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ICSV21, Beijing, China, 13-17 July 2014 1
The 21st International Congress on Sound and Vibration
13-17 July, 2014, Beijing/China
NOISE INSULATION FOR OPERABLE BLAST COVER
Wilson Ho, Morgan Cheng and Jinhao Cao
Wilson Acoustics Limited (WAL), Unit 601, Block A, Shatin Industrial Centre, Shatin, Hong
Kong SAR, China
e-mail: who@wal.hk
Ken Wong
Nishimatsu Construction Co.(NCC), Ltd, Room 508, Star House, Salisbury Road, Tsim Sha
Tsui, Kowloon, Hong Kong SAR, China
Gap sealing of traditional blast cover is easily damaged by daily opening operation and blast
overpressure such that high noise insulation of blast cover is difficult to achieve, especially
for large area cover. An operable blast cover has been installed on top of a vertical shaft of
750m2 cross-sectional area to be excavated to 83.6m deep for railway tunnel construction of
MTR South Island Line. 24-hour drill and blast excavation is employed. The blast cover is
required to be opened for moving in and out of heavy equipment and removal of excavation
rubble, and closed for blasting protection and noise insulation for daily operation. The shaft
is located at 10m from British Council and 40m from Shangri-La Hotel. Blast overpressure is
required to control below 168dBL (5kPa) at the blast cover and 120dBL at residences. Apart
from blasting noise, rock drilling and breaking is the noisiest activity, where predicted noise
levels without noise enclosure are 102dB(A) at British Council and 82dB(A) at Shangri-La
Hotel. The operable blast cover is required to provide minimum 40dB(A) noise insulation to
achieve the statutory criteria, and finally it provided 44dB(A) insulation by specific cover
closing and gap sealing mechanism, material selection and good workmanship. With shaft
entrance double door system, checklist record and regular noise monitoring to ensure the
overall noise insulation performance, the 24-hour drill and blast excavation was improved
from 2 to 3 blasts per week and the shaft excavation process was completed within 8 months.
1. Introduction
For MTR South Island Line (SIL) tunnel construction, a vertical shaft of 83.6m depth and
750m2 cross-sectional area is required to excavate through hard rock geology (Grade III or better
volcanic tuff) at Central District of Hong Kong. Drill and blast excavation is employed from 8m
deep and downward. The shaft is close to British Council and Shangri-La Hotel (Figure 1). Blast-
ing overpressure, rock drilling and breaking noise are the major environmental concern. This paper
presents acoustic design, verification tests and management measures for obtaining and maintaining
24-hr excavation for 8 months without adverse environmental impact on the surrounding.
21st International Congress on Sound and Vibration (ICSV21), Beijing, China, 13-17 July 2014
ICSV21, Beijing, China, 13-17 July 2014 2
2. Site conditions and noisy construction equipment
The shaft is located at a congested area, where British Council is at 10m from the shaft and
Shangri-La Hotel at 40m as shown in Figure 1. 24-hour drill and blast excavation is required where
jumbo drills and hydraulic breakers are the noisiest construction equipment.
Figure 1. Location of the construction site
3. Statutory noise criteria and effective noise control process
Statutory noise criteria are stipulated in “Technical Memorandum on Noise from Construction
Work other than Percussive Work” under Noise Control Ordinance1 (NCO). Acceptable noise levels
are defined based on area sensitivity rating, operation period and multiple permit correction. For 24-
hour construction works, the noise levels are required to be control within 62dB(A) during evening
(1900 to 2300 hours) at British Council and 47dB(A) during nighttime (2300 to 0700 hours) at
Shangri-La Hotel, including -3dB(A) multiple permit correction.
Before conducting evening and nighttime construction works, the Contractor is required to
obtain a Construction Noise Permit (CNP) from the Environmental Protection Department (EPD).
The CNP approval process normally takes 18 to 28 days subject to the complexity of the application
and adopts a conservative noise assessment method under worst-case scenario. The assessment as-
sumes that all applied construction equipment are to be simultaneously full-power operating at a
notional location close to the concerned buildings, without considering the practicability of such
congested equipment operation at a single point. CNP will be issued only when the worst-case sce-
nario still achieves statutory noise criteria. The CNP approval process is highly effective to ensure
the noise climate at the sensitive receivers would not be adversely affected by the construction
works during evening and nighttime, and minimise any adverse impact on nearby the residents.
In addition, a blasting permit is also required where blasting overpressure is controlled below
168dBL (5kPa) at the blast cover and 120dBL at residences. Such overpressure would be audible
as low-frequency rumbling for a few seconds, but would not create long-term noise disturbance.
