creep (also referred to as time dependency) can be a significant contributor to the overall behaviour of a geotechnical structure. (Bjerrum, 1967; Karim et al., 2011; Yin and Graham, 1999; Kelln et al., 2007) A number of Elastic-viscoplastic (EVP) soil models (Kutter and Sathialingam, 1992; Karim et al., 2010; Yin, 2001; Kelln et al., 2008) have been proposed in the literature to deal with the physical evidence for creep. However, in order to use an EVP soil model, it is important to determine whether the soil is deforming due to the pwp dynamics which induces efffective stress changes, or it undergoes significant viscoplastic straining simply due to elevated stress conditions at rest, or both mechanisms are responsible. An attempt has been made in this paper to better understand the time-dependent behavior of Northern Irish glacial till. This paper outlines the stress path tests carried out on reconstituted samples of the fine fraction of glacial till samples, collected from a site near Loughbrickland, Northern Ireland, under compressive stress conditions. It also presents an evaluation of the samples response to rest and cyclic pwp changes when subjected to deviator stresses close to critical state. Observations indicate that, once the soil reaches a stress level causing irrecoverable deformations, the material will continue to undergo significant viscoplastic straining under a constant deviator stress and demonstrates that creep related progressive deformation could play a significant role in the behaviour of glacial till cut slopes.

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R. Harley; V. Sivakumar; D. Hughes; M.R. Karim; S.L. Barbour (2014) Progressive deformation of glacial till due to viscoplastic straining and pore pressure variation in GEO2014. Ottawa, Ontario: Canadian Geotechnical Society.

@article{GeoRegina14Paper252,author = R. Harley; V. Sivakumar; D. Hughes; M.R. Karim; S.L. Barbour,title = Progressive deformation of glacial till due to viscoplastic straining and pore pressure variation,year = 2014}