4. NOISE BARRIER TYPES

This section describes the differences between the following two basic types of noise barrier systems, as well as special features associated with each:

Ground-Mounted
and
Structure-Mounted

4.1 Ground-Mounted

Ground-mounted noise barrier systems are barriers constructed into or placed on top of the ground. This section will discuss the features of the three basic types of ground-mounted noise barrier systems:

• Noise berms (Section 4.1.1);
• Noise walls (Section 4.1.2); and
• Combination noise berm and noise wall systems (Section 4.1.3).

4.1.1 Noise Berms.
Noise barriers constructed from natural earthen materials such as soil, stone, rock, rubble, etc. in a natural, unsupported condition are termed, noise berms (see Figures 26 and 27). These types of barriers are typically constructed with surplus materials available on the project site or from materials transported from an off-site location. The source and availability of such material are factors which can significantly affect the cost of such systems. Noise berms generally occupy more space than a wall type of barrier. This is mainly due to the sloping sides of the berms which must be gradual enough to maintain stability of the structure. For most berms, side slopes of 2:1 "run:rise" (i.e., 2 m horizontal to 1 m vertical) are typical, although on occasion steeper slopes (1½:1) may be acceptable. For berms constructed from rock (in an unsupported condition) side slopes as steep as 1:1 may be acceptable. The top of the berm may be of minimal width (with normal slope rounding) or it can be designed with a relatively wide plateau. While the level plateau area results in more space required to construct the berm, it provides for easier maintenance of the berm and offers an area for placement of such features as plantings, a right-of-way fence, or even a noise wall which could be used for improving the acoustical effectiveness by effectively increasing the height of the barrier system.

Figure 26. Noise berm photo #2584	Figure 27. Noise berm photo #2712

Other factors to consider in selecting a berm as a noise barrier system include:

• Right-of-way requirements - Is the existing right-of-way width sufficient, or is additional property required for its construction?
• Location of the berm in relation to the right-of-way line - Should the berm be constructed entirely on the roadway right-of-way, the adjacent owners' property, or with the right-of-way line running down the center of the berm?
• Visual Implications both residential and highway side - Is the berm going to be too ominous and overshadowing in a small residential back yard?
• Destruction of existing features for construction of the berm - Will it be necessary to remove older trees or other aesthetic features?
• Maintenance and accessibility requirements - Will the berm be left in a natural state or will it be mowed/landscaped and, if so, by whom?
• Drainage implications - Are special features necessary to avoid disrupting natural drainage patterns and possibly flooding adjacent lands or the roadway?

4.1.2 Noise Walls.
Most noise wall systems are fabricated off-site, i.e., all of the components for this type of noise wall (foundation components excluded) are fabricated in a plant, then transported to the project site, and assembled on-site. Noise wall systems fabricated on-site include only cast-in-place concrete walls. This section includes a discussion of the following types of noise wall systems.

• Post-and-panel noise walls (Section 4.1.2.1);
• Brick and masonry block noise walls (Section 4.1.2.2);
• Free standing noise walls (such as precast concrete, planted/bin type, and stone crib) (Section 4.1.2.3);
• Direct burial panels (Section 4.1.2.4);
• Noise walls used to partially retain earth (Section 4.1.2.5); and
• Cast-in-place concrete noise walls (Section 4.1.2.6).

4.1.2.1 Post and Panel.
This type of system consists of noise barrier panels mounted between foundation-supported posts (see Figure 28). Primary elements of this type of barrier include: post and post/foundation attachments, panels, and panel to post connections.
	Figure 28. Post and panel noise wall photo #27

• Post and Post/Foundation Attachments - The type of post and post to foundation anchorage system is typically determined by the responsible organization's structural criteria for the particular type of barrier selected and the requirements of the system itself. The following describes some of the most common types based on currently constructed noise barrier systems:

  - Reinforced concrete foundation with post bolted to top of foundation - This foundation may be either concrete cylinder (caisson), spread footing, or continuous footing with either steel, reinforced concrete, or wood posts (see Figures 29 and 30).

Figure 29. Post and panel attachments: concrete cylinder photo #528	Figure 30. Post and panel attachments: continuous footing photo #529

- Reinforced concrete foundation with partial-depth embedment of post - This foundation is limited to a concrete cylinder with posts being either steel or reinforced concrete. The posts are usually embedded in the unreinforced concrete foundation to a point where the overlap of the post and the reinforcing is sufficient to achieve the desired structural strength in the foundation connection. - Unreinforced concrete foundation with full-depth embedment of post - This foundation is limited to a concrete cylinder with posts being either steel or reinforced concrete. The posts are usually embedded in the reinforced concrete foundation to within 300 mm (1 ft) from the bottom of the footing (see Figure 31). - Wood posts embedded in augured cylindrical hole with stone backfill. - This type of foundation is typical of wood noise barriers systems.

Figure 31. Post and panel attachments: embedded post photo #5

• Panels - Noise barrier panels (or their components) are normally prefabricated and shipped to the project site. Size and configuration of panels vary depending on project application. With the exception of wood post systems and certain proprietary noise barrier products, the type of panel used is not generally dictated by the post type. For instance, steel posts are used to support wood, steel, and concrete panels; concrete posts can support concrete, wood, or steel panels. However, wood posts, commonly used for wood panel installations are seldom used for metal panels and never for concrete panels. Panels can be generally grouped into two categories - full height panels (including panels pre-assembled into full-height size) and panels stacked on site (see Figures 32 and 33).

