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Foundation Design - Raft Slab

Stiffened Raft Foundation

Raft Foundations or Waffle Slabs as they are sometimes called were developed in the 1980’s, as a response to more traditional strip footing foundation failures in expansive clays.

Expansive clay soils shrink and swell significantly throughout the year in response to moisture levels and are common in Northern New Zealand. This shrink / swell cycle puts large pressures on in-ground foundations and is responsible for foundation cracking and movement which can cause damage brick facades and internal plaster board walls.

The raft foundation avoids many of these issues by sitting on top of the ground and having a lattice of steel reinforced concrete ribs and beams to provide support and stiffness. Because they do not require excavation and are specifically designed for each building they can be more robust, quicker to build and often less expensive than traditional foundations.

As they are specifically engineered for each structure it is not “one size fits all”. Accurate numerical loading calculations and rib placement are critical.

Ashby Consultants has spent time developing the tools and expertise to design strong, cost effective stiffened raft foundations for domestic and commercial projects.

Contact Ashby Consultants.


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