Councils in London and elsewhere are increasingly asking for detailed assessments of ground-borne noise where developments are located close to railways and other sources of vibration.
Ground or structure-borne noise is noise in the audible frequency range (approximately 20Hz – 16 KHz) than is transmitted as vibration and then reradiated into rooms as noise.
With railways, ground and structure-borne noise tends to be dominant at around 100Hz. This is perceived as a low frequency rumble as trains pass by. Most of us will have experienced this at one time or another when in a building near a railway.
For new residential developments, this rumble has the potential to disturb future occupants. Particularly during the night-time when residents will be most sensitive to noise.
Unfortunately, there are currently no UK or international standards for the assessment of ground-borne noise affecting new residential developments. The most relevant guidance is that given in the Association of Noise Consultants guideline – Measurement and assessment of ground-borne noise and vibration. The ANC guideline summarises current research and guidance from elsewhere.
One of the guidance documents referenced in the ANC guideline was published by the American Public Transit Association (APTA). This suggests criteria for acceptable maximum levels of ground-borne noise affecting various building types, including a criterion of 35 dB LAmax for ground-borne noise affecting residential properties. This criterion is increasingly being adopted (as 35 dB LASmax) by Councils when defining acceptable ground-borne noise levels for new developments.
In our view 35 dB LASmax represents a good standard but it may actually be acceptable for ground-borne noise levels to exceed this limit, depending on other factors such as the frequency of events (e.g. train passes) and the range of maximum noise levels that occur.
There are various treatments that can be adopted to reduce ground-borne noise affecting new developments. However, where significant ground-borne noise levels are present, full structural anti-vibration bearings are likely to be required. These are typically rubber or spring bearings that are used to isolate the building structure near ground level.
Spring bearing on a steel framed building
Elastomeric rubber bearing on steel framed building
Images provided by Mason UK (bearing manufacturer and supplier)
Such treatments carry significant cost, risk, design and program implications. It would not be unusual for the treatment of a medium sized block of apartments next to a railway in London to cost £100-150K in bearing costs alone. There is also the associated costs in design work, added costs to the structure and additional construction work onsite. Bearings also add complexity to the design that increases the risk of construction errors or timetabling issues. It is therefore important to ensure that bearing treatments are only used where strictly necessary.
Given the implications of such treatments, it is always advisable for developers to establish ground-borne noise levels and Council requirements early on where development sites are located near to railways or any other significant sources of vibration. In this way any required treatments can be assessed and properly costed and budgeted for.