Figure 32. Full height panel photo #2946	Figure 33. Stacked panel photo #2680

When considering the type of panel (full height or stacked), the following factors should be considered:

- Post spacing - Larger post spacings may be more economical than shorter spacings but may dictate the use of stacked panels due to shipping constraints. Longer, narrower panels are, typically, more susceptible to warping (in all directions) than shorter panels for some types of materials. The effect of such warping must be considered in light of stacked panel joints, post-to-panel connections, and visual implications (such as shadows caused by uneven joints).

- Shipping requirements - Due to highway vertical clearance restrictions, large full-height panels shipped in a vertical or near vertical position may have to be laid on their sides on trucks. This may dictate the placement of lifting inserts on both the top and sides of panels. If these panels are shipped in a horizontal position (flat on the truck), oversized load permits may be required with restrictions of horizontal clearances of highway underpasses and overpasses (see Figure 34).

Figure 34. Post and panel: shipping requirements photo #527

- Weight and Size Limitations - Some manufacturing facilities may be limited in terms of their capacity to manufacture, handle, and store larger panels above a certain size or width.

- Acoustical considerations - When designing noise barriers using stacked panels, careful consideration must be given to the design of the horizontal joints between panels. Connections must be such to preclude sound leaks due to gaps (see also Section 3.4.2).

- Aesthetics - Stacked panels by their nature create horizontal joints. The relationship of these joints to the aesthetic pattern of the barrier system must be considered (see Figure 35). A well-designed stacked panel system can integrate the panel joints into the aesthetic pattern of the barrier. Use of stacked panels with exposed aggregate surface treatment may emphasize the inherent variations between one panel and another. Use of stacked panels with a vertical form liner finish may require a deep pattern to help "hide" the horizontal joints. Care must also be taken to ensure proper alignment of one panel on top of the other when vertically oriented designs and textures are used such as fluting. This alignment may become compromised with time due to differential settlement of the ground. Stacked panel usage may limit the choices of aesthetic treatments on one or both sides of a concrete barrier. However, the joints may enhance the appearance if they are incorporated into the overall aesthetic design of the noise barrier wall.

Figure 35. Post and panel: panel aesthetics photo #2979

- Installation implications - For any given post spacing, heavier capacity equipment (crane, lift, etc.) is usually required for a full height panel (see Figure 36) than for a stacked panel system. In certain restricted areas (such as, when a barrier must be placed under a bridge where access may only be available for smaller pieces of equipment, or where lifting height is restricted by overhead power lines) the use of stacked panels may be the only option. On the other hand, the use of a full height panel requires only one lifting and setting operation for each bay of the barrier system and negates the need to align, seal, or caulk any horizontal joints between panels.

Figure 36. Post and panel: installation implications photo #6049

- Maintenance considerations - The possibility of a barrier panel being damaged and needing replacement should be considered when choosing a panel type (see Figure 37). A damaging collision on a full height panel would require the replacement of the entire panel. A similar collision on a stacked-height panel bay may only require the replacement of one or a few damaged panels. If the damage occurs at the bottom of the panel bay, it is likely that all of the stacked panels would need to be removed and then reset. Matching the new replacement panel (texture, color, etc.) with the remaining existing panels is a factor requiring consideration with either the full-height or stacked panel system, although the possibility for variance is multiplied with the use of a stacked panel system. In addition, the possibility of damaging intact panels is increased during the removal and reinstallation operation. Since replacement of barrier elements may be required throughout the life of the noise barrier, the availability of replacement parts becomes a critical issue. To address this concern, some agencies have instituted a stock piling policy where the contractor/ manufacturer, at the time of construction, supplies additional components to the organization responsible for maintenance. This concern is discussed further in Section 12.2.

Figure 37. Post and panel: maintenance considerations photo #5572

• Panel to Post Connections - A variety of techniques are available and have been utilized to attach or secure panels to posts. In selecting these connections, there are important acoustical and structural criteria that need to be considered. It is also essential that any factors related to the specific materials used in both the post and panel construction be evaluated in the design of the connection. This is especially critical in the design of barrier systems using dissimilar materials such as steel and aluminum and specialized materials, such as the connection between steel posts and transparent plastic or glass panels. Panel to post connections must be designed to eliminate any significant sound transmission leaks. As such, they should have a snug fit along the entire post to panel contact area or sufficient panel embedment into the post. These acoustical requirements may be accomplished by a variety of techniques related to the physical "fitting" of post to panel connection points, such as the use of backer rods and/or caulking and use of sound absorption material on the panel at the connection point. Any packing material used to ensure a snug fit at the connection point between the post and panel must be designed to remain in place and be functional for the life of the barrier. The connection should also be designed to eliminate or minimize any damage that could be caused through normal movement or vibration to either the post or the panel.

