A Novel Framework for the Analysis of Low Factor of Safety Slopes in the Highly Plastic Clay Shales of the Canadian Prairies – A Case History of the Battlefords Bridges

Jasyn A. Henry, Jitendra S. Sharma, Allen J. Kelly, Harpreet S. Panesar

In the proceedings of: GeoMontréal 2013: 66th Canadian Geotechnical Conference; 11th joint with IAH-CNC

Session: Landslides I

ABSTRACT: This paper describes the development of a novel framework for the analysis of low-factor-of-safety (FOS) slopes in the highly plastic Cretaceous clay shales that affects critical infrastructure such as bridges and foundations throughout the Canadian prairies. The south abutments of the old and new Battlefords bridges were used in the case study because of the abundance of site investigation and historic monitoring records. It is widely known that slopes in Cretaceous clay shale undergo slow deformation at factors of safety greater than 1.0. As such, limit-equilibrium (LE) methods of analysis do not capture the effect of these slow movements on structures built on such slopes. The new framework correlates movement with mobilized shear strength by establishing mechanisms of deformation using a set of rigorous finite element (FE) analyses. This paper presents a synthesis of existing work at the bridges, detailed site characterization, back analysis of the old bridge failure using LE, application of the back analyzed properties to FE models of the new bridge, explanation of the mechanism of failure and mobilized shear strength and a comparison of the results of FE and LE models. This framework will provide owners and consultants with a way of analyzing low FOS slopes.

RÉSUMÉ: Cet article décrit le développement d'un cadre nouveau pour l'analyse de la faible facteur-of-sécurité (FOS) pentes dans les schistes argileux du Crétacé très plastique qui affecte les infrastructures critiques telles que les ponts et fondations à travers les prairies canadiennes. Les piliers sud de la vieille et nouvelle ponts Battlefords ont été utilisés dans l'étude de cas en raison de l'abondance de l'enquête sur les lieux et les registres de surveillance historiques. Il est bien connu que les pentes de schiste argileux du Crétacé subissent une déformation lente des facteurs de sécurité supérieur à 1,0. En tant que tel, la limite d'équilibre des méthodes d'analyse (LE) ne tiennent pas compte de l'effet de ces mouvements lents sur les structures construites sur les pentes. Le nouveau cadre est en corrélation avec la résistance au cisaillement mouvement mobilisé en établissant des mécanismes de déformation en utilisant un ensemble d'éléments finis rigoureux (FE) des analyses. Cet article présente une synthèse des travaux existants sur les ponts, la caractérisation détaillée du site, le dos analyse de l'échec de l'ancien pont en utilisant LE, l'application des propriétés de retour analysés pour les modèles FE du nouveau pont, l'explication du mécanisme de défaillance et mobilisé la résistance au cisaillement et une comparaison des résultats de modèles FE et LE. Ce cadre fournira aux propriétaires et aux consultants avec une méthode d'analyse FOS faibles pentes. 1 BACKGROUND The Battlefords bridges are located approximately 140 km northwest of Saskatoon, SK on Yellowhead Trail (Federal Highway 16) and connect the city of North Battleford and the Town of Battleford, SK (Figure 1). The bridges span the North Saskatchewan River and are vital transportation links for travel across the Canadian prairies. The North Saskatchewan River valley walls have a history of landslides (Stauffer et al., 1990). Historically, slope instability at the Battlefords bridges has been isolated to the south valley wall (Clifton et al., 1999). In October 1967, the older (east) of the two bridges experienced a major slope failure at the south abutment just prior to its being opened to the public. The landslide was remediated with the construction of a large toe berm, extensive slope flattening, and sub-surface drainage improvements. Since remediation, although there has been no catastrophic failure, slow movements in the south valley wall have continued as recorded by the Saskatchewan Ministry of Highways and Infrastructure (SMHI). The new bridge was constructed in 2001 and opened to the public in November, 2002. The foundation materials below the Battlefords bridges are Late Cretaceous clay shales. They are highly plastic and are found near surface along the valley walls of the North Saskatchewan River. This valley was a melt water channel during glacial retreat and, as such, most of the stratified drift and glacial till was eroded from the valley slopes, leaving the highly plastic clay shale near surface. The slopes in these shales are considered filow factor of safetyfl (FOS) slopes because several of them continue to the exhibit slow movement, averaging a few mm/year (Clifton et al., 1999 and Sauer, 1984), without experiencing total failure.

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