Raft Foundation Design

Raft foundation designs are mostly used on construction sites with filled ground or with soft clays or they are often used on construction sites affected by trees. NHBC chapter 4.2 places further design requirements on raft foundations on sites affected by trees and often due to the stringent design rules – mainly the permitted depth of the stone cushion below the raft – rafts may not be possible and piling would be recommended Bullivants are a respected national piling engineering company amongst others. The benefit of raft foundations is that they have a characteristic stiffness and load spreading capability which standard strip foundations do not offer on filled ground or poor load bearing capacity strata, which makes raft foundations more suitable in the above scenarios such as construction sites with filled ground or weak strata. A raft foundation is essentially a continuous slab of reinforced concrete of varying thickness governed by design that sits under the entire building footprint, including walls and columns. As its name suggests it acts like a “raft” floating on the ground, distributing design loads over a larger area to prevent overloading the strata.

Raft foundations are not straightforward to construct; reinforcement cages can be fabricated off site and are a common and quick way to install the main beams. However, we would definitely recommend using contractors who are experienced with raft foundation construction as mistakes could prove to be very costly and, in some cases, could result in abortive work – which we have seen. Don’t skimp and use a professional.