Johnsonville Tunnel clearance assessment, New Zealand

With the new Matangi passenger trains confirmed for the Wellington network, there was a need to upgrade the Johnsonville Railway line to allow the new, larger units to replace the 1930s’ English Electric trains, which had provided service for the last 80 years.

The Johnsonville Railway was constructed in 1880 and is unique in New Zealand, as well as in world terms, by being considered a "mountain railway". The railway features tight 150m radius bends, maximum grades of 1 in 40 and seven tunnels, including many with curves. Initial evaluations by KiwiRail confirmed that by lowering the rail level and enlarging the tunnels, the new Matangi units would be able to use the line and achieve the much-needed passenger unit upgrade.

The challenge for Aurecon was to accurately model the dynamic performance of the proposed Matangi trains through the tight and winding route to achieve the minimal tunnel upgrade works, as the line was to be kept largely operational throughout the upgrade.

Aurecon worked closely with KiwiRail to define the operational clearance parameters. Then, using 3D terrestrial laser scanning, Aurecon built an accurate, spatially correct model of the tunnels comprising four million scanned points per tunnel.

Applying specialist software, Aurecon refined the outputs to produce visual representations of the constraints, the rail height, cant (camber) and alignment. The findings were fully optimised to achieve the required minimal upgrading works.

The work undertaken was exceptionally complex and involved multiple iterations, as the grades and cant of the line had very limited scope for change, due to the desired 40km per hour design speed on the line.

The final alignment was selected following clearance assessments at 1m centres. It involved lowering the rail in the order of 350mm through the tunnels and introducing a number of complex rail curves within the tunnels to achieve the required clearances, which were down to 65mm at the floor level of the unit.

The project has set new benchmarks for rail clearance assessments in New Zealand by providing innovative outputs to demonstrate the constraints and exceed quality assurance requirements. Considerable time and cost savings for physical tunnel works were achieved compared with the initial expectations.

In October 2010, the first Matangi unit was tested on the Johnsonville line and proved that the modelling had been a major success. KiwiRail was delighted with the performance by Aurecon, which accomplished an exceptional result.

In 2011 the Johnsonville Rail Tunnel Clearance Assessment received a silver award at the ACENZ INNOVATE NZ Awards of Excellence.