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This week, guest contributor and geotechnical engineer, Michael Diez de Aux, is taking over our 'Explainer'. This is the first part of a series that will look at shoring, or the supporting of walls in an excavation site.
For most developments of any height, underground levels, often several of them, are needed for programmatic aspects of developments. Buildings connect to services such as water, gas, and hydro, and mechanical equipment for that is normally underground. In suburban areas, or more traditional downtown buildings, garage space for storing motor vehicles is large. (Newer buildings downtown, and those close to higher order transit, are no longer needing such large garages as the number of cars being purchased these days is dropping.) Servicing by waste disposal tracks or moving vans can take up a lot of space, and if the servicing area cannot be fit in at ground level, it normally needs to go underground. Bicycles are being allotted increasing amounts of space in buildings now too, while space for storage lockers is needed too. If these can't fit on a podium level, then underground is where architects normally located them. Finally, buildings need support from below, and various types of foundations require varying amounts of space.
Depending on how much depth is needed for all of those items, (we will look more at some of that in a future Explainer), suffice it to say, a dig is normally necessary. In less confined environments, shoring may not be required; an excavation can be made easily by cutting the sides of the hole back to form slopes. This is best and cheapest when there is enough room, as side-slopes don’t need special engineering design or monitoring.
But what if your property is immediately beside a road, a building, or a subway tunnel? If you are digging in soil, you can’t dig right beside a road or a footing without undermining it, causing the founding soils to lose their load-bearing capacity. That would be like pulling the rug out from under a structure. Not to mention the fact that vertical holes made in soil tend to collapse quickly. So, a temporary shoring wall is needed to hold up and support the structure behind it, as well as maintain a safe excavation for the construction crew to work in.
So, how do you make a hole in the ground that is several storeys deep with perfectly vertical walls, directly beside other buildings? And how do you do that without damaging all of the things around it? A shoring wall is basically composed of two elements: the columns and the filler between the columns. The columns are basically steel beams which give the wall most of its strength. The filler between the columns can be wood or concrete, depending on how stiff and how strong and how water-tight the wall needs to be.
Shoring walls that are made to support buildings, tunnels, foundations, and other critical structures that can’t be allowed to move at all are usually caisson walls. Caisson walls have strong columns of steel inside concrete, and concrete filler between the columns. This kind of wall can be designed to not move at all because it is very strong and very stiff.
However, it’s not always necessary to have a wall that doesn’t move at all. A more flexible wall that moves about 10 to 15 millimeters in total (i.e. less than the width of a thumb) will still do a great job of supporting an asphalt pavement and a sidewalk without damaging anything. These walls are typically made as soldier pile and lagging walls, which have steel beam columns (with concrete only at the base to form the foundation of the column) and wood planks bridging the gap as filler. With no concrete, the wall is obviously less stiff, and it will move a bit more. But it will still do the job in many cases, and so it is the economical choice where there are no structures to support.
There are other shoring system elements which keep a wall stiff and prevent movement, that we’ll discuss in future Explainers. These include tiebacks, rakers, struts, and other mid-wall bracing.
In the next Explainer, we will delve into how these walls are constructed.
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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