2755
Research on the Load-Bearing Behaviour of Bored Piles
with Different Enlarged Bases
La recherche sur le comportement portante de pieux forés avec diverses bases élargies
Herrmann R.A.
University Siegen, Siegen, Germany
Löwen M.
University Siegen, Siegen, Germany
Tinteler T., Krumm S.
University Siegen, Siegen, Germany
ABSTRACT: According to DIN EN 1536 overboring of the bottom of drilled piles to increase the EC7-1: bearing resistance is permissible up
to three times the pile diameter. The former German standard DIN 4014, however, permitted only an overboring of up to double the pile
diameter. The bearing resistance was reduced to 75% in order to take into account the EDZ (excavation disturbed zone) caused by the
overboring process. This special area of foundation engineering has been largely unexplored until the early tests by FRANKE/GARBRECHT,
and reliable data is not yet available for the new possible pile-base enlargement according to DIN EN 1536. Therefore, model tests at a scale
of 1:25 were performed. The experiments were also based on the geometry of the new-type overboring excavation method by BAUER. Thus,
new knowledge was obtained on the bearing and settlement behavior of large overbored pile bases with enlargements of one-, two-, and three-
times the pile diameter. The tests first showed that an enlargement contributes significantly to an increase in bearing resistance or in limiting
pile settlement. More efficient pile foundations are technically possible. A further result is that by the use of a reduction coefficient of 0.75 for
the bearing resistance, covering the effects of the drilling the enlargement, the bearing resistance is underestimated. The results were
evaluated against the available results of large 1:1 drilled pile experiments. The results of the full-scale tests were assessed regarding their
Serviceability Limit State. The available results are to be evaluated by further investigations.
RÉSUMÉ: Selon la norme DIN EN 1536 l'élargissement du fond des pieux forés pour augmenter la capacité de support de charge est
permis jusqu'à trois fois le diamètre du pieu. L'ancienne norme allemande DIN 4014, cependant, est premise seulement pendant un
alésage allant jusqu'à doubler le diamètre du pieu. Le palier de capacité a été réduite à 75% afin de tenir compte de l'EDZ (excavation
zone perturbée) causée par le processus d'élargissement. Étant donné que cette région est en grande partie inexplorée à part les essais
antérieurs de FRANKE/GARBRECHT et que les études pour l’élargissement de la nouvelle base de pile possible selon DIN EN 1536
ne sont pas encore disponibles, des essais selon un modèle à l'échelle 1:25 ont été effectués. Les essais se sont également basés sur la
géométrie de la nouvelle méthode de type d'excavation de BAUER. Ainsi, de nouvelles connaissances relatives à la capacité de charge
et au comportement de déformation, en cas d’élargissements du diamètre du pieu d’une, de deux et même trois fois ont ainsi été
obtenues. Les essais ont montré, en premier lieu, qu’un élargissement selon cette nouvelle méthode contribue à une augmentation de
la capacité portante ou à une limitation de tassement du pieu. Techniquement, des fondations plus efficaces des piles sont possibles.
Une autre conséquence est que le coefficient de réduction de 0,75 pour la résistance de roulement, justifiée par l'influence de la
technique de forage dans la production de l'expansion, conduit à une valeur calculée qui contient un facteur de sécurité élevé. Les
résultats ont été évalués en fonction des résultats disponibles des essais de forage 1:1 des pieux. Les résultats des grands essais ont été
évalués quant à leur fonctionnalité. Les résultats disponibles doivent être évalués par des enquêtes ultérieures plus approfondies.
KEYWORDS: Drilled Pile, Pile foot expansion, reducing coefficient, pile diameter, EDZ excavation disturbed zone
1
INTRODUCTION
Specialized foundation engineering literally lays the groundwork
for almost any construction project. On poor, inadequate subsoils,
the use of shallow foundations is not acceptable due to excessive
settlements. Wooden piles were already being used more than 4000
years ago for foundation construction in weak soils. This made it
possible to transmit loads into deeper soil layers with a higher load-
bearing capacity. Today wooden piles are only used occasionally
for small, temporary buildings.
Along with various other pile foundations, as for example driven
piles, displacement piles and micropiles, bored piles are a current
standard for deep foundations, especially when high loads must be
supported. Special types of the bores piles are bored piles with
bottom enlargement according to DIN EN 1536. With this system
an increase of the load- bearing capacity is achieved through an
enlargement of the supporting cross-sectional area. As a result of
the larger area of the base in addition to the skin friction, larger
bearing resistance can be attained where the area of the bottom, D²,
is larger or the enlargement will preserve the assigned
Serviceability Limit State (SLS). Normally a pile with a base
enlargement would first have the point bearing with the skin friction
determined and then be designed predominantly based on a point-
bearing pile (see also Herrmann, Lauber).
The aim of this test series was to test the load-bearing capacity
the afore-mentioned pile system with different diameters. This need
for the tests arose from the issuance of the European standard DIN
EN 1536. This standard permits an expansion of up to twice the pile
diameter, whereas DIN EN 1536, which replaces the old German
standard DIN 4014, permits a cross-section enlargement of up to
thrice the pile diameter (see DIN 4014 and Franke/garbrecht).
In order to obtain and afterwards compare the differences in the
load-bearing behaviour due to the increased base areas, static axial
tests (compression tests) were carried out on scale model piles
(smooth probe rod (d= 36 mm) with turned pile foot (hard plastic))
at a scale of 1:25 (see Figure 1).
Figure 1. Pile models without (1xD), and with 2 - (2xD) and 3 -(3xD)
times the pile diameter bottom enlargements
2
PILE SYSTEM
2.1
General
A drilled pile with base enlargement functions like a large
drilled pile in accordance with DIN EN 1536 which is
manufactured with or without suspension support depending upon
the stability of the ground. Subsequently, the bottom of the borehole
will be enlarged by the assistance of a special drilling tool, the
Belling bucket (see Figure 2).
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