Story: Bridges and tunnels
Page 4 – Notable bridges
At 322 kilometres, the Clutha River/Mata-Au is New Zealand’s second-longest river. Running from Lake Wānaka to the sea, it traverses areas settled during the 1860s Otago gold rushes. Bridging the Clutha was a considerable challenge, which became clear in the great flood of 1878. The Bannockburn bridge on the Kawarau River was swept away, and its wreckage destroyed the Clyde bridge on the Clutha, which in turn ruined the Roxburgh bridge, a laminated wooden arch, only erected in 1875. Further downstream the Beaumont bridge was washed away and it in turn destroyed the Balclutha bridge.
The flood began a period of bridge-building:
- Balclutha, 1881 – a wooden truss bridge to replace the lost one, but it did not last either. A concrete bridge with bowstring arches opened in 1935.
- Alexandra, 1882 – a handsome suspension bridge resting on two schist masonry towers.
- Roxburgh, 1887 – a new suspension bridge, and a truss bridge at Beaumont, the first iron bridge made in the colony.
- Millers Flat, 1899 – a steel bowstring arch
- Horseshoe Bend, 1913 – a handsome swing footbridge.
In 2009 a river punt remained at Tuapeka Mouth. It had operated since 1896, but in the 2000s most cars used the Clydevale bridge, 10 kilometres downstream.
The Millers Flat bridge was due to be opened by Premier Richard Seddon in January 1899. He had another engagement and asked for the opening to be delayed. But a couple had arranged to be married on that day so they could be the first to drive over the new bridge. Flags flew, bands played, beer was poured and the newlyweds drove across. Then the bridge was padlocked to await the premier a month later. In his speech Seddon won no friends by attacking the locals for their unofficial opening.
Christchurch has long been known as a city of bridges because the Avon River flows through its centre. In the 2000s there were 26 road bridges and seven footbridges on its 22-kilometre length. In the 1850s wooden bridges were built for carts or pedestrians – but most rotted. The first to be made of permanent materials was the Victoria Street bridge, built of stone and imported cast iron in 1864.
New, wider bridges were erected in the next decade – five in 1875 alone. Most were wood – though some had handsome iron balustrades. Many lasted until the 1950s, when six new concrete-slab bridges were opened, followed in the 1960s by 13 road bridges and three footbridges, mostly of pre-stressed concrete.
Main trunk line viaducts
In most western countries, a viaduct is a structure with many arches and small spans. In New Zealand the term usually applies to high railway structures – probably because of the remarkable series of viaducts which cross the steep ravined country in the middle sections of the North Island main trunk railway line, completed in 1908. Of more than 20 viaducts on the line, nine were substantial, including:
- the Makōhine viaduct (1902), just south of Ōhingaiti – 228 metres long and 73 metres high
- the Mangaweka viaduct (1903) – 288 metres long and 47 metres high, and originally the longest on the line, it was bypassed and demolished in 1981
- the Makatote viaduct (1908), 12 kilometres south of National Park – 78.6 metres high, it was the highest on the line at the time it was built
- the Hāpuawhenua viaduct (1908) – 284 metres long, it was the second-longest at the time it was built.
Died in the course of duty
The major main trunk viaducts were designed by Peter Hay, an engineer in the Public Works Department. But he did not see the fruits of his work – the year before the main trunk opened he suffered from exposure while inspecting the line, developed pleurisy and died.
In 1981 the Mangaweka deviation was built. It included the north Rangitīkei and south Rangitīkei and Kawhatu viaducts, all made of pre-stressed concrete. At 81 metres the northern viaduct became the highest on the line. Six years later the Ōhakune to Horopito deviation involved a new Hāpuawhenua viaduct. It was 414 metres long – the country’s longest rail viaduct.
Auckland Harbour Bridge
The most spectacular bridge in New Zealand is the Auckland Harbour Bridge, built between 1955 and 1959. The need for better transport between Auckland city and the North Shore had long been the subject of inquiry and agitation. Finally the Auckland Harbour Bridge Authority was set up in 1950 to raise funds and organise construction. The bridge’s ‘coat hanger’ design, with lattice girders above the road on the 243-metre span between the first and second pier, allows ships to pass beneath.
Building the bridge involved clever cantilevering of the steel girders, and staff working 33 metres below sea level preparing the foundations of the reinforced concrete piers. The bridge is 1,017 metres long, and used 5,670 tonnes of steel, 17,160 cubic metres of concrete and 6,800 litres of paint.
The ‘pick-a-back’ span
When constructing the Auckland Harbour Bridge, the span between the second and third piers was constructed on top of the span between the fifth and sixth piers. The two spans were floated into position together, the top one was fixed to its piers and the bottom span moved back to its original position. The ‘pick-a-back’ span weighed 1,000 tonnes and was 176 metres long.
Originally the bridge was four lanes, but this quickly proved inadequate. In 1969 the ‘Nippon clip-ons’ – two lanes on each side, pre-fabricated in Japan – were added. At the time this was pioneering technology, but 15 years later fatigue was discovered in the weld of the splice joints and several thousand joints had to be replaced. Tolls were charged on the bridge until 1984.