Slopes in the Highly Plastic Clay Shales of the Canadian Prairies - A Case History of the Three Borden Bridges

Harpreet Panesar, Rob Bushman, Allen Kelly, Wayne Clifton

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

Session: Landslides and Geohazards

ABSTRACT: The three Borden Bridges cross the North Saskatchewan River along Highway 16 near Borden, Saskatchewan approximately 50 km west of Saskatoon. The east abutments and piers of the three Borden Bridges were built on old landslide debris. This paper presents the summary of annual visual inspections, instrumentation monitoring data (horizontal movements and pore-water pressures) and updated numerical stability analysis (which includes as-built abutment topography, changes in the shoreline and river bathymetry, monitoring results and state-of-the-art software optimization techniques). The paper also includes data gap analysis, sensitivity analysis, evaluation of preliminary repair options and recommendations for future study. RÉSUMÉ Les trois ponts traversent la Rivière Saskatchewan Nord le long de l'autoroute 16 près de Borden, Saskatchewan, et 50 kilomètres à l'Ouest de Saskatoon. Les contreforts et les piles de pont du cote Est furent construits sur les débris d'un ancien glissement de terrain. Ce rapport présente une synthèse des inspections visuelles annuelles, des données d'instrumentation de surveillance (incluant les mouvements latéraux et la pression d'eau interstitielle), ainsi que d'une nouvelle analyse numérique de stabilité (qui inclus la topographie originale des contreforts, les changements dans la bathymétrique de la rivière et de la rive elle-même, les résultats de surveillance et les techniques avancées d'optimisation de logiciel). Ce rapport inclus également une analyse d'absence de données, une analyse de sensibilité, une évaluation des options de réparations préliminaires, ainsi que des recommandations pour de futures études. 1 INTRODUCTION The three Borden Bridges cross the North Saskatchewan River along Highway 16 near Borden, Saskatchewan approximately 50 km west of Saskatoon. The three Borden Bridges were built in 1937, 1985 and 1997 within a distance of approximately 35 metres from each other. The first Borden Bridge was built in 1937 to carry vehicular traffic until 1985, but is now an abandoned bridge that is open to foot traffic only. This old bridge has the longest arch span of any similar design in North America and may have been the last substantially reinforced concrete bowstring arch bridge designed and built in Canada, which adds to its heritage value (modified after Neufeld 1984). Extensive landslides exist along the river valley due to the erosion of the valley into highly plastic clay shales. The Cretaceous shale that underlies a large area of the central plains of North America is montmorillonitic and exhibits high plasticity. In the unweathered, intact shale, these materials are hard and exhibit high apparent cohesion and friction angles consistent with the nature of the clay minerals. Upon softening, the loss of strength, particularly through loss of suction (apparent cohesion), is substantial. When subjected to shear strain, these materials exhibit marked residual behaviour with a reduction in the effective angle of internal friction and reduction of apparent cohesion to near zero (modified after Clifton et al. 1995). ˘ˇ Figure 1 Œ Aerial photograph showing various landforms at Borden Bridges site. The east abutments and piers of the three Borden Bridges were built on old landslide debris (see Figure 1). The landslide style of the valley slope is a typical multiple retrogressive failure on a near horizontal shear zone. The structure resulting from the original landslide consists of a series of blocks sliding on a common slip surface. Individual blocks control the potential instability at the east abutments of all three bridges.

RÉSUMÉ: s in the Highly Plastic Clay Shales of the

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Harpreet Panesar; Rob Bushman; Allen Kelly; Wayne Clifton (2014) Slopes in the Highly Plastic Clay Shales of the Canadian Prairies - A Case History of the Three Borden Bridges in GEO2014. Ottawa, Ontario: Canadian Geotechnical Society.

@article{GeoRegina14Paper345,author = Harpreet Panesar; Rob Bushman; Allen Kelly; Wayne Clifton,title = Slopes in the Highly Plastic Clay Shales of the Canadian Prairies - A Case History of the Three Borden Bridges,year = 2014}