Behaviour of a compacted cohesionless till under cyclic simple shear loading

Vincent Castonguay, Jean-Marie Konrad

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

Session: Laboratory and Field Testing

ABSTRACT: A cyclic simple shear apparatus allowing the application of cell and back pressures was used to study the behaviour of a compacted cohesionless till. The till samples were compacted at different water contents (both wet and dry of Proctor optimum) to study the influence of fabric and degree of saturation. The till samples were also sheared under different cyclic stress ratios (CSR). It was found that samples compacted dry of Proctor optimum reached failure after more than 1000 cycles while samples compacted wet of Proctor optimum generally reached failure after 100 cycles (for constant CSR of 0,3). For samples compacted wet of Proctor optimum, the cyclic resistance curve (relation between number of cycles until failure and CSR) was determined to study the liquefaction potential of the till, following the simplified NCEER method. Under the confining stress considered, the till was found non liquefiable for a northern Québec characteristic earthquake. RÉSUMÉ Un appareil de cisailement simple cyclique permetttant l'application d'une pression cellulaire et d'une contre-pression a été utilisé pour étudier le comportement d'un till compacté. Les échantillons de till ont été compactés à différentes teneurs en eau (en-dessous et au-dessus de l'optimum Proctor) pour étudier l'influence de la texture et du degré de saturation. Les échantillons de till ont aussi été cisaillés sous différents ratios de cisaillement cyclique (CSR). Les échantillons compactés du côté sec de l'optimum Proctor ont nécessité plus de 1000 cycles pour atteindre la rupture, alors que les échantillons compactés du côté humide ont atteint la rupture en environ 100 cycles. La courbe de résistance aux charges cycliques (nombre de cycles avant l'atteinte de la rupture en fonction du CSR appliqué) a été déterminé pour des échantillons compactés du côté humide de l'optimum afin d'étudier le potentiel de liquéfaction du till grâce à la méthode simplifiée NCEER. Pour les conditions de confinement étudiées, le till s'est avéré non liquéfiable face à un séisme caractéristique du nord du Québec. 1 INTRODUCTION Since the 1950's, tills have extensively been used for the construction of dam cores in the province of Québec (Canada). In addition to being a material combining low permeability and high resistance, tills are most of the time naturally available close to the province's northern construction sites. Since such material is commonly used for the construction of earth dams, it is of vital importance to understand its behaviour in as many engineering situations as possible. The herein presented study focuses on the dynamic response of an unsaturated cohesionless till under cyclic simple shear loading. Such situation can be found in embankment dams subjected to seismic loading for example. The article is divided in four sections. First, different topics relevant to the project are reviewed in section 2. The methodology of testing is then presented in section 3 (containing a presentation of the apparatus used and of the soil studied). Results for one test are presented in section 4. Finally, results are gathered and discussed in section 5. An example of application (liquefaction potential evaluation) of such results is also presented in section 5. 2 BACKGROUND This paper deals with three major subjects: cyclic simple shear testing, tills and liquefaction susceptibility evaluation. Each of these topics is quickly reviewed in the next sub-sections. 2.1 Cyclic simple shear testing It is well known that cyclic loadings such as earthquakes are better represented by the cyclic simple shear test than by the popular cyclic triaxial test (Peacock & Seed, 1968). The smooth rotation of the principal stresses axis offered by the cyclic simple shear test more closely represents an earthquake's shear waves propagation than does the cyclic triaxial test. Many different apparatuses have been developed through the years to fulfill the need for cyclic simple shear testing. Cubical samples were first used (Peacock & Seed, 1968) in the Cambridge apparatus. Cylindrical shaped samples (Silver & Seed, 1971) were then used in NGI like apparatuses to allow easier sample mounting procedures. More recent apparatuses have include an acrylic chamber to allow the use of confining and back pressures during consolidation of samples (e.g. Boulanger et al., 1993), just as in triaxial testing.

RÉSUMÉ: iour of a compacted cohesionless till under

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Vincent Castonguay; Jean-Marie Konrad (2014) Behaviour of a compacted cohesionless till under cyclic simple shear loading in GEO2014. Ottawa, Ontario: Canadian Geotechnical Society.

@article{GeoRegina14Paper222,author = Vincent Castonguay; Jean-Marie Konrad,title = Behaviour of a compacted cohesionless till under cyclic simple shear loading ,year = 2014}