  Important structural considerations related to the post to panel connections involve the following:

  - Panel-to-post dead load transfer - For a typical post and panel system, this load transfer occurs as point loads at the base of each post (if the panel is supported by the bottom of the post) or at the top of each caisson (if the panel is supported directly by each caisson). For stacked panel systems, all dead loads should be assumed to be transferred to the bottom panel. Further, the bottom panel should be assumed to have no support underneath its entire length except for the two endpoints. Similarly, no support (except at endpoints) is assumed to exist along the bottom of full height panels in a post and panel caisson supported system. However, if the post and panel system is supported by a continuous footing, then dead loads are assumed to be transferred uniformly through the panel (or bottom panel in a stacked panel system) along the entire length of the footing. Loads are transferred differently with systems using nailed, screwed, or bolted panel-to-post connections.

  - Panel-to-post wind load transfer - The important consideration here is that the actual constructed panel-to-post connection is consistent with the design assumptions related to wind load transfer. If the wind load design assumed that loads were uniformly distributed from the panel to its two supporting posts along the full height of each post, then sufficient physical contact must be provided in the constructed system to ensure even load distribution from the panel to the post. If panels are wedged or blocked to the post at several locations, a full length load transfer may be assured for loads applied in only one direction. However, the point loading resulting at the wedge points must be considered for loads applied in the opposite direction against the wedges or blocks. Full length load transfer may be provided by use of appropriate grouting, caulking, or mechanical devices which will ensure even distribution of loads. In the case of free floating panels between posts, the gap must be kept to a minimum and only sufficient enough to allow the panels to be installed without damage to the post or the panels and to assure a continuous load transfer from the panel to the post.

4.1.2.1.1 Tilted Post and Panel.
This type of system is a specialized type of post and panel system (see Figures 38 and 39) used in areas where sound reflections (either single or multiple) could be a problem if the more standard vertical noise walls were used (see Section 3.5.4). This type of system is most commonly manufactured using precast concrete elements, but at least one known system uses wood.

Figure 38. Tilted post and panel noise wall: community side photo #36	Figure 39. Tilted post and panel noise wall: highway side photo #34

Aside from the specialized loading treatments related to the tilted design, the considerations for post and panels discussed above apply to tilted post and panel systems. Since the angle of tilt is generally in the range of ten degrees, the issue of the wall being flat enough for people climbing it does not appear to be substantial. Because of aesthetics, care should be taken where such tilted walls transition to vertical walls or end abruptly. Similarly, the proximity of such tilted walls to residences and other areas of public use is probably more significant from a visual standpoint due to the possible perception of the wall "falling down."

4.1.2.2 Brick and Masonry Block.
This type of system includes barriers constructed of fabricated brick or masonry block units (see Figures 40-43). 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. However, in certain instances, such barriers may be constructed on a base beam supported at the ends by the posts or by the top of the concrete caissons for the posts.

Figure 40. Brick noise wall photo #8014	Figure 41. Brick noise wall photo #560
Figure 42. Masonry block noise wall photo #2454	Figure 43. Masonry block noise wall photo #2457

4.1.2.3 Free Standing Noise Walls.
This type of barrier system includes barriers which support themselves. Such barriers, constructed to date, can be grouped into the following general categories:

• Precast concrete (Section 4.1.2.3.1);
• "Planted" or Bin Type (Section 4.1.2.3.2); and
• Stone crib (Section 4.1.2.3.3).

4.1.2.3.1 Precast Concrete.
These systems generally obtain their stability from the combination of their "zig-zag-like" or "trapezoidal" configuration and their system mass (see Figures 44 and 45). Depending upon soil conditions, precast free-standing walls may be supported by compacted soil and a well-drained stone base, a plain cement concrete leveling pad, or a continuous reinforced concrete footing.

Figure 44. Precast free standing concrete noise wall photo #1206	Figure 45. Precast free standing concrete noise wall photo #1218

4.1.2.3.2 "Planted" or Bin Type Barriers.
These systems obtain their stability from a type of structural shell, typically either concrete, wood, or plastic, which is filled with soil and then planted (see Figures 46 and 47). These systems are most often supported by some form of continuous concrete leveling pad or footing. However, depending on the design and the type of plantings, these systems may be set directly on top of the existing ground with little or no preparation other than minor leveling.

Careful consideration needs to be given to the type of planting selected and to the means for providing adequate watering of the plant material during all seasons. Maintenance requirements can be significant on such systems, particularly related to items such as weeding, removing large saplings which grow from blown-in weed seeds (if not removed they can adversely affect the structural integrity of the barrier), and replacing pockets of washed out soil. Safety, security and liability issues such as the ability to climb the steps of the planted wall should also be considered.

Figure 46. Bin type noise wall: plastic photo #247	Figure 47. "Planted" noise wall: concrete photo #2567

4.1.2.3.3 Stone Crib.
This special type of barrier system (also referred to as a gabion system) is comprised of crushed rock contained in large rectangular baskets made of heavy wire mesh (see Figure 48). For aesthetic purposes, these wires can be coated with vinyl which are available in various colors. The baskets are stacked on top of each other in a pyramid fashion to obtain the required barrier height and stability. The baskets are typically placed on well draining, compacted ground. Their structure is flexible enough to tolerate some settlement. This type of system is only feasible if sufficient quantities of suitable rock material are readily available close to or on the project. Little, if any, plant life can be expected to grow on or within this barrier system. The system adapts well to rolling topography.
	Figure 48. Stone crib noise wall photo #549

4.1.2.4 Direct Burial Panels.
The direct burial panel type is a special panel system which involves burying a portion of one end of the panel (either precast concrete or wood) directly into the ground with no other means of foundation support (see Figure 46). With this type of system, the panels are usually full height and the connection to adjacent panels are typically designed as a tongue and groove system. Since differential settlement of the panels will most likely occur, a smooth top-of-wall profile cannot be expected. Therefore, a jagged profile should be considered during the design of this system (see Figure 47). In some cases, this differential settlement may not be even throughout the length of each panel, thus causing tilting of the panels and ultimately resulting in separating and gapping at the vertical tongue and groove joints.

