Pandrol Technical Director Steve Cox looks at how the application and maintenance of rail fastenings can be adapted for use on the precast slab trackform selected for High Speed 2.
“High speed tracks around the world have been built with ballasted and non-ballasted track forms. Both are proven and can work well, but the characteristics of HS2 in the UK have led to the selection of a non-ballasted track form for the plain line sections of phase 1 construction. HS2 will operate up to 18 trains per hour, at speeds of 330 km/hr, with traffic amounting to >60 MGTPA.”
The further decision has been reached that the track will be built with pre-fabricated slab panels, which are usually between 5 and 6.5 metres long and typically pre-fitted with 16 or 20 fastenings. Panels can be built off-site, in a controlled factory environment, mitigating the labour required at the construction site and risk to the workforce. Damaged panels can be replaced to ease track repairs and renewals more efficiently. The ground conditions along the HS2 route are such that significant levels of settlement and heave can be anticipated. While adjustment of individual slabs is possible, the easiest way to re-establish alignment is by adjusting the fastening systems. The HS2 requires fastenings to provide up to 70mm of vertical adjustment
In a non-ballasted track form, the resilience, otherwise given by the ballast, needs to be provided elsewhere. HS2 has specified a track stiffness of 64 kN/mm that can be seen at the rail head. On rigidly mounted slab, all the resilience in the track must be provided by the rail fastenings. With typical fastener spacings and a 60kg rail section, each fastener needs a stiffness in the range of in the range 20-25 kN/mm.
All the above are basic requirements of HS2. A reliable supplier with a good track record on high speed is vital for a major project of this nature. But against that background, how can proven technology be configured to provide the very best possible track fastening to meet the particular needs of HS2? And is there any scope for innovation? These are questions that Pandrol has been addressing. The Pandrol High Speed FASTCLIP baseplate is well suited to the task.
Delivering flexible adjustment
Adjustment is a fundamental requirement that involves several aspects including: the range over which directional adjustments can be made both horizontally and vertically, the accuracy in which these directional adjustments can be done, the ease and speed of the adjustment, along with the number and complexity of any additional or exchange parts.
Typically, these adjustments are made twice. Segments of curved track consisting of short straight panels must place its fastenings in an offsetting position to achieve a smooth alignment. This applies particularly to the lateral baseplate position and becomes a pressing issue as the curve sharpens. The fastenings must be positioned very accurately to achieve the tight tolerances on track gauge required on high speed track.
Pandrol’s baseplate is infinitely adjustable in the lateral direction. It can be tightened down and held firmly in position at the exact location required. The baseplates can also be slightly skewed relative to the axis of a slab, aligning the baseplate towards the centre of the curve and forming a part. This means that every slab can be identical to every other slab, and every fastening is identically configured relative to every other fastening on initial track construction. Only the exact positions of the baseplates fitted to any one slab differentiate it from other slabs in the track. So ‘spare’ slabs needed for repairs are universal, and do not need to be purpose constructed with the associated difficulties and lead times. Nor are any bespoke fastening configurations required to achieve exact track alignment.
Increasing the speed, reliability and quality of installation
The combination of universal slabs with universal fastenings is extremely attractive but not new. However, what is new and innovative is the way that Pandrol proposes to fit its baseplates to the slabs. Transferring the expertise that it has built up from their use in its clip manufacturing production lines, robots will be used to pick baseplates and place them in the exact positions and orientations required. The combination of baseplate positions on a given slab can be selected at the touch of a button. Using RFID tags, scanning is used to assist and confirm the baseplate positioning. All tags are affixed to each slab before it leaves the factory so that the curve, where it is meant to be installed, can be recalled at any time.
This robotic installation increases the reliability and quality of the installation on which the track alignment depends. It also increases the rates of production and reduces the risks of delays or interruptions.
Robotic installation of baseplates is greatly facilitated by the fact that the Pandrol baseplate can be largely pre-assembled before it is installed on the slab in the factory. This is an advantage when it comes to the second area, where adjustment is essential in maintaining the track over its operating life. A damaged baseplate could be replaced as a self-contained unit. Lateral adjustments are particularly beneficial as they require no additional parts or need to largely disassemble the fastening. The base plate simply needs to be loosened off, moved to the correct position, and retightened. Vertical adjustments are likewise. The baseplates are loosened off, any additional shims that are required are slid into place, and the baseplate is retightened. The height adjustment shims themselves are a very simple planar design, easily manufactured to whatever precise thicknesses is required. There is no danger associated with assembling shims of different thicknesses in the wrong order.
While high speed lines built in earthquake zones, such as Japan and Taiwan, have led to a need for relatively higher levels of vertical adjustment in the past (typically +50mm), the HS2 requirement for a +70mm is greater. The difference may not seem large, but the overturning moment that acts on the fastening is greater and any concrete upstands provided to react lateral loads are further distant from the top level of pre-stressing or reinforcement in the base slab. Pandrol has tested the new maximum height adjustment requirement very thoroughly. We conducted a very successful test 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.
As vertical adjustments are made on slabs with rail seats, 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 determining 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. This adjustment is 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.
Enabling maintenance efficiency
Speed of construction and maintenance are important. The Pandrol FASTCLIP system allows machines to switch the clips between the parked and installed positions enabling 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 help keep the workforce off the track and increase safety.
As has been noted, the global stiffness of the track has been specified by HS2, and this 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 influences 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 that Pandrol understands well. 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.