What developers can do to assess and mitigate ground-borne noise and vibration impacts.
WHILE TRANSPORTATION systems are the arteries and veins of a city, when it comes to new development, their presence can cause a developer’s heart to skip a beat. One particularly significant challenge relates to ground-borne noise and vibration. Air rights construction invariably places new facilities in close proximity to highways and/or rail lines, both of which produce noise and vibration that may adversely impact building occupants and sensitive equipment. Fortunately, there are ways to evaluate the potential impacts and mitigate them if necessary.
What Is the Concern?
Anyone who has dined in a quiet restaurant near an underground subway line has experienced ground-borne noise. That low-frequency rumble you hear when a train passes by is actually related to the vibration produced by the train; it is not a muted version of the airborne noise in the tunnel. If the vibration is sufficiently severe, you might also feel it. More likely, however, you will “hear” it. The vibrating walls, floor and ceiling of the restaurant radiate sound like giant loudspeakers. Because the vibration and noise are transmitted through the ground, they are referred to as ground-borne vibration and ground-borne noise.
In the context of new development, vibrations arriving from outside are typically referred to as “environmental vibrations.” As one might expect, not all sources of environmental vibrations are created equal. In general terms, the order of concern is as follows: 1) freight rail or commuter rail with diesel locomotives; 2) electric rail, subway or light-rail transit; and 3) highway or street traffic. Vibrations from street traffic are strongly related to road condition; potholes and other discontinuities can produce significant vibrations, but fortunately, in most cities, these are only temporary.
One challenging aspect of environmental vibrations is that the developer usually has no control over them. Transportation agencies may be willing to work with a developer to reduce vibrations, but most often the onus is on the developer to deal with their impacts.
When Should a Developer Be Concerned?
Because vibrations are site specific, there are no hard and fast rules to determine how close is too close. Vibrations are affected by a number of factors. For trains alone, these factors include wheel and rail condition, vehicle speed, track support, soil conditions and the response of the building to the vibration. To offer some rough guidance, a developer should give environmental vibrations serious consideration if a proposed building is within 300 feet of a rail line or 100 feet of a street or highway.
Assessing the Impact
Once a building footprint is established, it is a relatively straightforward exercise to measure the site vibrations. From there, it is possible to calculate how much vibration will enter the building, usually at the foundation, and how it will propagate up through the building structure. (Vibration dissipates as it rises, so a problem on the first floor might not be an issue on the 10th.)
Established techniques also make it possible to predict the amount of radiated noise a building will experience, based on the measured vibrations. As long as one knows how much vibration there is, one can estimate the noise.
Once the exposure is known, the potential impact can be assessed. The impact on people is the most common concern, but the sensitive equipment used in labs or medical facilities may also be an issue. Electron microscopes and MRIs, for example, are very sensitive to vibration. In either case, criteria can help establish whether or not there may be an issue.
It is always a good idea to conduct the assessment very early in the design process. If mitigation is necessary, it can have a significant impact on design and development costs. The cost of an assessment will vary depending on the project, but a typical study will cost between $10,000 and $20,000.
What About Mitigation?
If mitigation is needed, the first step should always be to consider the source of the vibrations. A developer might not be able to do anything about it, but it never hurts to ask. Particularly if air rights are involved and the responsible agency has a vested interest in ensuring that the development project goes forward, the agency may be willing to take steps to decrease the vibrations.
Resilient pads can be installed below a building’s foundation to isolate vibration from transit lines and highways; this is a typical neoprene base isolation pad for a single column.
Otherwise, the developer is left with solutions that can be applied at the building itself. The starting point is usually the building layout. Since vibrations and noise decrease with increased distance from the source, it might be possible to locate building occupants and sensitive equipment in the parts of the building that are farthest away from the vibration source. For example, a designer could locate parking on the lowest, most-affected levels and place retail and office space above, topped by residential units — typically the most sensitive users — on the uppermost floors. The cost of moving these uses around in the design stage is minimal.
The next and most invasive method is vibration isolation. This can be done at several different scales. At the foundation level, it can be done by installing resilient pads below the foundation. Essentially, the building or its columns rest on rubber or similarly resilient pads. The cost, approximately $0.05 per pound of isolated load, is not insignificant. A pad for a 1,000-kip (1 million-pound) column could increase the project cost by $50,000. At the room scale, elements like floated floors and resiliently hung ceilings and walls can be used to mitigate vibration and noise. A “resilient” room incorporating one or more of these features is typically two to three times as expensive to build as a conventional room. Neither of these solutions is trivial, and both introduce complexities into the building design.
Some vibration isolation can be installed after a building is completed, at a relatively high cost. For example, the cost of adding supplemental vibration isolation to an electron microscope could amount to $30,000 or more.
Knowing how much environmental vibration a proposed development project near a highway or transit line will experience is clearly important, both to determine whether a project is feasible and, if it is, to mitigate the expected impacts as early and as cost-effectively as possible. Early analysis gives developers and designers the information they need to make informed decisions about these issues.
Jeffrey A. Zapfe