Why do we design foundations at SLS and not ULS?
If we use ULS wouldn't we be taking a higher factor of safety?
SLS = Serviceability limit state. It considers how the structure is actually expected to behave, generally by using partial factors of 1.
ULS = Ultimate limit state. It considers complete structural failure, utilising partial factors to increase loads and decrease strength.
No. In fact, SLS frequently leads to a higher factor of safety than ULS.
This is because ULS deals with whether or not the structure will collapse. It does not care whether the structure will stand by generating absurd deformations, massive cracking (in the case of reinforced concrete structures), or if it will vibrate like a guitar string whenever you take a step.
SLS, on the other hand, does care about such things. It makes it so that your structure must not only stand upright, it must also not have excessive deformations, cracks or vibrations. This frequently means that a larger cross-section or more steel reinforcement is necessary to satisfy SLS than would be necessary according to ULS. This therefore leads to a larger safety factor.
Obviously, this isn't always the case. After all, ULS uses a reduced material strength and increased loads, while SLS uses the characteristic values for everything. Though SLS's underlying assumptions are quite conservative, in many cases ULS is still the controlling factor.
Foundations, however, deal with soil. The main concern here tends to be how the soil will behave under load. Once fully loaded, the structure will sink into the soil due to the applied compression. That is an unavoidable fact. The question is whether your structure will sink by one meter or one millimeter. That is the sort of question that is best answered by SLS. After all, so long as the structure is still standing, sinking by one meter is fine according to ULS.
And it's not as if foundations have no factor of safety. In fact, theirs is usually the highest safety factor in the entire structure. One usually defines that the soil's safety factor (ultimate strength divided by applied stress) must be greater than 2, at the very least. Concrete structures (other than their foundations) usually have a total safety factor of around 2, and steel structures of around 1.7.
