The depth of the tunnel varies along this route from 10 metres to 60 metres; it is closer to the surface near the tunnel portals (e.g. Kirkwood St, Wakehurst Parkway, Seaforth shops) or at low points topographically.
The maps below from the EIS show the properties impacted by tunnelling noise and vibration. Those impacted most severely are indicated in purple (greater than 50dB(A)), moving down to blue and white for those less affected. (Beaches Link EIS, Appendix G Noise and Vibration – Part 2, Annexure I, Pages 137-139, Click here to download PDF chapter)
The EIS states they have assessed that 853 properties in Seaforth / Clontarf will experience noise from tunnelling above 35dB(A) (the night-time limit) of which 373 are also over 45dB(A)(3).
They have also assessed that 325 properties in Seaforth / Clontarf will experience vibration above the criteria for human comfort(4).
These exceedances will not occur during the whole construction period, but are predicted when tunnelling is beneath these properties, or when certain loud equipment is used.
The government has told residents that the impact of tunnelling under their houses will take roughly 2 weeks to pass, and will sound and feel similar to a refrigerator running. Whilst this may be true for some residents less affected, others will experience noise and vibration above exceedances set for the project and may need mitigation strategies such as providing overnight accommodation as homes are not conducive to sleep.
Vibration that is just detectable to humans typically does not cause damage to property. However, when levels become too high, particularly at shallow depths of tunnelling both cosmetic and structural damage can occur.
Residents on similar tunnelling projects in Sydney have experienced this. At a site in Carrington St, North Strathfield, many residents suffered severe cracking in their homes from tunnelling 40 metres below them, and had difficulties ensuring contractors and the government accepted responsibility for extensive repairs(1).
Additionally, last-minute changes to the depth of tunnelling well after the final design were announced for residents of Leichhardt and nearby suburbs; depths understood to be 40 metres below homes were changed up to 20 metres shallower(2).