Roadway Noise Barrier Design

4177 words (17 pages) Essay in Engineering

18/05/20 Engineering Reference this

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1.    Introduction

A noise barrier is an exterior structure designed to protect residents living near Highways, Industries, etc. from Noise pollution. Noise barriers are the most effective method of reducing roadway, railway, and industrial noise sources. They help in reducing the noise from the Highways from reaching the people residing near the highways.

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Sound reacts to the noise barrier in three ways. It gets reflected, absorbed and/or transmitted. The taller the wall, the more surface area it will have to effectively do all three of these things. Since no wall is infinitely tall or long, a certain percentage of sound passes over the top of the wall and a certain percentage passes around the wall. If a wall is effectively textured, it can also diffuse sound, causing it to scatter in a variety of directions, which can further aid in the reduction of noise. This is why newer-generation sound walls feature brick-like texturing, corduroy concrete, or other embossed patterns.

2.    Types of Roadway Noise Barriers

Basically, there are two types of Road Noise Barriers.

2.1        Ground Mounted

Fig 1:  Ground mounted roadway noise barrier

Ground-mounted noise barrier systems are constructed into or on top of the ground. There are three types of ground-mounted noise barrier systems. They are as follows: –

a)      Noise Berms- Noise Barriers created from natural materials such as soil, stone, rock, rubble, etc. in a natural unsupported condition are called as noise berms.

b)     Noise Walls: – Noise Wall systems are generally fabricated in the plant, transported to the project site and assembled on site. There are 6 types of Noise Walls. They are as follows;

  • Post and Panel: – This type of system consists of noise barrier panels mounted between foundation-supported posts. The main components of this setup include post and post/foundation attachments, panels and panel-to-post connections
  • Brick and Masonry Block: – This type of system includes barriers made up of fabricated brick or masonry block units. Typically, these types of systems are constructed by laying the brick or masonry block in a conventional fashion using a continuous spread footing as a base.
  • Freestanding walls: – This type of barrier system includes barriers which support themselves.
  • Direct burial panels: – The direct burial panel type is a special panel system which involves burying a portion of one end of the panel directly into the ground with no other means of foundation support.
  • Noise walls used to partially retain earth: – These noise barrier systems utilize the bottom portion of a noise wall system to retain earth from either the residential or roadway side. Such applications have been successfully employed where barriers are constructed near the slope hinge point of a highway on fill and near the top of a highway cut section.
  • Cast-in-place concrete Noise Walls: – These types of barriers are constructed at the project site. The construction process includes excavating for the footing, erecting formwork, setting reinforcement steel, pouring concrete, surface finishing, and curing.
  • Combination of Noise Wall and Noise Berm Systems: – Many noise barrier systems consist of a portion of the barrier height obtained through use of an earth berm with the remainder of the required height achieved by placing a noise wall on top of the berm.

2.2        Structure-Mounted Noise Walls

Fig 2: Structure mounted roadway noise barriers

These types of Noise walls are constructed on structures such as bridges or over retaining walls.

a)      Types of Noise barriers on bridges

  • Post and Panel Noise Barriers

    • On top of parapet
    • Inserted into parapet
    • On outside face of parapet

      • Mechanical anchoring system
      • Chemical anchoring systems
      • Bolt through system
      • Cast-in-place bolts
  • “Post-less” Panels

    • On top of parapet
    • On outside face of parapet
  • Masonry Block Noise Barriers
  • Cast-in-place integral with parapet wall
  • On parallel supporting structure adjacent to parapet

b)     Types of noise barriers on retaining walls

  • Combination cast-in-place retaining wall and noise barrier wall
  • Noise wall behind cast-in-place retaining wall
  • Noise wall on or behind retained earth system type retaining wall
  • Noise barrier walls in combination with or behind pre-manufactured retaining wall

3.    Selection of materials

One of the key features in all structures is the material ultimately chosen. Despite the above categorization, the materials could largely be categorized as reflective and absorptive. In general, these materials are used for construction of the noise barrier. They are as follows;

