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Influence of multiple helix configuration on the uplift capacity of helical anchors
Influence de la configuration des hélices sur la résistance à l'arrachement de pieux hélicoïdaux
Tsuha C.H.C., Santos T.C.
University of São Paulo at São Carlos, Dept. of Geotechnical Engineering
Rault G., Thorel L., Garnier J.
Université Nantes Angers Le Mans, IFSTTAR, Centre de Nantes, Département GER, Physical Modelling in Geotechnics
Group
ABSTRACT: The uplift capacity of multi-helix anchors usually depends on the helical blades configuration (including the number
and the diameter) and the soil characteristics. An evaluation of those parameters is based on the results obtained from two different
experimental programs. The first experiments were performed in centrifuge on dry Fontainebleau sand. For the second testing
program, tension load tests were carried out in field at São Carlos in Brazil in a tropical soil. The geometrical effect (cylindrical or
tapered helices) is also presented.
RÉSUMÉ : La capacité portante en traction des pieux hélicoïdaux dépend de la configuration des hélices (dont le nombre et le
diamètre) et des propriétés du sol. Deux programmes expérimentaux permettent d’éclaircir l’influence relative de ces paramètres.
L’un est réalisé sur modèles réduits centrifugés dans du sable sec de Fontainebleau, l’autre est mis en œuvre
in situ
sur un site test à
Sao Carlos au Brésil, constitué de sols tropicaux. L’effet de la géométrie (hélices inscrites dans un cylindre ou dans un cône) est
présenté.
KEYWORDS: helical anchor, tension capacity, centrifuge modeling, field load tests.
1 INTRODUCTION
Helical anchors have been employed in the construction of
structures to sustain tension loads. Uses for helical anchors
include transmission tower foundations, utility guy anchors,
pipelines, braced excavations, retaining wall systems, etc. They
are composed of helical bearing plates welded to a steel shaft,
and installed into the ground by application of torsion to the
upper end of the shaft (Figure 1).
The most common methods to estimate the uplift capacity of
helical anchors are two: individual bearing and cylindrical shear
methods. The individual bearing method assumes that the total
capacity of a multi-helix anchor is equal to the sum of the
individual capacities of each plate, estimated using the
Terzaghi’s (1943) general bearing capacity equation.
Figure 1. a) Helical anchors; b) Anchor installation.
The cylindrical shear method, described in Mitsch and
Clemence (1985) and Mooney et al. (1985), supposes that the
failure mechanism consisting of the bearing capacity failure
above the top helix and of a cylindrical failure zone developed
along the perimeter section between the helices.
The failure mechanism of helical anchors depends
principally on the helix spacing ratio (ratio of helix spacing to
helix diameter). Kulhawy (1985) stated that if the helices are
widely spaced, the multi-helix anchor behaves as a sum of
various single-helix anchors. According to the results of a field
investigation on the behaviour of multi-helix anchors in clay,
presented in Lutenegger (2009), there is no distinct transition
from cylindrical shear to individual bearing behaviour. For
helical anchors in sand, Lutenegger (2011) found that this
transition occurs at a helix spacing ratio of about three.
For the application of these two prediction methods, used in
helical anchor design, reductions in the values of some soil
parameters have been suggested in the literature to consider the
effect of the soil disturbance above the helices caused by the
anchor installation.
As reported by Kulhawy (1985), significant disturbance does
occur within the cylindrical installation zone of the helical
anchor. Mitsch and Clemence (1985) cited that the installation
of helical anchors induces significant stress changes in soil due
to the disturbance produced by screwing the anchor into the
sand and that these changes influence the anchor uplift
behaviour.
Tsuha et al. (2012) mentioned that when a helical anchor is
installed into the ground, the soil traversed by the helices is
sheared and displaced laterally and vertically. According to
these authors, the disturbance caused by the anchor installation
is normally more pronounced in the soil above the upper plates
than above the lower plates, because the upper soil layers are
penetrated more times.
Some experimental investigations on helical anchors
(Clemence et al. 1994, Sakr 2009, and Lutenneger 2011), with
relative helix spacing of three times the plate diameter, have
demonstrated that, the amount of increase in the uplift capacity
of helical anchors with the increase in the number of helices is
not as expected. The gain in the uplift capacity of helical
anchors due to the addition of one more plate is variable, and
depends of the anchor configuration and soil characteristics.
For this reason, considering that a thorough understanding of
the influence of helices configuration on the uplift behaviour of
helical anchors is fundamental to give accurate estimates of the
helical anchors capacity, the purpose of this paper is to evaluate
the geometry effect on the soil disturbance due to anchor
installation and its influence on the anchor capacity. Two
