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UrbanToronto regularly brings you news of construction across the city, and the great majority of the buildings that we cover start with deep excavations. While developers of suburban sites where there is lots of space can go a simpler route and dig out sloped walls around their pit, most of the buildings going up here are on constrained sites and require sheer walls straight down into the city's substrate. Often dug to property lines—the pits normally require vertical retaining walls to safely hold back materials when digging multiple levels below grade.
Two types of shoring systems are commonly used on Toronto sites; watertight caisson wall systems needed for sites with waterlogged soils, and the quicker and more cost-efficient pile and lagging systems for drier sites. Now, photographs from a number of Toronto construction sites with pile and lagging shoring walls—Artists' Alley, 859 The Queensway and Rush Condos—offer a close-up look at how this type of shoring wall is formed.
The process begins with a cleared and graded site. With flat ground upon which they can work, drilling rigs drill a series of boreholes around the perimeter of the site, into which steel I beams known as soldier piles can then be lowered. As seen above at Artists' Alley, a shoring rig with a drill bit is ready to create another borehole, while soldier piles line the edge of the site to the right of the photo, and another two opt out of the earth in the foreground. In loose soils, the piles go deeper into the ground than the excavation will, and it's below the eventual floor level of the pit to which concrete will be poured to tightly secure the pile at the bottom of its borehole. If the excavation will go into the bedrock, in most cases the pile needs only to go deep enough to be secured into the bedrock.
Soldier piles act as vertical bracing against which timber lagging is slotted. The lagging limits the horizontal movement of the soil behind. At the Rush Condos site above, bundles of lagging have been delivered on flatbread trucks in anticipation of being slotted between the piles.
Once soldier piles are in, a couple metres worth of earth are excavated, exposing the soldier piles and freeing up space for the first rows of horizontal lagging. The timber is then positioned against the inner flange of the I-beams, section by section. Once placed, excavation work moves deeper to reveal more of the buried I-beams, priming the pit for the next row of lagging to be placed. The closer look into the excavation pit at 859 The Queensway shows this process in action: excavation recently progressed below sections of already-placed lagging, leaving a void below around the exposed soldier piles. One of the workers in this photo can be seen shovelling soil through the gap in the shoring wall to firm up the surrounding soil behind it. The gap will be sealed by placing a thinner piece of lagging into it, or forcing down the boards from above.
As the pit deepens, the process continues.
You can learn more from our Database file for the associated projects, linked below. If you'd like to, you can join in on the conversation in the associated Project Forum threads, or leave a comment in the space provided on this page.
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