DHARMSHALLOW FOUNDATIONS 639
case of clay, minimum pressure occurs in the middle and maximum (in fact, sometimes, very
high) pressure occurs at the edges. It is also interesting to note that the pressure under a raft
on clay may vary with time (Teng, 1949), and the worst conditions expected are to be consid-
ered for design.
It is unlikely that the edge pressure will exceed twice the average pressures.
As an alternative to the relatively high cost of a stiff raft of large-size above a compressible
deposit, substantial economy can be realised by designing a flexible raft and superstructure
that can deform without damage into the shape corresponding to the compression of the subsoil.
It may often prove preferable to accept the deformations if the cost of a stiff foundation can be
avoided. The design of a flexible raft foundation cannot be readily base on the calculation of
stresses in the slab. Instead, it is necessary to estimate the maximum curvature to which the
raft may be flexed, and to select the thickness of the slab and the quantum of reinforcement
such that the slab will not develop cracks large enough to permit a serious leakage of ground
water. As an approximate guideline, 1% of steel may be provided in each of two directions at
right-angle to each other, equally divided between the top and bottom of the slab. The thickness
of the slab should not be generally greater than 1% of the radius of curvature, though local
increases of thickness near columns and walls mey be required to prevent shear failures.
15.7.6Construction of Raft Foundations
Raft foundations are invariably constructed of reinforced concrete. They are poured in small
areas such as 10 m × 10 m to avoid excessive shrinkage cracks. Construction joints are care-
fully located at places of low shear stress—such as the centre lines between columns. Rein-
forcements should be continuous across points. If a bar is spliced, adequate lap is provided.
Shear keys may be provided along joints so that the shear stress across the joint is safely
transmitted. If necessary, the raft may be thickened to provide sufficient strength at the joints.
*15.8 FOUNDATIONS ON NON-UNIFORM SOILS
It is generally assumed that the subsoil is relatively uniform either to a very great depth or
else to a limited depth where a firm base is encountered. In reality, such situations are so
uncommon as to be considered rare exceptions. The procedures of foundation design are not
often directly applicable to practical problems; but these may be modified to give reliable indi-
cations of the probable behaviour of foundations on non-uniform deposits.
Most subsoils consist either of definite strata or more or less lenticular elements. On the
basis of preliminary information, such as that from exploratory borings together with stand-
ard penetration tests and simple laboratory tests, it is possible to identify deposits which are
sufficiently strong and incompressible. This would enable one to concentrate on the weaker or
more compressible strata, so as to ascertain their influence on the behaviour of the proposed
foundation. The load-carrying capacity of the doubtful materials is ascertained and based on
failure or permissible settlement. Usually this information is adequate for a selection of the
proper type of foundation. Sometimes, more elaborate exploratory procedures and soil tests
may be required to provide the basis for a sound decision.
Stresses may be computed using Newmark’s chart or by some simplified procedure.
Although the chart is based on the assumption that the material is homogeneous, the errors