1490
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
the mean values of
approaches to the values suggested by the
API for coring condition.
3.3
End bearing factor N
q
The effect of soil plugging on unit end bearing was investigated
using the end bearing factor
N
q
in Eq. (7). As mentioned
previously, IFR is a better indicator for quantifying the effect of
soil plugging on end bearing. However, IFR was not measured
during pile driving because it is difficult to measure the change
of the plug length over a certain interval in the massive piling
work without interrupting the pile driving process. Additionally,
Fig. 2 shows that the soil plug length generally increased
linearly with the penetration depth, implying that IFR may not
vary significantly during pile driving. Therefore, we used PLR
to investigate the effect of the soil plugging on the unit end
bearing. We first obtained the unit end bearing values
q
b
by
dividing the tip resistances from the CAPWAP analyses by the
gross cross-sectional area of the pile
(
B
o
2
/4). We then divided
q
b
by the vertical effective stress
v
′
at a depth of pile tip to
obtain the end bearing factor
N
q
. In reality, the operational area
for the end bearing may be in between the steel and the gross
areas for partially plugged pile. Therefore, the unit end bearing
values obtained using the gross area may under-estimate the
true unit end bearing. However, considering that it is very
difficult to identify the actual operational tip area, it may be
better to use the gross area to back-calculate
N
q
and use these
values in conjunction with the gross area for pile design.
Fig. 5 shows back-calculated
N
q
versus average PLR values
of PDA-tested piles.
Range for dense to
very dense sands
(API 2007)
0.5 0.6 0.7 0.8 0.9 1
Average Plug Length Ratio (PLR)
0
50
100
150
200
250
300
End Bearing Factor (N
q
)
406 mm
508 mm
762 mm
914 mm
Mean value
This study
Figure 5. End bearing factor
N
q
vs. average PLR of PDA-tested piles.
We obtained the following equation for
N
q
by fitting for the
mean values:
8.4
12.3
q
N PLR
(11)
where PLR = plug length ratio (0.76
≤ PLR ≤ 0.91).
Eq. (11)
indicates
N
q
decreases with increasing PLR. Mean
N
q
values
vary from about 134 for 406-mm-diameter piles to 35 for 914-
mm-diameter piles. API recommends using
N
q
= 40 for dense
sand, and
N
q
= 50 for very dense sand regardless of pile
diameter. Similar to the shaft resistance factor, API
recommendations for end bearing factor seem to be appropriate
for large piles such as 762-mm and 914-mm-diameter pile, but
significantly underestimate that for 406-mm-diameter piles.
Even though Eq. (11) indicates that
N
q
may increase drastically
for very low values of PLR because of power relationship with
PLR, in reality, the
N
q
values for the 406-mm-diameter piles
obtained in this study may be close to the upper bound for pipe
piles because these piles were most likely fully plugged at the
end of pile driving. According to CGS (2006), the upper bound
of
N
q
values for piles driven in dense to very dense sands is 120,
which is indeed close to
N
q
values (=134) of 406-mm-diameter
piles. In addition, the
N
q
value suggested by Paik and Salgado
(2003) for fully plugged open-ended pipe piles (IFR = 0) in
dense sand is 326
K
0
., which yields 130 with
K
0
= 0.4 and 163
with
K
0
= 0.5.
It should be noted that Eqs. (10) and (11) are obtained from
the piles driven in dense to very dense sands with PLR values
ranging between 0.76 and 0.91. Therefore, these equations are
not valid for loose or medium dense sands. We are performing
similar study for the piles driven in medium dense sands. Eqs.
(9) through (11) will be updated by including the effect of
relative density of soils after the on-going study is completed.
4 SUMMARY AND CONCLUSION
In this study, we investigated the effect of the soil plugging on
the capacity of open-ended steel pipe piles in dense to very
dense sands. We measured the soil plug lengths at final
penetration of 1,355 open-ended driven pipe piles with pile
diameter ranging between 406 mm and 914mm. The pile
penetration depths ranged between about 10 m and 30 m. The
average PLR increased from 0.76 for 406-mm-diameter piles to
0.91 for 914-mm-diameter piles. Based on these observations,
we suggested an equation from which PLR can be estimated
based on pile inner diameter.
We estimated the skin friction and end bearing of the piles
by performing PDA tests and performing CAPWAP analyses on
99 piles. These results indicated that the unit skin friction and
unit end bearing values increased with decreasing PLR at the
similar or equal relative density. We also compared the field
pile capacities from CAPWAP analyses against the capacities
estimated following API procedures. The results showed that
the use of the API procedures might under-estimate the unit skin
friction and unit end-bearing values for small diameter piles.
We suggested new equations to estimate the skin friction
factor
and end bearing factor
N
q
, based on PLR for dense to
very dense sands.
5 REFERENCES
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planning, designing and constructing fixed offshore platforms
–
working stress design, Errata and Supplement 3
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