The Newlands Interchange Project

Introduction

Newlands interchange is at the top of the Ngauranga Gorge.a steep grade of about one in twelve, over a short 1.8km length of State Highway One situated 8km North of Wellington City.

The Newlands Interchange Project was a huge civil engineering challenge which involved removal of an existing set of traffic lights and conversion of the T-intersection with the State Highway into a grade-separated interchange - the aim being to reduce both the amount of traffic congestion and the number of vehicle accidents.

The construction phase began in January 1997 and was completed, with a minimum disruption to road users, two months ahead of schedule in August 1998. The cost of $16.7 Million was $600,000 below allocation.

This is a overview of the development of the project.

The need

The Ngauranga Gorge section of State Highway 1 carries large volumes of commuter traffic between Wellington City and Northern areas including Johnsonville, Newlands, Porirua and the Kapiti Coast, as well as being the main route for travellers North from Wellington. The traffic flow is approximately 60,000 vehicles per day and this is growing at approximately 3% each year. About 13,000 vehicles a day use Newlands Road , which previously formed a signalised T-intersection with the State Highway. The very high morning and evening weekday peak directional flows on State Highway 1 resulted in considerable peak hour congestion and an unacceptably high number of traffic accidents.

A number of options were considered during the investigation and design phases to resolve the problems at this intersection. The Interchange proposal was finally adopted after extensive consultation and assessment of the traffic, economic and environmental issues.

Major groups involved in the project

Client: Transit New Zealand
Consultant: Opus International Consultants
Main Contractor: Fulton Hogan Ltd.

Disciplines involved in the consultancy work for the investigation, design and construction phases included engineers, scientists, planners, draughting staff and lawyers.

Key features of the new interchange:

• The Newlands overbridge providing grade-separation of the interchange
• A fourth lane northbound to the Newlands off-ramp
• Upgrading of signage
• Separate provision for pedestrians and cyclists, including two subways
• Upgrading of highway lighting
• Extensive reshaping and trimming back of the steep rock bluffs on both sides of the gorge
• Strengthening and rehabilitation of an existing major culvert carrying the Ngauranga stream and buried up to 17m below the intersection
• Extensive landscape rehabilitation and revegetation

Specific features and innovations relating to the project

• Adoption of a formal Partnering Charter at the beginning of the construction phase to foster a team approach between the client, consultant, contractor and other parties involved. This was one of the first applications of this approach on a Transit New Zealand project.
• A very high standard of temporary traffic control to maximise the safety of workers and motorists whilst also minimising disruption to the public
• Cuttings up to 50m high in highly fractured rocks with residential properties at the top of these slopes and a very busy highway at the bottom required risk assessment and decision analysis to optimise costs and risks
• Adoption of a steel box girder form for the bridge superstructure to enable erection with minimum traffic disruption
• Architectural input into the bridge design to produce an aesthetic structure in a location where the bridge has high visual significance as a gateway structure to Wellington
• A demanding site which is exposed to high seismicity and in close proximity to the Wellington fault, with variable ground conditions across the site required a range of special features to provide significant earthquake resistance
• The use of new technology zinc/aluminium thermal metal spray for corrosion protection of the bridge box steel girders to provide an expected life to first maintenance in excess of 40 years
• Resolution of potential conflicts between vehicles, cyclists and pedestrians through, for example, provision of subways
• Strengthening and rehabilitation of the major culvert carrying a stream beneath the intersection using an innovative system employing internal sleeving and grouting
• Measurement of noise and light levels prior to commencement of construction to provide a benchmark against which to compare these conditions as they arose both during and after construction, and monitoring of noise levels during construction
• Extensive landscaping to ensure revegetation and to blend the interchange structures into the natural landscape and environment
• Extensive community consultation throughout the duration of the project, including regular meetings with community representatives to provide information and promptly address any complaints as they arose.

The design and construction of the overbridge

Ngauranga Gorge is generally acknowledged as the gateway to Wellington , and consequently an overbridge was seen as having the potential to act as a natural entrance feature. A subway under the highway had been considered during the scheme assessment phase but the tender submitted did not meet acceptable geometric standards.

