Sheet pile tuned mass damper for construction noise control Wilson HO 1 , Wylog WONG 2 , Eric CHU 3 Acoustics Innovation Limited Unit 106, Block A, Shatin Industrial Centre, Sha Tin, N.T., Hong Kong Aldous LO 4 , Allen WONG 5 Build King Civil Engineering Limited 6/F., Tower B, Manulife Financial Centre, 223 Wai Yip Street, Kwun Tong, Kowloon, Hong Kong

ABSTRACT Noise impact from sheet piling by vibratory hammer is one of the major concerns during the base- ments construction. It is also a challenge to conventional mitigation methods such as noise barrier or enclosure due to the size of the sheet piling wall. The results from acoustic imaging by acoustics camera shows that the major noise source of sheet piling event is the radiated noise from a row of sheet piles vibration with dominant frequencies were ranged from 630Hz to 2000Hz. Vibration fre- quency response of sheet pile is also measured with handheld force hammer. A tailor made Sheet Pile Tuned Mass Damper (Sheet Pile TMD) is invented to dissipate vibration energy of sheet pile by tuned mass damping mechanism hence the noise raising from the sheet piling work would be miti- gated. The on-site noise and vibration reduction performance of total 6 sets of 5m long Sheet Pile TMD, installed on 12m high x 4m wide sheet pile wall driven by vibratory hammer, were determined. The Sheet Pile TMDs was tuned to absorb vibration energy from 630 Hz to 1250 Hz by adjusting the stiffness of the resilient layer and the size of the oscillation masses. From the on-site trial results, the Sheet Pile TMDs achieved ~9-14dB vibration reduction at the 1 st , 2 nd and 3 rd piles next to the driven pile and ~7dB noise reduction at distance 7m and 22m from pile. 1. INTRODUCTION

Sheet piling is a conventional technique widely used in infrastructure projects due to its incompa- rable advantages. It is a proven technique which can quickly can be installed in all ground conditions . Other than the flexibility, sheet pile wall is more cost effective and environmental friendly compared to D wall and pipe pile wall, in most of the circumstances the sheet pile wall could be extracted for reuse after completion, while D wall and pipe pile wall cannot. Also, the use of concrete and cement grout are not necessary, which could minimize the use of material. In the construction of West Sewage Pump Station (WSPS) of the Tung Chung New Town Extension – Major Infrastructure Works in Tung Chung East, type VL sheet pile with ICE 815 were used, the vibration hammer could produce a maximum centrifugal force of 1250 kN. Inevitable, there is a common concern that the noise and vibration accompany with the sheet piling work create nuisance to the nearby resident. Recently,

1 who@aihk.hk 2 wylog.wong@aihk.hk 3 eric.chu@aihk.hk 4 aldouslo@buildking.hk 5 allen.wong@buildking.hk

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Acoustics Innovation Limited developed a patented sheet pile tuned mass damper to mitigate the vibration level of sheet pile hence the re-radiated noise from sheet pile wall would be reduced.

The sheet pile tuned mass damper has been designed and implemented in Build King Civil Engi- neering Limited construction site of Tung Chung New Town Extension (CEDD Contract no: NL/2020/03 - Tung Chung New Town Extension – Major Infrastructure Works in Tung Chung East) and achieved ~7dB noise reduction. This paper presents the design tuned mass damper for sheet pile and in-situ measurement result.

2. SHEET PILE TUNED MASS DAMPER MECHANISM

The design of Sheet Pile TMD and installation method is illustrated in Figure 1 - Figure 3 . Each damper comprises of multiple oscillation masses made by stainless steel block sandwiched between resilient layers and magnet. Most of the vibration energy of the sheet pile is transferred to the oscil- lating masses of Sheet Pile TMD and dissipated by hysteresis in the resilient layers. The natural frequencies of the oscillating masses can be tuned independently to provide effective damping. Tuned mass damping mechanism is effective for sheet pile vibration in the frequency range of 630Hz to 1250Hz.

Magnet

Oscillation

mass

Resilient

Layer

Figure 1 : Configuration of Sheet Pile TMD Figure 2 : 5m long Sheet Pile TMD

Figure 3 : Sheet Pile TMD installed on sheet piles

The Sheet Pile tuned mass damper incorporates three key features to enhance its performance,

- The oscillation mass is oscillating along the shear directions of the resilient layers. It pro- vides clear oscillation modes for vibration absorption at target frequencies ( Figure 1 ) - Multiple masses with different target frequencies from 630Hz to 1250Hz are installed on a 5m long aluminum tube ( Figure 2 ) - Lightweight design with total weight <20kg and magnetic attachment on sheet pile allows easily transport, install and dismount without any machine ( Figure 3 )

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3. NOISE SOURCE IDENTIFICATION

Characteristics of sheet pile noise was investigated by the use of acoustic imaging, analytic esti- mation, Finite Element Analysis (FEA) and on-site hammer impact test. It is founded that re-radiated noise at ~1000Hz from sheet pile vibration is the major noise source during piling work.

