NZTA project Project Roading

Rai Saddle 
realignment

A key risk-spot on SH6 between Blenheim and Nelson has undergone a challenging realignment. Hugh de Lacy reports.

NO FEWER THAN 20 people have died and 93 have been seriously injured in the decade to 2016 along the length of SH6 at the top of the South Island, and the Rai Saddle, one of the two most challenging stretches on that picturesque road, is getting an upgrade that is proving to be distinctly more difficult than first envisaged.

The Rai Saddle is at an altitude of 250 metres, half the height of the better-known Whangamoa Saddle further west, but the five bends at its summit have seen 25 crashes, five of them serious, in the decade to 2015.

It was to cut this toll that NZTA announced back in 2013 that one of the five sharpest corners on the Rai summit would be eliminated, and the others straightened and widened.

If the weather permits, the project is due for completion in July of this year, and the realignment, mostly on the Nelson side of the summit, will lower the Rai Saddle’s elevation by about six metres, and offer a safe speed environment of between 75  kilometres per hour and 85 kilometres per hour.

This stretch of highway is only about 1.4 kilometres long, and NZTA originally anticipated the cost, including the geotechnical investigations, at around $6.6 million.

However the complexity of the site, and particularly the need to remove weak sub-soil, saw the cost revised upwards, with batters having to be flattened and larger volumes of waste and replacement accounted for.

The changes in batter slopes affected the approach of the successful tenderer, Downer, with the resultant narrowing of the site requiring the incorporation of no fewer than three temporary road alignments.

Compounding these challenges was the 10-metre height separation between the old and the new roads, the former being the high, requiring batters and temporary geotechnical engineering on the downside to ensure stability.

“When we found out the material was a bit weak, requiring some of the batters to be cut further back, we had to re-design our temporary diversions and push into the hillside to create some room at the bottom,” Downer’s project manager, Nathan Hodges, tells Contractor.

This did, however, have an upside in that it provided more good cut-to-fill material to replace the additional site waste.

Two design-and-build culverts had to be installed, the first two metres in diameter and 100 metres long, to accommodate one of the two streams that encroached on the site.

This culvert sat below a 17-metre fill embankment, and because the pipe was made of plastic, the trenching and groundwork to support it was one of the job’s main challenges.

Both the inlet and the outlet required the building of special stormwater structures to accommodate and smooth out the energy of expected floodwaters.

These were part of the design-and-build package.

“Groundwater was a bit of a problem as we dug down and trenched out that embankment, but we managed to contain it without too many problems,” Nathan says.

The second culvert, though only 900mm in diameter but 105 metres long, carries up to 13 metres of fill and has three vertical drop-shafts within it to create the required fall.

The two streams both had to be temporarily diverted to allow the work to go ahead.

Native fish were collected from both streams by locals and the Nelson company Fish and Wildlife Services before work could start.

The larger culvert had to be fitted with special prefabricated polyethylene baffles to enhance fish passage.

These were screwed to the floor of the culvert to create eddies and whirlpools through which the fish could navigate in either direction.

The relative isolation of the site means that workers travel from both Blenheim and Nelson every day, so an hour’s travel time each way has to be fitted into the daily work schedule for the workforce that usually numbers between 20 and 30.

The site is worked six days a week and the potential fatigue hazard is mitigated by staggering the Saturday work between two crews.

Downer’s own staff make up the bulk of the workforce, but local contractors are brought in as necessary, with the Pelorus firm of Mike Edridge Contracting favoured with the main subcontract to supply pavement aggregates, some of the earthmoving machinery and to complete a minor drainage job.

Edridge is sourcing most of the aggregates, comprising 8000 cubic metres of sub-base and 4000 cubic metres of base course, from the Pelorus River.

Paving will cover 20,000 square metres, and the wider project involves 245,000 cubic metres of cut-to-fill and 90,000 cubic metres of cut-to-waste.

With trucks having to cross back and forth across the temporary carriageways during the early stages, traffic management was a significant component of the job, Nathan says.

This was in the hands of Downer’s Nelson-based subsidiary Integrated Traffic Solutions (ITS), which was stopping traffic briefly to allow trucks to cross, and  maintaining a 30 kilometres per hour speed limit during working hours, raised to 50 kilometres per hour otherwise.

“All our temporary alignments had to be constructed within a 50 kilometres per hour design speed,” Nathan says.

As well as the environmental considerations, especially relating to protecting the life forms in the streams, there were initially also cultural and archaeological ones.

Staff were trained to look out for middens, greenstone, argyllite or any other sign of early human habitation, and for the first few weeks an iwi monitor was on-site full-time, but in the end nothing of cultural significance was uncovered.

The increasing numbers of cyclists using the country’s open highways, even on roads as hilly as SH6 and with its settlements so widely spaced, had to be taken into consideration in the design of the Rai Saddle realignment.

On its completion though, cyclists no less than motorists will find this section of the highway between Blenheim and Nelson considerably less challenging and dangerous than it used to be.

This article first appeared in Contractor’s February issue.

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