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The influence of bond stress distribution on ground anchor fixed length design. Field
trial results and proposal for design methodology
L'influence de la répartition des contraintes sur les tirants d'ancrage de longueur fixe. Résultats de
planche d'essais et proposition de méthodologie de conception
Vukotić G.
Kellerterra, S.L.
González Galindo J., Soriano A.
Universidad Politécnica de Madrid
ABSTRACT: This paper presents a brief analysis and comparison of different recommendations for a ground anchor fixed length
design and a load transfer capacity at grout-ground interface, comparing it with the full scale test results recently carried out in Spain.
Simple methodology for ground anchor routine design is proposed, incorporating efficiency factor as a conceptual control of anchor
capacity, and fixed length criteria to determine range of application of conventional anchors, with single fixed length unit, or Single
Bore Multiple Anchors (SBMA). Ground anchors discussed in this paper are cement pressure grouted, formed by pre-stressed strand
tendons that are installed in soil or rock.
RÉSUMÉ: Cet article présente une brève analyse et la comparaison des différentes recommandations pour la conception de tirant
d’ancrage de longueur fixe et la capacité de transfert de charge à l’interface coulis-sol, en le comparant avec les résultats d’essais à
échelle réelle obtenus en Espagne au cours des dernières années. Une méthodologie simple pour la conception de tirant d'ancrage est
proposée; elle intègre le facteur d'efficacité comme un contrôle conceptuel de la capacité de l’ancrage et les critères de longueur fixe
pour déterminer le champ d'application des tirant d’ancrage conventionnels, d’unique longueur fixe, ou d’ancrages multiples en un
unique forage (SBMA pour ses sigles en anglais). Les tirants d'ancrage décrits dans ce document sont injectés à pression au moyen
d’un coulis de ciment et sont formés par un faisceau de câbles d’acier précontraints qui sont installés dans le sol ou la roche.
KEYWORDS: anchor, fixed length, bond stress, efficiency factor.
1
INTRODUCTION
Nowadays anchors represent a key medium to sustain and
strengthen slopes formed by instable soils and fractured rocks,
and to ensure the stability of various types of gravity structures.
Regarding bond stress and load transfer capacity at the
grout-ground interface, most procedures for anchor design are
empirical values or formulas derived by local experiences, very
difficult to extrapolate for different locations or execution
systems. The design procedure is often simplified, considering
direct proportionality between fixed anchor length and its load
capacity, as it is prevailing practice in Spain and South
America.
However, since late 1960s numerous authors demonstrate
bond stress or skin friction distribution to be highly non-
uniform at all stages of loading, with high bond stress
mobilization along reduced fixed length. In the following
chapters, based on presented references and analysis of the field
trial results recently performed in Spain, the methodology for
anchor fixed length design is proposed.
2
DESIGN OF THE FIXED ANCHOR LENGTH
2.1
Current practice
Design assumption of uniform load distribution along the fixed
anchor length is not only limited to usual methodology and
standards in Spain and South America but is internationally
generally adopted. Considering this hypothesis the ultimate or
capacity of the anchor is commonly expressed as follows:
ult
fix
ult
Ld
T
·
· ·
(1)
where: d = anchor diameter, L
fix
= anchor fixed length and
τ
ult
= ultimate bond stress.
This formula differs from experimental and theoretical
evidence that corroborate that there is no linear dependency of
ultimate capacity on fixed anchor length.
2.2
Non-uniform bond stress distribution
It is fully acknowledged by numerous researchers that the
distribution of stress along the fixed anchor length is non-
uniform, both at low stress levels and at failure. This
phenomenon results from the general incompatibility between
elastic modulus and corresponding deformation of the anchor
strands, cement grout and ground.
Field tests on instrumented conventional anchors, reported
by Muller (1966), Berrardi (1967), Ostermayer (1974),
Ostermayer and Scheele (1977), Mastrantuono and Tomillo
(1977), Barley (1995) and Briaud et al. (1998), showed that
when applying the initial load the bond stress is concentrated
over the proximal length of the fixed anchor, leaving a
significant part of the fixed length towards distal end unstressed.
By the evolution of the load the bond stress concentration zone
is transferred along the fixed anchor as the bond stress along
either the tendon/grout or grout/ground interface is exceeded.
Figure 1. Development of bond stress distribution along a fully bonded
fixed anchor length (Barley 1995).
Simultaneously, due to progressive debonding, stress at the
proximal end reduces to residual values. The bond stress
concentration zone reaches distal end of the anchor just before
the failure, as it can be seen in Figure 1.
