Over the last two weeks, geotechnical engineer Michael Diez de Aux has contributed special guest editions of UrbanToronto's 'Explainer.' While the first week's feature offered a detailed introduction to the shoring process, last week's took a step-by-step look at how shoring is constructed. This week, Diez de Aux looks into support systems that ensure structurally sound walls during excavation.
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A shoring wall is a massive vertical construction wall that needs to do two things. One, it needs to hold an excavation open. Two, it needs to support whatever building or road is directly on the other side of the wall. This is the only way to dig deep in an area where space is at a premium and there are adjacent structures everywhere. But what makes these walls structurally sound? How are they braced so that they don’t buckle and bend? In this week’s Explainer, we look at shoring support systems and how they brace excavations.
Shoring walls that are made for the construction of only one basement can be made short enough that they can be cantilevered. A cantilevered beam basically acts like a diving board. You have the end that’s fixed, and then you have the flexible end. For a cantilevered shoring wall, the soldier piles are drilled much deeper than the base of excavation (up to two to three times as deep), which provides enough lateral (sideways) support that this essentially fixes the beam in place. The part of the pile that is exposed is the part acting like a diving board, which supports the excavation and can bend a little without moving too much.
However, shoring is more frequently used to go two or more basement levels deep, and in this case the wall will need to be structurally reinforced some other way. Basically, we need to pin the middle of the wall in place, where it is most prone to buckling. You can do this with tiebacks, rakers, or struts.
Tiebacks are usually preferred because they keep the excavation clean and open. Essentially, a tieback is a massive soil or rock anchor. A tieback machine drills an angled hole through the wall, puts a large steel cable in the hole, and then pumps concrete into the hole to backfill it. The concrete bonds to the cable and acts in friction against the soil/rock to resist being pulled out. Then the tieback cable is attached to the soldier pile, and prestressed (loaded) so that it is actively holding the soldier pile back.
Tiebacks are great if your next-door neighbour will allow you to drill into their land and install them. The City of Toronto usually allows this, for example. They’ll even allow it within a very small distance of a subway tunnel! There’s no real harm as long as the cable is destressed (cut) after the excavation is filled with the new building. But, some neighbours just won’t let you encroach, or their building may be too close. Then you need to look at alternative ways of bracing your shoring wall in the middle.
A raker is basically an enormous kickstand for your shoring wall. These are easy to build, but they sit inside your excavation (tiebacks extend outside the excavation) and make life difficult when you need to make a building around them. They are a necessary evil when tiebacks are not possible or not allowed.
You can also brace your shoring walls against each other, and avoid the massive kickstand. A strut is a beam that braces opposite sides of an excavation against each other, so that as they flex into the excavation they are pushing on each other and propping each other up. This is less popular because the struts do get in the way of putting equipment and concrete into the hole, but they are popular support methods in places where you might otherwise need rakers on all walls (which would make a very messy excavation).
There are other kinds of shoring, like slurry walls, which we won’t explore today. Needless to say, if you’ve read this far please let us know if you’d like a follow up on alternative shoring systems!
Guest contributor Michael Diez de Aux is a geotechnical engineer with Grounded Engineering.
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From 2015 to 2017, UrbanToronto and its sister publication, SkyriseCities, ran an occasional series of articles under the heading Explainer. Each one took a concept from Urban Planning, Architecture, Construction, or other topics that often wind up in our publications, and presented an in depth look at it. It's time to revisit (and update where necessary) those articles for readers who are unfamiliar with them. While you may already know what some of these terms mean, others may be new to you. We are publishing or updating and republishing Explainer on a weekly basis. This article is an update of one that first appeared in 2016.
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