CHRIS WEBB reviews a sizeable restoration project that has given an old classic and important viaduct a new lease of life.
LOCATED 12 KILOMETRES south of National Park, the distinctive 262 metre long and 79 metre high Makatote Viaduct is the third highest rail bridge in New Zealand and a Category One heritage structure.
This centenarian, opened for business in 1908, was the last and highest structure constructed on the North Island Main Trunk (NIMT) line, and is now undergoing a $13 million strengthening and refurbishment designed to prolong its life by at least another 50 years.
Built at a cost of £53,369 by Christchurch-based J & A Anderson between 1905-08, Makatote was the first significant steel structure in New Zealand and opened for business in 1908. Between 400 and 500 worked on the project, designed by Glasgow, Scotland-born Peter Seton Hay. Yet he was never to see it completed, as he died in his mid-50s in 1907.
According to the Dictionary of New Zealand Biography, he was the first graduate of the newly established University of Otago, obtaining a BA in 1877 and an MA with first-class honours in mathematics in 1878. A highly influential engineer in the Public Works Department between 1879 and 1907, he helped to plan major railway works including the North Island Main Trunk (NIMT) line, and reported on the possibilities of hydroelectric power generation.
Construction of Makatote involved the erection of five steel piers, five 30-metre long steel trusses and steel plate girders with the help of twin six-inch (150mm) aerial cables. It took two-and-a-half years to build, utilising a fabrication workshop on the north abutment and a water turbine-powered aggregate crushing and concrete plant. Some 4600 cubic metres of concrete were used in foundations and 975 tonnes of steel were used in construction of its superstructure.
Exposure to the elements has taken its toll on the structure, and KiwiRail was faced with the options to refurbish or renew it.
Replacement would have cost an estimated $35 million, says Mike Keenan, KiwiRail’s acting head of structures engineering.
New Zealand’s rail operator chose instead to retain the historic structure for posterity. But along with that decision came some challenges, including removing what is left of the current coating system, replacing corroded members, strengthening the truss spans, installing a new inspection walkway and applying a new protective coating. All this, while maintaining scheduled rail services. The project went to open tender in 2010, while resource consents were approved by Horizons Regional Council in September 2013.
In spite of the viaduct’s advanced years, says Mike, the concrete foundations themselves are in remarkably good condition, though some scour has taken place at the base of the stream piers. These issues were partly addressed in the 1980s when one pier was underpinned, and later in 2006. “We tested the concrete bases, and generally the concrete is in the region of 40-60MPa.”
On the whole, says Mike, the viaduct is in good shape.
“These old structures are very stable; they go wide out at the bottom and can stand up to hurricane wind loading. Remember they carried very, very heavy steam trains in their day. Generally corrosion has occurred in the bracing members of the trusses, where the sun rarely gets to dry them out. There are also indications that steel quality was variable.” Some repairs have been carried out by rope access, he says.
The Makatote Viaduct paint system was last renewed in 1959 and, common for the time, it featured a lead-based system. A full survey of the structure, including pull-off tests, determined there was no longer adequate cohesion between the steel and the paint. Other problems included pooling water and suspect rivets. In order to preserve the structure it was decided to remove the failing cover, add strengthening and replace corroded steel components, and apply a new epoxy-based protection system.
Not only is this a harsh environment in which to work – at an elevation of 780 metres above sea level it typically has 200 rain days per annum – the sensitive environment makes this a difficult project requiring a specialised approach that only a few organisations in the country are able to deliver, explains Mike. KiwiRail is using New Zealand company TBS Farnsworth to undertake the project, which is now well into its two-year, 105,000 man-hour contract.
Each section of the viaduct is being progressively scaffolded and covered in shrink wrap.
“In this way we can remove the old coating material by blasting,” says Mike.
Some 280 tonnes of semi-precious garnet stone abrasive is being used, recovered by vacuum, the contaminated remains are collected in sealed bags and subsequently removed from site for disposal at a managed waste site to protect the local environment. The entire system is thoroughly checked each day prior to blasting works commencing to ensure there is no effect on the environment.
TBS Farnsworth was awarded the contract to undertake the refurbishment in June of last year; consultant Opus carried out research and a review of the planned work. Well versed in this type of work, TBS was part of an alliance that completed an $85 million refurbishment of the Auckland Harbour Bridge in 2011, a 600,000 man-hour project which employed 200 at its peak. The company has also put its skills to work on electricity transmission towers and other similar structures and prides itself on its asset management capability.
Graham Matthews is currently group technical director and Makatote Viaduct project director for TBS. “We have $6 million of equipment at Makatote, which is a lot for a coatings company. We see ourselves as asset managers.”
One of the biggest single costs on the project, he explains, is that of providing the access necessary to carry out the work on the structure.
“We have 350 tonnes of scaffold providing our painting crew with access. We’re suspending the scaffold off the structure, and we’ve had to take account of transmission of wind loads. We have a box scaffold through a box lattice and the design has been to maximise ease of access,” he says.
Steep valley sides and uneven ground, along with a stream below meant crane access was not possible. The contractor has instead used a system of suspending scaffold frames from the permanent works, and providing ‘cocoons’ or weather-tight enclosures in which the teams of painters can work and contaminated arisings do not escape into the surrounding environment. “These areas were heated throughout the winter, maintaining a constant 15 degrees [Celsius], even though the temperature outside may have been freezing. This ensured good curing conditions for the application of coatings.”
Graham says the addition of gas heaters to the contained areas has enabled temperature and dew point to remain “in the positive region” for applying coatings.
“Blasted surfaces are holding up well and require minimal attention to get them ready for paint the following day.”
Around 36.2 tonnes of new steel has been used in strengthening, along with 4500 bolts, replacing original rivets which, in many cases are hard to reach in the complex laced structure.
The project team has been working from South to North, replacing parts of the steel trusses and towers and adding strengthening steel elements before a new oxide-based paint system is applied coating an area of some 13,500 square metres using an estimated 14,610 litres of paint. Altex Carboline products are being used, including an epoxy holding primer (nominally 50µm), a Rustbond penetrating sealer, stripe coat and full prime post structural work epoxy total build (150µm). An intermediate coat epoxy layer (150µm), epoxy fill on pooling faces and full top coat urethane application brings the total cumulative film build to 375µm.
The contractor has put in place a robust system to protect the environment during the works, including a 15 cubic metre per second dust collector and abrasive vacuum recovery and recycling plant. For personnel there is a strict decontamination process and blood lead level monitoring.
The viaduct is expected to be completed in late 2016, extending its ‘fit for purpose’ life for at least the next 50 years. The new lightweight aluminium inspection system will also ensure that engineers gain easier access to assess the structure’s condition in the years to come. As a bonus, says Mike, the environment will be left in better shape after the removal of potentially hazardous lead.
One wonders what the viaduct’s designer, Peter Seton Hay, who died in 1907 of pneumonia prior to completion of one of his most notable engineering achievements, would have thought of its new look.