Raft foundation

[Jake2021]

Hi all,

Could someone explain a raft foundation to me? I sort of get the just that it’s a concrete slab with rebar but wondered if anyone can go into more detail.

I’m currently looking at PD for a two story extension however as the soil is more soggy clay would a raft be better then trench fill?

If so how deep do I need to go down for the slab etc?
Thanks
Jake

 

[^woody^]

The selling point of a raft is that it can spread the building load over a wide area, so may be good for ground with poor load bearing capacity. Think snow shoes.

It's not so good for ground that moves seasonally. Think a raft on rough sea.

It's a engineering decision anyway, not really a custoner choice from a list. Rafts can be required to be placed quite deep, not just near the surface.

 

[noseall]

Hi all,

Could someone explain a raft foundation to me? I sort of get the just that it’s a concrete slab with rebar but wondered if anyone can go into more detail.

I’m currently looking at PD for a two story extension however as the soil is more soggy clay would a raft be better then trench fill?

If so how deep do I need to go down for the slab etc?
Thanks
Jake

A raft is an designed/engineered foundation, that is deeper at the edges (where walls sit above) than in the centre. It's all locked together via mesh and rebar, strategically placed and wired together. It's meant to act as one unit.

Raft is NEVER preferred over trench fill. Its too expensive and semi-specialist.

Clay isn't prohibitive regards trench foundations. But you do need to look at local shrubbery or trees etc.

 

[George_engineer]

If the house is built off a raft then the extension should also be.

Conversely, if the house is built of strip footings then the extension should be too.

Yes, simply put rafts are concrete slabs with additional reinforcement bars to counteract the additional forces that the slab will experience due to the superstructure being built off of it (slabs are normally just a floor load). It is the specification (size, layers, orientation, edge details) of the reinforcement that needs engineering input. Much of the extra cost is the steel fixing time.

In clay you would still expect a substantial amount of ground work to prepare for a raft, as they are still somewhat susceptible to heave. This is usually 50% of the depth of a strip footing. So if Building Control say you need 2.0m deep strip footings, the raft will need shrinkable subsoil taken out to a depth of 1.0m, which is also a significant cost.