A massive refurbishment of an historic viaduct demands the best technological and safety approach.
LOCATED ON THE southwestern slopes of Mount Ruapehu, the Makatote Viaduct bridges the Makatote Gorge 12 kilometres south of National Park.
Opened in 1908 it is 262 metres long, 78 metres high, is 700 metres above sea level and is a Category 1 heritage structure.
Until 1981, it was the tallest structure on the North Island Main Trunk and remains our third tallest railway viaduct.
Concerns over its deteriorating paint system, continuing corrosion of the century-old steel and the need to increase live load capacity – prompted KiwiRail to commission TBS Farnsworth to undertake design and physical works to repair and strengthen the steel work, upgrade the live load capacity, and refurbish the paint system.
Graham Matthews is TBS Farnsworth’s technical director, and at the recent Australasian Corrosion Association’s (ACA) international conference in Auckland he outlined the challenges to deliver what Horizons Regional Council allows as the only site in the region to obtain an ‘exceeds compliance’ result.
In early 2012, TBS Farnsworth was engaged through an Early Contractor Involvement (ECI) agreement to carry out the refurbishment, says Graham.
A lump sum contract was awarded to the company in June 2014 for the full refurbishment, including strengthening and structural works to be completed by January 2017. TBS worked with Opus International Consultants to generate a brief on the current condition of the structure, the environment, and a coating design life of at least 50 years before full replacement would be required. Physical works started in September 2014 and were completed in October 2016.
Refurbishment was always going to be an environmental challenge with the the area recording an average of 200 days of rainfall a year. During winter snow is common and frosts very severe. Being in a pristine natural environment also meant unique environmental concerns had to be addressed, including protecting the surrounding area from hazardous dust and protecting the endangered whio (blue duck) colony below.
TBS and KiwiRail worked closely with the Department of Conservation throughout the project including developing an erosion and sediment control plan, and the careful storing of hazardous substances.
The viaduct is a riveted structure with girders on the piers, and components of the trusses had been built up using a lot of small section lacing. In total, there are 15,400 square metres of steel and, by completion, 15,300 litres of paint had been used on the project.
Many elements were replaced during the refurbishment both in the towers and trusses, because significant corrosion had occurred through water pooling, combined with a lack of maintenance – aggravated by the harsh environment.
During this work trains continued to use the viaduct.
The coating system had previously been based on red lead primer (RLP), which worked very well – even when surface preparation was poor. It had performed adequately since the last full blast and paint of the viaduct in 1959, but there was now significant coating breakdown and heavy corrosion.
Unsurprisingly, a reassessment in 2010 confirmed that the RLP would have to be removed and a complete new coating system applied. Removing this required full encapsulation so no lead could escape to the atmosphere.
Reaching the piers and trusses required significant temporary scaffolding works, the design process for this proving a significant project in itself. Due to the limits in allowable wind loadings, sequencing was carefully managed to ensure the encapsulated area/s did not risk toppling the bridge structure.
Each scaffolding tower had bespoke footings – both concrete pads and steel beams. Beams were used when concrete foundation piers were well off the ground and where the dead weight of the scaffold needed to be transferred to the foundation, thereby reducing the chances of the structure overturning in high winds.
The largest scaffold, at 76 metres high, was for pier six. It contained 270 tonnes of scaffold equipment and consumed 4500 man-hours to build. It was fitted with a man-riding 500 kilogram power hoist with internal access using stairways. No ladders were used as they were deemed unsafe, especially for helmet-clad abrasive blasters, and inefficient when carrying materials and equipment.
Working from south to north, TBS Farnsworth progressively encapsulated the sections. Trials had demonstrated that all cleanliness standards were easily achieved by abrasive blasting, using about 200 tonnes of garnet in total.
Encapsulation was double wrapped to form an insulation system during winter, and diesel indirect-fired heaters pumped air into the system so that paint continued to cure on the bridge while snow was falling outside.
“Heating the piers was more difficult because we couldn’t fit a second skin over those and with the hot air rising, height of the heater spaces had to be reduced,” says Graham.
“We completely sealed floors every 10 metres to contain the heat, which made painting through the winter possible but was much slower and more costly.”
Resource consent requirements specified that 99 percent of the waste had to be captured. Part of the compliance solution was erecting an abrasive blast plant building that also allowed the project to proceed year round. It was eight metres high and had a lifting, eight metre-wide door that sealed. The space was connected to a large dust collector to control any lead-containing dust from escaping the recovery/recycling plant.
Two dust collectors were used on site; a larger 15 cubic metres per second one (30,000cfm 100hp 99.95 percent retention of 0.5 micron and greater particle size) at the top of the bridge. A 700mm diameter duct was run 260 metres along the western side of the bridge with ‘Ts’ at each pier. A second 10 cubic metres per second (20,000cfm 65hp) dust collector was placed at the base of the larger piers, as ducting runs from the larger unit were getting too long.
A stationary vacuum recovery unit was also inside the building. Gravity chutes were used to bring the recovered abrasive down the piers, with the vacuum system used to clean the containments themselves.
The recovered abrasive was then loaded into a recycler capable of processing three tonnes per hour. The waste stream from the recycler had a proprietary leaching reduction additive manually blended into it, and when there was sufficient waste the bulk bags were emptied into 15 three cubic metre hook-bins and fully blended using a small digger.
“The leachable lead levels were so low, the spent abrasive could be dumped as non-hazardous waste,” says Graham.
The deteriorating paint system had led to significant corrosion and section loss of key elements on the viaduct.
“The intricate lattice work connected by hundreds of thousands of rivets made painting especially difficult.
“A spray application would normally be the most efficient way to apply paint, but the low film build produced around the edges and the back of rivets had led to their premature failure. All had to be stripe coated, by hand, three times.
“All surfaces were sprayed, down to the smallest steel sections. The steel plate girders had some bigger flat areas, but they weren’t sufficient to warrant using airless spray equipment.”
Graham says one of the biggest challenges of the project was to provide a watertight rail deck between the four rails that would withstand the expansion and contraction due to the temperature, vibration and rail movement as each train passed over the bridge.
“With the very regular rain this proved to be highly troublesome, as the rain affected the blast and painting operation which made recovery of wet, spent garnet very difficult.
“We used a drier to recycle it.”
The bottom level of the scaffold was lined with linoleum to produce an impervious, tough deck that garnet could be swept and shovelled off.
Health and safety
Graham recalls that when the viaduct was originally constructed seven workers fell to their deaths. While working conditions have vastly improved, this time work was largely determined by the need to manage the risks associated with lead.
“There were no lost-time injuries during the project and just three medical treatment injuries over the two years of work on a very hazardous and challenging site.”
Personnel were given a baseline heath check including blood lead levels, and were tested every month for any change. Everyone remained clear throughout the duration of the project.
“With the harsh weather, wet decontamination wasn’t feasible so we used dry air washing instead, and all staff wore dust masks in the transition area,” says Graham. This proved to be very successful.
“This work has proved that a project involving the removal of large amounts of lead need not affect the environment adversely.
“It can be controlled with simple, innovative methods that leave the environment better than when we started.”
This is the second, and more technical, story on the viaduct. The first was published in March 2016 and can be viewed at: www.contractormag.co.nz/contractor/makatote/
Encapsulation was double wrapped to form an insulation system during winter, and diesel indirect-fired heaters pumped air into the system so that paint continued to cure on the bridge while snow was falling outside.