Stabilization of Railway Bridge Foundations, Fairchild Creek Viaduct near Brantford, Ontario

Colin Alston, Mario Ruel, Kianoosh Hatami, David M. Howett

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

Session: Transportation Geotechnics

ABSTRACT: A 170 m long viaduct carries the double track CN Dundas railway subdivision across the Fairchild Creek Valley, East of Branttrack over the 15 m deep valley. The bridge consisted of steel superstructures supported on shallow masonry pad piers was erected to provide support to the additional superstructure. The soil profile at the site consists of a thick soil deposit of layered silty clay above the bedrock stratum which lies at a depth of approximately 25 m below the tableland. Over the past several decades the soil supporting foundations experienced downslope creep movement with corresponding foundation movement. As a consequence, the base of the steel towers supporting the tracks became considerably eccentric with respect to the centre of the pad foundations. Accurate monitoring revealed that the rate of the downslope movement was accelerating as a result of erosion from the stream flow in the channel at the base of the slope. Back calculation of the stability of the pad foundations with respect to bearing capacity and global stability indicated that the reserve of stability had progressively decreased in time to the point where the factor of safety was marginal with respect to current standards. The existing pad foundations which are positioned on the valley side slope were stabilized by an array of soil nails drilled into the slope on a 25 degree angle beneath each pad to secure the supporting silty clay soil from both local and global shear movements. Additional micropile underpinning was needed around the most vulnerable primary foundation pad which is located at the edge of the eroding channel. The micropiles were drilled to attain embedment in the underlying rock formation and connected to a ring beam enclosing the old masonry pad: the ring beam was tied back into the slope with an array of soil nails. Large rip rap protection was then installed along the creek banks as erosion control. Installation of the soil nail and micropile appurtenances has resulted in significant improvements to the bearing capacity of the foundations and the overall stability of the valley side slope. These appurtenances have been in place for more than ten years. The results of monitoring show that deleterious movements in the foundations have been arrested, as has global slope movement.

RÉSUMÉ: Un viaduc ferroviaire de 170 m de long supporte le chemin de fer à voie double de la subdivision Dundas du CN et traverse la vallée Fairchild Creek, qui est située à l'est de Brantford, en Ontario. La structure originale du viaduc avait été érigée à la fin des années 1800 afin de supporter la ligne ferroviaire à voie unique par-dessus la vallée, profonde profondes. Au début des années 1900, le pont a été modifié pour accommoder une deuxième voie ferroviaire, et un ensemble de piles de pont supplémentaires ont été construites afin de supporter la superstructure additionnelle. Le profil du sol sur place est composé d'une couche épaisse d'argile limoneuse au-dessus de la strate rocheuse, à environ 25 m sous le plateau. Au cours des dernières décennies, la solifluxion du sol supportant les fondations a provoqué un mouvement correspondant des fondations. Par conséquent, la base des tours en acier qui supportaient les voies ferroviaires est devenue considérablement excentrique par rapport au centre des fondations. Une Des calculs à rebours, effectués sur la stabilité des fondations en ce qui a trait à la capacité portante et à la stabilité globale, ont démontré qu'au fils des années, la réserve en stabilité avait progressivement diminué et que le facteur de sécurité était rendu à la limite par rapport aux normes de sécurité.

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