DHARMSHALLOW FOUNDATIONS 613Floating foundation
The floating foundation is a special type of foundation construction useful in locations where
deep deposits of compressible cohesive soils exist and the use of piles is impractical. The con-
cept of a floating foundation requires that the substructure be assembled as a combination of
a raft and caisson to create a rigid box as shown in Fig. 15.10.Fig. 15.10 Rigid box caisson foundation using floating
foundation concept (McCarthy, 1977)
This foundation is installed at such a depth that the total weight of the soil excavated
for the rigid box equals the total weight of the planned structure. Theoretically speaking,
therefore, the soil below the structure is not subjected to any increase in stress; consequently,
no settlement is to be expected. However, some settlement does occur usually because the soil
at the bottom of the excavation expands after excavation and gets recompressed during and
after construction.15 .3 Choice of Foundation Type and Preliminary Selection
The type of foundation most appropriate for a given structure depends upon several factors: (i)
the function of the structure and the loads it must carry, (ii) the subsurface conditions, (iii) the
cost of the foundation in comparison with the cost of the superstructure. These are the princi-
pal factors, although several other considerations may also enter into the picture.
There are usually more than one acceptable solution to every foundation problem in
view of the interplay of several factors. Judgement also plays an important part. Foundation
design is enriched by scientific and engineering developments; however, a strictly scientific
procedure may not be possible for practising the art of foundation design and construction.
The following are the essential steps involved in the final choice of the type of founda-
tion:
- Information regarding the nature of the superstructure and the probable loading is
required, at least in a general way.