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The New Zealand Railways Magazine, Volume 4, Issue 5 (September 1, 1929)

Modern Bridge Building on The New Zealand Railways

page 7

Modern Bridge Building on The New Zealand Railways.

In a statement to the Press the Hon. W.B Taverner (Minister of Railways) gives some interesting particulars regarding the rebuilding programme of the Department in the matter of bridges.

The Government's policy of giving New Zealand industries as much support as possible in contracts for public works is exemplified in the recent acceptance of tenders for the new Ngaruawahia and Whenuakura bridges. The steel plates and shapes (a total weight of 500 tons) will be imported from Great Britain, but the whole of the fabrication of the steel work will be done in the Dominion.

In addition of the encouragement of local industry, these works have another interesting aspect as they mark the beginning of an extensive programme of reconstruction of the old original timber bridges.

Altogether, the New Zealand railway system has about 53 miles of bridges, including a total length of about 32 miles of timber construction. Most of the old wooden bridges were made of native timber, chiefly kauri or totara. They served their purpose well under the lighter traffic of the past, but they have to be replaced now by stronger structures to meet the new needs of the much-changed times. Increasing traffic has demanded the adoption of heavier types of engines, exceeding the weights for which the bridges were originally designed, and native timbers in bridges have been wholly replaced with the stronger and more durable Australian hardwoods. These, in turn, have a limited life, and as the need for heavier engines still persists, and timber increases in cost, it becomes uneconomical to maintain the original types of bridges in service. Therefore reconstruction in steel and concrete to a high standard of strength is found advisable.

Attention is being focused first of all upon important main lines, so that restrictions on the running of some of the heavier engines can be removed, with the resulting economies in traffic operation.

The Ngaruawahia Bridge.

The Ngaruawahia bridge carries the Main Trunk line over the Waikato River at Ngaruawahia, near Frankton. The existing bridge (timber and iron with cast iron cylinder piers) was built about the year 1876. Practically all the original native timber have since been replaced with ironbark in the course of maintenance, and much of the ironwork has also been replaced or strengthened. Present-day engine loadings now far exceed those of former days, however. Therefore, the old bridge, with its materials rapidly deteriorating in spite of careful maintenance, has now reached the end of its useful life.

The new bridge will have three 120ft. spans and three short approach spans. The main shore piers will be of mass concrete resting on piles, and the river piers will be reinforced concrete cylinders, ten feet in diameter at the base, sunk to a depth of about 40 feet below water level. At both Ngaruawahia and Whenuakura bridge sites, borings have been put down near each new foundation, and test piles driven and loaded with test loads in order to determine as far as possible the conditions to be met with in constructing the foundations.

The Whenuakura Bridge.

The Whenuakura bridge crosses the Whenuakura River between Rangikura Patea, on the Marton-New Plymouth section. The new bridge will have a single main span of 142 feet resting on reinforced concrete cylinder piers, and three 60ft. plate girder spans.

The original bridge, constructed of native timber, in about the year 1880, consisted of two 60ft. trusses and nine 20ft. beam spans, all carried on high timber piers. One of the main piers and the two 60ft. truss spans were carried away in a heavy flood in January 1922. A temporary reconstruction has served to carry the traffic at a reduced speed up to the present. The old timbers are now worn out, and the extra temporary piers, constructed in mid-stream, have caused heavy scour and weakening of the foundations which can only be relieved by the removal of the temporary piers now obstructing the waterway, necessitating the construction of a long main span. The wash-out in 1922 was mainly due to a log jam between the bridge piers, and the new main span has been made long enough to prevent any such blockage in future.