Acoustics and tunnel boring – what are the challenges?

This article was published in Bygg & teknik nr 3/23 (website). Top photo: Tomas Ärlemo/Svenska kraftnät

Tunnel boring may cause disturbances for residents and businesses. However, the acoustic impact of tunnel boring involves other challenges besides measuring and monitoring noise levels.

About the tunnel boring machine:

The tunnel boring machine is called Elektra and is referred to as TBM, which stands for “Tunnel boring Machine”. Elektra is a smaller model than the machine used for drilling traffic tunnels, but it still reaches 240 meters. The machine is constructed like a train set with 20 carriages and weighs 1000 tons. The carriages contain generators, hydraulics, workshops, cement mixers, and a rescue chamber that can be used in case of an accident, among other things. At the front, there is the drilling head. It measures five meters in diameter and weighs 50 tons. On average, the machine advances 60-70 meters into the rock per week and about 100 meters per week under favorable conditions. During operations, Elektra consumes three megawatts, which is equivalent to the electricity used by a small community.

The article is based on experiences from the Anneberg-Skanstull Tunnel project, owned by the public authority Svenska kraftnät. The project aims to build a cable tunnel to reinforce the electrical network in the Stockholm region. The tunnel is 13,4 km long, from Mörby to Hammarby Sjöstad in Stockholm, and is being constructed using a tunnel boring machine. The tunneling work began in early 2020, and currently, half of the distance has been drilled. The tunnel is drilled between 50 and 100 meters below ground level. The geology along the route can be described as “good and hard rock” with granite, gneiss, and metamorphic rock with traces of weak zones.

How are noise requirements and working hours managed during tunnel boring?

Before an infrastructure project such as this begins construction, extensive work is required for permit and environmental assessment. Once the environmental permit for the project has legal force, the conditions for managing environmental impact and tunneling are set.

• Environmental conditions include:
• Conditions for groundwater impact and seepage of groundwater into the tunnel facility
• Working hours for noisy or structural noise-generated work
• Noise requirements – reference values for noise and structural noise
• Under what conditions temporary accommodation can be offered to residents if noise requirements are exceeded
• The roles of supervisory authorities in monitoring conditions in the environmental permit

In the environmental permit for the Anneberg-Skanstull tunnel project, indoor noise requirements and tunneling working hours are specified in general condition No. 9:

Airborne noise and structural noise during TBM operation and the construction of connecting tunnels between the cable tunnel and ventilation shafts must be limited so that the equivalent indoor sound level does not exceed the reference values.

45 dB(A) in residences and workplaces for quiet activities and 40 dB(A) in schools on weekdays from Monday to Friday, 07:00-19:00

40 dB(A) in residences on weekdays from Monday to Friday, 19:00-22:00

40 dB(A) in residences on Saturdays, Sundays, and holidays from 09:00-18:00, except for weekends with two or more consecutive holidays, during which 35 dB(A) must not be exceeded

35 dB(A) in residences on Saturdays, Sundays, and holidays from 19:00-22:00

30 dB(A) in residences every day from 22:00-07:00

Activities using exceedances of reference values are only allowed on weekdays from Monday to Friday, 07:00-22:00, and on Saturdays, Sundays, and holidays from 09:00-18:00. Under special circumstances, other deviations may occur only after approval from the supervisory authority. Noise-producing activities that do not exceed the reference values are allowed to occur at any time every day.

In summary, noise requirements according to general condition 9 imply that structural noise from tunneling may be carried out on non-holiday weekends between 07:00 and 22:00 and on holidays between 09:00 and 18:00, with exceptions for long weekends. The noise requirements, or reference values, may also be exceeded during these times. However, during all days of the week and nighttime, work can only be carried out if equivalent noise levels are lower than 30 dBA in residences. This requirement corresponds to NFS2004:15, Naturvårdsverkets allmänna råd om buller från byggplatser (Naturvårdsverkets general advice on noise from construction sites). Therefore, more significant consideration is given to residents’ nighttime sleep, but higher noise disturbances may be allowed during other times. If the reference value for residences during nighttime is to be exceeded, approval must be obtained from the supervisory authority. In this case, the supervisory authority is the municipality through which the tunnel boring machine passes.

What factors affect the spread of noise and vibrations?

Tunnel boring can cause noise in buildings from structure-borne vibrations. This noise is referred to as structural noise. Louder structural noise occurs in buildings founded on rock since rock has low material damping. For buildings founded on soils, especially clays with high material damping, structural noise is lower and sometimes not even perceivable by humans. This is because soils act as acoustic filters that reduce noise levels. The parameters that affect structural noise from tunnel boring are:

• Distance from the tunnel boring to the building
• Geology, type of rock or soil, and thickness of soil layers
• Building foundations
• Number of floors in a building
• Sound absorbers in a space
• To some extent, the way the tunnel boring machine passes a building (towards or away from the building)

Vibration impact is primarily determined by:

• Distance from the tunnel boring to the building
• Geology, type of rock or soil, and thickness of soil layers
• Building foundations

Thus, many parameters affect the transmission of structural sound and vibrations. These parameters may contain a significant uncertainty, especially regarding a building’s foundation.

How are structural noise and vibrations monitored?