Figure 49. Direct burieal panels photo #348	Figure 50. Direct burieal panels photo #351

4.1.2.5 Noise Walls Used to Partially Retain Earth.
In certain instances it may be necessary and advantageous to 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 (see Figure 51).
	Figure 51. Noise walls used to partially retain earth photo #8027

In such applications which have employed a post and panel system, the normal depth of the foundation for the noise barrier wall is usually sufficient to retain fills up to 500 mm (1.5 feet). Otherwise, careful structural analysis of the system is required to assure that the bottom portion of the panel (if full height) or the bottom panels in a stacked panel system are sufficiently strong to retain the soil (see Figure 52). Panel to post connections and the joints between stacked panels require similar analysis. In these systems, special drainage designs may be needed to assure that water does not "pond" or saturate the soil retained by the noise wall. In addition, the design should allow for proper drainage of the soil via weeping holes in the walls or via other suitable means.

Figure 52. Noise walls used to partially retain earth photo #480

Other types of noise walls (concrete block, free standing, cast in place, stone crib, etc.) may be considered for such earth retaining applications, but only after the same careful consideration of the above factors. Combination retaining wall/noise barrier systems (generally requiring more significant depths of soil) are discussed in Section 4.2.2.1.

Potential also exists for the adjacent land owner to re-grade their property in the vicinity of the noise barrier. If the surrounding topography has the potential for regrading by adjacent owners, then it should be assumed that this will occur and the impact of this action must be considered in the design of the noise barrier.

4.1.2.6 Cast-In-Place Concrete Noise Walls.
These type of barriers are constructed at the project site (see Figure 53). The construction process includes excavating for the footing, erecting form work, setting reinforcement steel, pouring concrete, surface finishing, and curing. Except for certain foundation elements, such systems are significantly different from the noise wall systems discussed in the previous sections in terms of construction techniques, architectural and aesthetic treatment processes, and inspection and quality control procedures. All material testing and inspection procedures must be done in the field both during construction and on an as-erected product. All casting and curing occurs under a variety of weather conditions as compared to more controlled conditions for many of the components of prefabricated barrier systems.
	Figure 53. Cast-in-place concrete noise wall photo #1054

Form liners and architectural inserts must be placed on vertical or near vertical surfaces of the form work which may lead to a significant increase in imperfections in the wall surface when compared to precast components usually cast in a horizontal position. In addition, the application of concrete retarding chemicals to the vertical form work surfaces for the purposes of obtaining an exposed aggregate finish is significantly more difficult than in precast operations. Therefore, obtaining a consistent and acceptable exposed aggregate surface may not be possible. Surface textures obtained through raking, brushing, or stamping of concrete are not possible with cast-in-place walls.

4.1.3 Combination Noise Berm and Noise Wall 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 (see Figures 54 and 55). The foundation, post, and panel considerations for these wall portions are similar to those discussed above.

Figure 54. Combination noise berm and noise wall system photo #27	Figure 55. Combination noise berm and noise wall system photo #993

Additional considerations with this combination type of system relate to the following factors:

• Reactive loadings on the berm's soil due to the wall portion - The soil design parameters for earth in berms are substantially different than those of undisturbed soil. Factors such as the frictional interaction of the foundation and compressive loads on the berm's soils need to be considered in detail. For example: the area between the wall's foundation and the berm's side slopes need to be thoroughly analyzed in relation to the shear slip circle.
• Plateau area on top of the berm - An adequate level area on top of the berm should be provided to assure stability of the soil at the base of the noise wall structure. Generally, a minimum plateau width of 2 m (6.5 ft) should be provided for foundation stability. This minimum requirement will help alleviate erosion of the berm resulting from rain hitting the wall and flowing down onto the berm.
• Need for and location of right-of-way fence - It is possible that the placing of a noise wall on top of a berm may negate the need for a right-of-way fence. However, it is also possible that a right-of-way fence may still be required at the right-of-way line or another location. This then would require consideration of maintenance and accessibility issues related to the area between the right-of-way fence and the noise wall commonly known as "no man's land" or "dead man's zone."

