1,787 Rail Bridges to Traverse New Zealand's Fast-Flowing Rivers and Difficult Terrain
New Zealand's bridge structures have earned public admiration for their scale and boldness
In the 140 years since New Zealand's first small public railway was constructed, the development of the national rail system has seen a variety of bridge types.
Many of the 1,787 bridge structures have earned public admiration for their scale, boldness, and not least for the grandeur of their setting, Today some rail bridges have become a major tourist attraction. The longest freight bridge is the Rakaia river bridge (1,743 m) and longest passenger bridge spans the Waiau river (700 m).
We need to remember the skill of the engineers as designers and the construction feats of those who built them. These bridges form a very significant part of the engineering heritage of our country.
Early Rail Bridges Linked Isolated Rural Communities of the 1860s, Paving the Way for Economic Prosperity
To appreciate the significance of rail bridges in New Zealand it is necessary to understand the nature of the country's development by the late 1860s.
In the South Island the gold rushes and high prices for wool and wheat had seen prosperous conditions, especially in Otago and Canterbury. In the North Island rural development was hindered by the vast tracts of virgin bush, the steep and broken topography, and the wars in which Maori and European concepts of land and its ownership were so much at odds. These factors confined the bulk of the population to the more accessible coastal regions that could be served by small ports. Communication by road was still primitive.
1852 : Provincial Councils Commence First Public Railways Using Three Different Railway Gauges
The development of the country was largely under the control of the provincial councils established by the 1852 Constitution. Some were more successful than others in promoting public works to open up the country, and so the first public railways were begun. However, there was no consensus between the provinces for a standard gauge, and this was to cause problems. For a country that had a population of only a quarter of a million by the late 1860s the haphazard approach adopted was not in the best interest of national development. Only 80 kilometres of railway, but of three different gauges, was operating.
1870 : Central Government Embarks on Ambitious Project to Build National Railway
Because of the over-commitment of the provincial councils on railways and their strained finances, central government decided that it should build and operate all railways. They would be on a national rather than a provincial basis, with a uniform 3 ft 6 in (1067 mm) gauge. As a means of improving the stagnant economy, in 1870 Julius Vogel, the Colonial Treasurer, proposed a bold policy of public works and immigration requiring an 'outrageous' loan of £10,000,000 over the following ten years. The emphasis was on the construction of railways, with assisted immigrants providing the labour.
Bridging Problems of Difficult Topography, Unskilled Labour and Suitable Permanent Materials
The challenges to railway construction were many. New Zealand has a great many fast-flowing rivers, some very wide with meanders and others with deep rocky gorges. Floods, and scouring of banks, often exacerbated by the clearing of bush, were also continuing problems. Hence bridging was a major factor throughout the entire country.
The resources initially were meagre, especially so in money, human skills and suitable permanent materials. The reliance on native timbers, particularly in wet areas, saw remarkably rapid deterioration through decay. Only totara was found to be long-lasting, although it had the limitation of brittleness. It should be remembered that engineers also had to design and build numerous road bridges urgently required as well as all manner of civil engineering works. The demands made on them were heavy. Many contractors were neither skilled not competent in bridge construction.
The central government therefore arranged for the English firm of John Brogden and Sons to undertake a number of railway contracts for considerable lengths of line in both islands. This firm failed to perform satisfactorily in the conditions at the time: the result was that its contracts were terminated. The government therefore decided on the implementation of public works by the Public Works Department, a move which subsequently produced immeasurable benefits to New Zealand in having competent trained professionals, and in time a body of engineer assistants and overseers to ensure uniformly high standards of workmanship.
Standardised Bridges to Help Overcome Shortages of Skilled Labour and Save Time
To overcome the problems of incompetent private contracting the government introduced the cooperative contract system. This proved successful in undertaking many miles of road and railway construction. Just as standardised buildings had been introduced in 1869 by W H Clayton, the first Colonial Architect, so too bridges were standardised by the Public Works Department wherever practicable to help overcome shortages and to save time.
For suitable bridge technology engineers looked to the USA, where railway construction had boomed in the latter half of the 19th century.
Construction Materials : From Wood to Steel to Reinforced Concrete
At first timber trestles with timber stringers were used. Then came a period when for larger structures wrought iron was the favoured material, either as built-up plate girders or as trusses being supported on piers of cast-iron cylinders filled with concrete.
By the 1890s mild steel was replacing wrought iron and cast iron. On the main lines many of the steel viaducts had lattice trusses for the main spans, plate girders over the pier heads supported on trestle piers, and concrete piers for the land spans.
Masonry bridges were a rarity because of the scarcity of good building stone in most districts, and likewise brick was seldom used. Although plain concrete was in use for piers in the 19th century, reinforced concrete did not appear as the entire structural material until 1939 and then in only two examples [which ones?]. In more recent years it has found favour for major railway viaducts. It is especially esteemed for its seismic resistance and its relative maintenance-free qualities.