The need for adjustment
High speed lines built in earthquake zones such as Japan and Taiwan have in the past led to a need for relatively high levels of vertical adjustment – typically +50 mm – and requirements for HS2 in London, the requirement is even greater at +70 mm. The difference may not seem large, but the overturning moment that acts on the fastening is greater and any concrete upstands provided to react to lateral loads are further from the top level of prestressing or reinforcement in the base slab. Pandrol has tested the new maximum height adjustment requirement very thoroughly against the relevant European CEN requirement, running 3 million load cycles with a block configured so we could test adjacent assemblies at installation heights of 0 mm and +70 mm.
On slabs with rail seats, as vertical adjustments are made, the lateral position of the gauge face of the rail changes too. In order to maintain close control of track gauge, vertical adjustments may mean that the components that determine lateral alignment need to be replaced unless, like the Pandrol baseplate, the position of the baseplate itself can simply be adjusted. A +70 mm height adjustment on a 1:20 rail inclination as is the case for HS2 results in a 7 mm change in gauge, much greater than the 2.5 mm change that results from a +50 mm maximum height adjustment on a 1:40 track, as, for example, in China.
Speed of construction and maintenance are important and the Pandrol Fastclip Baseplate system allows machines to be used to switch the clips between the parked and Pandrol Fastclip Baseplate installed positions to allow for rail change and de-stressing. The rates at which the clips can be applied and extracted are exceptionally high and well known in the UK. Train-mounted optical track inspection systems that allow the positions and surety of non-threaded Fastclip system to be verified are readily available, and can operate at relatively high speeds – typically up to 160 km/hr. As well as speed, these maintenance and inspection systems also help to keep the workforce off the track and increase safety.
The global stiffness of the track controls several aspects of the behaviour of the vehicle-track system, but most track fastenings used on high speed lines incorporate at least one baseplate or steel plate, so that in principle at least two resilient layers can be introduced – one below and one above the plate. Even for a predetermined and specified global stiffness of the whole fastening, the selection of the stiffness of these two individual elements can affect overall dynamic performance.
This in turn may influence level of wayside airborne noise, as well as the mechanical behaviour of the system in response to the loads applied to it – rail roll, dynamic gauge widening, and so on. This is a complex area where Pandrol has great understanding. But to confirm that its designs provide the best possible mitigation of airborne noise within the given constraints, Pandrol is working closely with the Institution of Sound and Vibration Research (ISVR) in Southampton to test different detailed design options.