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Beyond the Tracks: Our Otira Tunnel Scanning Project

The 100-year-old Otira Railway Tunnel running under the Southern Alps between Arthurs Pass and the West Coast is an important piece of NZ’s transport infrastructure, carrying around 70 trains every week - mostly transporting coal from the mines of the West Coast to the port at Lyttelton.  

When it opened in 1923 the 8.5km tunnel was the longest in the British Empire and was one of the engineering marvels of its time. Fast forward 100 years and some modern-day technological wizardry was required to keep this vital link open.  


The Arthurs Pass portal under construction in 1910 (Wikipedia) 

Engineers Tonkin+Taylor needed a 3d model of the steep face rising from the Arthurs Pass end of the tunnel in order to undertake geotechnical and structural assessments. The extreme slope, dense vegetation and working railway corridor ruled out traditional surveying and on-ground investigation methods but having often worked with Fox & Associates in the past, T+T knew who to call in for this challenging task. 

The scope of works agreed was a 3d laser scan of the Arthurs Pass tunnel portal (entrance) and an aerial LiDAR survey extending 280m up the side of the valley above. The work required permits from both Kiwirail for work within the railway corridor and also the Department of Conservation since much of the survey area was within Arthurs Pass National Park – normally a no-fly zone for UAVs.  


Laser scanning the tunnel portal. 

Having successfully negotiated the bureaucracy, the next obstacle to overcome was the weather. The tunnel portal is 740m above sea level and spends extended periods of time each winter surrounded by a picturesque blanket of snow that would render our 3d model hopelessly inaccurate. When it isn’t snowing the weather can be highly changeable with high winds and driving rain arriving at short notice. After a number of postponements work eventually got underway on a brisk day in August 2020.  


3d ‘point cloud’ of the tunnel portal. 

Working on the ground on site presented its own challenges since access to the railway corridor is across the Bealey River from the village, and the laser scanning work had to be completed in the brief window of time between two trains using the tunnel. The aerial survey had to be flown at a low enough altitude to enable the LiDAR to ‘see’ through the vegetation while keeping the UAV safely clear of the tops of the trees. To meet these requirements the automated flight plan flown by the UAV was designed to ‘step’ up the slope away from the railway line. The cold temperature and high altitude meant that the UAV consumed batteries at a far higher rate than we normally experience – fortunately we always build plenty of safety margin into our flight plans.  

Having completed the two surveys, the data from the two surveys passed through our now well-oiled processing system to produce the required 3d model and aerial photography. Tonkin + Taylor were amazed at the level of detail achieved, which far surpassed anything that could have been achieved by more traditional surveying methods. As always, at Fox we are always looking for a better way, and in this case our innovative use of technology played an important role in keeping this vital piece of infrastructure up and running.   


A) Coloured point cloud of survey area, with the rainway running into the tunnel at bottom-centre 

B) 3d model of the same view 

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