Geotechnical Investigation, Design and Construction for the Blackwater River Crossing, NWT

Entzu Hsieh, Alexandre Tchekhovski, Kevin Spencer, Mike Charko, Ann Lanteigne

In the proceedings of: GeoManitoba 2012: 65th Canadian Geotechnical Conference

Session: T2A - Foundations III

ABSTRACT: The Blackwater River crossing involved construction of a single-lane multi-span steel bridge, about 305 m long and 7 m wide, and construction of approximately 5 km of approach road. The geological profile at the crossing consisted of fluvial and glacial deposits with inclusions of cobbles and boulders. Relict permafrost was encountered in till at the south abutment. The sand and gravel at the north abutment and flood plain area contained groundwater under high hydrostatic pressure. On the basis of the encountered subsurface conditions, the bridge structure was designed to be supported on 762 mm and 1,067 mm diameter open-ended steel pipe piles driven to depths ranging from 28 m to 32 m. Challenges for the pile design included driving issues through coarse materials and permafrost and penetrating into the artesian aquifer. Dynamic tests were conducted on selected piles to confirm the driving criteria and pile capacity. Construction of a 13 m high approach embankments under winter conditions using frozen fill material required an observational technical approach. The project also involved sourcing construction materials including granular backfill and of rip rap aggregates for erosion protection on the abutment slopes and piers.

RÉSUMÉ: une voies et à travées multiples fluviale ou glaciaire comprenant des cailloux et des blocs. Une zone de pergélisol discontinu existe à la culée sud, tandis que des sols sableux soumis à de fortes pressions artésiennes sont présents à la culée nord. Pour tenir compte des conditions de sol et de pergélisol rencontrées, la structure de pont conçue repose sur des pieux tubulaires de 762 mm et 1 067 mm de diarencontrés lors de la conception comprennent la mise en place de pieux au-travers de matériaux grossiers et de sous pression artésienne. Des essais dynamiques ont été u moyen de remblais argileux des talus et des piliers. 1. INTRODUCTION The Blackwater River Bridge project, located about 90 km north of the hamlet of Wrigley, is part of an on-going plan to extend the all-weather Mackenzie Valley Highway from Wrigley to Norman Wells, NT. The project involved construction of a single-lane multi-span steel bridge, about 305 m long and 7 m wide, and approximately 5 km of approach road leading to the bridge. The current pier design, with some modification, is able to support an extra lane to accommodate future traffic loads. Geological, permafrost and hydrological conditions at the crossing were assessed to be complex due to presence of cobble/boulder inclusions in surficial soils, discontinuous permafrost combined with artesian groundwater conditions having hydrostatic head several meters above the existing ground surface. Based on the encountered subsurface conditions, the bridge structure was designed to be supported on 762 mm and 1,067 mm diameter open-ended steel pipe piles driven to depths ranging from 28 m to 32 m. The pile design and installation were adjusted considering driving issues through coarse materials, permafrost and artesian aquifer pressures. Specific geotechnical considerations were required for winter pile driving and construction of a 13 m high approach embankment using frozen fill material under winter conditions. The project also involved identifying sources of construction materials including granular backfill for embankment construction and rock riprap for erosion protection on the abutment slopes and piers. Construction of the project commenced in 2008 and was completed in 2010. 2. BACKGROUND INFORMATION During conceptual design, various bridge alignments between the existing Enbridge pipeline (to the north) and the existing winter road alignment were considered. The final bridge alignment was located approximately 450 m north of the winter road (Figure 1). In addition, various bridge span configurations and deck options were

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