4.2 Structure-Mounted Noise Walls
This section discusses the types of noise walls used on structures, the concerns related to structure-mounted noise walls, and the general design and construction techniques used to address these concerns. There are two primary types of structure-mounted noise walls: Noise walls on bridges (Section 4.2.1); and Noise walls on retaining walls (Section 4.2.2).
	Figure 56. Noise wall on a bridge photo #2374

4.2.1 Noise Walls on Bridges. 4.2.1.1 Types of Noise Walls on Bridges.
A number of techniques have been successfully employed to attach noise walls to bridges (see Figures 56 to 59). While somewhat different procedures and operations exist for attaching noise walls to existing bridges as compared to attachments to new bridges, the resultant attachment types are similar enough to be discussed under the following general categories:
	Figure 57. Noise wall on a bridge photo #5231

• Post and Panel Noise Barriers

  On top of parapet - Such attachments usually include high strength bolts anchored to or embedded into the top of the parapet. On new construction, such bolts are often set in the parapet form work prior to the concrete pour. In existing parapets, bolts may be anchored by mechanical fastening or chemical bonding (epoxy grout) methods. Depending on the type of noise wall material, these high strength anchored bolts and nuts are used to secure either a continuous horizontal beam (or angle) or vertical posts to the parapet. Noise wall panels or other components are then secured to the beam or posts to create the in-place barrier. Obtaining a smooth or desired top of barrier profile with such a system may require each panel to be custom made if the top of parapet profile is not smooth and/or consistent. Any bottom of barrier jaggedness or gapping, can be concealed by flashing.

  Inserted into parapet - This method should only be considered for new bridges. Although not as common an attachment technique, posts have been inserted into the parapet itself (either prior to casting of the parapet, or after parapet casting) via insertion into precast holes within the parapet wall itself.

Figure 58. Noise wall on a bridge photo #1717	Figure 59. Noise wall on a bridge photo #5090

On outside face of parapet - Although suitable for existing and new bridges, it is particularly suitable for retrofitting of existing bridges. A rather common practice is to mount noise barriers onto the outside face of the parapet (Note: special consideration should be given in the situation of two parallel bridges, where a sizable gap between the bridges might compromise barrier performance). The barrier posts are usually attached to the parapet by one of four methods:

-Mechanical anchoring system - This type of anchor system consists of a wedge shaped nut which is inserted into a drilled or cast hole in the concrete parapet wall. As the bolt is turned, the nut is forced to spread and is wedged in the hole providing a solid anchor for the bolt to be sufficiently tightened. This system is limited in it's use and should only be considered for use in concrete, and should not be used in situations where the anchor will be exposed to constant vibrations from traffic and wind loading. In addition, any drilling into the parapet walls may diminish its bearing capacity, particularly if reinforcing bars are severed during the drilling operation.

-Chemical anchoring systems - This system basically consists of a two-part epoxy mixture adhesive inserted into a drilled or cast hole in the concrete wall and then mixed by spinning the bolt inside the hole. This method is more suited for older structures and for areas where the anchors are routinely exposed to vibrations. However, the same concerns regarding the severing of the reinforcing bars during the drilling operation (see Mechanical anchoring system above) should be considered before using this method. When this product first came onto the market, concerns were expressed regarding it's durability and long-term performance. These concerns appear to have been addressed by the industry, and use of this type of anchoring method is not restricted to specific applications.

-Bolt through system - The bolt through system uses long bolts which are inserted into holes either cast or drilled completely through the parapet walls. This method addresses most of the concerns associated with the durability of both the mechanical and the chemical anchoring systems. However, it is more destructive to existing structures and may diminish the bearing capacity of portions of the wall.

-Cast-in-place bolts - Although a less commonly used method, this anchoring system is considered to be the most effective and least destructive of all methods. However, this method should only be considered for new structures or where key areas of the structure are being rehabilitated. There may also be some difficulty in maintaining bolt location tolerances due to movement of the forms during pouring.

Additional barrier anchorage may be provided via angle iron mounted to the top of the parapet. On barriers constructed as part of a new bridge construction, the bridge slab may be extended beyond the outside edge of the parapet, providing additional dead load support for the barrier.

• "Post-less" Panels - Such systems use either concealed posts or no posts with the panels, typically mounted in the following manners:

On top of parapet - For concealed post systems, the post to parapet connections are similar to those discussed above for the post and panel systems. For "post-less" systems, the panels (typically constructed of relatively lightweight materials)are attached via bolts to two parallel angle iron pieces mounted to the parapet.

On outside face of parapet - Such systems are mounted in manners similar to those for post-and-panel systems listed above except that the panels themselves are bolted to or through the parapet. With this type of system, additional detailed care should be taken in the design of the horizontal joints between panels to assure a leak-free noise condition and to maintain the consistent alignment of adjacent panels.

• Masonry Block Noise Barriers - These barriers are "laid up" in a manner similar to ground-mounted masonry block barriers except that their anchorage is to the protective concrete bridge parapet wall, which usually has the same shape as the standard concrete traffic barrier walls, i.e., Jersey barriers. The anchoring is via reinforcing bars extending out of the top of the parapet wall. The noise barrier wall can be further strengthened by inserting reinforcing bars and concrete within the voids of the masonry blocks.

• Cast-in-place integral with parapet wall - On occasions it may be necessary and appropriate to construct noise barriers integral with the bridge parapet wall. This type of structure-mounted noise barrier wall is more suitable where short height barriers can provide the desired noise attenuation or in situations where it may be the only possible option due to restrictions in erecting any other types of barrier systems.

