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.