Tahuna Road Bridge
Owner: Transit New Zealand
Contractor: Fletcher-Higgins Joint Venture
Consultant: Peters & Cheung Ltd.
Completed: Opened to Traffic 3 May 2003
The Tahuna Road Bridge crosses State Highway 1 at Ohinewai, 100 km south of Auckland, New Zealand. Innovative design aspects include stabilisation of the liquefiable sand beneath the site by vibro-compaction, galvanised Keystrip ladders and single slope TL4 traffic barriers. The bridge site is underlain by a seismically liquefiable sand layer.
The major challenges in the project are summarised below:
Design and Construct Project - The project was an open tender design and construct project with five tenderers bidding for the project. The contractor's consultants had very little room to build any conservatism into the design without jeopardising the contractor’s chances of winning the tender.
Seismically Liquefiable Ground Conditions - The site is underlain by a seismically liquefiable sand layer of over 20m thick. Although the cost of the bridge is relatively low, a specialist engineering seismologist was called in to develop a site-specific seismic model to optimise the design. The seismic model was used to determine the seismic induced porewater pressure within the sand for various earthquake magnitudes, ground accelerations and return periods.
Ground Improvement - Extensive analyses indicated that a Ground Improvement depth of 6m was sufficient to limit the permanent horizontal bridge abutment displacement to less than 200mm even when the ultimate limit state design earthquake was exceeded. The minimum 6m deep improvement was also required to maintain the static stability of the abutments following severe earthquake events during the period that the seismic induced pore water pressure had not been fully dissipated.
Resonant Vibro-Compaction Ground Improvement - The insitu sand generally has a fines content of less than 3-5% and is suitable for resonant vibro-compaction. A vibro-probe, comprising three steel plates of 500mm wide and 10m long was specially made for the job. The contractor had a suitable vibration hammer readily available. The resonant frequent of the ground was continuously monitored to optimise the densification efficiency. It is believed that this is the first time that ground densification by vibro-compaction has been used in New Zealand.
Mechanically Stabilised Earth (MSE) Retaining Abutments - MSE walls have excellent seismic performance, exhibiting extremely good ductile behaviour. KeySystem (TM) steel ladder reinforcement with KeyStone facing was selected for the project (note in changes to for). High tensile strength geotextiles were also employed to enhance the structural ductility of the abutments.
Bridge Foundations - The bridge is founded directly on strip footings at the two MSE abutments and the central pier. The bridge footings at the abutments have approach slabs which are tied into the abutment fill with steel ladders to ensure that the bridge responds elastically under the design seismic loading. Transverse seismic loads are be resisted by friction between the abutment footings and approach slabs and the underlying granular material in the abutment fill.
Bridge Deck - The deck comprises prestressed double hollow core beams connected with transverse prestressing, designed to distribute traffic and parapet impact loadings. The two spans and the central pier and the abutment footings are tied together by galvanised reinforcement.
Limited Permanent Displacement Design Concept - The bridge and abutment design extended the ductility concept within the framework of the Ultimate Limit State design for structures to include Foundations on seismically liquefiable ground. Significant investigation of the bridge deck and seat interface was undertaken to evaluate the acceptable level of seismic induced movement of the abutments and their influence on the bridge deck. First, below the 450 years Ultimate Limit State earthquake loading condition, the acceptable abutment movement was set to 50mm. The bridge deck has been detailed to tolerate the movement without damage. Second, if the Ultimate Limit State design earthquake loading is significantly exceeded, the abutment movement is limited to less than 200mm. The passive soil wedge behind the bridge seat is designed to fail and act as an energy absorption mechanism to dissipate the earthquake energy transferred from the bridge deck to the abutment so that the integrity of the entire bridge structure is maintained.
Innovations
The development of the "Limited Permanent Displacement" design concept was used for the first time for a bridge designed in New Zealand. The adoption of the concept led to significant savings in the typically high costs of ground ilmprovement to sites which are susceptible to liquefaction.
This is the first time that ground densification by vibro-compaction has been used for a bridge in New Zealand
This is the first time that single slope reinforced concrete bridge parapets were adopted to resist the new TL4 parapet loading
