2791
Full scale rapid uplift tests on transmission tower footings
Tests grandeur nature d'arrachement rapide sur les fondations d'une tour relais
Levy F.M., Richards D.J.
Geomechanics Research Group, Faculty of Engineering and the Environment, University of Southampton, UK
ABSTRACT: This paper describes results from a series of full scale tests on transmission tower footing at a London clay site in Kent.
The testing series investigated the effects of rapid loading at field scale with footings founded directly onto clay or coarse granular
material, with a clay or coarse granular material backfill. It was shown that footings founded on London clay mobilised significantly
greater uplift capacities at smaller displacements than those with a breakaway condition. In a suite of centrifuge tests this enhanced
capacity was shown to originate from suctions that formed on the base of model footings (Lehane et al. 2008). Back analysis of the
field tests revealed that inferred normalised suctions were similar to those generated in the centrifuge tests. However the enhanced
uplift capacity was not sufficient to cause the uplift resistance to reach design capacity by the displacement serviceability limit state of
10mm.
RÉSUMÉ : Cet article présente les résultats issus d’une série de tests grandeur nature sur les fondations d’une tour relais basées sur un
terrain en argile de Londres (London clay), dans le Kent, Angleterre. Ces séries de tests ont été menées pour étudier les effets d’un
chargement rapide en conditions réelles avec une tour remblayée et fondée dans un sol argileux ou un matériau granuleux. Il a déjà été
prouvé que, pour de petits déplacements, des fondations basées sur de l’argile de Londres présentaient une résistance à l’arrachement
plus importante que des fondations placées sur une base en gravier. Des essais en centrifugeuse ont montré que cette résistance à
l’arrachement accrue provenait de succions sur la base de la maquette (Lehane et al. 2008). De nouvelles analyses sur le terrain ont
révélé que les forces de succion calculées étaient similaires à celles générées durant les tests de centrifugeuses. Cependant cette
meilleure résistance à l’arrachement n’a pas été suffisante pour satisfaire la limite de résistance imposée par le cahier des charges pour
un déplacement fonctionnel de 10mm.
KEYWORDS: clays; footings; foundations.
1 INTRODUCTION
In the UK there are 22,000 high voltage transmission tower
pylons supported by a pyramid (or pad) and chimney type
footing under each tower leg. The majority of these towers have
cable bundles (conductors) that are reaching the end of their
design life (Clark et al. 2006) and due to demand increases and
changing power generation patterns the majority of these cable
bundles require uprating to transmit higher voltages. This will
require larger cable bundles increasing the loads transmitted to
the tower support foundations.
Recent studies undertaken by Southampton University for
National Grid UK have shown that the design basis for
transmission tower foundations may not be reliable. The uplift
capacity derived from the conventional UK design practice is
higher than that predicted by other models (e.g. American IEEE
design methods (IEEE 2001)). The ultimate reliability of
National Grid foundation systems in terms of their uplift
capacity is therefore uncertain, particularly as the imposed loads
transmitted to the foundations are likely to increase in both
magnitude and frequency as climate change produces more
extreme storm/loading events.
However, the failure of tower foundations/footing systems in
service is extremely rare suggesting that although the design
methodologies may be unsound as they are not based on actual
failure mechanisms there are additional factors not considered
in the simplified design methods used that are providing
increased resistance to uplift, particularly under rapidly applied
loading conditions. To examine these issues a series of full scale
rapid uplift tests were carried out at the Building Research
Establishment’s London clay test site at Chattenden, Kent.
1.1
UK design and construction practice
The uplift capacity of transmission tower footings in the UK is
calculated using the frustum method. This method assumes that
there is no transfer of tensile resistance from the soil on the
founding plane to the base of the footing and that a breakaway
condition exits.
The design uplift resistance of a footing is derived from the
footing mass (Wf) and the soil mass (Wf), contained within an
inverted frustum extending to the surface from the base of the
footing. The geometry of the frustum is governed by the in-situ
soil properties. In accordance with TS 3.4.15 Issue 2 (National
Grid 2004) in ‘strong’ soils (SPTN>20 or su>50 kN/m2) the
angle of the failure plane to the vertical (frustum angle) is 25o.
In all other cases the frustum angle is set to 15o.
The serviceability limit state (SLS) displacement criterion of
shallow foundations is considered to be approximately 10% of
footing width (B). However the serviceability displacement
criterion of individual tower footings is very low to prevent
buckling failure of the tower support structure members. Both
the UK (National Grid 2004) and the United States (IEEE 2001)
design codes set minimal SLS vertical footing displacements
(w) regardless of footing size – 10mm and 13mm (0.5")
respectively. These values are based on the assumption that the
lattice tower has enough flexibility to redistribute load as a
result of these maximal differential movements but will weaken
considerably thereafter.
A truncated pyramid base with an inclined chimney is the
most common footing type for lattice transmission towers in the
UK. The chimney is constructed from steel reinforced concrete
with the reinforcement extending into the base of the pyramid;
