Unmitigated
and Mitigated Noise Levels of Spur line Railway Operation
Tables E.2 and E.3 provide details on the mitigation measures proposed for meeting the operation noise criteria for different times of the day. The Lmax criterion is 85 dB (A) for night-time (2300 – 0700) operation. The noise criteria in Leq30min[1] for 0700 ~ 2300 and 2300 ~ 0700 are 60/65/70 and 50/55/60 for Area Sensitivity Ratings of A/B/C respectively.
The structure re-radiated noise component of the overall noise levels at the viaduct section will be mitigated by installing Floating Slab Track (FST) on viaduct section between Sheung Shui Station and the at grade section at Kwu Tung and Low Vibration Track (LVT) on viaduct section between the at grade section and Lok Ma Chau Terminus.
The operation of Spur Line is from 0600 till one hour after the close of Lok Ma Chau Terminus Customs kiosks. To reduce air borne noise level using sound reduction plenum system at the hours when the noise criteria are more stringent, especially during 0600 ~ 0700 when the morning traffic peak begins, train service will be provided using 6 skirted trains in each direction. From 0700 onwards, 12 unskirted East Rail refurbished trains will be operated in each direction.
A 1.2 m tall track side parapet and walkway plenum structure will be installed as part of standard walkway design for all viaduct sections and at grade, ballasted embankment / cutting sections of the alignment.
At chainages 30+680 to 30+880 north of Tsung Pak Long, at the uptrack track side an approximately 200 m long and 2 m tall noise barrier is proposed.
A full enclosure will cover the points and crossings just north of the existing Sheung Shui Station, to overcome the reduced effectiveness of the sound reduction system due to the loss of the inboard plenum.
Cumulative Noise Assessment
The detailed values of the cumulative noise assessment as described in Chapter 3 of this EIA report are shown in Tables E4 to E8.
Details of Low Vibration Track (LVT)
As part of the TS-900 and follow-on DD901 environmental studies of West Rail during 1997 and 1998, the vibration mitigation of candidate trackforms was considered with respect to the viaducts, where structure radiated wayside noise was a major issue. Tunnel, at-grade and station sections of the alignment were also evaluated, as regards adverse impacts from ground and structure borne noise and vibration. Where maximum vibration reduction is required, floating slab track was specified; lower cost low vibration trackform (LVT) was chosen for track sections where either no or moderate vibration isolation is necessary. Based on vibration measurement data taken on a number of other transit systems, and detailed numerical modelling of the specific implementations under consideration for West Rail, the TS-900 Study found that the use of FST would be unambiguously required on the viaduct sections, if the maximum wayside noise levels stipulated under the Hong Kong Noise Control Ordinance (NCO) were to be satisfied. FST was also recommended along limited sections of non-viaduct track where: 1) there is special sensitivity to N&V (Kwai Tsing Theatre); 2) property development occurs above stations; and 3) source levels are expected to be higher than along the plain line (switches, turnouts and crossings).
In late 1998 and early 1999, Wilson, Ihrig and Associates, Inc. (WIA) performed a study of the structure borne noise from the Tsing Yi Viaducts for the MTRC. The trackform along the section of viaduct considered in that study was LVT, similar to the type specified for West Rail. One finding of the study was that the high frequency vibration isolation, and consequent levels of structure radiated noise from the viaduct, was lower than expected at frequencies greater than 250 Hz, i.e., in the frequency range which has a significant effect on the noise measure (LAeq30) specified in the Hon Kong Noise Control Ordinance (NCO). This benefit was attributed to the use of soft rail pads in conjunction with the LVT system, which appears to provide additional high frequency vibration isolation beyond that expected from the LVT system with standard (stiff) rail pads.
This beneficial effect was not enough to eliminate the need for FST on the West Rail viaducts; however, use of an optimised variant of the LVT system will provide an adequate substitution for FST on the western viaduct section of the Sheung Shui to Lok Ma Chau Spur Line, where less severe reductions in structure radiated noise are required, on account of the lower speeds, longer headways and locations of sensitive receptors. WIA has assessed design improvements put forward by Sonneville (Designer of LVT) with regard to the vibration isolation characteristics of the LVT system and has established a reference structure radiated wayside noise level which is used in the assessment of noise impacts in the Spur Line EIA. The development of this reference structure radiated noise level is described in the following.
