In last week's Explainer, we looked at the construction work that takes place underground to support and prepare for the ascent of a structure. We touched on foundations in that post, but this time we describe a specific type of foundation known as a raft slab, as suggested by Forum member Daveography.

The beginning of a three-metre thick raft slab, image by Craig White

A raft or mat slab is typically utilized when the soil has low bearing capacity and needs to support heavy structural loads. A stiffened raft foundation is a thick slab of steel-reinforced concrete that covers the structure's surveyed footprint. Usually at least 100 millimetres thick, a stiffened raft slab is most suitable for fairly flat sites and contains both edge beams and internal beams.

Internal steel reinforcement, image by Edward Skira

Put simply, a raft slab is cast atop the soil, like a raft on the water, with no other foundations piercing the ground into the subsoil. It is often implemented as an alternative to a pile foundation, which requires deep excavation. Once the concrete has cured and the slab has been properly reinforced, further construction can commence.

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