1488
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
different relative densities, as the relation was developed based
on the calibration chamber test data where soil conditions are
relatively homogeneous. In reality, it is not easy to measure
even PLR for routine piling work. For this reason, Lehane et al.
(2005) proposed a formula for estimating IFR
avg
, averaged over
20 pile diameters of penetration, as a function of the pile
internal diameter as follows:
0.2
min 1,( /1.5)
avg
i
IFR PLR
B
(3)
where
B
i
= pile inside diameters in meters. It should be noted
that the average of the IFR measured with the same penetration
increments over the entire pile length is equal to the PLR.
Yu and Yang (2012) determined that the ratio of unit end
bearing of soil plug to tip resistance from the cone penetrometer
tests (CPT) depends on PLR. They collected PLR data from
literature and suggested the following equation to estimate PLR
from internal pile diameter:
0.15
min 1,
i
PLR B
(4)
1.2
Pile design
The total axial capacity
Q
t
of open-ended steel pipe pile is the
sum of the limit skin friction
Q
s
and the ultimate end bearing
Q
b
:
t
s
b
si si
b b
Q Q Q q A q A
(5)
where
q
si
= limit unit skin friction for soil layer
i
;
q
b
= ultimate
unit end bearing;
A
si
= pile shaft area interfacing with soil layer
i
; and
A
b
= area of pile base. There exist many methods to
estimate
q
s
and
q
b
. Among others, the method proposed by
American Petroleum Institute (API) is one of the most widely
methods used for design of on- and off-shore foundations in
USA.
According to the API (2007), the unit skin friction in sandy
soils can be derived from the normal effective stress acting on
the pile shaft and the frictional properties between the pile and
soil interface as:
tan
s
v
q K
(6)
where
K
= coefficient of lateral earth pressure;
v
'
= vertical
effective stress in the center of the soil layer; and
= soil-pile
interface friction angle. The
K
and
are often incorporated
together as shaft friction factor
, and Eq. (6) then becomes:
s
v
q
(7)
According to the API (2007),
values vary between 0.37
and 0.56 for open-ended pipe piles driven in unplugged mode
for medium dense to very dense sands. The API suggests
increasing
by 25 percent for full-displacement piles such as
open-ended pipe piles driven in fully plugged mode or for
closed-ended pipe piles.
The unit end bearing for piles installed in sandy soils is
given as:
b q v
q N
(8)
where
v
'
= vertical effective stress at the base of pile;
N
q
= end
bearing factors ranging between 20 and 50, for medium dense to
very dense sands.
2 SITE CONDITIONS AND PILE DRIVING
2.1
General site conditions
The project area is about 270-m-wide and 400-m-long. A total
of 10 soil borings with Standard Penetration Tests (SPT) and 1
Cone Penetration Test (CPT) were performed within the area.
The existing site grade at the time of exploration varied from
about El. 354.5 m to about El. 359.5 m. At the time of pile
driving, the finished grade was at about El. 356 m, with typical
groundwater level at 6 m below the finished grade. The project
area generally consists of dense silty sands (typically consisting
of 60 % of sand, 33 % of fines, and 7% of gravel) from below
the finished grade to a depth of about 6 m. The SPT N-values
within this stratum generally ranged between 13 and 49 blows
with an average of about 31 blows. Below this layer, dense to
very dense sands (typically consisting of 89 % of sands and
11 % of fines) were encountered to a maximum depth of about
40 m. The SPT N-values within this stratum generally ranged
between 41 and greater than 50 blows. CPT also shows similar
soil conditions with average cone resistance
q
t
of 25 MPa to a
depth of about 6 m, and 42 MPa thereafter.
2.2
Pile driving and dynamic testing
A total of about 3,000 steel pipe piles were driven at the project
site. We measured plug lengths for about 1,355 piles. The pipe
piles driven in the project area consist of 406 mm x 9.5 mm
(pile outer diameter
B
o
x wall thickness
t
), 508 mm x 12.7 mm,
610 mm x 12.7 mm, 762 mm x 15.9 mm, and 914 mm x 19.1
mm open-ended steel pipe piles (corresponding to inner
diameters of 387, 483, 585, 730, and 876 mm, respectively).
APE D46-32 diesel hammer was used to drive the 406-, 508-,
and 610-mm-diameter piles. APE D62-42 and APE D80-42
diesel hammers were used to drive the 762- and 914-mm-
diameter piles, respectively.
We measured the hammer blow counts required for driving
piles each 0.25-m interval to the final penetration depths. After
completion of the pile driving, we measured the depth of the top
of soil plug using a wire-connected weight lowered inside the
pile and rested on top of soil plug. We then calculated the soil
plug length from the pile penetration depth and stick-up length
above the ground.
In order to estimate the pile capacity, dynamic load tests
were performed on 99 piles using the PDA. The pile capacities
were estimated based on signal matching analysis using
CAPWAP.
3 EXPERIMENTAL RESULTS
3.1
Soil plug length ratio
As mentioned earlier, we measured the plug lengths of a total of
1,355 piles. The pile penetration depths ranged between about
10 m and 30 m. The lengths of the soil plugs at various final
pile penetration depths are presented in Fig. 2(a) through 2(e).
The dashed lines in these figures represent the fully coring state,
and the solid line is a fitted line through the measured data. It is
clear that the solid lines become closer to the dashed line as the
pile diameter increases, indicating that the large diameter piles
were driven in close to fully coring mode.
We performed histogram analysis of PLR values at final
penetration depths. A summary of the statistical data of PLR is
presented in Table 1.
Table 1. Summary of statistical data of PLR.
Outer
Dia.,
mm
Total
No. of
Piles
Penetration
Depth, m
Mean
PLR
Standard
Deviation
Most
Frequent
PLR Range
406
83 10.4 - 16.8 0.761 0.065
0.80 - 0.84
508 585 10.3 - 26.5 0.842 0.048
0.84 - 0.88
610 113 13.5 - 23 0.868 0.039
0.84 - 0.88
762 373 10.5 - 29.8 0.874 0.051
0.88 - 0.92
914 201 10.3 - 29.8 0.905 0.023
0.90 - 0.92
As seen in Table 1, the mean value of PLR increases from
0.76 for 406-mm-diameter pile to 0.91 for 914-mm-diameter
pile. Data from 406-mm-diameter piles show the most scatter
and 914-mm-diameter piles show the least scatter.
