Construction on the 14-kilometre LRT line from Lewis Farms to downtown Edmonton is well underway. If you’ve spent any time in the Lewis Farms area and along 87 Avenue, you’ve probably noticed a lot of earth being moved.
One of the largest parts of the project is an elevated guideway that will house two elevated stations at the West Edmonton Mall and the Misericordia Hospital.
Before construction of the elevated guideway can begin, Marigold Infrastructure Partners (MIP) must first drill holes and install caissons to create the foundation. The foundation is a concrete structure below the ground that will act as a base to support the guideway. Crews are busy completing this work between 170 Street and 175 Street, along 87 Avenue. Most of the activity right now is taking place near the old West Edmonton Mall Transit Centre.
So, what are caissons, you ask?
Caissons—borrowed from a French word meaning “large box”—are hollow cylinders put into the ground and filled with concrete to create a deep foundation. They become large, reinforced concrete columns that will support the piers for the elevated guideway. This construction will transfer heavy loads from the elevated guideway deep into the ground and reduce noise and vibration when trains cross the elevated stations.
Each caisson (pronounced “kay-sohn”) measures 2.5 metres in diameter, which is about 8 feet. When lying on its side, a caisson would hover over your tallest friends.
In October, a drilling rig was assembled and drilling began. In the picture above, crews are using a large auger to drill into the ground. An auger is a steel spiral drill bit that resembles a huge screw. The drilling rig screws the drill bit into the ground, digging a deep hole while moving soil to the surface.
The hole for the first caisson along 87 Avenue was drilled near the end of October. Crews will drill 35 holes between 165 Street and 178 Street. Each one will be about 25 metres deep (about 82 feet) and will establish a solid foundation for the future guideway.
After the holes are drilled and the caissons are inserted, the crews will use a crane to install reinforcing steel (also referred to as “rebar”) cages into the holes. Once all the elements are in place, the holes will be filled with concrete. The rebar cages and concrete work together as “reinforced concrete,” which will allow the deep foundation to sustain heavy loads from the elevated guideway and to resist big storms, powerful winds, and heavy snowfalls. This design plays a role in ensuring the new LRT system is climate-resilient and achieves a 100-year service life.
Once the foundations are complete, MIP will build piers for the superstructure to rest upon. A superstructure is built above the foundation and, in this case, refers to the bridge structures of the guideway that will hold the tracks for the trains.
The drilling and foundation work is expected to take several months. The project team will work to limit disruption to the public as much as possible; however, you can expect some noise and vibration, traffic detours, and lane and sidewalk closures. Also, watch out for activities around construction sites and workers. Always make sure to follow road rules and posted signage.