INTRO: Detailed design of trains, track and structures on Hong Kong’s West Rail line has achieved an unprecedented reduction in lineside noise and vibration. Development and implementation of a multi-plenum system has been key to the control of under-car noise, notably from the wheel/rail interface
BYLINE: Alan Crockett and John Carlisle *
* Alan Crockett is a Director of Hong Kong-based acoustic and vibration specialist Wilson Ihrig & Associates. John Carlisle is now working for John Holland Ltd, but was previously Permanent Way Manager for Kowloon - Canton Railway Corp. The authors would like to thank the Directors of KCRC for their support and permission to publish this article.
THIS AUTUMN is due to see the opening to passengers of Kowloon - Canton Railway Corp’s West Rail line between Kowloon, Yuen Long and Tuen Mun. The 30·5 km route includes 13·5 km of viaduct, much of which traverses urban areas, and 9 km of tunnel.
West Rail is effectively a high-capacity heavy metro designed to accommodate 33 trains/h, worked by nine-car EMUs. The trains have a maximum axleload of 18·3 tonnes, and will run at 130 km/h on parts of the route. In the longer term, total traffic is expected to reach 90 million gross tonnes per direction per year.
The Hong Kong Noise Control Ordinance stipulates stringent statutory noise limits for railway operations. During night hours (23.00 to 07.00), operational noise levels at most locations along West Rail cannot exceed an equivalent of 55dB(A) over a 30min period. To meet this limit on the viaduct sections, at peak headways, the noise level during the passage of a single train must not exceed 64dB(A) at 25m from the track centreline. Assuming a reference lineside noise level of 88dB(A) for a conventional unmitigated viaduct, the NCO limit requires a 24dB(A) reduction.
A study of possible noise mitigation measures was conducted in 1997, before the start of detail design of the West Rail viaducts, as part of the statutory Environmental Impact Assessment1. It was found that edge barrier walls alone would not adequately attenuate the direct train noise, and significant reductions would also be required in structure-radiated noise. Use of full enclosure, except at isolated locations, was ruled out because of its visual appearance.
A concurrent study2 indicated that it would be possible to achieve the 24dB(A) reduction through integrated mitigation measures on the trains, the trackform and the viaduct structure. This meant that design of the trains, trackform and viaduct would have to be oriented significantly towards the noise control objective, meaning that the interfaces between these systems would have to be highly co-ordinated.
Noise control design
Noise control measures employed on the West Rail viaducts are summarised in Fig 1. The basic approach to controlling direct noise from under the trains, in particular wheel/rail noise, is to employ mitigation measures in stages close in to the source.
Vehicle skirts along the length of the train, including in front of the bogies, leave a minimum gap between the derailment constraint and the edge of the skirt (left). Sound-absorbing material is applied inside the skirts and under the car floor around the bolsters, creating a noise plenum beneath the train.
Walkway edges have been brought as close as possible to the train body without infringing the kinematic envelope. Sound absorption beneath the walkway and on the surface of the adjacent parapet creates a noise plenum beneath the walkways. A similar plenum is created between the tracks on a double-track viaduct by building an absorptive median wall with a platform on top.
A low-resonance floating mini-slab trackform with high-attenuation baseplates has been installed to minimise the transmission of vibrations from the track to the viaduct and the noise radiated from the floating slabs. This required a robust viaduct structure to provide a stiff and massive support to optimise the attenuation of the trackform.
Absorptive parapet walls nominally 1200mm high above the walkways block line of sight between the train/walkway gap and the noise receiver. Provision has been made to extend the parapets to high noise barriers or full enclosures to give additional mitigation at highly sensitive locations or to accommodate future property development.
Thus the under-car plenum, the under-walkway plena and the absorptive edge wall or parapet barrier consecutively reduce the direct noise as it is radiated through the gap between the vehicle skirt and derailment constraint, and subsequently through the gap between the walkway and the side of the car.
Permanent way measures
This multi-plenum system reduces direct noise to the statutory limit. But without trackform attenuation there would still be significant lineside exceedances caused by structure-radiated noise. Many different trackforms were considered, but only a high-attenuation floating mini-slab trackform with high-attenuation baseplates was found to reduce both structure and slab-radiated noise sufficiently.
Extensive use has been made of floating slab track on viaducts and in tunnels. In total West Rail has 88 km of track, all laid with continuously-welded UIC 60 rail inclined at 1:20. Of this, 34 km is carried on floating slab, 27 km is non-ballasted on resiliently-supported dual-block sleepers, and the rest, which includes the depot, is ballasted. Of the 120 items of special trackwork, predominantly turnouts and crossovers, 38 are carried on floating slabs.
Because of the many sensitive locations along the route, the need to provide for future development and the desire to minimise the number of trackform changes, all plain line viaducts on West Rail have floating slab track. The same solution is applied to special trackwork on the viaducts, except that larger slabs are used. High-attenuation bonded baseplates are used to support the track through turnouts and crossovers.
