Fulton Hogan needed to monitor roughly 160m of track on the North Island Main Trunk Route project and three existing rail overbridges carrying State Highway 1 in the North Island of New Zealand as part of the enabling works package. This package involved building a crash wall and piling for three new overbridges for the state highway being constructed as part of the Pukekohe to Papakura rail electrification project.
Vitruvius partnered with Aptella to implement 92 tilt nodes, with 10 of those nodes installed on three existing bridges and the remaining 82 tilt nodes installed on the tracks, giving Vitruvius the ability to report on both the track movement and movement of the bridges.
“There was about 160m of track which was going to be potentially affected by the enabling works carried alongside the existing North Island Main Trunk rail line,” said Mr Foot.
“Fulton Hogan came to us and asked how we would do it and I proposed we use tilt sensors fixed to the sleepers. I was already relatively familiar with the technology as Vitruvius has worked with Aptella on other rail and wind farm projects in New Zealand and Australia and that it will be a good site to use that technology on because we could actually do the installation very quickly.”
One of the challenges that Mr Foot noted with this project was that three quarters of the 160m track that need to be monitored was on relatively degraded timber sleepers that Kiwi Rail intended to replace during the timeline of the monitoring project.
“We were going to have to secure the sensors onto timber sleepers laid down in the 1960s which were coming to the end of their service life; that did cause us a little bit of concern,” said Mr Foot.
“We worked together with Aptella to come up with a fixing scheme, which consisted of using Selley’s ArmourFlex (an adhesive sealant) to secure the track mounting plates on to the concrete sleepers and ArmourFlex coupled with construction screws to secure the plates to the timber sleepers. This fixing method worked really well.”
On the remaining quarter of the track Vitruvius were required to monitor, there were concrete sleepers. These seemed an easier proposition to monitor but as the project progressed it became apparent that due to a number of factors the rail on the concrete sleepers was experiencing track corrugation.
“Track corrugation is an undulatory wearing of the track caused by the train and although this type of rail defect does not pose any safety risk to train operation, it does have some major consequences such as increased track wear, increased ride noise and higher levels of vibration. For the tilt sensors the high levels of vibration turned out to be an issue it’s a vicious circle which gets worse the more it happens. What we found was the track corrugation effected the sensors on the concrete sleepers more than it did on the timber sleepers and this is potentially due to something called track elasticity,” said Mr Foot.
With the sensors picking up irrelevant movement and vibration Mr Foot and Aptella developed a scale factor to dampen the x-axis values that were being reported in order to get the settlement being calculating from tilt to reflect what would have been achievable from a precise traditional level survey.
“We were able to achieve that, it’s something that we would probably want to look at again, but it gave us confidence to tell Fulton Hogan that their works was not causing any settlement,” said Mr Foot.
Fulton Hogan was initially considering a monitoring system based on total stations and precise 3D targets and Mr Foot noted that implementing the Senceive monitoring solution from Aptella instead probably saved Fulton Hogan half of the total cost. At the same time the Senceive monitoring system was able to be installed a lot faster than a traditional optical system.
“The speed of deployment was very quick. We were able to install 83 sensors on the track within about five hours using a team of three surveyors, that’s significantly quicker than what would have been possible if we had to install optical targets to the rail,” said Mr Foot.
“The other great advantage we found was when Kiwi Rail carried out spot re-sleepering during the lifespan of the project we were able to remove and then re-deploy the tilt sensors during a nightshift very easily and have the track monitoring system back up and running before train services resumed. Spot re-sleepering is where KiwiRail remove individual broken or very worn sleepers by unclipping a limited length of track and sliding the sleeper ear-marked for replacement under the rails.
Mr Foot also noted that the removal of the Senceive sensors was equally as fast as the implementation: “We got a phone call from Kiwi Rail and within 48 hours we were able to get onto site to remove all of the sensors so that they could start re-grading the formation and replacing the track and because we’d gone for this track mounted system meant we could just unclip the sensors and hand them to Futon Hogan for safe keeping and possible re-deployment,” said Mr Foot.
“We did that in one afternoon and the material is all salvageable, other than the track mounting bracket fixed to the sleeper. It’s such a flexible system that if Kiwi Rail insists the next contractor or Fulton Hogan continues to monitor the track we can use those same sensors, put them back on the track and start monitoring again.”
Vitruvius has worked with Aptella previously in Australia and Mr Foot found the responsiveness of the team to be an essential benefit.
“If I had a technical query put to me by the client, Kiwi Rail, that I couldn’t answer or I just felt that I needed a second opinion before I responded, I knew that I could ring up Alin or Dylan and within a couple of hours I could get some technical advice,” said Mr Foot.
“The responsiveness was brilliant and essential – this was effectively the largest tilt monitoring system installed in New Zealand to date.”
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