1836
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
some time and is now the subject of systematic review, prEN
ISO 22477-5 has yet to be published.
2 DESIGN REQUIREMENTS OF EC7
The limit state framework adopted by EC7 requires that
anchors are designed to ensure that:-
•
Neither an ultimate limit state (ULS) nor a
serviceability limit state (SLS) occur within the
anchored structure or other supported structures.
•
That an anchor has the required ULS and SLS
resistance corresponding to these limit states.
The requirements of the new draft of Section 8 which are
designed to satisfy these limit states for the anchor and structure
are discussed in the following section.
2.1
Ultimate limit state (ULS) design force
Anchors are required to have an ULS design capacity (
R
ULS,d
) to
resist not only the force required to prevent an ULS in the
anchored structure and supported structure (
F
ULS,d
), but also
must have the capacity to resist the maximum force that could
be transferred to the anchor during its service life (
F
serv,k
), with
an adequate margin of safety. Thus the design of the anchor
must consider the prestress or lock-off force applied and also
any additional force attracted to the anchor during its design
life. These safety requirements are expressed as Eq. 1 to 3
where
γ
serv
is a partial factor.
;
≤
;
(1)
where
;
= (
;
;
;
)
(2)
;
=
;
(3)
2.2
Serviceability limit state (SLS) design force
Anchors are required to have the design capacity (
R
SLS,d
) to
resist the
F
serv,k
such that the limiting creep or load loss for a
SLS are not exceeded. This requirement is not explicitly stated
in all countries and may be covered in a ULS requirement.
Assuming that the appropriate partial factor for this SLS is
unity, this requirement is expressed as Eq. 4.
;
≤
;
(4)
2.3
Geotechnical ULS anchor resistance
EC7 requires that anchor tests be carried out to confirm that
they have the resistance to satisfy Eq. 1. The value of the ULS
resistance,
R
ULS
, is defined as the “value of the resistance of an
anchor complying with ultimate limit state criteria”. This means
that tests must demonstrate that an anchor can provide a certain
resistance while satisfying specified criteria of creep or load
loss. The pull-out resistance will be greater than the value
determined from the test. The design value of
R
ULS,d
and the
characteristic resistance (
R
ULS,k
) are determined from the
minimum (
R
ULS,m
)
min
of measured values (
R
ULS,m
) in
investigation and suitability tests using the partial factor
(
γ
a,ULS
)and correlation factor (
ξ
ULS
) and Eqs. 5 & 6.
,
=
(
;
)
(5)
;
=
;
;
(6)
2.4
Geotechnical SLS anchor resistance
In those countries which require that SLS of the anchor
resistance be considered, it is necessary to verify that the
anchors have at least the capacity to satisfy Eq. 4, satisfying
SLS criteria of creep or load loss. Using the same symbols as in
2.3 but with SLS replacing ULS and a correlation factor of
unity, the design SLS resistance (
R
SLS,d
) and characteristic SLS
resistance are given by Eqs. 7 & 8.
;
=
;
(7)
;
=
;
;
(8)
3 CURRENT DESIGN PRACTICE
The amended Section 8 of EC7 only covers the design of
anchors from load tests, hence only this aspect of anchor design
is covered in this paper. Calculations using parameters derived
from ground tests are considered to be for the estimation of the
bond length only, and the design is then verified by load tests.
The discussion on current design practice is not as
straightforward as it might appear due to the way anchor forces
are determined for particular design situations in some countries
and to the current lack of agreement on what precisely
constitutes ULS and SLS failure criteria of an anchor. The
design aspects include not only whether the SLS or ULS
resistance of the anchor is verified but also whether the force
used in this verification process is derived from analyses using
factored ground properties or using characteristic values.
The derivation of ULS anchor forces is well developed and,
for embedded walls, typically involves some type of Limit
Equilibrium Analysis, although the use of finite elements is
becoming more common. However the design of anchors in
some countries is related to the ‘working load’. This practice
arose from the fact that earth pressures in the SLS condition
(unfactored and considering compaction and at rest pressures)
are greater than those at failure when the soil strength is fully
mobilised, consequently can give rise to greater anchor force.
Furthermore, as these are ‘working loads’, the anchor would be
required to satisfy more onerous creep criteria at such loads.
However, EC7 requires that in ULS design a more conservative
view is taken of the ground strength and resistance, together
with unexpected excavations and higher surcharges than
considered for SLS and this situation must also be considered.
The methods used in the past to determine SLS forces, which
are also called working forces, were very approximate for
embedded walls. Typically the length required for ULS was
derived by considering the wall as a beam with a length
required for ULS, supported at the anchor and by passive earth
pressure, on which act the active earth pressures determined
using characteristic actions and soil parameters. Other
approaches for simple walls were to calculate the anchor force
using the characteristic actions and parameters but with the
shortened pile length that is required for equilibrium. The
advent of finite elements and other methods of analysis has
allowed deformations to be considered more realistically thus
providing a more reliable estimate of
F
serv;k
. The forces required
to limit the movement of the structure and the supported ground
are considered, including those forces attracted to the anchor
after lock-off.
France has perhaps a design practice that can be most easily
related to the proposed amendments of EC7 in that a
F
ULS;d
is
determined from an ULS analysis of the structure and a ‘service
load’, similar to
F
Serv;k
,
, is also derived using characteristic
values of actions and soil parameters. The testing is required to
verify that the anchors have the required ‘pull-out’ resistance to
satisfy the ULS requirements, including the required ULS
resistance to ensure safety under
F
ULS;d
, and that the creep
requirements are satisfied under the service load.
Germany also calculates a value of
F
ULS;d
, however this
value is calculated from characteristic values of the effects of
permanent and variable actions, which are termed
F
Gk
and
F
Qk
.
The anchor force for proof testing is related to
F
ULS;d
which is
the maximum of
1,35 F
Gk
+ 1,5 F
Qk
, or 1,35 times the anchor
force after lock-off if that is greater. The proof load has to
satisfy a limiting creep criterion which is discussed in the
