In this week’s Explainer, we jump back into shored excavations (not literally of course). Part One of this series is an introduction to shoring. Or more simply, how do you make a hole in the ground that is five storeys deep with perfectly vertical walls, directly beside other buildings? How do you do that without damaging all of the things around it?
Why dig a deep hole in the first place? Why are there so many of these deep holes downtown, where it’s hardest to dig? The answer is that the underground space is necessary. The depth could be driven by a requirement for a certain number of parking spaces from the City or prospective tenants. Sometimes, foundation systems also impose restrictions. More on that in a future Explainer.
In most non-urban settings, 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.
Now that we understand when we need a shoring wall, how are they constructed? 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 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 Terraprobe.