In the tunnels, a similar floating mini-slab trackform is used for plain line at locations where groundborne noise may be excessive in adjacent structures. The main difference is that the rail is fixed with more conventional bonded baseplates, since the attenuation of the floating slab is sufficient and noise radiated from the slab is not critical. Where groundborne noise is not a problem, resiliently-supported booted dual-block sleepers are installed.
For tunnel turnouts and crossovers, which are a concern from the standpoint of groundborne noise, large slabs are used, as on the viaducts, except that the resiliently-booted dual-block sleepers are taken through the special track for simplicity and ease of construction.
The planned intensive operation of West Rail and short overnight maintenance periods dictated that safety, robustness, reliability and minimal maintenance requirements were over-riding issues in the selection of track components. The plain line was designed, detailed and specified in full, and special trackwork specified to define clearly the form, key elements and concepts, while leaving the detailed manufacturing design to the specialist S&C subcontractor.
The track design was continually refined during the procurement and installation phase, as the detailed design of special trackwork and final choice of plain line components was made, with particular emphasis on buildability and future maintenance. The same approach is now being employed on KCRC’s East Rail extensions programme.
Floating slab track design
The principle parameters for rail deflection are determined by fatigue and safety at a rail break. These were set at 4mm lateral and 7mm vertical for continuous rails, and 13mm lateral and 25mm vertical at a rail break, under maximum loading conditions.
KCRC decided that the floating slab trackform (FST)should be designed first to meet the particular viaduct requirements, especially deck movements. The system would then be adapted for the tunnels, where FST has to mitigate the transmission of vibration to integral and adjacent structures, generally at stations.
As part of the design process, an indicative installation method was developed to assist in the assessment. It was concluded that the optimum solution was to use pre-cast mini-slabs placed directly on the structure, with the final rail alignment accommodated by a ’top-down’ second stage concrete infill. This ensures that viaduct deck tolerances and deflections under the trackform dead load are consolidated in the final alignment. Final rail installation only took place after all dead loads were in place. In-situ slabs were used for special trackwork, both S&C and expansion joints.
An early decision on the trackform depth was required to enable the civil design to proceed. This was set at 770mm. The design is based on the principle of coarse tolerances for initial wet works, and tight tolerances for the final rail alignment.
On plain line, combined derailment and side restraint concrete upstands, 400mm wide, are constructed with the inside face 700mm from the gauge face of the rail. The upstands are positioned vertically to match the designed rail levels. The low upstand is set to the level of the grade rail, and the high upstand is raised to match the plane of the rails when on canted track, maintaining the relationship between the upstand and vehicle skirt.
The modular pre-cast slabs are roughly 1200mm long, to support two sleepers, and weigh 2·9 tonnes. Cant is achieved by a combination of pre-cast slabs providing 40mm or 80mm of built-in cant, with final adjustment using in-situ fibre-reinforced concrete between the baseplates and the slab. This concrete fill is also used to achieve the required vertical and horizontal track alignment. Shims below the baseplate provide for future maintenance adjustment, with lateral adjustment by serrated washers on the anchor studs (Fig 2).
Vertical bearings are positioned under each rail fastening. The bearings sit directly on the viaduct deck, and no attempt is made to adjust the vertical levels. To ensure that the four bearings of each slab are planar during installation, a four-legged ’Zimmer frame’ was placed at the bearing positions, and a hard rubber shim inserted under opposite legs if rocking occurred.
The FST units are positioned, and the side bearings pre-compressed, by jacking the slabs to either side and installing shims behind the bearings. One bearing each side is used on straight track and curves until the sum of cant and deficiency is greater than 100mm. After this, one or two side bearings are provided, depending on the relative values of cant and deficiency. End bearings are used to restrain longitudinal movement, and are also pre-compressed.
In tunnel applications, a stiffer and thinner bonded-compression baseplate allows a slightly thicker and heavier slab, with lower resonance frequency. Otherwise the tunnel slabs and bearings are identical to those used on the viaducts, standardising both the bearings and the construction techniques.
With trains braking or accelerating at the same places around each station, acceleration and deceleration rates are high and considerable forces are generated. Longitudinal restraints are required through the platforms and extending on each side to prevent the track units creeping. A special pre-cast restraint unit was designed with projecting side arms that fit into a gap in the derailment upstand, anchored by longitudinal bearings.
The slabs for special trackwork are continuous for the length of the switch and crossing unit, generally between 20m and 36m long, with 50mm wide joints and vertical bearings positioned on a rectangular grid pattern. There is some flexibility in positioning the bearings, to accommodate drainage holes and the drainage channel cast in the viaduct deck. The slab thickness is 400mm, giving a mass of 1000 kg/m2. Extra bearings are added at the ends of the slab to provide increased stiffness, such that deflections at slab ends and middle, and between two adjacent slabs, are approximately equal.
