Foundation, Concrete and Earthquake Engineering

How are Raft Foundations Used to Reduce Settlement?

In the previous post we discuss the construction method of raft foundation and have an over all idea of the raft foundation. In this post we will share some information about settlement reduction raft foundation. as we learned earlier rafts are nothing but a flat slab reinforced with steel. In the design purpose equivalent frame method of flat plate/slab is used as recommendation of some codes. When C.G. (center of gravity) of loads arrived from individual columns or share walls coincide with raft’s centroid, the upward reaction from soil becomes uniform. The uniform upward reaction is equals to the sum of all loads from the columns and other members divided by area occupied by raft. It is important to note that in structural design self weight of raft is not considered as self weight is supposed to rest directly on the subsoil. Here no consideration for differential settlement (moment and shear form differential settlement) is accounted and the effects of differential settlement are compensated by using heavier reinforced section than that required from above analysis.

In case of compressible soils, sometimes the total raft foundation is made compensated foundation.

Now question is what is compensated foundation?

Raft Foundation: Thick raft slab to absorb differential settlement as well vibration of machine to supported on basement
The answer is the raft slab is provided at such a depth that the weight of soil excavated is equal to the weight of raft slab plus that of structure to be supported.

Theoretically there should be no settlement as no pressure is increased at that level. Practically the settlement is also insignificant. But total compensation is often impossible or impractical as weight of structure often becomes too large. In this situation partial compensation is used so that the settlement is in tolerable limit.
Raft foundation with stiffening T beam to form a cellular foundation
So far we considered differential properties of soils. But if loads from different columns are not distributed uniformly the raft should be stiffened by T-beams connecting the raft with the stems. In case of subsoils that are susceptible to large differential settlement this stiffening method is also applicable.

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