1971
Experiences with SBMA ground anchors in spanish soils
Etude expérimentale avec les tirants d’ancrage SBMA dans le sol espagnol
Fernandez Vincent J.M.
Universidad de Buenos Aires, Universidad Catolica Argentina, Geotecnika SRL
ABSTRACT: The load transfer mechanism of steel, subjected to axial tension, adhered to a soil or rock through a cement grout is not
through a uniform stress distribution. This results from the general incompatibility between the modulus of elasticity of steel, grout
and soil, causing the phenomenon known as progressive debonding, as it causes the increase of the load of the anchor. The
investigation of this phenomenon, the development of this knowledge and its application to industry has resulted in the SBMA
System. This system applies much more efficiently the bond stresses available in the field through the use of various units within a
single anchor borehole. We present brief guidelines for their design, and case histories within different geotechnical units in Spain.
RÉSUMÉ : Le mécanisme de transfert de charge d'un tirant en acier, soumis à une traction axiale, scellé dans le sol ou le rocher par
un coulis de ciment ne se fait pas avec une répartition uniforme des contraintes. Ceci résulte de l'incompatibilité générale entre le
module d'élasticité de l'acier, le coulis et le sol, provoquant un phénomène de décollement progressif et d'augmentation de la charge
de l'ancrage. L'étude de ce phénomène, le développement d'un savoir et son application à l'industrie sont synthétisés dans le système
SBMA. L'utilisation de plusieurs éléments dans un seul forage permet d'augmenter l'efficacité de l'ancrage. La communication
présente des directives de conception, et des cas historiques dans différentes unités géotechniques en Espagne.
KEYWORDS: design – ground anchor – multiple anchor - load transfer – efficiency factor – fixed lenght
MOTS CLES: Conception, tirants, ancrage multiples, charge de transfert, facteur d'efficacité, longueur de scellement
1 INTRODUCTION.
Anchor design codes allow engineers to assume load is
distributed uniformly through the length of an anchor, but
experts acknowledge that the ultimate load is not proportional to
the anchor’s fixed length.
Anthony D. Barley’s (1995, 1997, 2003) research and
development for over a decade confirmed and extended existing
independently works coming to the same conclusion of the non-
uniformity of the distribution of adhesion in the fixed length
anchor, but finally introducing the efficiency factor for
calculating the length of the fixed length of an anchor
considering this phenomenon. This research was then applied to
the development of a new technology for ground and rock
posttensioned anchors called Single Bore Multiple Anchor
(SBMA).
In 2004 this technology was introduced in Spain, in which
the author was responsible. This was done with the technical
support of A. Barley.
2.
DESCRIPTION
The system involves the installation of multiple units in a single
anchor borehole. Each unit has its own single tendon, his own
free length, length of bulb and is loaded using its own
tensioning unit. The tensioning of all anchor units is performed
simultaneously by a hydraulic jacks synchronized equipment
that ensure that the load applied to the various units is always
the same. The sum of the loads of the various units loads totaled
the anchor total of the SBMA. With the design conception, there
is no theoretical limitation on the overall bond length used (the
sum of the fixed lengths of the various units), while for the
conventional anchors little increase is expected load over fixed
lengths of 8 to 10 meters.
Another advantage of the system is the opportunity to take
account of the varying strata within the ground, as each anchor
within a bore can be designed for different ground conditions.
Experts agree that the SBMA system is most effective in weak
soils either to enhance capacity or to reduce the total number of
anchors. However, they are not economically viable where the
structural loading requires only low load anchors at wide
spacings.
Figure 1. Adhesion stress distribution in several efficient units
3.
DESIGN
3.1
Efficiency factor
The relationship between the stiffness of the fixed anchor
(controlled by the steel tendon) and the stiffness of the ground
governs the rate of progressive debonding as an anchor is
loaded and hence affects fixed length efficiency.
The nonlinearity of the mobilization of the value of the
grout/ground bond stress (
ult
vs.
res
) along the fixed length
could be accounted by the efficiency factor for ground anchors.
f
eff
1.6
L
0.57
(1)
