Except for berms and brick or masonry construction, most noise barriers are of post-and — panel construction, that is, vertical posts spaced a distance apart with horizontal or vertical panels running in between. Rails or girts may also run between the posts to support the panels. Posts are embedded in the foundation soil to design depth, which depends on wind loading, soil properties, and frost depth. Brick and masonry walls generally require spread footings, underlain with uniform layers of soil.
According to a 2006 FHWA survey, the main materials that have been used for noise wall construction, in order of usage, are the following:
• Concrete
• Block and brick
• Wood
• Metal
• Earth berms
Other materials sometimes used include plastic, glass, composites, and gabions (rock- filled wire baskets). Glass and clear plastic are alternatives where it is desirable to not block scenic views.
Concrete. Users indicate that selection has been based on cost, durability, low maintenance, surface treatments available, and acoustical properties. Concrete walls can be precast, cast in place, or of post-and-panel construction. Precast concrete panels may be of either prestressed or reinforced construction. Various surface finishes such as texturing are available and are relatively inexpensive. A 4-in-thick (100 mm) wall provides a relatively high transmission reduction of 32 dBA.
Block and Brick. Brick and masonry construction is also popular, mainly because of its pleasing appearance and acoustical properties. However, initial cost is likely to be higher, depending upon the geographic location, as well as repair cost if damaged. Slump block, cinder block, stone, and brick have all been used. Units can be arranged to produce various patterns. The typical transmission loss is 33 dBA, and this can be improved by the addition of mineral wool or fiberglass to the wall interior.
Wood. Attributes that favor selection include favorable cost, ease of construction, aesthetic appeal, and availability. Disadvantages include shrinkage, warpage, deterioration, difficulty of quality control, discoloration around fasteners, and low resistance to vandalism. Wooden walls have been constructed from timbers, planks, plywood, and laminated products. Often, these materials are used for the panels or facing and concrete or steel is used for the posts. Tongue-and-groove construction should be used for panels running between posts to eliminate gaps. The durability of wooden walls can be enhanced by using materials that have received a pressure preservative treatment. Wood provides a transmission loss of 18 to 23 dBA/in (0.72 to 0.92 dBA/mm) of thickness.
Metal. Metal walls, primarily of cold-formed steel sheet, can be used as stand-alone barriers or in combination with berms. Low cost, maintainability, and ease of construction favor use of steel. Disadvantages include vibration problems, denting, and ineffectiveness in the low-frequency range. For steel construction, the panels are fluted (have rectangular corrugations) vertically or horizontally, with a channel-shaped cap at the top. Prepainted galvanized sheet and weathering steel have been used, and other durability treatments are available. The transmission reduction is generally between 10 and 22 dBA.
Earth Berms. Earth berms or mounds are preferred by some. Natural appearance, favorable cost, ready availability of the material, low maintenance cost, and acoustical efficiency favor their selection. A disadvantage is the space needed for construction, particularly in view of safety requirements. Sometimes soil is used in combination with a wall where space is limited. For example, if there is not enough space to achieve the full desired height with a berm, a noise barrier can be located on top of a berm of lower height. Berm side slopes of 4:1 or flatter are desirable on the basis of considerations of safety (see Art. 6.2), roadside maintenance, and wall stability. Some states permit up to 3:1, depending on lateral location. Both acoustics and aesthetics can be improved when the berm is combined with a dense planting of vegetation. Vegetation with a minimum depth of 100 ft (30 m) (perpendicular to roadway), height of 15 ft (4.5 m), and density such that there is no clear path between the highway and the adjacent land use areas can result in a noise level reduction of up to 5 or 6 dBA. Existing soils must be capable of supporting the added berm load.
Proprietary Systems. There are a number of proprietary systems on the market. Some products have included recycled materials such as tire rubber, wood processing waste, and plastics. Of course, steel and aluminum products contain a very high level of recycled metal.
