Each of the various districts has its own peculiar bridging problems. These are set out as follows:
Southland and Otago: There are some large rivers in these provinces. Many of the early bridges spanning the deep rocky gorges in which these rivers flow were finely proportioned suspension bridges. The towers from which the steel cables were suspended were usually made of attractive local stone, and deck and stiffening trusses were usually of timber. The Kawarau River Suspension Bridge is a fine example, with a span of 300 ft and a deck some 160 ft above river level. This bridge, built 80 years ago, has only recently been replaced by an all-welded steel-box girder arch structure. Deep wrought-iron and timber trusses were also built; for example, the bridges across the Clutha River at Cromwell and the lower Shotover River. The wrought iron used (even unpainted) has hardly corroded in the dry air of Central Otago. Steel is therefore the chosen material for the new bridges to replace the old; for example, the new tied-steel-arch bridge over the Clutha River at Alexandra.
Canterbury: The bridging of the many wide gravel beds of the rivers in the Canterbury Plains has posed quite different bridging problems. In normal flows the river channels are braided and sweep in serpentine courses across the gravel beds, which are up to a mile wide. During summer floods, fed by the melting snow, the waters reach from bank to bank and flow swiftly on an average slope on one in 200 for some 40 miles across the plains. Long multispan structures are thus needed to bridge these river courses.
|Canterbury Plains: Typical Multispan Bridges|
|River||Bridge Length||Number of Spans||Built|
|Rangitata N||2,122||53||c. 1930|
|Rangitata S||1,042||26||c. 1933|
At all times (but particularly during floods) there is much movement of gravel in these rivers with heavy scour, or build up, at obstructions in the channels. Bridge piers therefore have to be founded either on piles driven deep into the beds or else on cylinders. The superstructures of the early bridges were of timber or of wrought-iron trusses and girders. Later bridges have been built of reinforced concrete or, more recently, of prestressed concrete. The beams are simply supported and the spans are up to 100 ft; for example, the Waitaki River highway bridge has 30 spans.
Westland, Nelson, Marlborough: In general, the many large and often wild rivers in these districts are rather short and steep and subject to severe flooding from heavy rainfall in the mountainous catchments. Many of the early bridges were washed away by floods which scoured the foundations, for the problems of design were not then appreciated. The difficulties were immense. For example, the Buller River, which enters the sea at Westport, has the greatest estimated flood flow of any river in New Zealand—600,000 cu. ft. a second. The bed material of the rivers in these districts varies from sand to coarse gravels and large boulders. This makes foundations generally difficult to construct. Often the only possible solution is to sink cylinders up to 10 ft in diameter, under compressed air, down to 40–70 ft. Such foundations were necessary for the Blackwater and Big Grey River bridges. These rivers flow through heavily forested country. During floods trees are washed downstream to catch on piers and block up waterways unless the spans are long enough and the decks high enough above flood level (about 8 ft) to clear the biggest probable flood.
North Island: The general topography of the North Island differs considerably from that of the South. Alluvial plains are built up of volcanic muds, silts, and fine pumiceous material. Large blocks of rock boulders and ash accumulate alluvially on the concave slopes of volcanic cones and there are also lava flows. These materials produce difficulties in connection with foundations.
The special problems facing New Zealand bridging engineers were well illustrated in the construction of the North Island Main Trunk railways. Many deep river gorges had to be spanned in the high central country. On the east coast a number of notable steel viaducts were designed and built, the longest and largest being the Mohaka Viaduct, 911 ft long and 312 ft above river level. Later viaducts have been built of reinforced concrete; for example, the Waiau Stream bridge on the Gisborne-Napier railway. Bridges in New Zealand are built to withstand moderately severe earthquakes and heavy floods. The Tangiwai disaster of Christmas Eve, 1953, shows what exceptional stresses bridges may be exposed to. The rail and road bridges over the Whangaehu River at Tangiwai gave way under a “lahar” (or high flood) which originated from a breaking away of part of the lip of Mount Ruapehu's crater lake. The released water, very heavily surcharged with ash and coarser materials, swept down the Whangaehu River bed and completely demolished these two bridges. A passenger train crashed into the gap and 154 people were killed. Any future high flood is now predicted by up-stream monitoring equipment which is connected to the Railways Departments' signalling and manning system. New bridges have now been built at Tangiwai.
Two Auckland city bridges deserve special mention. The first, the Grafton Bridge which was built in 1910, was for many years the largest reinforced-concrete arch span in the Southern Hemisphere (main arch span, 320 ft; overall length, 973 ft; width, 24ft of road, plus two 6-ft footpaths). The second is the recently completed Auckland Harbour Bridge, with a central span of 800 ft and overall length in the main spans of 3,348 ft. The minimum clearance under the main span is 142 ft. The piers of the main harbour spans are built on caissons founded in the Waitemata sandstones at depths of up to 110 ft below high-water level. The southern approach spans are steel; the 1,660 ft approach spans at the northern end are steel and reinforced concrete. The deck is 42 ft wide and has four traffic lanes.
Motorway Bridges: The development of some 35 miles of limited-access motorways in New Zealand has included the designing of many grade-separation bridges. Pains have been taken to design attractive bridges; for example, the pleasing designs of those at Johnsonville, Royal Road, and Alfriston Road, and the Number 3 Bridge on the northern approaches to the Auckland Harbour Bridge.