Civil Engineering Reference
In-Depth Information
The range over which the average is taken is extended up to eight pile diameters above
the level of the pile base. Thus, in homogeneous sand, the relevant cone resistance,
q
c
design
is estimated as (see Figure 4.5)
q
c
design
=
(
q
c
1
+
q
c
2
+
2
q
c
3
)
/
4
(4.6)
where
q
c
1
=
average cone resistance over two diameters below pile base.
q
c
2
=
minimum cone resistance over two diameters below pile base.
q
c
3
=
average of minimum values lower than
q
c
1
over eight diameters
above pile base.
Design values of cone resistance must be reduced to allow for the limited dis-
placements relevant for pile design. Adopting what is essentially a serviceability
displacement limit of 10 % of the pile diameter, typical ratios of
q
b
/
q
c
recommended
for closed-ended driven piles range from 0.2 (Jardine and Chow, 1996) to 0.5 (Kraft,
1990) or higher. De Nicola and Randolph (1997) noted a decreasing trend of the
ratio
q
b
/
q
c
ratios may be taken as 0.5 at shal-
low depths, decreasing to 0.3 for effective overburden stresses greater than 200 kPa.
Randolph (2003) has pointed out that the magnitude of end-bearing pressure mobi-
lized within displacements of 10%of the pile diameter will depend on the magnitude of
residual stress locked into the soil below the pile base during installation, but proposed
a design value of 0.4 for
q
b
/
q
c
with depth, and suggested that
q
b
/
q
c
unless high residual stresses could be demonstrated (see
Figure 4.6).
Further reduction values should be applied for other types of pile. For open-ended
driven piles, Foray
et al.
(1993) and Bruno (1999) suggest a 25% reduction relative
to a closed-ended pile. Lehane and Randolph (2002) have developed relationships
for the
minimum
end-bearing resistance that can be relied upon as a function of
1
Driven pile (suspect data point)
0.9
Residual load
corrections
0.8
0.7
0.6
Suggested design value
(diameter independent)
0.5
0.4
0.3
Jacked piles
0.2
Data after
Chow (1997)
0.1
Vibro-driven
0
0
0.2
0.4
0.6
0.8
1
Pile diameter (m)
Figure 4.6
Normalized end-bearing resistance for driven closed-ended piles.
(Annotations by
Randolph, 2003.)
