Civil Engineering Reference
In-Depth Information
ranging between a maximum value,
K
max
, at the pile tip, and a minimum value,
K
min
,
according to
K
min
)e
−
μ
h
/
d
K
=
K
max
−
(
K
max
−
(4.9)
where the rate of degradation is controlled by the parameter,
μ
, which may be taken
as
05.
K
max
may be estimated as a proportion of the normalized cone resistance,
typically 2% of
q
c
/σ
v
for closed-ended piles reducing to 1% for open-ended piles, and
K
min
lies in the range 0.2 to 0.4, giving a minimum friction ratio,
∼
0
.
τ
s
/σ
v
, of 0.1 to 0.25
(Toolan
et al.
, 1990).
Where data from a cone friction sleeve are available, the friction ratio may be used
to refine estimates of
K
max
for closed-ended piles, provided an appropriate value for
the interface friction angle,
between cone and soil is adopted. However, it should
also be noted that data from friction sleeves are very sensitive to wear of the sleeve,
and may therefore underestimate
K
max
.
Piles driven into loose to medium deposits of sand provide considerable compactive
effect. As will be discussed in Chapter 5, the capacity of individual piles in sand
tends to increase as neighbouring piles are driven in a group, due to this compactive
effect. For dense deposits, the greater tendency of the sand to dilate will limit the
amount of densification close to the pile shaft. Again, though, driving further piles
close by will have a beneficial effect on the shaft capacity of a pile, due to vibratory
compaction.
On the basis of instrumented pile tests, some authors have recommended that shaft
friction for piles loaded in tension should be taken as half that for piles loaded in
compression. However, many such pile tests took insufficient account of the residual
stresses which exist after pile installation, and considerably underestimated the end-
bearing capacity of the pile, thus overestimating the shaft friction in compression.
Differences between shaft friction for piles loaded in tension and compression have
been assessed by De Nicola and Randolph (1993), who noted two main effects. The
first of these is due to contraction or expansion of the pile shaft due to Poisson's
ratio effects, while the second (which dominates for short piles) is due to differences in
effective stress changes in the soil as the pile is loaded in either direction. They proposed
the ratio of shaft capacity in tension to that in compression may be estimated from
Q
s
tens
δ
1
2 log
10
100
1
2
Q
s
comp
≈
−
0
.
−
8
η
+
25
η
(4.10)
L
/
d
where
Q
s
ν
p
being respectively the average soil shear modulus, Young's modulus of an equiva-
lent solid pile and Poisson's ratio for the pile. Although other effects, such as local
stress changes due to dilation, will influence the shaft capacity ratio, the expression
in equation (4.10) provides a reasonable design basis for assessing the reduced shaft
capacity for loading in tension, compared with that for loading in compression, with
typical ratios in the range 0.7 to 0.85.
A simplified design approach linking the shaft friction of driven piles directly to
the cone resistance,
q
c
, was suggested by Lehane
et al.
(2005) in their review of the
is the shaft capacity and
η
=
ν
p
(
L
/
d
)(
G
ave
/
E
p
) tan
δ
, with
G
ave
,
E
p
and
