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Now that we’ve discussed what shoring is and why it might be needed, the next logical step is to ask: how does shoring get built? How do vertical walls get installed before the ground is even excavated? How do you hold up a building with such a wall? We dig deeper into shoring wall design (pun intended) in this week’s Explainer.
The special thing about a shoring wall is that it must already be in the ground before you start digging. It is not possible to excavate a five-storey-deep hole in the middle of a city, and then worry about holding it up afterwards. In fact, the Occupational Health and Safety Act prohibits access to unsupported excavations that are more than 1.2 metres deep unless they are appropriately sidesloped or otherwise supported. The supporting wall has to be made so that it is holding up your neighbour when the hole is one-storey deep, then two storeys deep, and so on. If you are excavating along a shoreline, it would also have to hold back the lake, river, or ocean. The shoring for Toronto's Living Shangri-La excavation was made to support operational TTC subway tunnels under University Avenue.
So how does one build such a wall underground, from the surface? Using a really big drill called a caisson drill rig. Depending on the size of the rig, a caisson rig can drill a hole up to five metres in diameter and around 40 metres deep. That’s up to two cars wide and 11 storeys deep! This also explains why demolition sites are made flat with surface, instead of just opening up the basement and leaving a big gaping hole. A working platform is needed for the drills.
After drilling, a big steel I beam is dropped into the hole. The bottom of the hole is filled with concrete so that the beam — which is now a 'soldier pile' — has a solid foundation. Similar soldier piles are drilled all the way around the excavation at some nominal spacing (usually about three metres). After the soldier piles are in, excavation begins. As the hole gets deeper, the soldier piles become progressively exposed, and wood lagging is hammered in between the piles to fill in the gaps. The soldier piles and lagging support the sides of the excavation in this way, and are simply known as a soldier pile and lagging wall.
However, wood lagging is not going to hold up a neighbouring structure, which might even be a heritage building. Regardless of the nature of the building, they all need their doors to open nicely, and that is not possible if the building shifts from rectangle to parallelogram. A soldier pile and lagging wall does a good job of holding back soil, and even roads and sidewalks, but this system is not stiff enough to hold back a building or subway tunnel. If you need a very stiff wall to prevent adjacent structures from moving, you’ll need a wall made of concrete. But the approach is much the same. Soldier piles are drilled, but they are now filled all the way up with concrete so that the steel beam is completed encased (a 'caisson'). Then, the gaps between the caissons are filled in by drilling filler holes. The filler holes bite into the adjacent concrete so that they overlap (or 'interlock'), like an infinite linear Venn diagram. The filler holes are also filled with concrete. Now, instead of soldier piles holding wood lagging, you have interlocking concrete caissons, known as a caisson wall. These walls are made from thick concrete, which will flex much less and can maintain adjacent structures at rest. They are also relatively watertight, which is useful for supporting things like lakes.
Regardless of construction type, shoring walls are usually supported with external elements if they are more than one level deep. At about one basement depth, these walls can be cantilevered so that the toe of the wall is resisting the lateral force. At about two basement levels deep, these walls could buckle and bend if left unsupported. The shoring designer could opt to use really big steel, very large boreholes, and very strong concrete, but it is usually more economical to build these walls with supports instead. Support systems for shoring walls will be explored in a future Explainer.
Guest contributor Michael Diez de Aux is a geotechnical engineer with Terraprobe.