3.1. Acoustic imaging

Acoustic imaging captured from acoustics camera located at 22m from sheet piles without and with the implementation of Sheet Pile TMD are shown in Figure 4 and Figure 5 . The major noise source is an elliptical shape which cover and match with the shape of sheet piles. The noise sources remain relatively stable at the row of sheet piles instead of following the movement of vibratory hammer. Therefore, it is expected that the major noise source is the radiated noise from the row of sheet piles vibration. By comparing Figure 4 and Figure 5 , the noise reduction at the centre point of noise source is ~7dB(A).

Figure 4 : Acoustic imaging of sheet piling without Sheet Pile TMD (Duration ~2mins)

Figure 5 : Acoustic imaging of sheet piling with Sheet Pile TMD (Duration ~2mins)

3.2. Vibration, Noise Prediction from FEA Simulation

The sheet pile model is FSP-VL, which has a dimension of 500mm (width) x 200mm (length) x 12m (Height) and a cross section thickness of 24mm. A cross section of the sheet pile is shown in Figure 6, the flange and the web are as labelled.

Vibration of the web will be largely excited when they vibrate in and out as a membrane on the steel plate surface, or as a 12m long bending beam. For the membrane vibration, the wavelength 𝜆 on a metal plate is given by Equation 1

(1)

1 4

𝜆 = 12𝜌 1 −𝜈 2

2𝜋

2𝜋𝑓 1/2

𝐸 ℎ 2

, where 𝜌 is the density, 𝜈 is the poisson ratio, 𝐸 is the Young’s modulus, ℎ is the thickness and 𝑓 is the frequency. Putting the values of standard industrial steel ( 𝜌= 7870 𝑘𝑔/𝑚 3 , 𝜈= 0.29, 𝐸= 210 𝐺𝑝𝑎 ) and 𝜆 to be 2 times the width of the web, we expect the web resonance vibration at ~1000Hz. The bending beam mode of the sheet pile is expected at low frequency <200 Hz. Among the 2 modes, the web resonance mode is of higher concern due to its high re-radiation efficiency.

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A FEA modal of a single sheet pile is setup in Strand7 ® for modal and frequency response anal- yses. The web resonance vibration of ~1000Hz and the bending beam vibration of ~150Hz are iden- tified, and their frequencies are found consistent with the analytic estimation. The typical modal shapes are as shown in Figure 7 .

The structure vibration will be converted into noise efficiently when the vibration frequency ex- ceeds the critical frequency. Physically, it happens when the wavelength of air ( 𝜆 𝑎𝑖𝑟 =

343

𝑓 𝑚 be-

comes smaller than the wavelength 𝜆 in sheet pile. The critical frequency is given by Equation 2

(2)

2

𝑓 𝑐𝑟𝑖𝑡 = 𝑐 0

2𝜋ℎ 12𝜌 1 −𝜈 2 /𝐸 1/2

, where 𝑐 0 is the speed of sound in air. The critical frequency for a single sheet pile is calculated to be 500 Hz, thus high noise level is expected at frequency higher than 500Hz during operation, and the noise level is expected to reach a peak at ~1000Hz due to web resonances.

The resonance frequency of the web resonance is about 2 times of the critical frequency, while the frequency of the bending beam vibration is significantly lower than the critical frequency. Therefore, our dampers are designed to cover frequencies from 630 to 1250Hz, the regime where radiation effi- ciency is high and web mode resonances is severe.

Figure 7 Modal shape of a low frequency (145Hz) beam bending mode (left), and a high frequency (1040Hz) web resonance mode (right).

Figure 6 Cross section of one sheet pile.

3.3. Vibration frequency response

Vibration frequency response of sheet pile was measured by the excitation of handheld force ham- mer as shown in Figure 8 . The measurement result shows that sheet pile dominant vibration reso- nance frequency is 1000Hz, which has a sub harmonic vibration peak at 2000Hz. This dominant peak at 1000Hz is due to the web resonance from FEA analyses ( Figure 7 ). The second peak at lower frequency corresponds to the beam bending mode at lower frequency of f~150Hz. The handheld hammer excitation agrees with the FEA analyses results.

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Figure 8 Vibration frequency response of sheet pile.

4. ON-SITE NOISE&VIBRATION REDUCTION PERFORMANCE

A row of sheet piles (as shown in Figure 3 ) was tested before and after installation of Sheet Pile TMD. A sheet pile was driven by vibratory hammer from 12m high above ground level to 6m above ground level. The duration of each sheet piling event driven by vibratory hammer is ~2minutes. The accelerometers were located ~2m above ground at 1 st pile, 2 nd pile and 3 rd pile next to the driven pile to measures the vibration level. The microphones were located at 7m and 22m from the sheet pile and 1.2m above the ground to measures the noise level.

4.1. Vibration Reduction Measurement

Vibration response of the sheet piles during sheet piling event driven by vibratory hammer without mitigation measures and with tuned mass damper implementation were measured as shown in Figure 9 . The vibration level peak at 1000Hz is due to web resonance as discussed in Section 3.2 .

