Removing the sting from Hihitahi

Removing the killer curves out of State Highway 1 at Hihitahi has been a very complex project. Nevertheless, thanks to the skill of those involved, it was a remarkably smooth-running operation, and the finished work has left the locals delighted.   BY HUGH DE LACY

For decades Mulvay’s Corner at Hihitahi was the undoing of dozens of motorists travelling between Waiouru and Taihape on State Highway 1, and the curse of the Mulvay family who seemed always to be untangling vehicles and crash victims from their farm’s roadside fence.

If the two sharp bends with their permanent 30 kilometre per hour speed limit weren’t hazard enough, unwary drivers on a winter’s night could fall victim to the thick layer of ice that spread itself across the road.

It ranked as one of the most dangerous spots on State Highway 1 in either main island, and efforts to do something about it began as early as 1964, only to be repeatedly abandoned because of the scale of the attendant problems – physical, environmental and cultural. The physical problems revolved around the Hautapu Stream, a pristine river rising on Mt Ruapehu 30 kilometres to the north, and winding its way down through precipitous papa and limestone gorges to Taihape before joining the Rangitikei River south of Utiku.

Sited 12.5 kilometres south of Waiouru at an elevation of 740 metre, Hihitahi sits on the southern edge of the central volcanic plateau, and is one of the coldest and wettest places in the North Island. What made the project even more problematic was that the existing road was carved into the shady side of a bluff 100 metres high, with the ravines congesting the site and making access difficult.

Opus International consulting engineer Bob Smith first became involved with Hihitahi in 1983 when he drilled a bore-hole at Mulvay’s Corner for Opus’ predecessor, the Ministry of Works and Development. But it wasn’t until Opus won a Transit New Zealand contract for a 1.4km realignment in 1994 that the project began to take shape – and to grow like Topsy.

Opus, at its own expense, surveyed a more cost-beneficial southward extension, involving just the two new bridges, which Transit subsequently adopted and funded retrospectively.

The project was then extended at the northern end to include the replacement of the Hautapu Bridge and a passing lane beyond which saw the final project grow to 3.5km, with work beginning in 2004.

The new alignment had to cross the old road no fewer than six times, all at different heights, and with the worst match-up being at the northern end where the old road was 7.5 metres higher than the new. The proximity of the Main Trunk rail line with its own bridges did not help either.

The environmental problems arose not only from the Hautapu being Taihape’s water supply and an outstanding trout stream, but because the Hihitahi realignment cut into areas of native bush. With the river capable of rising rapidly under the frequent heavy rain, silt fences had to be installed, and check dams built in concentrated channels and water tables to minimise erosion.

The cultural dimension to the project arose from the presence of three Maori burial sites in the area. These factors all contributed to a long list of stakeholders, including the Tangata Whenua, Transit New Zealand, the Department of Conservation, the Rangitikei District Council, Telecom, OnTrack and the Horizons Regional Council. It took an extensive consultation round to get all these ducks in a row.

The essence of the project from an engineering point of view, according to main contractor HEB Smithbridge’s David Loe, was simply to get the roadway out of the shade and into the sunlight.

“There wouldn’t have been the accident history if the road hadn’t iced up so much,” Loe told Contractor. In fact the last accident happened just before work started when a truck and trailer unit, with a small child aboard, took the safety barriers with it as it plunged 20 metres into the river – amazingly with no serious injury to either driver or child.

Forty years after remedial work was first considered, the project took on urgency when it became apparent that the piles on the single existing 30 metre concrete bridge could not be braced further against the river’s erosion. The scale of the project, and the reason for its long delay, is apparent in the fact that it took three bridges totalling 417 metres in length and spanning the river four times, to replace the existing 30 metre one. This took the total cost of the project to $19.2 million.

The largest and southern-most of the bridges was the 210 metre long Turangaarere, comprising nine spans at a maximum of 18 metres above the river, sitting on piles that were either 2.4 metres in diameter with two metre columns and three metre sleeves, or two metre piles with 1.65 metre sleeves driven to a maximum depth of 32 metres. The abutment piers are founded on 1.5 metre piles at a maximum depth of eight metres.

The original plans called for the Turangaarere bridge, which crosses the Hautapu twice, to be built first to provide a haul road for the earthmoving, but ground conditions were such that this strategy had to be abandoned and the existing road, carrying through-traffic, used for haulage.

Just up the road from the Turangaarere bridge is the six-span 132 metre Hihitahi Bridge. This middle bridge is 16 metres above the river, and its design was complicated by both a 3.75 percent south-to-north gradient and a constant 700 metre radius curve. It sits on piles and abutments similar to the Turangaarere’s.

The smallest and northernmost of the bridges, the Hautapu, is of three spans and 75 metres in length with a maximum height 12 metres above the river. The central piers are two metre diameter piles bored to a depth of 10 metres, with a 1.65 metre diameter single column to the pier head. The southern abutment is a pad footing on a keystone wall reinforced with steel ladders, while the northern abutment is concrete on two 1.65 metre diameter columns.

The Hautapu was the first of bridges completed, in February last year, followed by the Turangaarere (April) and the Hihitahi (October), with Richardson Drilling subcontracting the piling for all three.

Given the site complications, it was remarkable that the project’s completion just before last Christmas was five months ahead of schedule, and it could be credited to two factors.

Firstly there was the good run that earthmoving subcontractor Noel Magee’s Magnum Construction of Te Kauwhata enjoyed in the summer season up to April 2005. Magnum faced making a total cut of 380,000 cubic metres, of which 300,000 cubic metres was cut to waste.  It also had to blast out 50,000 cubic metres of the hard limestone rock lying in seams up to four metres thick between layers of papa clay.

Some cuts were as much as 50 metres deep, with the biggest being 100 metres long and requiring the removal of 250,000 cubic metres of material. One of the cuts had to be made from the top down, with a helicopter delivering the fuel until access could be finally gained from the base.

To accomplish all this Magnum used a fleet of three 40 tonne and two smaller dump trucks loaded by Magnum’s latest piece of new machinery, a Hyundai 450 loader. Bulldozers and a 30 tonne digger rounded out Magnum’s inventory that was backed up by an HEB Smithbridge digger and dump-truck doing odd jobs.

The second reason the project came through ahead of time was HEB Smithbridge’s speed of construction. This was achieved by casting the 64 I-beams on-site and using a 150 tonne Kobelco crane, backed up by a 60 tonne one, to lift the 25 metre long, 25 tonne beams into place. This turned out to be a master stroke, given the further complications that would have arisen had the beams had to compete with road traffic to get onto the site.

In the end traffic delays were minimal: hold-ups of only 10-15 seconds to let the dump trucks through, and up to 15 minutes when blasting was under way. The trucks had a maximum haulage distance of two kilometres, and to finish the job off, Works Infrastructure laid 44,000 square metres of new pavement. Re-grassing and planting was carried out by Evergreen.

The early completion is the more remarkable for the weather’s refusal to co-operate. It started with a white Christmas just as work was getting under way in the summer of 2004, and throughout the following winters the temperature seldom made it above 10 degrees during the day, and rain and snow provided almost daily complications. Matters reached their nadir last winter when the area copped its biggest snowfall in 20 years.

Opus’ Bob Smith actively involved the people of Taihape, the self-styled gumboot capital of the world, through their local newspaper and a shop-window display. The contractors signed off on the project by relocating the town’s iconic corrugated iron gumboot to a higher profile site on the northern approaches of State Highway 1.

For a project as complex at Hihitahi, it was a remarkably smooth-running operation, and the finished work left the locals delighted.

Contractor Vol.31 No.7 August 2007