Building a series of highway overpasses can be a challenge for contractors at the best of times. But when soil conditions are far from ideal for supporting heavy infrastructure, it adds a new wrinkle to the project.
That’s the situation facing Pacific Gateway Constructors General Partnership, the consortium working on a $260-million Highway 91/17 upgrade project in Delta, B.C. The consortium is upgrading an interchange and building three new ones. Work began last spring, and the project is scheduled to be finished in fall 2022.
The project is located between two environmentally sensitive areas—the Fraser River and Burns Bog Nature Reserve—where the soil is composed of silt and sand. Because of the soil conditions, the consortium must use special techniques to consolidate and stabilize the soil in order to make the ground suitable to bear the weight of heavy columns for supporting the interchanges.
“The construction techniques being used reflect these conditions, which are typical of construction within the Fraser River Delta area, and here surface soil is primarily silt, sand and peat,” explains Jody Deane, executive project director for the upgrade with the B.C. Ministry of Transportation and Infrastructure.
Because of the soil conditions, three specific techniques have been employed to stabilize the soil at the sites. This involves preloading and surcharging, that is, placing fill material, such as sand, on the ground prior to construction to consolidate and compress the underlying soil. Up to 600,000 cubic metres or 240 Olympic-size swimming pools worth of sand will be used for pre-load and embankment work.
Stone columns will also be used below ground to further stabilize the soil. A vibrating probe is used to push layers or columns of gravel into the ground. The vibrations compact the soil and create a solid foundation. Roughly 2,000 stone columns of varying depths will be installed in the project area, primarily in sections where roadway connections to new highway bridges are being built.
Concrete and steel piles, meanwhile, are being installed in areas where the ground can’t bear the weight of the structure without the possibility of the soil expanding, contracting or shifting. It’s anticipated that as many as 32 reinforced piles will be used in the construction of the bridges and overpasses, and 30 steel piles will be needed for highway overhead directional signage.
“Typically, a construction contractor will select which technique to use based on a number of factors, including ground conditions, loads, area accessibility, and the equipment available,” says Deane.
The consortium—consisting of Ledcor, Aecon and BEL Contracting—was awarded the design-build contract for the work in 2019. Presently, it is working on placing fill, construction embankments, installing the stone columns, and doing foundation and shaft drilling work for the construction of bridges and overpasses along the route. A small army of heavy equipment operators is on site, using excavators, earthmovers, compactors, cranes, sweepers and hydrovac trucks.
The project is designed to improve the operation of existing roads while making the routes safer and more efficient for local residents and commercial drivers. The fact it is an active highway complicated the matter more.
To keep traffic flowing, new roads, ramps and bridges are first built outside the existing highway. Once completed, vehicles are detoured onto the new alignments. The builder then modifies the existing highway roadway and opens it upon completion.
Maintaining the integrity of nearby Burns Bog adds another layer of complexity to the planning process. The nature reserve is a valuable natural space for local wildlife.
“The design-builder follows construction management plans based on best practices to avoid or minimize potential environmental impacts to Burns Bog, including air quality and dust control, soil erosion and sediment control, as well as protecting vegetation, wildlife and aquatic resources,” says Deane. “The design-builder has also obtained all necessary environmental permits for work in the area, which include obligations to protect and maintain adjacent environmental features.”
A number of techniques are being deployed to keep the bog safe during the construction period. For example, ditches on the project have been designed to drain stormwater away from the bog. Engineered berms will be constructed and planted adjacent to the bog to reduce the intrusion of non-bog water into the bog.
The transportation and infrastructure ministry is also monitoring the project during construction to ensure compliance with all environmental requirements and commitments.
“Post-construction monitoring will be undertaken to confirm the effectiveness of long-term environmental protection measures,” notes Deane.