It is widely recognised that very low maintenance requirements and long life are major advantages of non-ballasted track (NBT) over conventional ballasted systems. NBT has a higher first cost than ballasted track and is time consuming to construct, but once in place it provides a stable track system that could still be in use a century later. In theory…
In practice, the rigidity of NBT creates its own set of problems, particularly in urban areas. Once concrete has been installed, it is very costly to adjust or correct the track geometry.
When new housing developments spring up alongside a rail system composed of NBT, residents are prone to complain about noise and vibration issues. This leads to the prescribed performance requirements needing to be increased – and you have a problem. Without the flexibility to adapt to the new noise and vibration mitigation demands, the whole track has to be modified, or lifted and rebuilt from scratch.
Now, for the first time, Pandrol can offer an innovative solution to futureproofing NBT. The Common Interface System gives rail infrastructure owners the reassurance of knowing that, when they install NBT, they will be able to adapt it to meet any future changes in performance requirements, quickly and affordably.
Currently, the concrete elements used for NBT have interfaces that are aligned to specific fastening systems and anchorages. Pandrol’s Common Interface System (CIS), as the name suggests, provides a common connection point for a range of different fastening assemblies that can be easily swapped to tune the track performance. When vibration and noise mitigation requirements change, rather than having to modify or replace the whole track, the fastening system can simply be swapped to achieve the required performance level.
The Common Interface System consists of two engineering plastic plates with retaining features, as shown. Reinforced plastic dowels sit underneath the plates, enabling anchor screws to be installed.
The Common Interface System is compatible with all the main Pandrol fastening range, screwed and non-screwed.
Imagine that a new metro station is built in a non-residential area of town, where few people live and work. With noise and vibration mitigation requirements low, the Common Interface System can be fitted with a simple, medium resilience fastening solution, such as Fastclip FCA.
The Fastclip SGI shoulders locate into the plastic construction plates and are retained using anchor screws. A plastic insulator is then added to the front face of each shoulder and the Fastclips, with integral toe insulators, are installed in a parked position. A resilient rail pad is introduced into the rail seat area before the rail is threaded and the Fastclips are pushed into their installed positions. The Pandrol FCA system can provide vertical adjustment of up to 10mm with the standard shoulder through the addition of shims underneath the rail pad and the shoulder. Further adjustment is possible up to 20mm using a second, taller shoulder.
The load applied to the rail is transferred through the shoulders and dispersed into concrete. The system stiffness is limited to 40 kN/mm in the standard configuration, to ensure there is no detrimental effect on dynamic track gauge through excessive rail roll. Although the rail pad contributes to the reduction of vibration, produced by irregularities of both surfaces of the wheel and rail head, the resilience limits of a single rail pad still allow the transmission of some ground-borne vibration to surrounding structures.
So, what happens ten years down the line, when new housing estates, offices and a school spring up around the station and local residents start complaining about levels of noise and vibration?
Instead of having to make costly alterations to the NBT, the FCA system can simply be removed from the Common Interface and replaced with a different, more resilient set of components, as shown below.
Although this looks similar to the FCA system, the two cast iron shoulders are now part of a linked baseplate that sits on top of a highly resilient baseplate pad. Two cast iron clamps complete with nylon bushes are used to attach the system to the Common Interface. The baseplate is able to move vertically about the nylon bush as the resilient pad compresses under load. This protects the clips from excessive deflection and, crucially, ensures rail roll limits are not exceeded.
This stability means that a higher degree of resilience can be utilised safely to provide greater protection against ground-borne vibration and any associated secondary noise issues. The more resilient the fastening system the less vibration travels through the track, reappearing in people’s property. A stiffness of 22.5kN/mm is typical for this set-up, but it can range from 16kN/mm to 25kN/mm.
Should requirements for noise and vibration mitigation become even more extreme, for example if a hospital operating theatre is built over the metro tunnel, the existing system can simply be removed from the Common Interface and replaced with more resilient components.
For maximum attenuation, the FCA or DFC system can be replaced with the Pandrol Vanguard (or Bonded Rubber DFF) system. Both systems provide a high degree of isolation of vibrations being transmitted through the rail that would otherwise be free to be transmitted to the structures surrounding the track. The Vanguard system supports the rail against the rail web and under the rail head. This allows the rail to be fully supported by resilient elements that attach to a cast iron baseplate. These resilient elements operate in shear, ensuring a high degree of gauge control as the resulting rail deflection is almost entirely vertical.
Rather than sitting on a pad or the baseplate, the rail is held up by the resilient blocks, producing an air gap underneath of approximately 6mm.
This system is extremely resilient, providing a stiffness of as low as 5kN/mm and has a proven track record of reducing ground-borne vibration levels in excess of 10dB.
The Follo Line in Norway connects Oslo and Ski via twin 19.5km tunnels, the longest in Scandinavia. The tunnels pass through a granite mountain on which people live and work, and it was clear from the outset that vibration attenuation would be needed to limit any impact of the railway on those living in this area.
The decision was made to use the Common Interface System to enable the resilience of the track to be tuned to meet the strict requirements of the noise and vibration (N&V) study carried out by consultants. Three different stiffness levels were provided using the system, with post-installation measurements confirming that the N&V hot spots highlighted in the study had been effectively controlled and met the required specification.
The images above show the Common Interface System in use in the Follo Tunnel with two different fastening systems – Pandrol VIPA DFC (left) and Pandrol Vanguard (right). Sitting on the same common footprint, these provide the intermediate and advanced levels of ground-borne vibration mitigation required at different points in the tunnel. A combination of two DFC assemblies and a de-tuned Vanguard assembly were selected to meet the challenging noise limit of 32dB. These systems were able to vary the track stiffness between 22.5kN/mm, 16kN/mm and 11kN/mm using the same interface.
Opting for the Common Interface System offers the reassurance that the NBT will be able to adapt to any changing performance requirements in the future.
Pandrol’s Common Interface System is set to revolutionise the future of NBT, providing the first-ever solution to the inherent inflexibility of current systems. With the Common Interface System, rail infrastructure owners can invest in NBT in the confidence that they can adapt the track to any changing performance requirements, whether these are 10, 20 or 50 years down the line.
The cost benefits of this are clear – being able to simply replace the fastening system, rather than the whole track, can save millions in unnecessary costs. The process is much simpler and quicker, creating less disruption for the transport system and local communities. And with concrete’s high carbon footprint, increasing NBT’s lifespan has immense sustainability benefits. Once removed, fastening systems can be recycled or, in some instances, reused on another project.
Pandrol’s Common Interface System is unique, providing futureproofing for valuable, high cost, assets. It is compatible with all NBT systems. Come and talk to us if you would like to find out more about its potential to futureproof NBT.