3.1        Concrete

 Concrete is used in various ways in the construction of noise barriers. Precast planks slotted into H shaped uprights provide a rapid means of construction and can be easily repaired. One form of proprietary concrete noise barrier is constructed from linked precast panels set at varying angles so as to obviate the need for separate post supports. Concrete noise barriers benefit from low maintenance, but prefabricated noise barriers are relatively expensive. Specially designed surface features can be beneficially employed to reflect sound at a desired angle, away from noise-sensitive receivers. On a highway contract involving other concrete structures, it may be economical to use in-situ concrete to construct noise barriers. Concrete noise barriers are usually sufficiently good to withstand vehicle impact damage, but a corrugated beam barrier may be needed to prevent excessive damage to vehicles if the surface finish is heavily textured. Alternatively, concrete barriers could be used to form the lower portion of a noise barrier.

3.2        Metal

 Metal noise barriers can be painted or coated in a wide range of colors. Steel is commonly used for supports. Sheet metal can be formed into lightweight hollow sections, which may contain fiberboard or mineral wool absorbent materials. A number of profiled barrier systems, comprising horizontal panels spanning between galvanized steel posts, are commercially available. The metal sheeting on one side may be perforated to allow noise to interact with absorbent material within, and the corrugated profile provides structural rigidity. Aluminum is often used in noise barrier systems because of its high strength to weight ratio; large panels may be easily erected with fewer supports.

3.3        Transparent Materials

Transparent materials are useful because they allow light to pass through the barrier so that the place behind the barrier does not get dark from the shadow of the Barrier. At the top of a noise barrier, using transparent materials will reduce the visual impact of tall noise barriers and tinted material may enhance the appearance. “Windows” (i.e. incorporation of transparent panels at eye level of the noise barrier) may allow road users to orientate themselves by providing views of the surrounding area. Transparent materials are noise reflecting but they are restricted in places where there would be a lot of reverberation. Transparent panels may need to be protected from impact by errant vehicles. It has some disadvantages as well. Some transparent materials become semi-opaque due to deterioration. These materials also need more maintenance as they are more fragile.

3.4        Wood

Most Wood Noise barriers are made of pressure preservative treated lumber, plywood, and glue-laminated products. Many different species of wood have potential to be used in a noise barrier. Some wood Barriers can easily be dismantled if necessary.

3.5        Plastics

There are several types of plastic materials available for use as a barrier material, including Polyethylene, PVC, and fiberglass. The most useful feature of using plastics in Noise barriers is their versatility. They can be produced and used anywhere. They are also lightweight which improves handling.

3.6        Composites

Composite Noise Barriers are made up of more than one material. For example, plywood with a fiberglass skin, or wood mixed with concrete and then layered onto concrete, etc. By using more than one material for construction of noise barriers, it improves the durability, performance, and safety.

4.    Design Optimization

4.1        Noise Prediction

The relation between the sound pressure level and sound power level is given by;

Lp= Lw20logr11+DIϴ

Lp=Receiver noise level

Lw=Noise power level

DIϴ=Directivity index

4.1.1        Noise prediction model

Prediction model can be divided of two parts. First is the description of the sound and the second is the entire propagation model.

Lp= Lw

+ΣΔpropagation factors

Lw

=

Lr0

+l0

logr0

+3

ΣΔpropagation factors = Summation of total of all the attenuation and corrections due to propagation

r0=Distance from the sound source

 

Lr0= Reference point sound pressure level of

4.1.1.1       Noise resource

In general, the equivalent height is given in term of vehicle type, the mean height of light vehicle is 0.35m that of the medium vehicle is 0.85m, the heavy vehicle’s is 1.5m, and the overall equivalent height is lm. So, the location of source is described as being at the center line of the road at a height of lm in the road noise model, which is given in Figure 3.

Fig 3: Source Position

4.1.1.2       Noise propagation

The general set-up of a propagation model with the ΣΔpropagation factors is composed with the following parts: Δgeo is the attenuation as a result of the geometrical spreading, Δair is the attenuation as a result of air absorption, Δground is attenuation due to the ground absorption, Δbarrier is free-field-diffraction attenuation of a barrier, and Δrefection is the contribution of ground refection.