• On parallel supporting structure adjacent to parapet - This type of structure-mounted noise barrier wall is not as common as other methods mentioned previously. This mounting system is particularly suitable for older or weakened bridges, where the structure (parapet wall, deck, and/or superstructure) is incapable of supporting the loads of the desired noise barrier system. A parallel supporting beam or similar structure may be built immediately adjacent to the existing structure. This structure would support the full vertical dead load of the noise barrier wall and all or some of the torsion load, if the beam and/or the wall were attached to the adjacent existing structure.

4.2.1.2 Effect of Noise Walls on the Structural Characteristics of an Existing Bridge.
Placing a new noise wall on an existing bridge adds a significant amount of stress on a structure caused be the additional weight and rotational loading for which the existing structure may not have been originally designed. This may result in the need to add additional girders, beams, and diaphragms; strengthen the existing bridge deck; or modify the existing parapet. Additional solutions which should be considered are reducing the weight of the noise wall by using light weight material or, only if absolutely necessary, by reducing the height of the wall or, ultimately, eliminating the construction of the wall. The latter should only be considered under absolutely severe situations.

Besides the obvious additional costs of such structural modifications (above the noise wall cost), other issues related to modifying an existing bridge include:

• Maintenance;
• Protection of traffic (both on and beneath bridge);
• Accessibility to areas requiring modifications;
• Bridge vibrations due to existing traffic;
• Vibrations from construction operations; and
• Potential environmental mitigation measures (related to painting beams or working over waterways or wetland areas).

4.2.1.3 Effect of Noise Wall on the Structural Requirements of a New Bridge.
While additional costs are still incurred (compared to the same bridge without a noise barrier), the ability to design the noise wall as an integral part of the overall structure addresses most if not all of the loading and traffic-related concerns discussed above.

4.2.1.4 Potential for Damage to Noise Wall From Vehicular Impact or Airborne Debris.
The proximity of the noise wall to the traveled portion of the bridge usually makes the wall considerably more susceptible to damage (compared to most ground-mounted noise walls). Such damage (see Figures 60 and 61) may be caused by vehicle impact, airborne debris such as stones, vehicle parts, snow removal operations, or material from salt spreaders in areas subject to snow fall.

Figure 60. Noise wall damage from vehicular impact photo #5331	Figure 61. Noise wall damage from airborne debris photo #1281

4.2.1.5 Potential for Damage and Injury in the Event of the Noise Wall or Parts Thereof Falling From the Structure.
Factors considered in addressing this concern include:

• Type and proximity of land use adjacent to or beneath the noise wall;
• Location of the noise wall on the bridge;
• Noise wall-to-bridge attachment details;
• Weight, composition, and shatterability of the noise wall component parts; and
• Any mechanisms (either internal or external to the noise wall) designed to retain noise wall components.

While these factors are typically considered by the noise wall designers, the degree of consideration can vary significantly during the decision-making process. The use of barrier materials which tear (such as metal) rather than break/shatter into pieces should be considered this process.

4.2.1.6 Other Safety-Related Concerns.
The proximity of bridge-mounted noise walls to traffic has raised concerns related to issues such as vehicular sight distance, barrier shading which increases potential for highway icing, and adverse effects on highway lighting. These issues are discussed elsewhere in this document (see Sections 9.2 and Sections 9.8).

4.2.1.7 Maintenance Considerations.
Snow drifting and storage implications, restrictions to bridge inspection teams using bucket trucks to inspect beams, and maintenance of the noise wall itself including graffiti removal, noise barrier and structure damage repair, repainting, etc., are some of the concerns which should be considered during all stages of design and construction. These concerns, except for bridge inspection, are common to both ground-mounted and structure-mounted systems. These concerns are generally greater for bridge-mounted noise walls due to their proximity to the roadway and accessibility limitations.

4.2.2 Noise Walls on Retaining Walls.
Retaining walls are typically constructed to retain a highway fill section (where the adjacent ground is lower than the highway grade) or to retain the adjacent ground (where the highway is in cut in relation to the adjacent ground). In either case, the possibility exists that the installation of a noise barrier wall may be warranted either as a part of the retaining wall or in the immediate vicinity of the structure.

Although the required height of a noise barrier system could be accomplished by placing an earth berm behind the retaining wall, such a condition is relatively rare and would be evaluated in the context of loads on the retaining wall. Therefore, the discussion in this section focuses on the variety of techniques used to construct a noise barrier wall on or near a retaining wall (see Figures 62 and 63).

Figure 62. Noise wall on a retaining wall photo #2947	Figure 63. Noise wall on a retaining wall photo #531

4.2.2.1 Combination Cast-In-Place Retaining Wall and Noise Barrier Wall.
Where the retaining wall is cast-in-place (see Figures 64 to 66), the necessary noise barrier height may be attained in the following manners: By extending the height of the retaining wall as a continuation of the cast-in-place structure; By installing one of the noise wall systems, such as the post and panel system, the "post-less" system, a brick wall, or a masonry wall on top of the retaining wall as discussed in Section 4.2.1.1; or By anchoring a noise barrier system to the sides of the retaining wall, using similar methods, as described in Section 4.2.1.1, for anchoring noise barriers to bridges.
	Figure 64. Combination cast-in-place retaining wall and noise wall photo #1844

Whether the retaining wall is new or existing, the structure must be capable of accommodating the additional dead loads and torsion loads of the noise wall. In addition to structural considerations, other concerns with this type of a retaining wall/noise barrier system are discussed in the following sections as they relate to specific types of installations.

