Civil Engineering Reference
In-Depth Information
Consequently, in both cases a rational design method would take the critical fric-
tion angle
φ
c
to determine a value of
N
q
for pile design. However, experiments and
in situ
tests indicate that use of
φ
c
with the values of
N
q
in Fig. 23.3(b) leads to
overconservative designs and often a peak friction angle
φ
p
is used in practice.
The base resistance of a single pile may also be estimated from the
in situ
probing
tests described in Chapter 16. The end bearing capacity of a pile is often equated with
the cone resistance measured during a static cone test (sometimes with a correction for
the different sizes of the pile and the cone) or derived from the standard penetration
test
N
value.
23.3 Shaft friction on piles
From Fig. 23.4 resistance due to shaft friction on a circular pile, diameter
D
, is given by
D
L
Q
s
=
π
0
τ
s
d
z
(23.5)
where
τ
s
is
very difficult to determine; it depends on soil, on the pile material and particularly on
the method of installation. For undrained loading of piles in clay,
τ
s
is the shear stress mobilized between the pile and the soil. The value of
τ
s
=
α
s
u
(23.6)
where
α
must be in the range 0
≤
α
≤
1. Typically
α
is taken to be about 0.5 for both
driven and cast
in situ
piles. For drained loading,
τ
s
=
σ
h
tan
δ
=
σ
z
tan
δ
K
(23.7)
σ
h
/
σ
z
and must be
where
K
is the ratio of the horizontal and vertical effective stresses
in the range
K
a
K
p
(where
K
a
and
K
p
are the active and passive earth pressure
coefficients discussed in Chapter 21);
≤
K
≤
δ
is the friction angle for shearing between the
Figure 23.4
Shaft resistance of piles.
