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Safeguard OSH Solutions - Thomson Reuters

Safeguard OSH Solutions - Thomson Reuters


PETER BATEMAN describes how the solution to a tricky demolition project resulted in high levels of safety and productivity.

How do you safely demolish the ceiling of a major road tunnel which is so busy and such a vital communications link that closing it for a few weeks would be unthinkable?

This was one of the challenges faced by the Wellington Tunnels Alliance as it contemplated how to proceed with the job of giving the Terrace Tunnel its first major refurbishment since it opened in 1978.

The solution they came up with – a large mobile working platform which can be raised or lowered the full height of the tunnel as it travels along it – won the Kensington Swan best initiative to address a safety hazard category at the 2012 New Zealand Workplace Health and Safety Awards.

The 460m tunnel carries 45,000 vehicles a day on three lanes, two of them northbound. After more than 30 years’ operation the $50m project was designed to upgrade the firefighting, lighting, ventilation and drainage systems, make some earthquake improvements, and build new tunnel control buildings. Given the tunnel’s importance as a transport link it was decided that it could be closed for refurbishment work only between the hours of 8pm and 6am, in order to leave it open during the day.

Demolition of the modular ceiling was a key challenge. The ceiling had been constructed in a series of defined structural bays 7m long and stretching across the width of the tunnel. Whatever solution was adopted had to address a number of issues, not least the safety of demolition workers operating from elevated positions. Also, each bay’s stability depended on all its existing fixed points, so there was no structural redundancy – if a ceiling bay was only partially demolished during a shift then there was the possibility that the rest could be unstable and fall onto traffic below.

So the challenge was to devise a solution which minimised risks to workers removing the ceiling and to motorists below, and which enabled the ceiling to be removed as quickly as possible.

The option of closing the tunnel entirely for six to eight weeks during the Christmas holiday period was considered but ruled out as too disruptive. Given the need to demolish in shifts, three other options were sketched out:

  • • 
    Develop a moveable platform running on rails installed on the tunnel wall;
  • • 
    Develop a platform on brackets installed in the tunnel walls;
  • • 
    Develop a moveable platform which could be raised and lowered, running on wheels.

The last solution was adopted and the platform – known as TB7 – was built. Offering a working area of 100 square metres, the platform could be raised to the full height of the tunnel or any point in between, depending on work requirements. When lowered it could be moved on its own wheels and under its own power via remote control.

The deck could be raised independently on each leg, allowing for fine-tuning, or uniformly on all four legs. Each leg could be erected up to 2.5 degrees off the vertical.

Importantly, the deck could be elevated even if the electricity supply to the screw jacks in each leg was disrupted.

After each shift the platform was left in its fully raised position, following the contour of the tunnel, and acting as a shield against demolition debris falling onto motorists below. All three lanes were able to re-open each morning.

From a safety perspective, the demolition crew were able to work with confidence and speed on a large, stable platform without the use of elevated work platforms or scissor lifts. Manual handling risks were minimised by the ability of TB7 to be quickly adjusted to whatever working height was required at the time, be it waist height, foot height, or overhead. Similarly, the platform could be swiftly lowered to the level of the top of waste bins, which meant the bins did not require elevation to be filled.

Being confident of their own safety allowed the crew to exceed forecast demolition rates by 20 percent, with the ability to leave ceiling modules partially demolished in the knowledge that the platform would protect motorists. They were also able to devise more productive methods, such as separation of recyclables in-tunnel.

James Carter from SKM, the lead structural engineer on the project, says the ceiling was a metal framework with a lightweight cladding underneath, so people had to work at head height first to rip off the cladding, and then raise the platform so they could work at waist height to dismantle the framework. Then there was some head-height work to lower the old extraction fans. This was repeated for each bay.

“It was efficient and guys could stand at a comfortable working height.”

The platform’s ability to be swiftly set up for work each night and put away in the morning was another factor in the productivity of the project. “We’d shut the tunnel and within half an hour the guys would be working. We got very good at getting operational so we’d be at work 10.30pm to 5.30am.”


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