Heritage Trail to Arthur’s Pass

Of all the mountain roads in New Zealand, none matches Arthur’s Pass for the complexity of its engineering. HUGH DE LACY finds out why.

A FEW NOSES WERE put distinctly out of joint in the little engineering community of colonial Christchurch when a teenager fresh out of a local church school landed the job of surveying the wild West Coast of the South Island for a way to link it by road to the east.

The Otira Viaduct was completed in 1999, a 445-metre long pre-stressed concrete box girder bridge of four spans, of which the 134 metre main one is the longest in New Zealand.
The Otira Viaduct was completed in 1999, a 445-metre long pre-stressed concrete box girder bridge of four spans, of which the 134 metre main one is the longest in New Zealand.

It was 1859, two years before Gabriel Read would ignite the nascent country’s gold fever by discovering bits of the stuff shining “like the stars of the Orion” in his fabulous gully in Otago, thereby triggering the exploration that led to further phenomenal discoveries up the length of the West Coast.

The Otago gold all filtered out through the vigorous Presbyterian settlement of Dunedin, bringing untold development benefits with it, and the Anglicans of the Canterbury settlement were determined that the West Coast’s riches should similarly endow Christchurch.

But there were only isolated reports of gold on the island when the Canterbury Provincial Government sent 19-year-old Arthur Dudley Dobson, not long graduated from what would become upper-crust private school Christ’s College, to survey the almost unexplored northern West Coast to see what treasures it might hold.

The murmurings about his appointment arose because his father, Edward Dobson, was the Canterbury Province surveyor.

Nepotism and his youth notwithstanding, Arthur and his younger brothers George and Edward, similarly favoured with employment as surveyors by their father, all produced superb exploratory work, as had the Edward senior and his brother Alfred, who was surveyor for the Nelson province.

Young Arthur Dobson spent seven months on the Coast before returning to Christchurch to report, and was then sent back up the Waimakariri River in 1864 to see if he could find a way across the Southern Alps to link Canterbury with the West Coast.

Summit of Arthur’s Pass
Summit of Arthur’s Pass

Reaching the Great Divide, Dobson climbed a thousand-metre peak above the Otira Gorge, and saw it as a place where a road might be pushed through between the Waimakariri on the eastern side and the Taramakau on the western.

By the time he’d completed this task and again reported back, the Gabriel’s Gully goldrush was in full swing, and reports of both placer and quartz gold discoveries had begun flooding in from the West Coast.

Keen to emulate Dunedin’s effective clipping of the golden ticket, a group of Christchurch businessmen put up a stake of £200 ($400) for anybody who could find a suitable route to the Coast.

C_Feb_2015_Pg46_4Arthur Dobson’s younger brother George checked out every possible pass at the headwaters of the Taramakau, Waimakariri and Hurunui Rivers, and returned to say that the pass Arthur had discovered was by far the most suitable.

So it was by way of Arthur’s pass that the provincial government decided to build a road to the Coast over Porters Pass, west of Springfield, up the Cave Stream to Cass on the Waimakariri, up the Bealey River and over the pass, then on to Otira and down the Taramakau.

The youngest Dobson brother, Edward, then got the job of building the 200 kilometre road, and he set a precedent for extraordinary engineering feats at Arthur’s Pass by completing it to a standard fit for stage-coaches by July 1866.

This was just a few weeks after the middle brother, George, working at the western end of the road, was mistaken for gold-buyer Edwin Fox and murdered by a gang of four cut-throats – they also killed four other men just a fortnight later in what became known as the Maungatapu murders, the country’s only incident of Australian-style bushranging.

Where a road crossed the Alps to the Coast, demands soon followed for a railway, leading to the building of the Midland Line between Lyttelton and Greymouth.

After decades of agitation, work on the railway line began in 1907.

Otira Tunnel railway from the west side, c1910
Otira Tunnel railway from the west side, c1910

The key passage over the great divide was to be by way of an 8.5 kilometre tunnel from Arthur’s Pass to Otira at a gradient of one in 33.

At the time one of the longest rail tunnels in a world obsessed with trains, it became the second great feat of engineering in Arthur’s Pass after the road, but taking 16 years to build and not opening until 1923.

In the meantime, the road through Arthur’s Pass depended at the eastern end on a zigzagging route to get it above a massive and constantly moving slip which saw everything from pulverised sand to massive greywacke boulders tens of metres thick plummeting into the stream bed below.

As ferocious seismic forces – including a major earthquake in 1929 – extended the slip further up the 700 metre mountainside behind it, new zigs and zags were added to keep the road above it.

By late last century, however, with the slip stretching ever higher up the mountain, the potential of the zigzagging strategy had been exhausted, and a new carriageway had to be built between Arthur’s Pass and Otira.

A long bridge would be at the heart of it.

C_Feb_2015_Pg46_5With the bed of the Otira River deep in avalanche rubble, more-or-less constant seismic movement triggering repeated rockfalls, the 800 metre elevation above sea-level ensuring extremes of weather that included five metres of rain a year, and that particular part of SH73 sitting within the Arthur’s Pass National Park and accordingly dotted with Maori cultural features requiring preservation, the challenges to designers Beca Carter Hollings and Ferner were without precedent even in the challenging New Zealand environment.

What they came up with, and what was built by contractor McConnell Smith, was a 445-metre long pre-stressed concrete box girder viaduct of four spans, of which the 134 metre main one is the longest in New Zealand.

The construction method was balanced cantilever, with the pier foundations of the elegantly curved structure driven 25 metres into the streambed, and the slender piers rising to 35 metres above them.

The piers had to be protected from rockfalls by deflector structures, and the viaduct rises along its length at a gradient of nearly 12 percent east to west.

And defying the Otira rail tunnel’s record of repeated delays and cost over-runs, the Otira Viaduct was completed in November 1999, a month ahead of its 36-month timetable and comfortably within its $25 million budget.

The outcome of this third of the great Arthur’s Pass engineering challenges is a beautiful ribbon of a man-made structure, seemingly floating above the rock-strewn riverbed, enhancing the savage natural beauty of the place and making it the highlight of the spectacular road trip across the island.

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