High density challenges

It’s taking four years to build a motorway extension just 4.5 kilometres long. But just look at what’s involved. It’s an example of how complex the answers to Auckland’s traffic problems can be.   BY GAVIN RILEY

MHX_1.jpgThere are two common denominators to most measures needed to solve Auckland’s massive traffic problems – they’re complex and they come at a high cost.

A typical example is the State Highway 20 Manukau Extension, a 4.5 kilometres stretch of motorway which when its four-year construction is completed in August next year will have cost $210 million.

This design-and-build project for the NZ Transport Agency will provide the southern entrance to the 48 kilometre Western Ring Route, one of the seven roads of national significance. The route will enable State Highway 1 through-traffic to bypass Auckland city between Manukau and the North Shore via State Highways 20, 16 and 18 when it is completed some years from now.

Given the Manukau Extension’s short length, it is a “high density” project. Covering a 64 hectare site and requiring more than 1,000,000 cubic metres of earthworks, 16.8 kilometres of drainage, seven sedimentation ponds, and 268,000 square metres of pavement, it will feature two lanes in each direction (with provision for third lanes), three major interchanges, 12 bridges, two sets of 100-metre-long twin culverts, and 240,000 plants and trees, consisting of groves of flaxes, totara and banks of pohutakawa.

MHX_2.jpgConstruction is being carried out by Leighton Works, a joint venture between Leighton Contractors and Downer EDI Works, which has engaged Sinclair Knight Mertz as the roading and structural designer. Golder Associates is responsible for the geotechnical design and EDAW Jasmax the urban design.

Two of the interchanges, at Cavendish Drive and Lambie Drive, will provide major entry and exit points for traffic travelling to and from the new motorway and local roads. State Highways 1 and 20 will join at the third interchange, which is a significant undertaking in itself with three levels and featuring a 240-metre curved flyover with 10 spans.

Building bridges

MHX_7.jpgThe project’s largest structure is a 124-metre-long, 34-metre-wide bridge over the main-trunk railway line. This bridge will have two decks, which will share piers along its length, and its six spans (requiring 72 concrete beams) will carry the motorway extension between two large embankments.

Other bridges will allow five roads to pass over the motorway and there are five motorway crossings over local roads and waterways, the tallest of which has required piers 12 metres high over the Puhinui Stream.

Roads and rail

Leighton Works is also carrying out improvements to the local roading network for Manukau City Council and has started construction of the Manukau rail link for Kiwirail and the Auckland Regional Transport Agency. The link, to connect Auckland’s southern line with Manuaku city centre, is part of Kiwirail’s $600 million upgrade of the Auckland rail network. The new station will be housed in a 300-metre-long concrete trench in Manukau city centre. The design work for this project has been done by Opus.

Using the same contractor allows the NZ Transport Agency and Kiwirail to utilise various synergies between the two important infrastructure schemes.


To cater for the construction of the motorway extension’s 12 bridges, a precast facility was set up at Takanini to manufacture the 345 T-beams and 750 bridge barriers required. Weighing up to 56 tonnes and up to 34 metres long, the beams had to be loaded onto transporters at the precast yard by special gantry cranes. The precast operation was completed earlier this year.

MHX_3.jpgPossibly the most visually impressive accomplishment of the project has been installing 10 beams across the spans of the SH1-SH20 flyover. This was carried out in May and required careful planning, co-ordination and a high safety priority as the beams had to be transported 11 kilometres from the precast yard to the flyover site then lifted into position by two 300-tonne cranes. Each beam had been made to fit precisely into its specific place in the flyover structure, so it was important that they left the yard and arrived on site in the same order they had to be installed.

Though this testing task was carried out in very wet weather, it was completed during a single overnight motorway closure.

More challenges

Building the bridge over the main-trunk railway is even more challenging, however. Installation of the beams, which began in the spring, requires extensive planning and careful co-ordination with rail-network manager Ontrack to ensure a high standard of safety is maintained and freight and passenger services are not disrupted.

MHX_6.jpgLeighton Works project manager Glenn Houpapa says the challenges of building the bridge over the railway are not limited to above ground, as a mix of basalt flow of varying thickness and strength overlies a weak saturated material.

