Four options for troublesome Manawatu gorge

On the eve of making a final decision, the NZTA has identified four alternative routes to the slip-prone Manawatu Gorge and none involves the existing road. Hugh de Lacy reports.

 EVERY FIVE YEARS or so the fractured and splintering greywacke rock that comprises the cliffs on the south side of the Manawatu Gorge gives a bit of a heave and dumps hundreds of thousands of cubic metres of spoil across the road that connects the east and west sides of the lower North Island.

It’s been going on pretty well ever since the first foot and horse track was punched through in the crumbly south side of the gorge in the late 1860s, followed by a road for wheeled vehicles in 1872.

And lately the landslides have been bigger and taken longer to fix: a monster came down in 2004 that closed the road for 70 days, then in 2010 the biggest ever until that time closed it for 450 days.

In April this year the gorge moved again with two 15,000 cubic metre slips coming down, and over the next three months further slips resulted in the road being closed indefinitely from July.

Now it’s been decided it’ll never open again – at least not as a state highway – and to replace it the NZ Transport Agency has put up four options for roads going over the ranges instead of through them by way of the gorge.

An announcement on the preferred option is expected early this month (December).

The decision to abandon the gorge road as a state highway was influenced in part by the failure of the mammoth effort to remove the 2010 debris when, for the first time, machinery was taken to the top of the cliff in the hope that cutting it back in the worst places would solve the problem long-term.

A pair each of bulldozers and diggers from Higgins Group, which had held the gorge road maintenance contract for 20 years, crossed miles of farmland to reach the clifftop and start pushing down rubble which previously could only be shifted by water-bombing from helicopters.

That single clearance operation cost the NZTA $16 million, plus another $5 million for upkeep of the Saddle Road.

And for the next five years, until they turned wayward again in April, the cliffs cooperated and 7000 traffic movements a day wound through the gorge.

The gorge looks like a giant fault-line running east-west through the ranges, but it’s not: there are plenty of fault-lines in the area but they run north-south, and it was the river, rising in the hills around Norsewood on the eastern side, which cut the gorge between the Ruahine Ranges to the north and the Tararua Ranges to the south.

The railway that runs along the north side of the gorge is nowhere near as prone to slips as the road on the south side, though one in 2010 closed it for 10 days, and in 2013 another closed it for three hours and badly damaged the milk train that ran into it.

The rail line’s relative stability over the years raised the suggestion that it was because picks and shovels were used to build it, rather than the dynamite that was liberally employed on the 17 kilometres of road on the other side of the river.

It’s a seductive theory but, even allowing for greywacke’s capacity to fragment, it’s an unlikely one.

Dynamite was in fact freely used on the rail line too – indeed the two big tunnels and the bridge approaches could not have been built without it – and the volumes of spoil that collapse onto the road are too vast to have been fragmented by explosives.

Instead, the difference between the two sides of the gorge is entirely geological: the structure of the rock on the rail side is far more stable than on the road side, and crucially is less steep and with fewer overhangs.

So after millions spent on its upkeep, the Manawatu Gorge road has been abandoned at least for the time being, and in the meantime east-west road traffic has the choice of either the Pahiatua Track for those headed for the Wairarapa, and the Saddle Road for those headed to Hawkes Bay.

Of the four options for permanent alternatives to the gorge road, three are to the north of the gorge and one to the south, and all will take at least five years to build.

They are:

Option One: A new road across the Ruahine Range north of both the Saddle Road and the Te Apiti windfarm, running for 15.7 kilometres, costing $350-$450 million and taking five to six years to complete.

Option Two: A major upgrade of the Saddle Road to bring it up to state highway standard, covering a length of 13.8 kilometres, costing $300-$400 million and with a completion time of five to six years.

Option Three: A new road south of the Saddle Road, costing $350-$450 million to build, extending 12.4 kilometres and with a five to six year construction period.

Option Four: A new road south of the gorge offering the most direct travel to and from the south-west of the island, 19.2 kilometres long, costing $450-$550 million and taking six to seven years to complete.

NZTA’s offering no hints as to its own preferred option, but on the face of it numbers two and four would seem the most likely, with the upgrading of the Saddle Road perhaps having the edge over the southern connection.

Excluded from the final options were a range of ideas with costs ranging up to $2.5 billion.

One involved running the road through a deep box cut in the gorge which would reduce its length to 12.6 kilometres at a cost of $1.9-$2.5 billion, but would take at least 15 years to build and posed damage to cultural and ecological sites.

A second was for a gorge viaduct, taking the road out from the cliffs for 14.2 kilometres at a cost of $1.1-$1.4 billion, with a completion time of six to seven years, but at the risk of both consenting delays and residual slips.

Then there were the long and short tunnel options for the road through the gorge at a cost of $1.7-$2.2 billion and a completion time of eight to 10 years for the long tunnel version, and $1.2-$1.5 billion with a completion time of six to seven years for the short one.

One suggestion that drew the early support of local authorities came from a consortium of civil contractors in the region who wanted to raise the road through the gorge by 15 metres, cutting a 20-metre wide fall zone between the road and the cliff at a projected cost of $165-$195 million.

In the end though it was the four less spectacular options that NZTA opted for after extensive consultation with local authorities, industry groups and the small town communities for whom a road over the ranges is key to their economic viability.

This article first appeared in Contractor December 2017.

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