Major Dunedin Re-Alignment Completed

29 January 2014 4:04PM

Re-alignment of an unstable section of track just north of Dunedin at the 365.85km Main South Line (MSL) presented significant challenges but sound planning and good team work saw the job completed over a weekend Block of Line (BOL).

Severe storms in June 2013 damaged a number of embankments on the MSL with the big embankment at 365.85km becoming dangerously unstable. To remedy this, a temporary re-alignment was made by cutting into the slope alongside it and pulling the track off the embankment. While this removed the immediate risk, it also tightened the curve the track was on and this increased the ruling grade.

"The ruling grade governs the tonnage that can be hauled up a hill. If you tighten curves, you increase the effort required and so reduce the tonnage that can be hauled. That of course increases both costs and operational risks. So it’s important that if at all possible you don’t make ruling grades worse," says Project Engineer Robert Storm.

Before the temporary re-alignment, this section of the line was already very steep at 1 in 50. The remedial work had made it by far the sharpest curve on the MSL at 100m radius. "It was actually outside our codes and we also had concerns about the long-term stability of the embankment. So while the issue of not running trains at all was solved by the temporary alignment, it still needed improving as soon as possible and the opportunity for a weekend BOL in early December provided us with the ability to do that."

The affected embankment was built in 1873 by simply filling a hole by tipping in barrow loads of material from an adjacent cutting. Robert says that while this may have been adequate for the ten tonne axle loads in use 130 years ago, axle loads today are nearer 20 tonne. The strength of earthworks is decreased when they get wet. Investigations led by Senior Engineering Geologist Daniel Rodriguez, revealed that the original embankment was particularly weak because even though it was several months since the storm the soil was still saturated.

"The earthworks were built a long time ago and were not to the standard they would be built today. The big storm meant enough water had entered into the embankment to destabilise it. The solution was to remove the saturated embankment material and replace it with modern engineering fill. This might sound straight forward but it presented significant challenges. Replacing a six metre depth section of a 70 metre long embankment with an operating railway running along the top is not easy, especially when you’ve only got a weekend to do the job in," says Daniel.

A staged design was adopted that involved keeping the track in place on its temporary alignment while removing as much material as was safe and practicable before the start of the BOL. Around 3,000 cubic metres of rock had to be stockpiled in a very confined site ahead of time and then moved once reconstruction work was started. This very tight timeframe meant the Dunedin Track Gang and contractors had to work double shifts over the weekend.

"We were able to remove and replace enough of the old embankment to bring the track back into its original alignment. We also improved the drainage as poor drainage had contributed to the original problem. To manage all this over a weekend required weeks of careful planning and great coordination between everyone. The job was a big "ask" for a small team but everyone made a tremendous effort and despite some poor weather, which is the enemy of any earthworks job, we got everything finished ahead of time. The line is now better aligned and the slope is stabilised and this should improve its efficiency and safety for years ahead. That’s pretty good for a weekend’s work and I’d like to thank all involved," says Project Engineer Robert Storm.