There are two methods to record structural noise:

• Unattended sound measurement – also called stationary sound measurement
• Monitored sound measurement – also called handheld sound measurement

The advantage of unattended sound measurement is the possibility of long-term measurement with limited effort. The disadvantage is that background noise can affect the reliability of recorded sound levels. Great care must be taken to select spaces with low background noise.

The advantage of monitored sound measurement is that background noise can be easily excluded from recording, and the measurement data is of high quality. The disadvantage is that the measurement period is limited because a technician is on-site or the structural noise-producing work is not performed when the technician plans to conduct the measurement.

 

 

 

 

 

 

 

 

 

 

 

 

Vibration measurement is often conducted by connecting a triaxial vibration sensor to the same data logger that records stationary structural noise, see image 1. Therefore, structural noise and vibration measurements are often performed in the same space.

 

 

 

 

 

 

 

 

 

 

 

 

How is a prediction of structural sound performed?

Structural noise forecast relationships are based on field measurements during tunneling. The conjunction below concerns equivalent structural noise levels for rock-founded buildings and unfurnished residential spaces on the first floor (usually the basement):

Structural noise level, LAeq (dBA)= −20,36 x ln(distance from the tunnel boring to the buidling, meters) +142,52

The rules of thumb for structural noise from tunnel boring are:

• Structural noise decreases by approximately 3 dBA per floor (range 2-5 dBA)
• Structural noise is approximately 2 dBA lower behind the tunnel boring machine compared to in front of it
• Structural noise from grouting drilling is about 15 dBA (range ±5 dBA) lower compared to tunnel boring – depending on the distance to the tunnel boring machine and ground type
• The distance to the tunnel boring machine is at least 200-250 meters to be below 30 dBA on the lower floors and in buildings on rock foundation
• The distance to grouting drilling, drilling to seal rock against seeping groundwater, is at least 100 meters to be below 30 dBA on the lower floors and in buildings on rock foundation
• Tunnel boring is performed “jerky during a work day”, but over longer periods, the progress is higher than for blasted tunnels. Progress, or the speed of rock tunnel production, is at least 3-5 times higher in drilled tunnels compared to blasted tunnels in urban areas. This means the speed is 50-100 meters per week for tunnel boring, compared to 15-20 meters per week for blasted tunnels.

 

 

 

 

 

 

 

 

 

 

 

 

 

How high do the vibrations get?

The environmental permit does not specify explicit conditions related to vibration requirements during tunnel boring. This is because vibrations from tunnel boring are significantly lower than those from blasted rock tunnels. The peak values are lower than circa 0,2-0,3 mm/s if the distance exceeds 75-100 meters. This approaches the human sensitivity threshold but is far below vibration levels that can damage buildings.

The purpose of vibration measurement during tunnel boring is linked to activities and businesses with extremely vibration-sensitive equipment. In practice, research environments with very low vibration requirements are affected.

How are noise and vibration analyses conducted?

Measurement data from unattended noise measurement is automatically reported to a web portal for evaluation or in-depth analysis. The project’s acousticians and measurement technicians continuously evaluate the measurements. The aim is to follow up on the environmental permit’s reference values, assess how the surroundings are affected, or as a basis for production-controlling measures for tunneling.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

What is required to create an understanding of disturbances?

Large infrastructure projects require ongoing communication with the surrounding communities. Successful communication is based on proactive efforts where the community is educated and prepared for upcoming impacts or disturbances before the project begins. These communications are vital to convey an understanding of the purpose and benefits of the project and create acceptance or understanding of disturbances, especially from structural noise. In the Anneberg-Skanstull tunnel project, information and communication efforts are organized through:

• A communication coordinator who collaborates closely with functions in acoustics and hydrogeology and issues regarding permits, project planning, and production technical matters
• Personal communication and quick feedback on questions via phone, e-mail, and meetings with the support of social networks
• Staffed hotline available 24/7 that records questions or reported disturbances
• A mobile exhibition which is staffed during daytime and located near the tunnel boring machine’s position

 

 

 

 

 

 

 

 

 

Reporting acoustic impact to the supervisory authorities includes registered structural noise levels, planned measurement efforts, and how the project handles external communication and disturbance issues. Supervisory meetings occur when needed but typically with intervals between 1-3 months.

Conclusion

Acoustic environmental impact from tunnel boring involves a focus on:

• Noise requirements and working hours
• Measurement and reporting of structural noise levels – sometimes also vibration levels
• Communication and information to businesses, activities, residents, and supervisory authorities

The challenges of tunnel boring are, therefore, more comprehensive than just monitoring noise and vibrations. Successfully managing the acoustic impact of tunnel boring requires well-crafted information and good relationships with the surrounding community. And, not least, adherence to the project’s environmental permit.

About the author

Carl Lind
Tilia Consult AB

Carl Lind is a civil engineer with over 20 years of expertise in both small and large construction projects. He has extensive experience in project planning and construction, having worked on various private and public projects in Sweden. Carl has dedicated many years to education, skills development, and mentoring of technical consultants within the industry.

References:

 Svenska kraftnäts web page svk.se/anneberg-skanstull
 Nacka tingsrätt/MD mål nr M 2772-15 dated 2016-11-30 (refers to Svenska kraftnäts project Anneberg-Skanstull Tunnel)
 Naturvårdsverkets författningssamling NFS 2004:15 – Naturvårdsverkets allmänna råd om buller från byggplatser

 

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