150

140

Vibration level, L eq,event ,dB/1n m/s

130

120

Without mitigation - 1st pile next to driven pile Without mitigation - 2nd pile next to driven pile Without mitigation - 3rd pile next to driven pile With damper - 1st pile next to driven pile With damper - 2nd pile next to driven pile With damper - 3rd pile next to driven pile

110

100

125 250 500 1000 2000 4000

Figure 9 Measured vibration spectrum at 1 st , 2 nd and 3 rd pile next to driven pile during sheet piling event driven by vibratory hammer (duration: ~2minutes for each piling event).

Frequency, Hz

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The designed sheet pile TMDs target at frequencies range from 630Hz to 1250Hz, where the vi- bration dominated by the web resonance. The results show that the sheet pile vibration ranged from 630Hz to 1250Hz was reduced by 9-14dB after implementation of Sheet Pile TMD, see Figure 9 .

4.2. Noise Reduction Measurement

The time history of the sheet piling events without mitigation measures and with implementation of the Sheet Pile TMD is presented in Figure 10 . The Sheet Pile TMD noise reduction performance of ~7dB(A) averaged noise level L Aeq,event and ~9dB(A) maximum noise level L A,fast,max were meas- ured as shown in Table 1 . The dominant frequency of the noise is at ~1000Hz due to the web reso- nance and high radiation efficiency as discussed in Section 3.2. The radiated noise from the sheet piles, ranged from 630Hz to 2000Hz, is reduced by 4-8dB after implementation of Sheet Pile TMD as shown in Figure 11 . There is no significant difference for the low frequency noise at <200Hz, by the implementation of Sheet Pile TMD.

Table 1 : Ave ra ged Noise Level of Sheet Piling During Piling Event (Duration ~2mins)

Averaged noise level

Maximum noise level

L Aeq, event , dB(A)

L A,fast, max , dB(A) Microp ho ne Position 7m from pile 22m from pile 7m from pile 22m from pile Withou t mitigation 110.4 104.2 114.6 108.7 With Tune d Mass Damper 103.5 97.0 106.1 99.4 Noise Reduction 6.9 7.2 8.5 9.3

Piling Noise with implementation

Piling Noise without

of Sheet Pile TMD

mitigation measures

115

7m from sheet pile

110

22m from sheet pile

105

100

L eq,1s Noise Level, dB(A)

95

90

85

80

75

70

65

60

15:25:00

15:27:00

15:29:00

15:31:00

15:33:00

15:35:00

15:37:00

15:39:00

15:41:00

15:43:00

15:45:00

15:47:00

15:49:00

15:51:00

15:53:00

15:55:00

Figure 10 Measured Time History at 7m and 22m from the Sheet Pile

Time, HH:MM:SS

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120

Without Mitigation - 7m from pile Without Mitigation - 22m from pile With Tuned Mass Damper - 7m from pile With Tuned Mass Damper - 22m from pile

110

Noise Level, L eq,event , dB(A)

100

90

80

70

60

31.5 63 125 250 500 1000 2000 4000 8000 16000 dB(A)

Frequency, Hz

Figure 11 Measured noise spectrum at 7m and 22m from the Sheet Pile

5. CONCLUSIONS

Piling Tuned mass damper was implemented to reduce re-radiated noise from sheet piling driven by vibratory hammer.

The results from acoustic imaging by acoustics camera shows that the major noise source of sheet piling event is the radiated noise from the row of sheet piles vibration. Vibration frequency response of sheet pile was also measured with handheld force hammer. 5m long Sheet Pile TMD set was designed to dissipate vibration energy of sheet pile by tuned mass damping mechanism.

With the implementation of 6 sets Sheet Pile TMD, sheet pile vibration reduction of 9-14dB from 630Hz to 1250Hz were measured at the 1 st , 2 nd and 3 rd piles next to the driven pile. The noise reduc- tion of ~7dB(A) averaged noise level L Aeq,event and ~9dB(A) maximum noise level L A,fast,max were measured at 7m and 22m from the sheet pile. 6. ACKNOWLEDGEMENTS

We gratefully acknowledge Build King Civil Engineering Limited, AECOM Asia Company Lim- ited and Civil Engineering and Development Department to provide the testing site and the mechan- ical equipment for testing the sheet pile tuned mass damper.

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7. REFERENCES

1. Hanus, Vincent & Zürbes, Arno & Maas, Silvia & Waldmann, Danièle. Acoustics During the

Vibratory Pile Driving of Sheet Piles: Measurement Conditions and Key Parameters of the Noise Generation. Acta Acustica united with Acustica . Vol 96 . 1104-1114 (2010). 2. Wilson HO, Wylog WONG, Max YIU, Ting CAI, Tuned Mass Damper for Breaker and Piling

Noise Control, INTER-NOISE and NOISE-CON Congress and Conference Proceedings , Inter- Noise17 , pages 5073-5079 (2017) 3. H.K. Environment Protection Department, A practical Guide for the Reduction of Noise from

Construction Works , 1989. 4. H.K. Environment Protection Department, Technical Memorandum on Noise from Percussive

Piling , 1997. 5. F.G Leppington, E.G. Broadbent, K.H. Heron, The acoustic radiation efficiency of rectangular

panels, Proceedings of the Royal Society of London A 382 (1982) 245–271

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