Monday, 31 December 2007

Nuhaka River Bridge

The Nuhaka River Bridge is no. 256 on the Palmerston North to Gisborne rail line, the route which runs up the East Coast of the North Island. It is located at kilometre peg 324.84, 1 km east of the Nuhaka township. The bridge was built in 1922 by the Public Works Department and is about 1 km inland from the coast. It originally had six steel girder spans of 60 feet (18.3 m) for a total length of 110 m. The intermediate piers were each made up of ten ironbark (hardwood) loadbearing piles and one cutwater pile. It was designed for steam locomotives of 14 ton axleload, which in the diesel era allowed a 16 tonne load, that of the DX class locomotives which ran on the line at the time. The bridge was not rebuilt at any time during its life, having only routine maintenance and replacement of decayed components where necessary.

The lower reaches of the river are tidal and the river outlet to the sea is frequently blocked and has to be opened by machinery. Salt water flows into the lower river reaches when the outlet is open, but as the outlet is closed by the sea, the salt water does not mix readily with the fresh water due to lack of flushing action. A layer of saltwater thus existing in the bottom of the river encouraged the habitation of untreated timbers by marine borer, a typical example being the teredo worm, which was known to exist in Gisborne harbour, 77 km further north. However, there was no knowledge of teredo being active at Nuhaka previously, and so it was not considered a risk factor for the wooden piles of this bridge.

In 1993 a report on the state of the rail network identified that there were still many wooden bridge components in use on the network and that the economic conditions pertaining to the rail operator since the early 1980s recent years had resulted in maintenance expenditure for such bridges being markedly reduced.  This situation continued through the 1990s when NZ Rail Ltd was privatised and it eventually resulted in a report being commissioned by the government in 2002 when serious structural problems were unexpectedly found in a bridge on the Midland Line coal route. It would appear from these circumstances that the economics of much of the remaining national rail network in NZ has been questionable for many years, because during the 1980s as rail was required under government ownership to become at least marginally profitable and incurred further substantial debts, the ability to properly fund the necessary maintenance was in serious question. The Midland Line was and still is one of the most profitable routes, yet even under privatisation the economic marginality continued at a time when that line was being built up for larger and heavier trains, and the inability for the maintenance program to keep up with actual need was reflected in a number of areas of the network, including the North Island Main Trunk. The Gisborne Line was a financially marginal branch which was almost closed in the late 1980s and the same general situation applied as elsewhere, but more strongly.

In March 2005, routine inspections by maintenance contractors of the Nuhaka Bridge detected that Pier 4 of the bridge had partially dropped, causing the track to be out of line from tilting of the bridge. Prior to this time, inspections had not detected problems at this bridge. Attempts to remediate the drop by lifting and packing the superstructure foundation on the piles resulted in the pier sinking further. Subsequent inspections confirmed that the pier continued to tilt in only one row of piles. The bridge was placed under speed restriction of 25 km/h which was later reduced to 10 km/h. Work was put in hand to drive new piles and get an underwater inspection of the existing piers. The latter task was not able to be carried out before a work train was sent to the bridge with two large diesel rail cranes each weighing 142 tonnes with a maximum axleload of 24 tonnes, significantly in excess of the bridge rating, and also requiring runner wagons to allow the train to cross certain weak bridges on route to the work site. While crossing the river on 6 May 2005 in preparation to carry out the new piledriving work, pier 4 of the bridge collapsed, dropping one of the cranes and some rail wagons into the water and the ends of the two adjacent bridge spans. The reinstatement of the bridge and recovery of the rail vehicles required a very large construction crane to be erected on a temporary causeway built into the river. Three new piers were built and the reconstructed bridge was opened for traffic on 19 July 2005.

Examination of the bridge showed severe marine borer infestation of pier 4 and 5 and lesser damage to pier 3. As seen in the map below the positioning of piers was changed and the length of some spans altered. In order to support the six spans of the original bridge there were two abutment piers and five intermediate piers. The Google Maps satellite photo, which also shows the temporary access road built to get the recovery crane to the site, shows that there are now six intermediate piers, therefore there are now seven spans. The outer spans at each end remain at 18.3 metres length. The next pair of spans heading in from each end are are approximately 11 metres long, followed by a pair of approximately 18.3 metre spans, and a centre span of about 12 metres.
The Nuhaka Bridge is a rare example of a railway bridge collapsing in New Zealand. There were a number of contributing factors leading up to its collapse and various deficiencies were identified in procedures applying to bridges of its type at the time.

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