While tunnels may in themselves be used as an effective noise mitigation measure, it is recognised that new noise sources arise as a result of their implementation. This arises in two key areas – the construction noise during the creation of the tunnel and, after completion, the traffic noise emissions once the tunnel is operational. Noise management should, therefore, be a critical part of planning and design, both during construction and to assess future noise levels, once the work is complete.
Noise Laws and Regulations
Noise is deemed a hazard when the affected party is exposed to levels above 85 decibels (dBA) for a continuous period; in lay terms, that’s the equivalent to the noise level of city traffic heard from inside a car.
The most obvious impact of exposure to excessive noise is hearing loss. The risk increases alongside the magnitude, exposure time and the frequency of the noise; with higher frequencies doing more damage. There are several other potential major health impacts including sleep disturbance, cardiovascular disease and stress, cognitive impairment and mental health problems. It can also cause more noticeable direct effects such as tinnitus.
As well as health and safety regulations, many countries have specific noise rules, such as the USA’s Noise Control Act, Canada’s Occupational Exposure Limits and the EU’s Environmental Noise Directive (Directive 2002/49/EC).
Business Costs
Along with the human costs, businesses need to be aware of the likelihood of litigation from workers whose health is affected if the correct steps haven’t been taken to protect them. In addition to the court costs and potential pay outs, there is the management time involved when dealing with issues such as absences and staff cover.
In relation to projects in close proximity to housing, the safety of people within the surrounding neighbourhoods must also be considered.
Accurately identifying emissions during the construction phase
Construction noise and vibrations during tunnelling operations are unavoidable but it can be minimised. Noise maps are a crucial element in tackling nuisance noise. They are used to identify the cause, location and propagation of sound. Planners can then test set-ups and assess the best options to reduce excessive noise and protect workers and residents from its harmful effects. Mapping software can also help to calculate the way that noise will be radiated from the tunnel into the surrounding areas once the works are complete.
By using the software, it is possible to assess the cost of each and every noise reduction initiative, before the project even begins. This means it is possible to work out how much it would cost to raise a noise barrier by an extra metre and how that would affect nearby premises. By testing different elements, such as layouts, speed limits, the height and positioning of barriers and the surfaces they are made from to optimise the noise environment, the software makes trial and error a much more straightforward process.
Before simulation software reached its current level of proficiency and computers were fast enough to do the simulations, noise control engineers had no choice but to measure noise levels and then use their best judgment to control noise.
Today’s noise mapping simulations have distinct advantages over measurements. A measurement yields only one noise level. It is impossible to find how much noise came from which source. Measurements cannot occur in the planning process; the problem must physically be there to measure.
When noise levels can be simulated, it opens up the possibility of introducing theoretical noise reduction measures and evaluating possible improvements.
Simulations allow one to work with ‘what-if scenarios’ and noise maps can be created to depict the different scenarios and show the best solutions to mitigate noise. Solutions might include scheduling the work for a different time of the day, using quieter machines or shielding the noise with berms or other barriers.
Emission Calculations
Noise from railway and road tunnels must be treated in a special way. Unlike regular railway noise where the emission is on the railhead, noise from tunnels is emitted over the entire mouth of the tunnel. The sound inside has many reflections that each absorb a portion of the noise.
Depending on the absorptive characteristic of the tunnel walls, the noise from the tunnel has a different frequency spectrum and a distinct directivity.
Using noise maps assists with an object such as a tunnel opening/mouth to reflect these peculiarities. This will automatically use the emission of the roads or railways crossing the tunnel and predict the noise emission into the surrounding areas.
The software is designed to be user friendly. Noise can be displayed in colored maps making it easy to understand noise sources and propagation. They can also be produced in 3D and animated form so that the problem is truly understood.
Each noise map is unique according to project size, the geography, the objective, but most of all, on what data is available and can be readily acquired to be imported and used for the noise map.
Road and Railway Modules
The road and railway sound map modules consist of two main parts; the emission calculation and the propagation calculation.
The emission calculation is always performed inside the Geo-Database where the vehicle numbers for various vehicle categories, the speed of the vehicles, and the road/rail surfaces are fed into a calculation that results in the emission level. The emission level can be the sound power or a sound pressure level in a given reference distance from the road/rail. Emissions are calculated for every section of the road/rail. Where conditions change in traffic speed, volume or surface type, a new emission value is computed.
The propagation calculation is executed inside the calculation core. Calculations can be performed for single receivers or various types of noise maps (grid noise map, facade noise map, triangulated noise map, horizontal noise map). The results from the calculations can be documented in the spreadsheets and in graphics.
Wall Design
Among the many features offered by sound mapping software, the wall design module in SoundPlan helps engineers to find the optimal structure and placement of a wall. On a pre-determined line along a road/railway line a proposed noise barrier can be pre-calculated to design and optimise the barrier. The module helps to document and visualise the benefits (for instance noise reduction, compliance with target level) and aspects including barrier height, the barrier length, openings in the barrier, the noise receiver height or the source height).
Wall design helps to generate graphic presentations to visualise the interdependence between the acoustical parameters, the cost and the noise reduction and can be used to find the breakpoint between the barrier cost and the achieved noise reduction. The cost-versus-noise-reduction breakpoint changes dramatically when for example the maximum height of a noise barrier is lowered. The maximum height of a barrier may be set by planning constraints or the visual impact.
Within a short period of time the user can compute 10 to 20 barrier scenarios and document how these scenarios compare to each other (barrier length, cost, number of receiver below the target levels, number of receivers above the target, cost versus noise reduction breakpoint, etc.).
Before and After Comparisons
The value of property can increase once new tunnel works are complete. For instance, in Seattle by the waterfront the majority of noise used to come from the Alaskan Way Viaduct, an elevated structure with two levels for car traffic. The viaduct was damaged by an earthquake in 2001 and, due to its vulnerability to potential future earthquakes, has been replaced with the Alaskan Way Viaduct tunnel.
Noise control is needed, not only for residents in the adjacent buildings, but also for the waterfront area itself.
Sound maps show where noise levels have improved in the waterfront area, and by how much, through noise simulations and mapping capabilities. Before, after and comparison noise maps show the improvements at street level. Facade noise maps, which place and calculates receivers along the facades of buildings, can show the improvements for buildings at all the various floor levels.
Historically, some road and rail tunnels have been a source of annoyance due to noise and vibration. In more recent years, these problems have been addressed and techniques to calculate the noise and vibration have been developed. Effective use of sound maps and mitigation measures can help ensure that its success is not drowned out by nuisance noise.