Acoustic Cover
21st International Congress on Sound and Vibration (ICSV21), Beijing, China, 13-17 July 2014
ICSV21, Beijing, China, 13-17 July 2014 3
4. Noise mitigation measures
4.1 Noise insulation criteria
Noise level from mechanical rock drilling and breaking (using 2 double-arm rock drills and 1
hydraulic breaker simultaneously) is predicted to be 102dB(A) at the British Consulate and 82dB(A)
at Shangri-La Hotel without noise mitigation. As a conservative approach for the enforcement of
the regulatory requirement, only 10dB(A) noise insulation of the closed façade glazing at the hotel
is assumed, while openable windows are considered negligible noise insulation. In order to mitigate
the noise level from 102 to 62dB(A) at British Council and from 82 to 47dB(A)at Shangri-La Hotel,
the blast cover is required to provide minimum 40dB(A) insertion loss (IL). Insertion loss refers
the noise level difference before and after the installation of the acoustic cover to ISO 10847.
4.2 Blast cover acoustic design
In addition to noise insulation requirement, the blast cover needs to sustain 5kPa blast over-
pressure. Blast overpressure is controlled by different depth, diameter and number of explosive drill
holes, delay at each holes and maximum instantaneous charge per delay, as well as the size of over-
pressure relief opening. The area for each blasting is gradually increasing as excavation downwards
(188m2 blasting area for depth of 10.5-16m, 250m2 blasting area for depth of 16-23.5m and 375m2
blasting area for depth of 23.5-83.6m).
The blast cover was made of STC 47 proprietary acoustic panels (121 kg/m2 surface density).
Additional 17mm thick sheet pile and 180mm thick concrete were installed on top of acoustic panel
for the loading part of cover to support heavy plants (total 275 kg/m2) as shown in Figure 2 & 3.
Two operable covers (5.3 x 12m each) are required to be opened every day (except Sunday and
public holidays) for spoil mucking out and equipment transportation during daytime, and tightly
closed for night-time rock drilling and breaking.
Effective and durable gap sealing is the most important to maintain high performance noise
insulation. Gaps between interfaces of two acoustic panels are sealed by elastomeric acoustic seal,
silicon sealant and metal plate (Figure 4). Gaps between operable covers and supporting beam are
sealed by 3mm thick elastomeric acoustic seal and 10mm thick gasket (Figure 5).
Figure 2. Acoustic cover at MTR SIL construction site
21st International Congress on Sound and Vibration (ICSV21), Beijing, China, 13-17 July 2014
ICSV21, Beijing, China, 13-17 July 2014 4
Figure 3. Drawing of acoustic cover
Figure 4. Gap sealing between two panels Figure 5. Sealing for operable panels sealing
4.3 Silencers
Overpressure from blasting is released through the intake and exhaust ventilation openings of
each 1.8m(W) x 1.8m(H). The two ventilation paths are each installed with 5.2m long absorptive
plenum box, 2.4m long silencer and 2m long absorptive right-angle turn to provide 50dB(A) noise
attenuation. Potential breakout noise through silencer casings was prevented by noise enclosure
made of STC 44 acoustic panels with 98kg/m2 surface density. Concrete was applied to seal the
gaps between silencer enclosure and the acoustic cover.
Figure 6. Intake silencer with concrete sealing
Metal plate and
silicon sealant
10mm gasket
3mm elastomeric
acoustic seal
21st International Congress on Sound and Vibration (ICSV21), Beijing, China, 13-17 July 2014
ICSV21, Beijing, China, 13-17 July 2014 5
4.4 Acoustic Lobby House
For the concerns of noise leakage caused by man access during restricted hours (1900 to 0700
hours), a lobby house with CCTV system was installed on the blast cover. The lobby house, con-
sisting of two acoustic doors and one lobby house, has at least one door being closed at anytime
during the restricted hours. The entire lobby house made of STC 44 acoustic panels with 98kg/m2
surface density. All door gaps at four sides are sealed by double gasket.
Figure 7. Lobby house with CCTV system
4.5 Gap Sealing of Cable Opening
Metal tubes were installed penetrating through the 125mm thick acoustic panel. All cables
will access the acoustic cover through these tubes. Concrete were applied to seal all the space be-
tween the tubes and cables to prevent any noise leakage and reduce resonance.
Figure 8. Cable openings sealed by concrete
5. Noise Insulation Verification Test
The test followed ISO 10847- In-situ determination of insertion loss of outdoor noise barriers
of all types2. The insertion loss of the acoustic cover was determined by comparison of noise levels
when opening and closing the operable acoustic covers. Noise levels at 2m from the acoustic cover
were measured when loudspeakers system was switched on inside the shaft. Rock drilling noise
was pre-recorded and played back through loudspeakers to simulate the loudest construction activi-
CCTV Monitor Inner Door
Outer Door
Lobby house
Cable openings
21st International Congress on Sound and Vibration (ICSV21), Beijing, China, 13-17 July 2014
ICSV21, Beijing, China, 13-17 July 2014 6
ty inside the shaft. The loudspeaker system was able to provide steady continuous noise sources of
around 128dB(A) sound power level for accurate noise measurement.
For noise measurement without the operable covers, the noise levels at 2m from the operable
covers (red points in Figure 9) was around 105dB(A). When the operable covers were restored to its
covering position, the noise levels at 2m from the acoustic cover were measured again at all speci-
fied locations (green points in Figure 9). Noise measurements at additional points were also con-
ducted to verify any noise leakage at possible noise leakage locations, including lobby house, si-
lencer surfaces and openings.
