Geotechnical Engineering

DHARM

SHALLOW FOUNDATIONS 635

strength for relatively large column loads. (c) is a raft with thickened bands provided along
column lines in both directions; this provides sufficient strength, when the column spacing is
large and column loads unequal. (d) represents a raft in which pedestals are provided under
each column; this alternative serves the same purpose as (b). (e) represents a two-way grid
structure made of cellular construction and of intersecting structural steel construction (Teng,
1949). (f) represents a reft wherein basement walls have been used as ribs or deep beams.

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(a) Flat slab type

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(b) Flat slab thickened under columns

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(c) Two way beam and slab type

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(d) Flab slab with pedestals

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(e) Celluar type

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(f) Basement walls as rigid frame
Fig. 15.28 Common types of raft foundations (Teng, 1976)
A raft foundation usually rests directly on soil or rock: however, it may rest on piles as
well, if hard stratum is not available at a reasonably small depth.

15.7.2Bearing Capacity of Rafts on Sands

Since the bearing capacity of sand increases with the size of the foundation and since rafts are
usually of large dimensions, a bearing capacity failure of raft on sand is practically ruled out.
As a raft bridges over loose pockets and eliminates their influence, the differential settlements
are much smaller than those of a footing under the same pressure. Hence, higher allowable
soil pressures may be used for design of rafts on sands.

Terzaghi and Peck (1948), as also Peck, Hanson and Thornborn (1974), recommend an
increase of 100% over the value allowed for spread footings. The design charts developed for
the bearing capacity from N-values for footings on sands may be used for this purpose. The
effect of the location of water table is treated as in the case of footings.