We know raft or mat foundation cover entire floor area of structure; it is a combined foundation which support all columns and walls (structural or non-structural). We will discuss here about advantage of raft foundation in weak soil i.e. soil having very low bearing capacity.
In such condition raft foundation serves three advantages like-
- Ultimate bearing capacity as proposed by different researchers have relation with width of foundation such that with increase in width ultimate bearing capacity is also increased which brings deeper layers of soil in effective zone.
- The settlement of foundation is also decreased with increasing depth.
- The erratic behavior of foundation soil, offers always differential settlement. This type of settlement is very harmful to structure as stresses concentrated at weak points of framing system which opens cracks and often lead to failure. Raft foundation minimizes differential settlement and offer act as a bridge over cavities.
Structures have a limiting value of differential settlement not to be undergone damage. It is found that differential settlement measured in different parts of elements supported by raft foundation is significantly lower than identical structure supported on individual footings. It is interesting to notice that maximum settlement or total settlement has found same.
Again for raft or mat foundation allowable settlement is also considered more than allowable settlement for structures supported on individual footing; in the other word, this foundation permits higher bearing capacity in this situation.
Now we have to consider depth of soil layer having poor bearing capacity. When deeper soil layers have very poor bearing property, increasing width of foundation (like raft) will not always offer higher bearing capacity. We know zone of influence of foundation is a function of foundation width. So where soil at shallow layer have comparatively fair bearing capacity than underlain deeper soil layers (having much weaker soils), it is recommended to use individual footings as zone of influence of foundation is kept within relatively stronger layer at the top. So, mat foundation is discouraged in this situation.