Pile Design and Construction Practice, Fifth edition

gravel layer, the variation in thickness of the layer should be determined and its continuity
across the site should be reliably established. The bearing stratum should not be in the form
of isolated lenses or pockets of varying thickness and lateral extent.
Where driven or driven and cast-in-place piles are to be installed, problems can arise when
piles are driven to an arbitrary ‘set’to a level close to the base of the bearing stratum, with
the consequent risk of a breakthrough to the weaker clay layer when the piles are subjected
to their working load (Figure 4.24a). In this respect the driven and cast-in-place pile with an
enlarged base is advantageous, as the bulb can be hammered out close to the top of the bear-
ing stratum (Figure 4.24b). The end-bearing resistance can be calculated conservatively on
the assumption that the pile always terminates within or just above the clay layer, that is, by
basing the resistance on that provided by the latter layer. This is the only possible solution
for sites where the soils are thinly bedded, and there is no marked change in driving resist-
ance through the various layers. However, this solution can be uneconomical for sites where
a dense sand layer has been adequately explored to establish its thickness and continuity.
A method of calculating the base resistance of a pile located in a thick stiff or dense layer
underlain by a weak stratum has been established by Meyerhof(4.37). In Figure 4.25 the unit
base resistance of the pile is given by the equation:

qb qo (4.35)

qlqo

10 B

H ql

190 Resistance of piles to compressive loads

Firm clay

Dense sand

Firm clay

(a) (b)

Figure 4.24Pile driven to end bearing into relatively thin dense soil layer (a) Driven pile

(b) Driven and cast-in-place pile.

Weak soil

~10B

Dense sand

Weak soil

Pile toe

D

H

ql

qp

qo

Figure 4.25End-bearing resistance of piles in layered soils.