Three-Dimensional Geotechnical Complexities of a Small Bridge Replacement

Storer J. Boone, Mrinmoy Kanungo, Glen Powney, Philip Loh, Henry Huotari

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

Session: Soil Mechanics

ABSTRACT: A 1950s-era three-span highway bridge was demolished and replaced with a new box structure and embankment infilling to allow recreational trail continuity. Ground conditions included fill, peat, and soft varved clay. Stability and settlement of the 8 to 10 m high bridge embankments challenged original construction and the bridge piers and abutments were pile supported. While soft varved clay beneath the existing embankments had consolidated since the 1950s, conditions between the embankments and piers remained unknown. This paper summarizes investigations, laboratory testing, innovative design and construction challenges associated with building the new structure within a rapid design-build contract and low-bid context. Monitoring demonstrated the difficulty in predicting settlement behaviour due to the complex three-dimensional former and new loads, influence of the remaining pier piles and the varved nature of the soft clay deposits. RÉSUMÉ Un pont d'autoroute, des années 1950, de trois portées, a été démoli et remplacé par un dalot, le remblai et du talus, pour maintenir l'accès aux sentiers récréatifs. Les sols présents incluaient du remblai, de la tourbe ainsi qu'une argile varvée molle. La stabilité et les tassements du talus d'approche du pont, d'une hauteur de 8 à 10 m, ont présenté un défi lors de la conception originale. Les culées et les piliers ont donc dû être supportés sur pieux. Alors que l'argile varvée molle en-dessous des talus consolidait depuis 60 ans, les conditions entre talus et piliers demeuraient inconnues. Cet article résume les études, les essais de laboratoire, la conception innovatrice et les défis de construction associés à la réalisation de la structure, dans un contexte de conception-construction rapide et de plus basse soumission. Le suivi a démontré la difficulté de prédire les tassements due à la complexité tridimensionnelle des charges originales et additionnelles, l'influence des pieux existants et la nature varvée de l'argile molle. 1 INTRODUCTION In 2011, the Ministry of Transportation Ontario (MTO) awarded a design-build (DB) contract for removal of a highway bridge over an abandoned railway and construction of a new trail access structure in its place. The original highway bridge was a four-lane, three-span, simply supported concrete slab on steel girder structure with span lengths ranging from 16 to 18 m. After abandonment of the railway the 50 year old bridge no longer served its purpose. Rather than maintain the aged bridge, the MTO decided to replace it with a smaller trail access structure akin to a large concrete box culvert. The existing piers, abutments, and pile foundations were to be left in place but cut-off about 1.5 m below the new highway pavement. Key elements of the DB contract included an aggressive construction schedule along with a 7 year warranty period. The project schedule was complicated by the need to minimize traffic disruption during the heavily traveled summer months. After a 5 week bid period, the project was to be awarded to the lowest bidder in mid-July and fully completed by mid-November of the same year. All lanes of traffic were to be maintained except for a maximum 5 consecutive days when the traffic could be detoured for bridge removal. This paper describes the contractual, design, and construction challenges for this project related to the subsurface conditions. 2 SUBSURFACE CONDITIONS FOR BID The original 1956 geotechnical report, prepared before the MTO Foundations Section was organized, was provided as part of the DB Request for Proposals (RFP) which identified some 2 m of peat underlain by 6 to 9 m of soft silty clay resting on loose silt and sand. Dense sand suitable for pile foundations was found about 15 m below the surface. The 1956 report recommended that, after peat removal, the 10 m high embankments be constructed in two 5 m stages, separated by one year for soil strength gain to maintain stability during the second stage. Alternatively, wide berms were suggested for maintaining stability and permitting single stage construction. Embankment settlement was estimated to be about 0.6 m. The design included 14 and 21 m long steel pipe piles for pier and abutment foundations.

RÉSUMÉ: -DIMENSIONAL GEOTECHNICAL COMPLEXITIES OF A SMALL

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Cite this article:
Storer J. Boone; Mrinmoy Kanungo; Glen Powney; Philip Loh; Henry Huotari (2014) Three-Dimensional Geotechnical Complexities of a Small Bridge Replacement in GEO2014. Ottawa, Ontario: Canadian Geotechnical Society.

@article{GeoRegina14Paper145,author = Storer J. Boone; Mrinmoy Kanungo; Glen Powney; Philip Loh; Henry Huotari,title = Three-Dimensional Geotechnical Complexities of a Small Bridge Replacement,year = 2014}