DHARM638 GEOTECHNICAL ENGINEERINGka and kb must be evaluated by at least two tests using two different sizes, say 300 mm square
and 600 mm square.
15.7.5General Considerations in the Design of Rafts
The conditions under which a raft foundation is suitable have already been discussed. In its
simplest form a raft consists of a reinforced-concrete slab that supports the columns and walls
of a structure and that distributes the load thereform to the underlying soil. Such a slab is
usually designed as continuous flat-slab floor supported without upward deflection at the col-
umns and walls. The soil pressure acting against the slab is commonly assumed to be uni-
formly distributed and equal to the total of all column loads, divided by the area of the raft.
The moments and shears in the slab are determined by the use of appropriate coefficients
listed in codes for the design of flat-slab floors.
On account of erratic variations in compressibility of almost every soil deposit, there are
likely to be correspondingly erratic deviations of the soil pressure from the average value.
Since the moments and shears are determined on the basis of the average pressure, it is con-
sidered good practice to provide the slab more reinforcement than the theoretical requirement
and to use the same percentage of steel at top and bottom (Peck, Hanson and Thornburn,
1974).
The flat slab analogy is valid only if the differential settlement between columns is
small and furthermore, if the pattern of the differential settlement is erratic rather than sys-
tematic. Also, even if deep-seated or systematic settlements are negligible, the flat-slab anal-
ogy is likely to lead to uneconomical design unless the columns are more or less equally spaced
and equally loaded. Otherwise, differential settlements may lead to substantial redistribution
of moments in the slab.
Under such circumstances, rafts are sometimes designed on the basis of the concept of
the modulus of subgrade reaction, which implies that soil is considered to be analogous to a
bed of closely and equally spaced elastic springs of equal stiffness in its stress-strain behaviour.
Evaluation of the modulus of subgrade reaction, k, for design is not a simple problem since k is
known to vary in a complex manner on the shape and size of the loaded area, as well as on the
magnitude and position of near-by loaded areas. [For IS procedure, refer ‘‘IS: 2950
(Part-I–1974 Code of Practice for Design and Construction of Raft Foundation—Part-I Design’’].
If a raft covers a fairly large area and significantly increases the stresses in an underly-
ing deposit of compressible clay, it is likely to experience large systematic differential settle-
ments. For these to be avoided, strength of the slab alone is not sufficient, but stiffness is also
required. However, a stiff raft is likely to be subjected to bending moments far in excess of
those corresponding to the flat-slab or subgrade modulus analyses (Peck, Hanson and
Thornburn, 1974). These moments may require deep beams or trusses. Thus, the raft in such
instances may be considered to consists of two almost independent elements: the base slab,
which may still be designed by the flat-slab analogy; and the stiffening members, which have
the function of preventing most of the differential settlement of the points of support for the
base slab.
It has been known that contact pressure distributions in sand and clay are different
from the uniform distribution commonly assumed in conventional raft design. In the case of
sand, maximum pressure occurs at the middle and minimum, if any, occurs at the edges; in the