Figure 65. Combination cast-in-place retaining wall and noise wall photo #5004	Figure 66. Combination cast-in-place retaining wall and noise wall photo #329

4.2.2.2 Noise Wall behind Cast-In-Place Retaining Wall.
Placement of a noise barrier wall behind a cast-in-place retaining wall (see Figures 67 and 68) requires careful consideration of the load transfers (both dead load and torsion loads) on both the earth behind the retaining wall and the retaining wall itself.

Figure 67. Noise wall behind cast-in-place retaining wall photo #1745	Figure 68. Noise wall behind cast-in-place retaining wall photo #1691

As compared to a noise wall mounted directly on top of a retaining wall, additional considerations related to the area between the noise wall and the face of the retaining wall with this type of a system include:

• Maintenance and landscaping;
• Drainage; and
• Safety and security issues related to access and fencing.

These issues are discussed in more detail in Section 6.2, Section 7.1, and Section 9.4, respectively.

4.2.2.3 Noise Wall on or behind Retained Earth System Type Retaining Wall.
Many instances exist where highway fills or cut slopes are retained by proprietary systems which use metal straps, grids, or other techniques to strengthen, reinforce, and/or retain the earth mass behind the wall system. In this type of retaining wall system, the "retained" earth mass is the retaining wall system's supporting medium, as opposed to the structure itself. While these systems typically incorporate a concrete facing of one form or another, this facing is not the primary structural element of the retaining wall system, and thus, cannot be relied upon to support the loads of a noise barrier. Without an independent footing (usually a spread footing which may be tied to the cap of the retaining wall) a noise wall cannot be placed on top of, attached to the side of, or installed directly behind the face of this type of retaining wall system.

The following is a discussion on two typical methods of overcoming these restrictions. These methods should not be considered all inclusive and are not intended to restrict innovative approaches.

• Offset noise wall - Any noise barrier wall installed adjacent to this type of retaining wall system should not be allowed to be supported, even partially, by the retained soil mass. Therefore, the noise barrier wall should be set back some distance from the retaining wall face.
• Independent Foundation - Caisson-type footings can be erected in the "original" ground prior to the construction of the retained earth system. Due to the amount of structural loading caused by this type of footing, the caissons are typically designed to assume that no support will be provided by the retained soil mass. To address this structural limitation, post length and caisson size and depth can be increased to allow the posts to be extended up to the ultimate height (the top elevation of the retained earth system) and then the retained earth system "laid up" with its straps, grids, etc. diverted around the locations of the noise barrier foundations. With this technique, the noise walls can be placed somewhat closer to the face of the retaining wall, although careful analysis of structural loading should still be performed.

With either of these two techniques, the same issues related to maintenance, landscaping, drainage, safety, and access referred to in Sections 6.2, Sections 7.1, and Sections 9.4 should be considered.

4.2.2.4 Noise Barrier Walls in Combination with or behind Pre-Manufactured Retaining Wall.
A variety of retaining wall systems exist which are comprised of pre-manufactured materials such as precast concrete, metal, and plastic components which are assembled on site (see Figure 69). Some of these systems may also incorporate earth within their structural facing. In general, noise barrier walls cannot be supported directly on top of the face of these systems. Thus, their construction must consider the same factors as discussed above for barriers located behind retained earth systems.
	Figure 69. Noise wall caisson foundations behind pre-manufactured retaining wall photo #6534

4.3 Special Features
Situations and conditions often warrant the incorporation of special features into the construction of noise walls. These features may be required because of engineering, acoustical, and/or aesthetic reasons.

4.3.1 Caps.
For the purpose of this discussion, caps are considered to be separate elements of the barrier system applied to either the top of noise walls or to the top of the noise wall posts. The "cap look," which is accomplished as an integral part of the fabrication/construction of the noise barrier wall panels, is discussed further in Section 6.1.3.

4.3.2 Emergency Access Openings.
Special modifications of noise wall systems are often required in areas where the access from one side of the wall to the other is required (or anticipated to be required) on either a regular basis (as per maintenance personnel and/or equipment), or on an irregular basis (as per emergency personnel and equipment). The locations and frequency of such access openings is, usually determined by the responsible organization (DOT, emergency response unit, fire departments, police departments, etc.) on a case-by-case basis. Emergency and pedestrian access openings and doors are usually required to be identified on both sides of the barrier by signs denoting mile marker, station, or some other distinctive identifying feature. Section 9.4 discusses in detail several techniques used to provide such access openings in noise barrier walls.

4.3.3 Drainage Openings in Noise Walls.
Normally, water falling in the vicinity of a noise wall is carried longitudinally along the barrier in a drainage swale to a catch basin or inlet from which it is piped either under the wall or into the main roadway drainage system. In certain instances, it is necessary to allow water collected in front of or behind a noise wall to pass to the other side of the barrier. Design of such openings in the noise wall must assure that their size and frequency are such as to not degrade the acoustical effectiveness of the noise wall system. A more detailed discussion is provided in Section 7.1.

