Support of Excavation for a Coal Dumper Pit in North Vancouver, Canada

The overarching upgrade project at a coal dumping facility in North Vancouver, BC, consisted of a new railcar dumper pit vault with an outfeed conveyor to increase coal handling capacity at the facility. The site is located 200 m (656 ft) from the Burrard Inlet with groundwater table close to grade and coarse sands and gravels, presenting high risk for both groundwater and soil ingress. An active rail yard immediately adjacent to the site was to remain operational throughout the project’s construction. Support of excavation for the multi-level coal dumper vault involved an internally braced three-cell cofferdam with pipe-pipe and pipe-sheet combi pile walls, connected to a four-cell trench that accommodated the dumper vault’s outfeed conveyor tunnel. The center cell of the main dumper pit vault was designed for a maximum excavation depth of 27 m (89 ft), hydrostatic pressures starting 3 m (10 ft) below grade, and surcharge pressures from six adjacent rail lines. Three levels of bracing were used with spans of up to 10.5 m (34 ft) to facilitate the buildout of the permanent structure without cold joints. An unreinforced tremie base plug with tie-down anchors created basal stability against hydraulic uplift. This paper will highlight the challenging design requirements and the detailed construction sequencing that was required for successful installation of the support of excavation system. In addition to complicated geotechnical conditions and constraints due to the geometry of the permanent structure, environmental and health and safety requirements were a driving factor of this project. Construction for the deep excavation was to occur in a live coal handling facility with confined spaces, several contractors, and sensitive waterways. The project was an exceptional example of innovative design and illustrates the potential of a job when coordination and collaboration between the construction team, design team, and owners is prioritized. Support of excavation for the railcar dumper pit vault and outfeed conveyor tunnel was accomplished with a design-build approach.