“The abutments for this bridge are constructed on piles, with driven steel piles on the western end and concrete piles in the east, while concrete pad foundations have been used for the central piers,” he says.

“Design of the bridge had to be mindful of the future rail link so that the new tracks pass beneath the spans as the rail lines continue towards Manukau alongside the new motorway. This required significantly modified crossheads offset to the other piers and was achieved through the use of post-tensioning.”

What’s left to do

Glenn says considerable work remains to be done before the project’s scheduled completion next August. Pavement construction is underway across much of the project; effort is being made to complete the earthworks around the western end where the new motorway has to tie in with the existing Southwestern Motorway; and services, drainage and lighting continue to be installed.

MHX_5.jpgThe westbound service road between Roscommon and Puhunui roads is the last part of the project to be completed and can only be constructed after the motorway is commissioned. Construction of the service road will be concurrent with the final alignment works being completed to the Puhinui Stream. The stream has been an important factor in the project as it crosses the new route at seven locations. It has been relocated for 750 metres in total, an engineering achievement, which also involved the relocation of hundreds of fish and other aquatic life.

Over the coming months, Leighton Works will progress with the four remaining bridges. Construction of Plunket Avenue is underway and work is proceeding to complete a single-span structure over the Puhinui Stream towards the western end of the project by Christmas. The beams for the final span of the

SH1-SH20 flyover were installed in November and work is continuing on the bridge over the main-trunk rail line.

Despite the complex variables associated with this bridge, Leighton Works is targeting completion of the structure by April.

“Although lacking the graceful lines of the higher profile SH1-SH20 flyover, the bridge over the main-trunk line will be an impressive engineering achievement for the team,” Glenn Houpapa says.

Impressive underpass

MHX_4.jpgHe describes the SH1-SH20 underpass at the triple-level interchange as another substantial achievement. The bridge is constructed using top-down techniques, with deep bored piles forming retaining walls on both sides and supporting a large reinforced concrete deck that forms the underpass roof. The structure carries the southern motorway overhead as southbound drivers from State Highway 20 pass beneath before joining State Highway 1.

Glenn says that to maintain a constant traffic flow along State Highway 1 the bridge was constructed in three stages, each stage’s completion allowing the motorway lanes to be shifted over it and permitting the construction of the next stage of the bridge. This was carefully co-ordinated with the construction needs of other aspects of the SH1-SH20 interchange.

Glenn says members of the Leighton Works engineering team have contributed to several construction innovations to enable works to be completed more efficiently and safely. Sand jacks have been used to support temporary works, detailed traffic staging plans implemented, precast crossheads used to reduce installation time-frames, and improved energy-efficient methods employed to cure concrete in the precast yard. Waste materials have been salvaged and reused to improve sustainability, and initiatives introduced to ensure works do not have an adverse impact on the environment or the community.

Who’s involved

Supporting the Leighton Works joint venture are a number of subcontractors and major suppliers, who Glenn says are making a significant contribution and are very much part of the team. They include Allied Concrete, Gleeson & Cox (earthmoving), Hirequip, Hiway Stabilizers, NZ Crane Hire, Porter Hire, Steel & Tube Reinforcing, McKenzie & Parma, Hauraki Piling, Hynds Pipes, ICB Construction and Tranzcarr Heavy Haulage.

Glenn describes the Manukau Extension as extremely complex and challenging, but rewarding. In the structures department alone, he points out, “virtually every type of bridge crossing is being built, with bridges being constructed over the motorway, under the motorway, over local roads, over water and over rail”.

Expensive, but worthwhile

MHX_8.jpgWhile the project cost is considered relatively high at an average of more than $7 million a lane kilometre, it is expected to produce benefits beyond being the indispensable southern entrance to the Western Ring Route. The extension will mean faster access to and from Auckland airport and central and western suburbs, more reliable and quicker commuter trips, and considerable traffic relief for local roads. It will also provide a key connection and support future growth at the Wiri and Auckland international airport business zones, as well as at the new suburb of Flatbush where a population increase of 40,000 is expected.   


Contractor Vol.33  No.11  December 2009 - January 2010
All articles on this website are copyright to Contrafed Publishing Co. Ltd.