Retaining walls make effective noise barriers

by Ofentse Sefolo
Retaining walls make effective noise barriers

By Karin Johns, freelance journalist and marketing manager at Terraforce

With the rapid densification of urban areas and ever more traffic on the highways, cities are becoming increasingly noisy – not just locally, but globally.
World Health Organisation (WHO) guidelines state that outdoor noise levels between 55 and 60 A-weighted decibels (dBA) can create moderate to serious annoyance for a person. Traffic noise pollution is considered as the greatest pollutant in the industrial world, by the WHO, whose research found that alongside densely travelled roads sound pressure levels were equivalent to between 75 and 80dBA over 24 hours.

Concrete blocks as acoustic barrier
According to British Precast, a trade body for the United Kingdom precast concrete industry, a very effective way to reduce levels of noise alongside a busy section of highway is by placing a barrier in the direct path between the source and receiver that significantly reflects the sound energy. Highway noise barriers can effectively reduce noise levels by five to ten A-weighted decibels (dBA), cutting the loudness of traffic noise by as much as 50%.

However, the pressure waves hitting the top of the barrier will be diffracted downwards and some of this sound will still be received. A small amount of the incident sound may also pass through the barrier, which needs to be sufficiently dense (at least 20kg per m²) and continuous to reduce directly transmitted sound to an insignificant level compared with the sound diffracted at the top of the barrier.

Concrete and masonry are therefore ideal materials for this, as their inherent mass will always meet this requirement.

The staggered retaining wall next to the Oasis Retirement Village almost resembles a natural hill more than a wall.

Height and length of the wall
The key acoustic considerations are the height and length of the required barrier. A noise barrier can achieve a 5dBA noise level reduction when it is tall enough to break the line of sight from the highway to the home or receiver. After it breaks the line of sight, it can achieve approximately 1,5dBA of additional noise level reduction for each metre of barrier height. In addition, its length must extend far beyond the area to be protected to avoid its performance being compromised by noise diffracted around the ends of the barrier.

Positioning of the wall
Barrier location is also a vital decision. Optimum performance can be achieved by maximising the difference between the direct line between the source and the receiver and the diffracted path over the top of the barrier. On level ground it is normally desirable to place barriers as close as possible to the noise source, but where roads and railways are in cuttings or an embankment, placing the barrier further away at the top of the slope is usually the best solution.

The maximum heights that can be achieved with different retaining wall noise barrier structures when exposed to wind pressure of 1,45kN and wind suction of 0,65kN.

Interlocking concrete block retaining walls
Pre-cast interlocking concrete blocks are ideal for the construction of cost-effective highway noise barriers since they are strong, durable, plant-friendly and therefore easy to maintain.

An example of such a wall is the Stanhope Road barrier in Cape Town, next to high-density residential units, which was constructed in 1999. The noise barrier works by reflecting noise back across Stanhope Road and to minimise an increase in noise on the opposite side of the road, the lower section of the retaining wall was constructed at an angle to reflect the noise upwards to pass over any possible receivers on the other side. A sub-set of this type of barrier includes a range of modifications to the top edge, such as wide flattops and multiple vertical edges to reduce the level of diffraction.

According to Adrian Jongens, who acted as acoustic consultant on the project, the tenth floor of the apartment building experienced a 5dBA reduction and the second floor a 12dBA reduction of noise, which is a substantial reduction in noise levels overall. “Below the tenth floor, daytime levels below 60dBA were recorded, which is well below the local daytime limit and approaching those of world best practice,” he says.

As a result of this drop in noise levels, the high turnover of flats subsided, with the demand for units growing.

A retaining wall that serves as noise barrier at the Flight Training Centre at the Cape Town International Airport.

Earth berms
Barriers can also be formed from earth mounds or berms along the road, or from a combination of earth berms and walls. Earth berms have a very natural appearance and they reduce noise by approximately 3dBA more than vertical walls of the same height. However, earth berms can require a lot of land to construct, especially if they are very tall.

One such example is the barrier that was constructed within the Century City complex next to Ratanga Road in Cape Town. In order to protect residents from excessive traffic noise and a negative visual impact from the double carriage highway, a 4,5m high and 500m long earth berm was created with the surplus site material.

To reduce the footprint of this structure and improve noise absorption capability, a steep retaining block embankment was built on the road-facing side. The retaining wall runs along the full boundary length of the road and instead of a standard single slope running at a 70% incline, it consists of three 1,5m terraces that each runs at its own curvature line, breaking down the facade to accommodate plant pockets at different levels to soften the overall appearance.

This noise barrier had won an award from the Concrete Manufacturers Association in 2006 and was described as “an innovative and ecological application which dampens the noise levels and prevents problems due to drifting sand dunes”.

Different types of retaining walls that can serve as noise barriers.


Noise needs to be top of mind
There is a general need to increase public awareness about legal rights pertaining to acceptable noise exposure levels and the detrimental effect of noise on productivity, learning and overall health, to name a few, as well as for professionals to step up and demand that noise levels should be taken into account when embarking on new projects.

Any new development that might increase road traffic noise in a residential area has to consider methods to reduce noise pollution and environmental noise barriers should become standard features around housing sites, being one of the more straightforward methods of cutting down noise effectively.

Tel: 021 465 1907
Website: www.terraforce.com

Caption main image: A planted retaining wall reduces noise from the busy road next to the Oasis Retirement Village in Century City.

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