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Plugging Effect of Open-Ended Displacement Piles
Prise en compte de l’effet de bouchon pour les pieux battus ouverts
Lüking J.
HOCHTIEF Solutions AG, Civil Engineering Marine and Offshore, Hamburg, Germany
Kempfert H.-G.
Institute of Geotechnics and Geohydraulics, University of Kassel, Kassel, Germany
ABSTRACT: During jacking an open-ended displacement pile the soil is entering through the pile toe into the profile. This plug
can close up the pile toe completely. Because of this the pile can be treated approximately as a fully closed-ended displacement pile
and is able to mobilize an additional base resistance. Indeed the soil-mechanical processes and the different factors of influence on the
plugging effect are mostly unknown. This report is based on research work and investigated the influence of different factors on the
plugging effect and hence the change in the load-bearing behaviour mainly in non-cohesive soils using experimental, numerical and
statistical methods. All investigations show that a fully plugged soil inside the pile could not be identified and disproved the classical
model representation of a fully plugged pile toe. The load transfer in the plug takes place by compression arches, which are mainly
influenced by the pile diameter and the soil density. Finally, based on these results a practical calculation method is suggested.
RÉSUMÉ : Lors de la mise en place d’un pieu battu ouvert, le terrain est susceptible de pénétrer dans le pieu par son pied de manière
plus ou moins importante. Suivant le degré de pénétration du sol dans le pieu, celui-ci peut être considéré comme ouvert ou fermé et
une résistance supplémentaire peut alors être mobilisée. Ce papier propose une étude des processus de pénétration du terrain dans les
pieux battus ouverts pour des sols non cohésifs. La variation de capacité portante des pieux induite par ces processus est analysée
selon des points de vue expérimentaux, numériques et statistiques. Toutes les investigations réalisées montrent que l’effet de bouchon
complet n’existe pas et qu’un pieu battu ouvert ne peut pas être considéré comme véritablement fermé. L’effet de bouchon correspond
à la formation de « voûtes » à l’intérieur du pieu. Enfin, une méthode de prévision de la capacité portante intégrant ces processus est
proposée.
KEYWORDS: open-ended displacement pile, plugging effect, pile bearing capacity, pile foundation.
1 INTRODUCTION
Open-ended displacement piles are piles, which are open at the
pile toe like pipe piles, H-profiles or composed of sheeting
piles. During the piling process (jacking, impact driving,
vibrating or pressing) the soil is entering into the pile tube.
Between the opposite inner shaft areas a plug can occur, which
is able to mobilize an additional toe resistance. This toe
resistance depends on the soil parameters, the pile geometry and
the stress distribution.
Open-ended displacement piles are often used in harbour
constructions or as foundations for offshore wind plants (i.e.
monopiles or jackets).
Technical standards like API or others assume a fully
plugged open-ended displacement pile and treat this plug in a
monolithic way. However the soil-mechanical process and the
different factors of influence on the plugging effect are mostly
unknown.
Starting with a short state of the art this paper summarizes
laboratory tests, numerical and statistical calculations and
recommends new experience values for the bearing capacity of
open-ended displacement piles.
These research results are based on the works described in
Lüking 2010 and also Lüking and Kempfert 2012.
2 STATE OF THE ART
The bearing capacity of the plug can be evaluated by the values
IFR (Incremental Filling Ratio) after Brucy et al. 1991 or the
PLR (Plug Length Ratio) after Paik and Lee 1993, see Eq.1 and
Eq.2.
e p
d h
IFR
(1)
e p
dh PLR
(2)
These values describe the incremental and the absolute ratio
of the height of the plug h
p
to the pile embedded depth d
e
.
An IFR = 1 means that the surface of the plug does not
penetrate into the soil during driving in comparison to the last
measurement. Only the pile penetrates into the soil. This means
that no plugging effect takes place.
In contrast an IFR = 0 means that the surface of the plug
penetrates into the soil with the same value as the pile. In this
case the pile is fully plugged and all the soil has to be displaced
sideways.
The IFR will be measured during driving by a sounding line.
The PLR is only measured after finishing the driving and gives
only an average value for the plug development. This is
problematic in layered soils.
The highest radial displacement u
R
and radial stresses
'
R
occur by an IFR = 0. In this case the soil is fully plugged which
means that the soil resistance is the same like the toe resistance
of the profile. Then the plug could be treated like a monolith
and is comparable to a closed-ended pile. With an increasing
IFR the radial displacement and the radial stresses are
decreasing. If the IFR lies between 0 and 1 the soil is partially
plugged. The changeover from a full plug to a partial plug and
no plugging is steady and the statuses cannot easily be
distinguished. Figure 1 gives an overview of the described
context after White et al. 2005.
The maximum pile diameter in which a plugging effect could
occur is about 1.5 m, see Jardine et al. 2005.
