Contractor NZTA project Project

Working against the elements: Mingha Bluff realignment on SH73

Rain, wind and snow have conspired to delay a major realignment on SH73 between Canterbury and the West Coast. Hugh de Lacy gets his wet gear on.

CONTRACTOR AND CLIENT ALIKE knew that the 5.2 kilometre realignment on SH73 taking in the Mingha Bluff, a little to the east of Arthur’s Pass, was going to be weather-dependent, but they never counted on the nine metres of rain that fell in the first 22 months of the $22 million project.

Indeed, the wild West Coast weather was deceptively placid during the first winter and summer after work started in May 2015, but it was saving its demons for the 2016-2017 summer, when it was demonic enough to postpone the project’s completion date till spring this year.

The project was originally scheduled to be completed by March, but by then the weather had allowed just 80 percent of it to be finished.

The Mingha Bluff to Rough Creek stretch of highway between Greyney’s Shelter and Arthur’s Pass alpine village comprises one of the hilliest and windiest stretches of road across the Southern Alps, and its realignment has been the biggest project on SH73 in 10 years.

Realignment on SH73

Funded through NZTA as one of the government’s five Accelerated Regional Roading Package projects, it realigns the road itself closer to the Midland Railway line.

That has required 450 metres of rail line to be moved into the adjacent Bealey River bed to accommodate the new road alignment, which includes extensive river protection works.

Hawkins Infrastructure, which on April 1 this year was acquired by Downer, has been the head contractor at Mingha Bluff, supported by Westland Contractors of Hokitika, Truline Civil of Greymouth and Smith Crane and Construction of Christchurch, and between them they have had up to 70 workers on-site at a time.

The original contract allowed for 190 wet days over the two-years life of the project, and that seemed more than adequate as a dry 2015-2016 winter and summer allowed the work to surge ahead.

But then came the 2016-2017 summer, and with it – as well as some untimely snow – came rainfall of up to 378mm a week that reduced work to two and a half days out of five.

The weekly rainfall exceeded 200mm no fewer than 12 times, and during a single month it topped the metre mark – 1085mm, to be precise.

The weather was one thing, but traffic control, even on such a remote site, has been a major challenge throughout the project.

Normal expectations of traffic flows along SH73 went out the window after the mid-November Kaikoura earthquake closed the Lewis Pass, forcing Coast-bound traffic over Arthur’s Pass.

On two occasions both SH7 (the Lewis Pass) and SH1 were blocked at the same time, once by quake-generated slips and the second time by a huge fire near Hanmer, forcing north-bound traffic out of Christchurch to go over Arthur’s Pass to Kumara, then up past Westport to get to the ferries at Picton.

Automatic lights were mainly used to control the traffic, but sometimes the lights had to be operated manually to stop traffic backing up too far.

Realignment on SH73To cope, Hawkins Infrastructure split the five kilometre realignment into 11 successive zones, each one representing a part of the project with different traffic management requirements.

Detours of two or three weeks’ duration were imposed twice to guide motorists through the complex work-site.

And that was only the road traffic.

While KiwiRail took responsibility for actually laying the tracks on the realigned stretch of line, Hawkins had the job of building up the ballast for them to sit on.

More importantly, Hawkins had to build no fewer than 25 culverts beneath the 5.2 kilometres of affected rail line – and another 31 on the road realignment – without ever interrupting the schedules of a dozen trains a day taking milk, coal and tourists from or to the West Coast.

This called for precise timing of the installation of the 2.5-metre square concrete segments, pre-cast in Greymouth, that comprised many of the culverts, as each one had to be built beneath the railway and made operational within seven to 12 hours.

The fact that the crews never had to delay a train throughout the installation of the rail culverts was “something we’re very proud of”, Hawkins’ contract director, Graham Crow, tells Contractor.

Back on the road realignment, the weather was a constant threat to the road surfacing: with no silt or clay to bond them, the aggregate was all too easily washed away in the rain.

With the Mingha Bluff otherwise likely to slip into the river, taking road and rail with it, no fewer than eight retaining walls of up to five metres high had to be installed.

Sub-contractor Smith Crane proposed using a proprietary brand of retaining wall called Stonestrong, comprising hollow concrete blocks 900mm high and 2.5 metres long, and this was adopted by the client.

Around 1800 of the blocks, mostly filled with concrete or local fill, but with some also strengthened by reinforcing steel, were used to construct a total of 1450 lineal metres of retaining wall.

Inevitably the management of stormwater was a major consideration on so challenging a site.

“We were changing the topography, and if you’re not careful you can change the water course and create a dam, or block culverts and cause shingle fans across the road,” Graham says.

Just in case things weren’t complicated enough, the project lies within a national park, so it drew in an unusually wide range of stakeholders besides the client NZTA, including Ngai Tahu, the Department of Conservation, Environment Canterbury, the Selwyn District Council, KiwiRail and the Arthur’s Pass alpine village community.

Graham is adamant that despite the challenges, including weather that was “mind-numbingly bad” at times, the project will be completed this coming spring, with work continuing over the winter come rain or – less likely – shine.

The last of the retaining walls will be installed over winter, along with guard-rails, wire rope barriers and the landscaping.

A temporary seal will be applied to hold the carriageway together, and the final seal and road-marking will be done in the spring.

This project, plus the Lincoln-to-Westgate roading job for NZTA in Auckland, will be Hawkins Infrastructure’s last big horizontal infrastructure projects.

However Hawkins, one of the country’s biggest vertical construction companies, will see its brand live on in that segment of the market as Hawkins Construction, albeit under the Downer umbrella.

This article first appeared in Contractor June 2017.

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