If the weight of the building is equal to the weight of the soil and water removed from the excavation, then it is evident that the total vertical pressure in the soil below depth D in Fig. 14.4(c) is the same as in Fig. 14.4(a) before excavation.
Since the water level has not changed, the neutral pressure and the effective pressure are therefore unchanged. Since settlements are caused by an increase in effective vertical pressure, if we could move from Fig. 14.4(a) to Fig. 14.4(c) without the intermediate case of 14.4(b), the building in Fig. 14.4(c) would not settle at all.
This is the principle of a floating foundation, an exact balance of weight removed against weight imposed. The result is zero settlement of the building.
However, it may be noted, that we cannot jump from the stage shown in Fig. 14.4(a) to the stage in Fig. 14.4(c) without passing through stage 14.4(b). The excavation stage of the building is the critical stage.
Cases may arise where we cannot have a fully floating foundation. The foundations of this type are sometimes called partly compensated foundations (as against fully compensated or fully floating foundations).
While dealing with floating foundations, we have to consider the following two types of soils.
Type 1: The foundation soils are of such a strength that shear failure of soil will not occur under the building load but the settlements and particularly differential settlements, will be too large and will constitute failure of the structure. A floating foundation is used to reduce settlements to an acceptable value.
Type 2: The shear strength of the foundation soil is so low that rupture of the soil would occur if the building were to be founded at ground level. In the absence of a strong layer at a reasonable depth, the building can only be built on a floating foundation which reduces the shear stresses to an acceptable value. Solving this problem solves the settlement problem.
In both the cases, a rigid raft or box type of foundation is required for the floating foundation [Fig. 14.4(d)]
Figure 14.4 Principles of floating foundation; and a typical rigid raft foundation