The total wayside noise measured adjacent to the Tsing Yi Viaducts during passbys of the MTRC Airport Express is given in Figure E.1, where levels have been adjusted to 130 kph (30Log[130/80]) and a setback of 25 m (10Log[25/65]). The maximum overall A-weighted level is 88 dB(A), which is consistent with the agreed reference noise level for train passbys on an unmitigated viaduct structure. In Figure E.2, additional noise levels are given for measurements taken beneath the viaduct structure, where lower frequency structure radiated noise dominates, on account of the shielding of the higher frequency direct train noise by the viaduct structure.
Simultaneous measurements of vibration were also taken on all of the noise radiating surfaces of the Tsing Yi Viaduct. These vibration levels were converted to noise using the methodology employed by WIA in the validated analyses of structure radiated noise for the Lantau Fixed Crossing, the West Rail EIA and the East Rail Extensions. The noise levels obtained from the vibration measurements were then energetically summed, and adjusted to 130 kph (25Log[130/80]), to obtain the total structure radiated wayside noise level given in Figure E.3. It can be seen in the comparisons of Figure E.3 that there is very good agreement between the measured noise levels and that calculated from vibration levels below roughly 200 Hz. This is expected, as wayside noise in the low frequency range is not influenced by the direct (airborne) train noise. Above 200 Hz, the measured noise levels are dominated by the direct train noise, even under the viaduct, where there is shielding. However, the structure radiated noise falls off rapidly with increasing frequency, on account of the track isolation system (notably, the soft rail pad) and attenuation within the structure. The maximum overall A-weighted noise level for the structure radiated noise is 71 dB(A).
A study has been performed by WIA of the static and dynamic stiffness (in the acoustic frequency range) of a mock-up of the LVT system installed on the LAR. The static stiffness of the system, comprised, in part, by the sleeper pad, the rail pad and the sleeper boot, under expected axle loadings, is 20 kN/mm. The acoustic dynamic stiffness, however, is 104 kN/mm. This large difference is primarily caused by the high dynamic to static stiffness characteristic of the synthetic (EPDM) sleeper pad and the tight fit of the sleeper boot. An improved sleeper pad made of natural rubber, with a low dynamic to static stiffness ratio, and an improved sleeper boot, which provides much greater decoupling, has now been tested. The acoustic dynamic stiffness of the new configuration was found to be 52 kN/mm, or half of that on the LAR LVT. Although further improvements are currently underway, this improved configuration is used as the basis of the reference structure radiated noise level assumed in the Spur Line EIA.
In Figure E.4, the structure radiated noise level determined for the LAR viaducts is again given, assuming reference conditions of 130 kph train speed and 25 m setback. The second curve appearing in Figure E.4 is obtained by adjusting the structure radiated noise level for the LAR viaducts to account for 1) a reduction in speed to 100 kph, the Spur Line reference speed (minus 3 dB); 2) an increase in radiating area by a factor of two to account for the twin track viaduct structure on the Spur Line (plus 3 dB); and 3) a decrease in acoustic dynamic stiffness of the trackform from 104 kN/mm to 52 kN/mm (roughly minus 4.5 dB above 80 Hz). The resulting maximum overall A-weighted noise level is 66.5 dB(A), the level assumed for the Spur Line EIA.
The estimate of the structure radiated noise for the Spur Line EIA is conservative in three ways: 1) it does not take into account improvements to the viaduct structure planned for the Spur Line compared to the LAR viaduct structure; 2) the final LVT design to be implemented will likely include further ongoing improvements pertaining to vibration isolation; and 3) the actual insertion loss for the improved LVT will likely exceed the assumed insertion loss, as it was estimated conservatively.
FIGURE E.1. TOTAL WAYSIDE NOISE MEASURED ADJACENT TO THE TSING YI VIADUCTS DURING PASSBYS OF THE MTRC AIRPORT EXPRESS – ADJUSTED TO 130 kph AND 25 m SETBACK LEVEL WITH TOP OF RAIL.
FIGURE E.2. TOTAL WAYSIDE NOISE MEASURED ADJACENT TO THE TSING YI VIADUCTS DURING PASSBYS OF THE MTRC AIRPORT EXPRESS – ADJUSTED TO 130 kph AND 25 m SETBACK.
FIGURE E.3. COMPARISON OF MEASURED TOTAL WAYSIDE NOISE AND STRUCTURE RADIATED NOISE DETERMINED FROM VIBRATION MEASUREMENTS TAKEN ON THE NOISE RADIATING SURFACES OF THE VIADUCT – ADJUSTED TO 130 kph AND 25 m SETBACK.
FIGURE E.4. STRUCTURE RADIATED NOISE FROM VIADUCTS WITH LVT INSTALLED – 25 m SETBACK.