DHARM640 GEOTECHNICAL ENGINEERING
due to stratification or other irregularities are not likely to be significant enough to invalidate
the predictions of the probable behaviour of the soil.
In the following subsections, the more important kinds of non-uniform soil deposits will
be discussed.
15.8.1 Soft or Loose Strata Overlying Firm Strata
This situation is relatively simple to deal with since an unsatisfactory character of the materi-
als is likely to be apparent and rarely overlooked. The important decision is whether or not a
footing foundation may be used. This may be determined by computing the safe load on the
basis of the soft deposit assuming that it extends to a great depth. If the computed safe load is
too small, or the computed settlement too great, footings should be eliminated from considera-
tion. Provision of piles or piers or using a fully compensated or floating raft foundation are the
two possible alternatives.
15.8.2Dense or Stiff Layer Overlying Soft Deposit
The implications of the presence of a soft deposit at some depth below firm strata are not very
obvious, as when the soft materials are at shallow depth. If the firm stratum is not sufficiently
thick, footings or rafts may exert sufficient pressure to break into the underlying soft soil.
Even if the overlying firm layer is of sufficient thickness to prevent such a failure, the settle-
ment of the structure due to consolidation of the soft deposits may be excessive.
If the loading does not exceed the safe capacity of the underlying soft deposit, failure by
breaking through the overlying stiff crust will be highly improbable. If the footings are widely
spaced and the firm layer fairly thin with respect to the width of the footings, the stress at the
top of the soft layer can be considerably decreased by increasing the size of the footings. On the
other hand, if the footings are spaced rather closely and the firm layer is comparatively thick,
the distribution of pressure at the top of the soft layer cannot be altered radically by changing
the contact pressure (Peck, Hanson and Thornburn, 1974).
Even if the safe load on the soft soil beneath the firm layer is not exceeded, the settle-
ment of a footing or raft may be excessive. The settlement may be computed according to the
procedures given earlier, and if it is excessive, one of the other types of foundations must be
adopted.
If the computed settlement is not excessive and if the firm layer is thick enough to
prevent a bearing capacity failure the footing can be designed as if the soft deposit were not
present.
15.8.3Alternating Soft and Stiff Layers
If a deposit contains a number of weak layers, bearing capacity and settlement computations
may be made for each. If the structure cannot be supported on footings, piles or piers may be
used to transmit the loads to one of the firm strata at sufficient depth to provide a satisfactory
foundation. This depth may be determined from computations. The choice between piles and
piers, or of the type of pile to be used, is likely to depend on the difficulty that may be experi-
enced in driving through the firm strata involved. Conclusions with regard to this aspect have
to be based on the results of driving test piles.