noise insulation for operable blast cover - … · noise insulation for operable blast cover ......

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: [email protected]

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.

mailto:[email protected]

21st International Congress on Sound and Vibration (ICSV21), Beijing, China, 13-17 July 2014


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



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



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



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



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.



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



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.

noise insulation for operable blast cover - … · noise insulation for operable blast cover ......

Documents