Civil Engineering Reference
In-Depth Information
100
Overland Corner
y
=
3
Laboratory grouted
driven pile data
Grouted Anchor
GST
y
=
2
RS
CNS
y
=
1
10
GDP- Esso
y
=
0.5
Data from Kulhawy and Phoon
1
Clay
Shale, mudstone
Shale, mudstone(rough socket)
Sandstone,
limestone, marl
0.1
0.1
1
10
100
1000
Normalized shear strength,
c
u
/
p
a
,
q
u
/2
p
a
Figure 4.16
Shaft friction data from piles in cemented materials (extended from Kulhawy and Phoon
(1993) by Randolph
et al.
(1998)).
piles, the value of
is close to unity, which also gives a conservative bound to the
data in Figure 4.16. A check should also be made in any design, that the assumed shaft
friction is not greater than 5% of the concrete strength.
ψ
4.1.3.3 Correlations with SPT in chalk
Hobbs and Healy (1979) have presented an extensive review of the performance of
piles in chalk, and have correlated shaft friction and end-bearing pressures for different
pile types with the SPT value of the chalk. For large displacement driven piles, they
recommend using an effective stress approach for estimating the shaft capacity, taking
a value for
K
of 0.06 N, and interface friction angles of 20
◦
(for steel piles) to 24
◦
(for concrete piles). Higher values of
K
(up to 0.15 N) were recommended for driven
cast-in-situ piles where high-slump concrete was compacted against the chalk, while
lower values of
K
(0.03 N) were suggested for low-displacement piles. End-bearing
pressures were shown to correlate with
N
values with ratios of
q
b
/
N
lying between
0.2 and 0.25 MPa. Figure 4.18 shows summaries of measured values of shaft fric-
tion and end-bearing pressure, plotted against SPT values, taken from Hobbs and
Healy (1979).
For bored piles in chalk, where interlocking and possible chemical bonding occurs
between the pile and the chalk, Hobbs and Healy found that the shaft friction cor-
related well with the compressive strength of the chalk, according to the relationship
given in equation (4.25) with
1. The shaft friction of CFA piles has been reviewed
by Lord
et al.
(2003) who suggest an effective stress approach, although the correla-
tion of shaft friction with vertical effective stress is very poor. A direct correlation of
ψ
∼