4.3.4 Attachments to Noise Walls.
A noise wall may, in certain situations, be the only possible location or the most feasible location for mounting or attaching other roadway-related elements. Examples of such elements include call boxes, highway speed limit or advisory signs, highway lighting fixtures, etc. Incorporation of such features should occur integrally with the design of the noise wall, not as an add-on element. Safety considerations related to clearances (both horizontal and vertical), electrical connections, sight distances, etc. are essential. This subject is discussed in more detail in Section 7.2.

Section Summary

Noise Wall Types.
Item#	Main Topic	Sub-Topic	Consideration	See Also Section
4-1	Noise Berm	Aesthetic	Consider the visual implications on both residential and highway side and the landscaping required.	4.1.1
			Consider the destruction of existing features for construction of the berm.	4.1.1
		Drainage and Utility	Provide for adequate drainage requirements.	4.1.1
		Safety	Consider right-of-way requirements.	4.1.1
		Maintenance	Consider accessibility to and around berm and landscaping requirements.	4.1.1
4-2	Post and Panel Noise Wall	Acoustical	Ensure that there are no sound transmission leaks between stacked panels and panel-to-post connections.	4.1.2,1
		Aesthetic	Coordinate texture treatments with stacked panels so that the joints are either concealed by the pattern or become a part of the pattern.	4.1.2.1
			Consider special concerns related to tilted post and panel designs.	4.1.2.1.1
		Drainage and Utility	Overhead wires and other utilities may preclude the ability to use full height precast panels.	4.1.2.1
		Structural	Provide for the impact of panel-to-post wind and dead load transfers.	4.1.2.1
			Provide specialized loading treatments related to tilted post and panel designs.	4.1.2.1.1
4-3	Free Standing Noise Wall: Precast Concrete	Maintenance	Consider landscaping access requirements.	4.1.2.3.1
		Installation	For precast : consider size limitations, shipping requirements, traffic implications, reusability of precast panels, quality assurance process.	4.1.2.3.1 5.1
			For cast-in-place: consider on-site material testing and inspection procedures during construction and on an as-erected product and weather concerns for on-site casting and curing.	4.1.2.6 5.1
4-4	Free Standing Noise Wall: Planted or Bin-Type	Aesthetic	Consider the type of plantings.	4.1.2.3.2
		Maintenance	Ensure landscaping upkeep.	4.1.2.3.2
			Consider litter implications.	12.7
		Safety	Consider implementing a deterrent for climbing on barrier girts.	4.1.2.3.2
4-5	Free Standing Noise Wall: - Stone Crib	Safety	Consider implementing a deterrent for climbing on barrier girts.	4.1.2.3.3
4-6	Direct Burial Panels	Acoustical	Consider possible separating and gapping at the vertical tongue and groove joints.	4.1.2.4
		Aesthetic	Consider a jagged top-of-wall profile rather than a smooth profile.	4.1.2.4
		Structural	Consider possible differential settlement of panels.	4.1.2.4
4-7	Noise Walls Used to Partially Retain Earth	Structural	Consider loading concerns on the retained soil.	4.1.2.5
			Consider the impact if adjacent land owners re-grade their property in the vicinity of the noise wall.	4.1.2.5
		Drainage and Utility	Ensure proper drainage so that water does not "pond" or saturate the soil retained by the noise wall.	4.1.2.5
4-8	Cast-in-Place	Aesthetic	Form liners and architectural inserts must be placed on vertical surfaces of the form work which may increase imperfections in the wall surface.	4.1.2.6
			Application of concrete retarding chemicals to the vertical form work surfaces for the purposes of obtaining an exposed aggregate finish is difficult.	4.1.2.6
		Installation	Consider on-site material testing and inspection during construction and on an as-erected product.	4.1.2.6
			Consider weather concerns for on-site casting and curing.	4.1.2.6
4-9	Combination Noise Wall and Noise Berm	Structural	Consider reactive loadings on the berm's soil due to the wall portion.	4.1.3
			Consider a plateau area on top of the berm.	4.1.3
		Safety	Consider the need for and location of right-of-way fence.	4.1.3
4-10	Noise Walls on Bridges	Structural	Consider weight stress and rotational loading.	4.2.1
			Consider bridge vibrations due to existing traffic.	4.2.1
			Consider bridge vibrations from construction operations.	4.2.1
			Consider the impact of any parapet attachments.	4.2.1
			Consider the type of anchoring system.	4.2.1
			Consider top of barrier profile if the top of parapet profile is not smooth and/or consistent.	4.2.1
		Safety	Ensure protection of traffic (both on and beneath bridge).	4.2.1.4
			Consider the potential for damage and injury in the event of the noise wall or parts thereof falling from the structure.	4.2.1.5
			Ensure adequate vehicular sight distance.	4.2.1.6
			Consider barrier shading resulting in highway icing, and adverse effects on highway lighting.	4.2.1.6
		Maintenance	Consider accessibility.	4.2.1.7
			Consider snow drifting and storage implications.	4.2.1.7
			Consider possible restrictions to bridge inspection teams.	4.2.1.7
4-11	Noise Walls on Retaining Walls	Structural	Consider any additional dead and torsion loads due to the noise wall.	4.2.2
		Drainage and Utility	Provide for adequate drainage requirements.	4.2.2