The design presumed that the slabs would be cast in-situ on permanent formwork, but the contractor elected to pre-cast the slabs for special trackwork on the viaducts in sections transportable by road, and stitch them together when positioned. This proved successful, and ensured rapid construction where headroom was available for lifting, given the inherent flatness of the viaduct surface.
Bonded resilient baseplates of the ’S&C Egg’ type are used on the viaduct trackwork, to ensure low static stiffness. These are large and expensive units. The turnouts are installed in a top-down process, with the baseplates levelled by fibre-reinforced concrete. Drilling of the anchor bolts is the critical factor in determining the progress of installation.
For the tunnels, the handling of large pre-cast slabs was not practical, and the special trackwork slabs were poured in-situ using profiled steel permanent shuttering. There was no acoustic requirement to use the large baseplates, so resiliently-supported twin-block sleepers as used on plain line were laid on the floating slab, with the infill concrete connected to the slab using shear links and fibre reinforcing.
Transition arrangements are required wherever vertical stiffness and the consequent vertical rail deflections are significantly different. Within the FST, transitions are provided at the interface between the pre-cast plain line slabs the and wide in-situ S&C slabs. In addition, transitions are needed at the interface between FST and other trackforms. Pre-cast FST transition units are of a standard size, with additional recesses in the soffit to allow up to 16 vertical bearings to be used.
The viaduct spans were designed to a modified BS5400 RL loading, and stiffened in respect to vertical live load deflection to meet the standards in UIC 776-3R. The track requirement for continuously-welded rail dictated the use of simply-supported spans, or a portal viaduct concept. Movement of the standard viaduct spans through temperature changes are accommodated in the shear of the FST support bearings. The limiting criterion is determined by rubber fatigue, with the maximum longitudinal viaduct movement relative to the rail restricted to 7mm.
Any irreversible longitudinal movements, such as deck creep and shrinkage, would be eliminated by unclipping the rails and allowing the floating slabs to return to their neutral position. The effects of creep within the viaduct span are expected to be negligible after the first year.
Special long-span structures where thermal movements at the viaduct movement joints are greater than the agreed limits require the use of rail movement joints. These are provided with switch-type expansion joints, where relatively large movements are possible, supported on an in-situ floating slab.
Measurements were taken when trains began running, to verify the lineside noise performance along the viaducts and the trackform attenuation within the tunnels.
Recorded lineside noise levels are shown in Fig 3, along with typical levels for a comparable viaduct where no noise mitigation measures have been installed. The overall A-weighted level of 64dB(A) measured beside the viaducts confirms that the design reduction of 24dB(A) has been achieved. In addition, the noise measured beneath the viaduct exhibits very low levels of ’rumble’, confirming the effectiveness of the FST design.
Vibration attenuation for the resiliently-supported dual-block sleeper sections and the plain line FST installed in the tunnels are shown in Fig 4. In addition to the installation and maintenance advantages of dual-block sleepers for routine trackwork, the trackform provides attenuation consistent with low-stiffness baseplates. The FST provides exceptional attenuation above 16Hz, providing a significant reduction in groundborne noise and vibration along both soil and rock-based tunnel sections.
CAPTION: The West Rail EMUs are fitted with continuous side skirts and under-car absorptive linings for noise mitigation
CAPTION: Fig 1. West Rail will have multi-plenum and trackform noise attenuation systems
CAPTION: Twin-track viaduct showing the floating mini-slab trackform, high-attenuation baseplates and under-walkway and central noise plena
CAPTION: Fig 2. Cross-section of plain line floating slab track, showing the vertical and horizontal elastic bearings, derailment upstands and the fibre-reinforced concrete infill with provision for subsequent adjustment of the rail location
CAPTION: LEFT: Making provision for the future West Rail branch to Lok Ma Chau, this complex double crossover north of Kam Sheung Road station is carried on floating slabs
ABOVE: Close-up of a viaduct turnout carried on a floating slab, showing the high-attenuation baseplates
CAPTION: Turnouts installed in the tunnel sections of West Rail use double-block resiliently-booted sleepers carried on a single floating slab
CAPTION: LEFT:Fig 3. Lineside noise levels measured 25m from the West Rail viaduct, level with the top of rail, and under the viaduct, during the passage of a train at 130 km/h, compared to an unmitigated viaduct
RIGHT: Fig 4. Measured attenuation of resilient dual-block sleeper and floating mini-slab trackforms installed in the tunnel sections
1. Marsden P R, Pyke J R and Bullen R. West Rail Initial and Final Assessment Reports, Environmental Impact Assessment, Contract TS900, ERM-Hong Kong Ltd, prepared for KCRC, July 1997 and January 1998.
2. Crockett A R and Pyke J R. Viaduct Design for Minimisation of Direct and Structure-Radiated Train Noise. Journal of Sound & Vibration, Vol 231 (3.2000), pp883-897.