In last week's Explainer, we looked at the different stages of building construction. But before a skyscraper can begin its ascent, the site has to be prepared in several ways. In today's Explainer, we explore the work that takes place underground to ensure the tower above is structurally possible. 

A typical ground breaking ceremony, image by Marcus Mitanis

The official first step of construction is ground breaking. The precursor to excavation, a ground breaking ceremony gathers the project partners and stakeholders together to celebrate the beginning of construction. A ground breaking ceremony does not always coincide with the physical disturbance of the soil however, it sometimes occurs well before or well after digging has begun. 

Caisson wall and a wood lagging wall, image by Marcus Mitanis

The building's foundation needs to be strong enough to support the tower's massive weight and transfer its load to the earth. Piling is the process of driving a vertical cylindrical support — made of concrete or steel — into the ground to stabilize the structure on top. For deep excavations, steel soldier piles are installed within a few metres of each other around the perimeter of the site for soil retention purposes. A lagging wall, usually made of wood, is placed between these piles. 

Soldier piles and wood lagging wall, image by Marcus Mitanis

Alternatively, caisson walls composed of interlocking concrete piles are used to minimize the flow of groundwater into the site. The walls can be braced with struts or tiebacks. This process of supporting the underground loads is known as shoring. Once the site is secure and the walls are stable, footings are installed to support the building foundation. These footings are typically comprised of concrete reinforced with steel rebar. 

Caisson wall utilizing tiebacks, image by Marcus Mitanis

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