Grafton Gully Bridge
The Central Motorway Junction project involved the upgrading of the motorway interchange in central Auckland to improve its operational efficiency. The interchange is a key element in both the national state highway network and the Auckland regional road network. It is extremely heavily trafficked, with a total bi-directional flow of the order of 200,000 vehicles per day.
Due to its size, complexity and importance, the project was split into several contracts. The first contract, completed under an alliance model, involved upgrading the state highways between the junction and the Auckland waterfront. The second contract, completed under a design and construct movel, involved the modification of existing structures and the provision of six new structures to facilitate motorway-to-motorway traffic movements not provided for by the existing junction.
BBO were engaged by the Alliance to design the Grafton Gully Bridge to resist dead load, live loads and seismic loads, in accordance with current New Zealand codes, and to produce construction drawings and technical specification.
The Grafton Gully Bridge is a 45m long, two span road bridge carrying four lanes of urban traffic over a state highway. The post-tensioned voided slab deck units are supported on, and integral with, bored piles. The bridge features high retaining walls and differential skew of the abutments to accommodate the state highway geometry. Construction utilised a top down process to provide traffic flow throughout construction.
BBO achieved the Alliance objective of facilitiating the earliest possible start to construction by delivering the design within a demanding timeframe even though changes to the state highway geometry necessitated significant redesign.
Sinclair Knight Merz and BBO were awarded the contract to develop the conceptual design and obtain statutory approvals to allow the design and construct contract to proceed. This competitively tendered contract was secured largely on the strength of an alternative geometric layout developed by BBO.
BBO were responsible for the development of the structural and geometric conceptual design, which involved large structures with complex geometric elements and multi-level bifurcating ramp structures, and the development of construction sequences to avoid delays to traffic. The scope of BBO's subconsultancy covered other non-structural activities including consultation with key stakeholders, procurement of statutory consent and tender evaluation for the design and construct contract.