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FEM-aided design of a novel device for soil anchoring
Conception assistée par éléments finis d’un nouveau système pour l’ancrage des sols
Prisco di C., Pisanò F.
Politecnico di Milano
ABSTRACT: In this paper the pull-out performance of an innovative system for soil anchoring is mechanically interpreted on the
basis of a preliminary finite element investigation. The system consists of a tie rod equipped with thick steel sockets, extruding into
the soil to increase the overall pull-out bearing capacity. The effectiveness of the anchorage comes from two correlated strength
mechanisms: a direct one, associated with the shear/flexural strength of the sockets themselves; and an indirect one, in the form of a
remarkable increase in the normal confinement on the tie rod and hence in the available shear strength. Finally, the numerical results
are exploited to conceive a design-oriented analytical model for the prediction of the pull-out bearing capacity.
RÉSUMÉ: Dans cette article, le comportement en tension de un nouveau système pour l’ ancrage dans les sols est interprété sur la
base de une analyse préliminaire avec les éléments finis. Dans le système il y a une bar en métal avec des puntons qui s’extrudent
dans le sol pour augmenter la capacité totale de l’instrument. L’efficience de l’ ancrage dérive de deux mécanismes résistants: un qui
peux être définis direct, associe a la résistance des puntons, et un indirect, associe à l’incrément du confinement sur la bar. Les
résultats numériques ont été utilisés pour définir un model interprétatif du fonctionnement du system d’ ancrage.
KEYWORDS: soil anchoring, pull-out, soil–structure interaction, finite element analyses, plasticity
1 INTRODUCTION
The analysis and the design of soil anchors are of major
interest to geotechnical engineers in many practical
applications, including retaining structures, transmission towers,
marine pipelines, etc. For these purposes, the employment of the
Finite Element Method (FEM) is progressively increasing, as it
overcomes the limitations of most empirical/analytical
approaches (Das 1990) in dealing with complex geometries and
material non-linearities. On the research side, only a few papers
present numerical results about soil anchoring systems,
compared to the available experimental data and analytical
predictions (see for instance Rowe and Davis (1982a,b),
Merifield and Sloan (2006)).
This paper summarizes a recent research activity (di Prisco
and Pisanò 2012) concerning the study of a novel device for soil
anchoring. This latter is formed by a tie rod equipped with thick
steel sockets, which are extruded into the soil to ensure a
remarkable pull-out capacity.
To investigate the soil—structure interaction (SSI)
mechanisms determining the effectiveness of the system, FEM
simulations of pull-out tests have been performed. Then, based
on the critical analysis of the numerical outcomes, an analytical
model for the estimation of the pull-out capacity has been set
up. Despite the approximations introduced, the good agreement
between analytical and FEM results is believed to represent the
starting point for the conception of a reliable design procedure.
2 FEM SIMULATION OF PULL-OUT TESTS
The novel setup of the anchoring device under consideration
is characterized by the presence of internal steel sockets along
the tie rod shaft, to be extruded - after the rod installation - into
the soil by means of a hydraulic system. Recent in situ pull-out
tests have highlighted how socket extrusion ensures a very large
bearing capacity (Santoro 2009), much larger than in the usual
case of grouted anchorages.
Figure 1 sketches the installation of the anchoring system,
composed of the tie rod and the extruded steel sockets, while
Figure 2 illustrates the telescopic structure of the sockets. The
system is rather flexible in terms of geometrical configuration
as the number, the location, and the orientation of the sockets
can be freely designed; its installation is extremely fast and
inexpensive.
Figure 1. The anchoring system.
Figure 2. The steel sockets.
2.1
FEM model
The geometrical configuration for numerical analyses has
been kept as simple as possible, but sufficiently accurate to
reproduce the most relevant structural details. If the four sockets
