1487
Effect of Soil Plugging on Axial Capacity of Open-Ended Pipe Piles in Sands
Gudavalli S.R.
Fugro Consultants, Inc., Houston, TX, USA
Safaqah O.
Imperial Oil Limited, Calgary, Alberta, Canada
Seo H.
Texas Tech University, Lubbock, TX, USA
ABSTRACT: A degree of soil plugging during pile driving affects the ultimate capacity of open-ended pipe piles. A few researchers
have attempted to quantify the effect of soil plugging on pile capacity by introducing PLR (plug length ratio), ratio of length of soil
plug to pile penetration depth. However, the majority of these attempts were based on model pile tests conducted in the laboratory that
may not be directly applicable to field conditions due to scale effects. In this paper, we measured plug lengths at final penetration
depths for 1,355 open-ended driven pipe piles with pile diameters ranging between 406 mm and 914 mm. These piles were driven
mainly in dense to very dense sandy soils. The pile penetration depths ranged between about 10 m and 30 m. Our analyses indicated
that the PLR values increased with the increase in pile diameter in the given soil conditions. We performed PDA (pile driving
analyzer) tests on 99 piles and estimated skin friction and end bearing values of the 99 piles from CAPWAP (CAse Pile Wave
Analysis Program) analyses. The results from our analysis indicated that unit skin friction and unit end bearing values increased with
decreasing PLR. We suggested new design equations for estimation of skin friction factor
and end bearing factor
N
q
for piles driven
in dense to very dense sands.
KEYWORDS: open-ended steel pipe piles; plug length ratio (PLR); pile capacities; pile design.
1 INTRODUCTION
1.1
Soil plugging
When an open-ended pipe pile is driven into the ground, it is
very likely that the soil enters inside of the pile in the initial
stage of pile driving at very shallow depths. If pile penetration
depth is equal to the soil plug length, this behavior is typically
referred to as “fully coring” or “unplugging” b
ehavior. As the
pile is driven deeper into the soil, the soil friction on the inside
of the pile wall increases until a “soil plug”
is formed, which
may prevent or partially restrict additional soil from entering the
inside of the pipe. This behavior is
referred to as “plugging”,
and the length of soil plug is less than the pile penetration depth.
Even though soil plugging behavior of open-ended pipe piles
has been a recognized issue (Kishida and Isemoto 1977; Klos
and Tejchman 1977), the efforts to quantify the degree of soil
plugging have been very rare. The two most widely used
indicators of soil plugging are plug length ratio (PLR) and
incremental filling ratio (IFR), respectively, defined as
/
PLR L D
(1)
/
IFR dL dD
(2)
where
D
= pile penetration depth;
L
= length of soil plug;
dD
=
increment of pile penetration depth; and
dL
= increment of soil
plug length corresponding to an increment of pile penetration
depth
dD
(see Fig. 1). By definition, IFR is a first derivative of
PLR, meaning that IFR is a slope of curve of plug length versus
pile penetration depth plot. As shown in Fig. 1, in the case of a
fully coring mode, PLR and IFR must be equal to 1.
L
1
D
1
L
2
D
2
D
3
L
3
(= L
2
)
PLR = L
1
/D
1
= 1 PLR = L
2
/D
2
IFR = L
1
/D
1
= 1 IFR = (L
2
-L
1
)/(D
2
-D
1
)
PLR = L
3
/D
3
IFR = (L
3
-L
2
)/(D
3
-D
2
) = 0
pile
soil
Figure 1. Soil plugging mode.
In the case of fully plugging mode, however, IFR at final
penetration must be equal to zero because additional soils do not
enter inside of the pile after the previous penetration, but PLR is
not necessarily zero at that depth. Typically, open-ended piles
for onshore applications are driven in partially plugging mode
in sandy soils (Paikowsky et al. 1989; Paik and Salgado 2003).
PLR is a good indicator of degree of overall soil plugging,
and researchers proposed to use PLR for estimation of limit unit
skin friction of open-ended pipe piles (Paik and Salgado 2003).
The better indicator of soil plugging on estimation of end
bearing values may be IFR, as it can represent the condition of
soil plugging at the final penetration depth from final pile
driving. Paik and Salgado (2003) proposed an equation, derived
from model pile tests in calibration chamber, for estimating the
unit end bearing value in sandy soils using IFR. They showed
that the unit end bearing normalized to horizontal effective
stress increases with increasing relative density and decreasing
IFR.
Lehane et al. (2005) suggested using FFR (final filling ratio),
defined as a value of IFR averaged over a distance of 3 pile
diameters above the pile tip, to relate the unit end bearing value
with the cone penetration resistance in sandy soil. In the field, it
is easier to measure PLR than IFR. Therefore, Paik and
Salgado (2003) proposed an equation to estimate IFR from the
PLR, when only the PLR is measured in the field. This
equation was derived from the results of model piles driven in
sands of various confining stress and relative density to a depth
of 760 mm. The model pile had an outside diameter of 42.7
mm, inside diameter of 36.5 mm, and length of 908 mm. Even
though the ratio of pile length to pile diameter of the model pile
was close to that of the piles driven in the field (in fact, close to
a the lower bound of ratio of length to diameter of piles
typically driven in field), the ratio of pile diameter to soil
particle size of the model pile test was far from that of the field
condition. Therefore, the relationship between PLR and IFR
suggested by Paik and Salgado (2003) may not be applicable to
field condition. Furthermore, as Lehane and Gavin (2004)
pointed out and Paik and Salgado (2004) agreed in a separate
discussion, the correlation between PLR and IFR are not
applicable near the interface of two sand layers with very
Effet de formation d’un bouchon sur la capacité d’un pieu ouvert dans le sable
Effect of Soil Plugging on Axial Capacity of Open-Ended Pipe Piles in Sands
