Design standards can address wall height limits, wall materials, and foreground plantings.
How it Works
Retaining walls are walls constructed to hold back a solid material, typically earth. We see retaining walls in a range of areas such as parks, along roads, and on commercial, institutional, and residential properties. These areas consisted of slopes that were leveled to accommodate features such as usable open space, road corridors, building pads, and parking lots. Because retaining walls are used to alter the character of a slope, their location, form, materials, and associated landscaping can play an important role in the overall design quality of a development.
Municipalities can establish design articulation standards for retaining walls in new development. Establishing standards ensures retaining walls are limited in quantity within a development and aesthetically pleasing as viewed from neighboring properties and the public right-of-way. Standards can address design treatments like visible wall height, wall materials, and landscaping.
Benefits
Improved Visual Interest and Variety
Design articulation techniques, such as offsets and terracing, minimize the mass and unnatural appearance of retaining walls.
Improved Walkability
Design articulation, such as height limitations and landscaping, helps soften the edge of retaining walls located near public rights-of-way and sidewalks and improves the perception of an area’s walkability.
Compatibility with Surrounding Environment
Requiring wall face materials that blend with the natural environment and surrounding built environment character, especially in historic contexts, improves the appearance and compatibility of the structures.
Protection of Natural Resources
Limiting the use of large retaining walls encourages development to adapt to existing site conditions, which can help preserve important natural features and processes, such as stormwater drainage and shade from existing trees.
Reduction of Potential Environmental Problems
Sensitive site planning and design utilizes the opportunities and reflects the constraints created by floodplains, slopes, soils, vegetation, and other physical features to limit potential negative impacts from extensive grading and large retaining walls.
Design standards can prevent tall retaining walls from occurring close to public rights-of-way, which create an imposing visual appearance and a negative pedestrian experience.
Get Started
Ideally, with any development, all site improvements should be planned to harmonize and accentuate the natural features of a site. Development should fit the topography, soils, geology, hydrology, and other existing conditions on a proposed site so that grading and other site preparation, such as retaining walls, is kept to an absolute minimum.
When retaining walls cannot be avoided, municipalities typically require professional engineering design to certify structural components including safety elements. An exemption from this requirement is usually allowable for small landscaping, garden, or similar type walls less than four feet in height which do not support buildings, parking facilities, or other structurally significant site features.
In addition to providing minimum retaining wall standards to safeguard persons and property and to protect and promote the public welfare, municipalities have a few options to effectively manage retaining wall design articulation in new developments:
Zoning or subdivision and land development ordinance
These ordinances are implemented through regulatory codes that provide uniform requirements that all properties must adhere to. Establishing provisions for retaining walls in these ordinances is an appropriate approach to ensure community retaining wall goals for items such as height and setbacks are met. This approach, however, may lessen the degree of flexibility in dealing with retaining wall features such as wall face materials.
Grading ordinance
This ordinance can ensure that grading enhances rather than detracts from or ignores the natural topography, resources, and amenities of the land, such as trees. It also can conserve the natural contours of the land by regulating and controlling the design, construction, quality of materials, use, location, and maintenance of retaining walls.
Design guidelines
Design guidelines are different than ordinances because they are advisory and do not dictate specific requirements or solutions. They are intended to provide guidance on how to evaluate options and make informed decisions about a variety of design issues. Property owners and design professionals are encouraged to consult design guidelines when determining how to approach a project but are not required to do so.
Section showing a terraced retaining wall with landscaping incorporated.
Turning the ends of walls helps integrate their form into adjoining slopes
Considerations
Height and Setbacks
Limiting retaining wall height from finished grade can minimize the potential for vertical walls out of scale with adjacent surroundings. Some municipalities limit retaining wall height to four feet but allow terracing. This maximum height limit ensures walls are never out of scale, especially in relationship to elements in the public right-of-way, including pedestrians. Some municipalities regulate retaining wall heights based on their location inside or outside of frontage areas. For instance, Lower Merion Township, PA has lower height requirements in the front setback (four to six feet depending on zoning district) than beyond the front setback (six to eight feet depending on zoning district). Note, retaining wall heights within the right-of-way of public roads are typically exempt from height limits.
Terracing
Terraced walls instead of one large wall minimizes the appearance of mass. In addition to height restrictions, limiting the number of terraced walls to a certain number of tiers (e.g., three) and identifying the minimum width (e.g., 10 feet) and maximum slope (e.g. 25%) of terraces can prevent an excessive use of retaining walls on a site.
Form
Retaining walls should blend with the natural topography by following existing contours and follow a curvilinear layout to the greatest extent possible.
Landscaping
Landscaping, including layered plantings and either structurally supported climbing or hanging plants, should be a requirement to soften and screen the appearance of the walls visible from a public right-of-way. Plantings should be installed at each base level of any terraced system. Requiring a minimum height for trees and shrubs at the time of planting is recommended.
Materials
Context should dictate what materials are used on the face of the wall. Retaining walls visible from a public street should blend with the natural environment and surrounding built environment character through veneers of natural stone or earth-colored materials, textured surfaces, and other similar treatments. A material compatible with the primary building material is also acceptable. It is also recommended that walls within a development be designed with a single material, style, and method rather than a mix of materials. Another acceptable alternative is Loffelstein, or similar living walls, which consists of interlocking units that create pockets that are filled with soil and plants. Once landscaping matures, this type of wall has a softer appearance than a hardscape wall surface. It is recommended that railroad ties, timber, and gabion-type retaining walls be prohibited within the view of public rights-of-way because of their utilitarian design.
Fall Protection
At the top of walls greater than four feet in height there should be some type of fall protection. Fall protection systems include landscaping and/or fencing.
Maintenance
Standards for retaining wall maintenance can help ensure retaining walls are maintained in good repair and do not become a hazard to public safety or a visual nuisance to surrounding land or public rights-of-way. For plans containing retaining walls located on or affecting multiple properties there should be adequate provisions for private access, maintenance, and repair.
Examples
Peters Township, Washington County, PA
The Peters Township Grading Ordinance prescribes a maximum wall height and a maximum slope between wall terraces.
Needham, MA
The Town of Needham MA Zoning Ordinance prescribes retaining wall height in relationship to yard setbacks in the Retaining Wall Standards.
Brea, CA
The Brea City, CA Zoning Ordinance prescribes materials, landscaping, and the form of retaining walls in Retaining Wall Standards.
Lake Forest, CA
Retaining Wall Design Guidelines by the City of Lake Forest address context compatibility, design features, and visual interest.