1489
Technical Committee 203 /
Comité technique 203
Figure 2. Pile Penetration Depth versus Soil Plug Depth for (a) 406-mm,
(b) 508-mm, (c) 610-mm, (d) 762-mm, and (e) 914-mm-outer diameter
piles.
Based on these observations, we propose the following
equation to estimate the PLR as a function of pile inner
diameter as follows:
0.19
( /1.4)
i
PLR B
(9)
where
B
i
= pile inner diameter in meters (0.387 m
≤
B
i
≤ 0.876
m). Fig. 3 shows the measured PLR versus pile inner diameter
B
i
from this study. We also plotted the estimated PLRs from the
equations suggested by Lehane et al. (2005) and Yu & Yang
(2012). Predicted PLR values from Lehane et al. (2005) show
good agreement with mean PLR values measured in this study,
especially for large-diameter piles such as 762- and 914-mm-
diameter piles. Perhaps this is because Lehane et al. derived Eq.
(3) based on pile driving data of large-diameter off-shore piles.
The equation proposed by Yu and Yang (2012) appears to form
the envelope for upper bound values measured in this study.
Figure 3. Pile inner diameter vs. measured PLR.
3.2
Skin friction factor
In order to quantify the effect of the soil plugging on the unit
skin friction, we back-calculated the shaft friction factor
in Eq.
(7) from the CAPWAP capacities (see Safaqah et al. 2012 for
detailed information on how skin friction and end bearing
values were obtained from CAPWAP capacities). We first
calculated the unit skin friction
q
s
by dividing skin friction
values from CAPWAP analyses by outer area of pile-soil
interface (
B
o
D
). We then divided
q
s
by the vertical effective
stress
v
′
at the mid-depth of the pile length to obtain the
average shaft friction factor
. Based on the back-calculated
and average PLR values of the PDA-tested piles with pile
penetration depths ranging between about 10 m and 30 m, we
established the following equation to estimate
from PLR and
D
:
0.023
(3.5 3.2 )
D
PLR e
(10)
where PLR = plug length ratio (0.76
≤ PLR ≤ 0.91) and
D
=
penetration depth in meters (10 m
≤ D ≤ 30 m).
Fig. 4 shows the back-calculated
versus the mean PLR
values of PDA-tested piles together with the predicted values
from Eq. (10) with pile penetration depths of 10 and 30 m.
0.5 0.6 0.7 0.8 0.9 1
Average Plug Length Ratio (PLR)
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
Shaft Friction Factor (
)
406 mm
508 mm
762 mm
914 mm
Mean value
This study (D = 10m)
This study (D = 30m)
Range for dense to
very dense sands under coring
condition (API 2007)
Range for dense to
very dense sands under
plugging condition
(API 2007)
Figure 4. Skin friction factor
vs. average PLR of PDA-tested piles.
The mean
value decreases from 0.83 to 0.42 with PLR
values increasing from 0.76 to 0.89. According to the API,
values are independent of pile diameter for open-ended pipe
piles. However, Fig. 4 clearly shows that
is a function of PLR,
which in turn is a function of the pile diameter. Furthermore, a
mean value of
for 406-mm-diameter piles from this study is
even larger than the value recommended by API for fully
plugging condition. It also appears that as diameter increases,