4.1.2        Noise evaluation

4.1.2.1       Sound pressure level

Per octave frequency of the source level and propagation level can be calculated by the below formula:

Lp= Lr0

+ΣΔpropagation factors +3

4.1.2.2       Equivalent A-weights sound level

Noise emission levels of the receiver are evaluated with the equivalent A-weighted sound level, and is calculated as:

Leqi

=10

logNi/Tvi

+

Lp

-16

Leq

= lO

log100.1Leqi+100.1LB

T=

Time measured in hour

Leqi=

Equivalent A-weighted sound level of vehicles type i at receiver

Leq = Overall equivalent A-weighted sound level of the traffic flow

Lb = Sound level of background.

Ni = Number of vehicles type i passing per hour.

4.2        Optimization design

4.2.1        Calculation of Insertion Loss (IL)

4.2.1.1       Insertion loss of infinite noise barrier

The optimization is based on the prediction model and the calculation of insertion loss (IL) of noise barrier. The overall IL is combined by L

δ = A + B + – S

Where ΔLi is Insertion loss of center-frequency of each octave band of spectrum range from 63 HZ to 8000 HZ, fi is the center-frequency of i octave band, 6 is path difference and showed in Fig 2.and Fig 3, and c is sound velocity in m/s. Owing to the large calculation amount, equivalent frequency is applied in current barrier design methods. In fact, equivalent frequency is not adaptable to calculate IL in optimization process, and the accuracy of calculation is low. IL is calculated strictly in term of each third-octave band of representative spectra of noise source in this paper.

The sound pressure level of receiver, Lp, is calculated by using equation, as

Lp=10lgΣ100.1(LPiΔLi)

LPi is the sound pressure level of reference point at fi without noise barrier and is calculated by noise propagation.

Fig 4: Cross-section of road noise barrier

Fig 5: Cross-section of elevated urban expressway noise barrier

The overall insertion loss, ΔL is calculated by using equation as;

ΔL= LPo Lp

4.2.1.2       Correction of finite noise barrier

Fig 6: Angel of sight

In the real world scenario, the roadway noise barriers have only finite lengths. The noise reduction is weakened by the diffraction effect from the two ends of the roadway noise barrier. Insertion loss of infinite noise barrier is corrected by calculated ΔL and ά/β.

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4.2.2        Optimization Model

4.2.2.1       Design of variables

The vertical distance between noise barrier and noise source and the barrier height could be considered as the most common design variables for a roadway noise barrier. The most commonly used approaches for designing of roadway noise barriers is by taking the distance and the height into account, and consider the length simply or make little account of the length. In the reality, the length plays an important role in noise reduction and cost, which has been proved through calculation and test. The practical noise reduction will be lower than theoretic noise reduction, if the height of the roadway noise barrier is too short to prevent a diffraction effect at the edges of the barrier.

So, the design variables chosen to be varied in this report are the height, the distance and the distance as illustrated the above three figures.

4.2.2.2       Objective functions

The objective functions’ focus is on the cost effectiveness of the roadway noise barrier. The cost of a roadway noise barrier is determined mainly by the surface area and the length of the barrier. The past statistics have revealed that the material cost is 45% of the total cost, construction cost relevant to length is 35% of the total cost [12], and the ratio is 1:0.8.

It is aimed to minimize the overall cost of the roadway noise barrier under these functions. Following are the functions formulated towards achieving the optimized cost.

FX=x2+h2×x+0.8x

FX=x2×x3+0.8+h2×x+(3.7x1)×x

x=x3+L

x1 = Distance from source to the barrier

x2 = Height of the noise barrier

x3 = Length of the noise barrier

h2 = Height the noise source is placed

L= Length of the polluted area

4.2.2.3       Constraints

The major constraints regarding a roadway noise barrier are design constraints and acoustic constraints. The design variables must satisfy the required engineering design standards for noise barrier, and the ranges of design variables are restricted by many factors, for example, type of noise barrier, carrying capacity of bridge, and local circumstance. Acoustic constraints involve the maximum sound level, the equivalent A-weighted sound level of day-time (07:00-20:00 h) and night-time (20:00-7:00h). Leqd , Leqn and Lmax (Leqn +15) should satisfy the request of the current noise legislation and the limit of noise emission levels.

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