Consolidation of an instrumented Champlain clay layer in the laboratory

François Duhaime, Robert P. Chapuis

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

Session: Soil Mechanics

ABSTRACT: ion of an instrumented Champlain clay layer in the laboratory François Duhaime Laboratory for Geotechnical and Geoenvironmental Engineering (LG2) Œ École de technologie supérieure, Montréal Robert P. Chapuis Department CGM Œ École Polytechnique de Montréal ABSTRACT A 68 cm thick layer of remoulded Champlain clay was consolidated in the laboratory over a period of 200 days. The paper explains how the clay was remoulded and put in place, and how it was instrumented to measure both total settlement and pore pressures at different elevations. During the consolidation phase, a total stress increment of 6.1 kPa was applied on the clay, which produced a settlement of 2.1 cm. The clay layer was sampled twice with a thin-walled sampler after rest periods of 3 and 28 months. Oedometer and Swedish cone tests were conducted to evaluate the clay undrained shear strength, stress-strain relationship, secondary compression properties and permeability. The shear strength results allowed the influence of thixotropy and changes in water content to be distinguished. A linear relationship was observed between the logarithm of time elapsed since remoulding and shear strength. If secondary compression is accounted for, both the settlement and pore pressure data can be modelled reasonably well from the oedometer test results. Globally, the settlement data highlight the importance of secondary compression analyses when estimating large scale settlements. RÉSUMÉ Une couche d'argile Champlain remaniée de 68 cm d'épaisseur a été consolidée au laboratoire pendant une période de 200 jours. L'article explique comment l'argile a été remaniée puis mise en place, et comment elle a été instrumentée pour mesurer à la fois le tassement total et les pressions interstitielles à différentes élévations. Pendant la période de consolidation, on a appliqué une augmentation de contrainte totale de 6.1 kPa sur l'argile, ce qui a produit 2.1 cm de tassement. La couche d'argile a été échantillonnée à deux reprises avec un tube à paroi mince après des périodes de repos de 3 et 28 mois. Des essais œdométriques et au cône suédois ont été réalisés pour évaluer la résistance au cisaillement non drainé de l'argile, sa relation contrainte-déformation, ses propriétés de tassement secondaire et sa perméabilité. Les résultats de résistance au cisaillement permettent de distinguer l'influence de la thixotropie et des changements de teneur en eau. Une relation linéaire a été observée entre le logarithme du temps écoulé depuis le remaniement et la résistance au cisaillement. Si l'on tient compte du tassement secondaire, les données de tassement et de pression interstitielle peuvent être modélisées raisonnablement bien à partir des résultats d'essais œdométriques. Globalement, les données de tassement soulignent l'importance des analyses du tassement secondaire lors de l'estimation des tassements à grande échelle. 1 INTRODUCTION Starting from October 2010, a remoulded Champlain clay layer initially with a thickness of 68 cm was consolidated for a period of 200 days in the hydrogeology and mining environment laboratory at École Polytechnique in Montreal. The instrumented clay layer was prepared in the course of a doctoral project aimed at getting a better understanding of the impact of clay deformation on permeability tests (Duhaime 2012). After a rest period of 3 months, a monitoring well was installed in the clay layer to conduct different types of field permeability tests. Unfortunately, the well could not be sealed properly in the clay layer and the experimental set up was never used as intended. Fortunately, the clay layer was also instrumented to gather pore pressure and settlement data during the consolidation phase that preceded the failed attempt at installing a monitoring well. These data allowed the consolidation of a relatively thick clay layer to be studied. Oedometer tests are known to produce results that are difficult to translate toward full scale field applications. In this regard, the data gathered during the consolidation of this 68 cm thick clay layer have an interesting value for geotechnical engineers. Differences between large scale consolidation properties in the field and those determined from oedometer tests have been reported previously in the literature. Two main causes have been suggested for these discrepancies. The first explanation regards creep and strain rate effects. Smaller strain rates (slower deformations) are associated with larger deformations. Olson (1998) observed that strain rates are much higher for oedometer tests compared to typical applications in the field. Furthermore, Leroueil (1996) explained that strain rates tend to decrease in the field as the distance between a clay sub-layer and the drainage boundaries increases. With this explanation, oedometer tests should tend to underestimate field deformations. The second explanation regards the heterogeneity of soil deposits. Heterogeneity (e.g., sand stringers) can shorten the drainage path and increase the apparent coefficient of consolidation, cv (Duncan 1993). The thoroughly remoulded and homogenized instrumented clay presented

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Cite this article:
François Duhaime; Robert P. Chapuis (2014) Consolidation of an instrumented Champlain clay layer in the laboratory in GEO2014. Ottawa, Ontario: Canadian Geotechnical Society.

@article{GeoRegina14Paper280,author = François Duhaime; Robert P. Chapuis,title = Consolidation of an instrumented Champlain clay layer in the laboratory ,year = 2014}