The architectural and urban design theme developed for the bridge led to the proposal of three alternative design concepts:

• a conventional multi-span bridge
• a through-arch bridge
• a cable-stayed bridge

Public consultation undertaken by Transit New Zealand favoured the through-arch bridge option. This was therefore adopted for further development and preliminary design to sufficient detail to confirm structural feasibility and develop accurate costings.

Site constraints reduced the arch-bridge options to three tied arch options which, while feasible were assessed to cost in excess of $5 million - approximately double the cost of a conventional multi-span bridge. The arch concept for the bridge was therefore discarded.

Options for a multi-span bridge were constrained by Transit New Zealand's requirements that traffic flows at peak times be maintained on the State Highway without disruption and piers intruding into the State Highway be minimised.

Consequently only two-span options were considered, in both steel and concrete.

Incrementally launched concrete box girder or T-beam continuous superstructures were not considered to be economic or viable, due to the superstructure depth required. An option using large precast U-beams was also considered. For the spans required, unit handling weights of about 80 tonnes were estimated. Availability of cranes to handle such units of which there are only a few in New Zealand was considered likely to constrain construction programming and inhibit the competitiveness of pricing.

The final solution adopted was a continuous superstructure consisting of four parallel trapezoidal steel box girders with a cast in-situ composite concrete deck. Of particular significance was the ease with which this could be erected by craning box girder sections into position with the maximum handling weight reduced to less than 45 tonnes.

The fact that the Newlands Interchange site is close to the Wellington fault posed major practical difficulties in designing for 'near fault' earthquake events. A feature of the developed design was the substantial anchorage required to stabilise the abutments against the very large forces. In order to achieve a practical design for the eastern approach embankment and the western abutment, potential outward movements of 50mm were accepted for these elements under design earthquake intensity response. The bridge is 'base isolated' for earthquakes at the pier and two abutments and is supported on lead rubber bearings. These will effectively act as shock absorbers in a major earthquake and protect the bridge from major damage.

Within financial constraints, and in recognition of its siting, an emphasis was placed on the aesthetics of the bridge. A sculptural quality and visual impact were sought through treatments to achieve visual lightness of the bridge superstructure, contrasted by visual heaviness of the supporting pier and abutments. Special lighting was also adopted to emphasise the bridge at night.

As a bridge spanning a high traffic volume state highway, the need to minimise future maintenance was an important consideration. The site is exposed to wind blown salt spray and so new technology for corrosion protection was utilised and visible exposed concrete surfaces treated with a clear anti-graffiti coating system .

The project gained a high level of community support . In view of the proximity of residential areas to the construction site, monitoring of noise levels was an important activity and restrictions on the timing of certain works imposed. A high standard of traffic control was maintained at all times to protect staff, the travelling public and to minimise delays.

Benefits from the new Interchange

• The project meets Transit New Zealand's main objective of providing a safer and more efficient highway. It has resulted in:
• Reduced delays, improved safety and an improved level of service for both highway users and the local community
• Improved pedestrian and cyclist facilities
• A benefit/cost ratio in excess of six (ie for every dollar spent on the project the benefits are expected to exceed six times that amount)
• Predicted accident savings estimated at $1.3 million per year adding up to a total saving of $12 million over a 25 year period
• Travel time and vehicle operating benefits of approximately $70 million over the same period of time due to the elimination of traffic signals
• An estimated reduction of 9% in CO2 emissions

The project won the 1999 NZ Contractor's Federation Award for projects over $3 million and was a finalist in the Infrastructure category of the 1999 Institution of Professional Engineers of New Zealand (IPENZ) Engineering Excellence Awards.

This has been adapted from material produced by . The case study will be published as part of the EG series - a resource designed to support Technology in the New Zealand Curriculum. EG is a joint initiative by IPENZ, The Royal Society of New Zealand and Technology Education New Zealand (TENZ).

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