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Optimisation of bridge approach treatment via staged construction.
Optimisation du traitement de remblais d’accès à des ponts par phasage des travaux.
His J.P., Carson D.J., Lee C.H.
SMEC Australia Pty Ltd, Australia
ABSTRACT: This paper describes an improved approach to bridge embankment transition design and construction staging that was
utilised to overcome financial and programme challenges associated with the proposed initial design solution for bridge approach
embankments. An alternative staged approach was developed for construction, comprising improvement of the strength and
compressibility characteristics of the soft soil foundation by surcharging techniques in combination with use of prefabricated vertical
drains (PVD) and high strength geotextile. Unreinforced continuous flight auger (CFA) columns were installed after surcharging to
achieve smooth transition at bridge approach embankments. During construction, the behaviour of the foundation under load was
closely monitored and back analysis of the performance of the improved foundation was undertaken. Construction stage design
optimisations were then made to satisfy the design criteria using actual monitoring data. This approach to bridge embankment
transition design provided ability for the entire subsurface profile to accommodate the applied embankment loading. As a result,
major cost, programme and environmental benefits were realised during construction by avoiding the installation of approximately
88,900 lineal metres of concrete foundation piles that were specified in the initial design.
RÉSUMÉ : Cet article décrit une approche améliorée pour la conception et le phasage des travaux de remblais d’accès aux ponts.
Cette approche a été utilisée pour répondre aux contraintes financières et de planning associées à la solution initiale proposée. Une
approche alternative en termes de phasage des travaux a été développée et comprenait l’amélioration de la résistance et de la
compressibilité du sol de fondation (argile molle) par l’installation de remblais de chargement, de drains verticaux préfabriqués et de
géotextiles haute performance. Des colonnes en béton ont été installées après la période de chargement pour assurer une transition en
douceur au niveau des remblais d’accès au pont. En phase construction, le comportement du sol de fondation sous la charge était
étroitement contrôlé et une évaluation de la performance en ce qui concerne l’amélioration actuelle du sol de fondation a été réalisée.
Sur la base des mesures effectuées sur chantier, certains paramètres de conception ont été optimisés en phase travaux pour satisfaire
aux exigences du projet. Cette méthode de conception des remblais d’accès aux ponts a fourni à l’ensemble du sous-sol la capacité de
supporter le chargement qui s’applique sur le remblai. Ainsi, de conséquents gains financiers, de temps et environnementaux ont été
réalisés en phase travaux puisque cette solution a évité l’installation d’environ 88,900 mètres de pieux en béton, spécifiés dans les
études initiales.
KEYWORDS: Ground improvement, bridge approach transition treatment, prefabricated vertical drain, CFA column, preloading.
1 INTRODUCTION
As a state government initiative, the AUD $1.88B Gateway
Upgrade Project in Brisbane Australia involves the design,
construction, operation and 10 year maintenance (DCOM) of a
new Gateway Bridge, existing Gateway Bridge refurbishment,
12km of motorway upgrade and 7km of new motorway.
Located along Brisbane’s north south arterial transportation
corridor, the project provides improved connectivity to
infrastructure such as Brisbane’s Trade Coast region, Airport
and the Port of Brisbane. Construction completion for the entire
project occurred during November 2010.
Delivered by Queensland Motorways Limited (QML) in
partnership with Leighton Abigroup Joint Venture (LAJV) and
principal designers Maunsell SMEC Joint Venture (MSJV), the
project involved construction and refurbishment of 30 bridge
structures. Fourteen (14) of these bridges are located within the
Brisbane Airport Interchange precinct, which is characterised by
soft, compressible foundation soils up to 20 m in thickness, with
road embankment heights up to 13m.
Initial design for the bridge approach treatment in this area
comprised use of various forms of piled embankment supported
by a mixture of approximately 4,900 continuous flight auger
(CFA) piles, displacement auger piles, pre-stressed concrete
piles and dynamic replacement columns.
Following cost and program analysis, an alternative staged
ground treatment approach was proposed and adopted for the
construction of 14 of the 28 bridge approaches within the
Airport precinct. This paper focuses on one such abutment
(denoted as BR25A) within this area. Site based geotechnical
characteristics are identified together with key aspects of the
initial and alternative design approach, summary of the
alternative design methodology, comparison between predicted
and actual ground settlement and outcomes successfully
delivered through utilisation of a staged approach to ground
treatment.
2 GEOTECHNICAL CHARACTERISTICS
2.1
Subsurface conditions
Geotechnical investigations indicated that the Airport
Interchange is underlain by up to 20m Holocene (upper and
lower) and Pleistocene alluvial deposits.
Upper Holocene alluvium within the Airport Interchange
area was characterised by variable deposits of clay and silt (UH-
C) and sands (UH-S). Lower Holocene alluvium (LH-C) was
found to be of more uniform composition, comprising
compressible silty clay to up to 20m depth.
pti isation of bridge approach treat ent via staged construction
pti isation du traite ent de re blais d’accès à des ponts par phasage des travaux
Hsi J.P., Carson D.J., Lee C.H.
SMEC Australia Pty Ltd, Australia