Figure 9. Measurement locations for insertion loss measurement of the acoustic cover.
After the operable covers were closed, the noise levels at 2m from the acoustic cover were all
within the range from 57 to 61 dB(A). Noise levels at 2m from the operable covers were around
61dB(A). With conservative background noise correction (using the minimum of all measured
background noise level among all measurement locations), the insertion loss of the acoustic cover
was found to be 44dB(A), 4dB(A) better than the minimum requirement of 40dB(A).
Construction Noise Permit which allows overnight rock drilling was then obtained after sub-
mission of the measurement report to the Authority.
21st International Congress on Sound and Vibration (ICSV21), Beijing, China, 13-17 July 2014
ICSV21, Beijing, China, 13-17 July 2014 7
6. Management measures on noise control
6.1 Briefing of Construction Noise Permit (CNP) requirements
In order to ensure that staff at different levels (include engineers, foremen & workers etc.) ful-
ly understand the CNP requirements, briefing was arranged to explain details of the CNP conditions
and how different noise mitigation measures (e.g. close covers and keep at least one door closed for
man access lobby house after 1900 hour) as required by the CNP were properly implemented and
maintained.
6.2 Daily checking of compliance of CNP requirements by qualified engineers
Daily checking is required to ensure the CNP requirements are complied with, and so a check-
list was developed for this purpose. A qualified engineer was assigned to conduct a comprehensive
check on a daily basis on the site conditions against the CNP requirements, and was required to sign
on the checklist for record. In addition, the duly signed checklist and the CNP copy were displayed
at appropriate locations including vehicular site entrances/exits and the man access lobby house.
Figure 10. Display CNP copies & Checklist record.
6.3 Banner showing CNP requirements
For easy reference of the staff, simplified CNP requirements in bilingual language illustrated
with photos were posted on a 3x4m banner displayed on-site.
Figure 11. Banner with CNP requirements
21st International Congress on Sound and Vibration (ICSV21), Beijing, China, 13-17 July 2014
ICSV21, Beijing, China, 13-17 July 2014 8
7. Conclusion
For the tunnel construction of MTR South Island Line, a vertical shaft was excavated with 24-
hour drill and blast excavation in a congested area. A daily opening shaft cover has been installed
on top of the vertical shaft of 750m2 cross-sectional area to be excavated to 83.6m deep. The cover
is functioned both for blasting protection as well as noise insulation. Rock drilling and breaking
noise level is predicted to be 102dB(A) at British Council and 82dB(A) at Shangri-La Hotel without
the cover. In order to minimize environmental impacts, the shaft cover is closed for blasting at any
time and for rock drilling and breaking every night, while being opened for spoil mucking out and
equipment transportation during daytime.
The blast cover provided 44dB(A) noise insulation by on-site insertion loss test to ISO10847
and complied with the blasting requirements. With the operable blast cover, environmental impacts
from the construction site were minimized, and rocking drilling and breaking noise was controlled
within the statutory noise limit. After obtaining the Construction Noise Permit, the blasting cycle
was increased to 3 blasts per week from 2 blasts per week, and the shaft excavation was completed
within 8 months by 24-hour rock drilling and breaking. The success was attributed to appropriate
design of operable acoustic cover, good workmanship and effective site management.
ACKNOWLEDGEMENT
The authors wish to express sincere thanks to Albert Yau of Nishimatsu Construction
Co.(NCC) for his support on acoustic cover design, Hammer Tsui of Marion Acoustics (MYG) for
his support on acoustic cover installation, S.H. Law and W.Y. Yiu of Environmental Protection
Department (EPD) for their advices in addition to statutory noise limits, and Felice Wong of MTR
Corporation Limited (MTR Corporation) for her continuous support throughout the project.
REFERENCES
1 Environmental Protection Department of Hong Kong, “Technical Memorandum on Noise
from Construction Work Other Than Percussive Piling” and “Technical Memorandum on
Noise from Construction Site in Designated Areas”, under Cap 400 - Noise Control Ordi-
nance (NCO). 2 In-situ determination of insertion loss of outdoor noise barriers of all types, ISO 10847,
first edition 1997-08-01. 3 Isaac Ng, Edmond Wong and Wilson Ho, Co-existence of Construction Work and Tran-
quility – A Myth or Reality, Proceedings of the 8th International Conference of Sound
and Vibration , 1509-1516, (2001). 4 Wilson Ho, Isaac Chu, Etienne Baranger, Richard Kwan. Noise control for 24-hour rock
drilling in urban area, Acoustics 2012 Hong Kong Conference and Exhibition. 5 Wilson Ho, Isaac Chu, Richard Kwan, Henry Leung and Alain Hervio, Express Rail Link
HK Section – Construction Noise Control – the 5th Cross-straits Acoustical Meeting,
(2012). 6 Ken K.F. Kwok, Andy Raine, Adman Chu & Wilson Ho. Environmental Paper Award
Submission - GREEN CABLE TUNNEL CONSTRUCTION AT CASTLE PEAK, HKIE
Journal in Year 2009, and awarded the 1st runner up paper.
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