Cuts and fills along the edge or end of a roadway occur when the existing ground and proposed elevations differ. A retaining wall system is required if the elevation difference cannot be maintained with a maximum allowable ground slope per design standards. The intent of the wall limit shall be clearly defined on the drawings, including details for the wall ends.
All retaining walls shall be at least 2 feet clear of the sidewalk and 3 feet clear of the curb face where there is no sidewalk, unless otherwise approved by the Director of Transportation. Barriers, railings or fencing at the top of these walls may be required to provide safe passage. This requirement will be determined by the Traffic Division of the Department of Transportation. Refer to Std Plan 141 for the standards.
All design shall be performed by or under the direction of a professional structural engineer.
The following design standards shall be used:
- City of Seattle Right-of-Way Improvements Manual
- City of Seattle Standard Plans for Municipal Construction
- City of Seattle Standard Specifications for Municipal Construction
- Washington State Department of Transportation (WSDOT) Design Manual
- WSDOT Bridge Standards Drawings.
- AASHTO LRFD Bridge Design Specifications, 4th edition, with 2008 and 2009 interims.
- International Building Code (case by case basis).
Wall Types and Details
Erosion facing/non-structural walls: When the soil is determined to be stable under static conditions by a geotechnical engineer, the erosion facing wall, such as rock facing or decorative stackable masonry blocks may be used. These are considered non-structural walls. Refer to Chapter 4.21.1 Rock Facing Wall Standard plans. The maximum allowable height for this wall system is 8 feet.
Alternate erosion facing may include stackable masonry blocks, jut matting, etc. Any near vertical erosion facing needs to meet the same slope and clearance criteria as rock facing and requires the approval of the Director of Transportation.
Structural Walls: When soil is unstable under static conditions, a structural wall is required to support the soil permanently. SDOT accepts both the standard and non-standard structural wall types. For proposed structures that are non-standard and lack the long term performance history, pre-approval of the concept is required before design work should proceed.
An example of a non-standard wall is structural soil (geoweb soil wrap) wall. This wall system may be designed where no utilities or excavations would be anticipated. Permission to use these types of walls is rare, due to future utility uses and the no-dig zone space required for the soil mass.
Proposed City-owned and maintained structures require the approval of the Roadway Structures group of the Department of Transportation.
- If a retaining wall is not designed to resist hydrostatic pressure, install a 6-inch diameter subsurface drain and 3-inch diameter PVC weep holes spaced 12 feet apart. The weep hole shall be placed 6 inches above the finished grade at the toe of the wall, per City of Seattle Standard Plans. The subsurface drain shall be tied into the City drain systems, as approved by SPU. Refer to the standard plans in Chapter 4.21.1 for weep hole detailing.
- If a retaining wall is designed on top of an existing retaining wall, the new retaining wall should be designed with the assumption that the lower wall is not contributing to the lateral support to the slope below the newly designed retaining wall unless approved by Roadway Structures. Similarly, if the wall is designed at the toe of a slope below an existing retaining wall, the toe wall shall include the surcharge of the existing wall unless approved by Roadway Structures. (See Standard Plans 800 and 801.)
- Wall details that are water and debris traps should be avoided.
- Pedestrian guardrail openings shall not exceed 4 inches wide and the general layout shall discourage climbing activity.
- Private and public retaining walls shall not be built integrally. Total structural isolation is required for adjacent walls.
- Concrete walls that are prone to graffiti shall be coated with a moisture barrier and anti-graffiti paint.
The following are types of structures common to the City:
Reinforced Concrete Cantilevered Retaining Walls: Standard WSDOT cantilever reinforced concrete retaining wall design is acceptable. For walls longer than 30 feet, those must have expansion joints at a maximum of 24 foot spacing.
Soldier pile walls: Soldier pile walls are applicable when the soil types are unsuitable for other wall systems and where the right-of-way is unavailable for a standard cantilever retaining wall. This wall system shall have a reinforced concrete face and may be constructed with shotcrete or cast-in-place methods. Precast concrete panels are also acceptable on case by case basis. Timber laggings are not considers as a permanent structure. Weep holes and drainage shall be provided behind the wall. Unless enclosed by structural concrete, steel wide-flange piles should be coated with zinc rich primer and coals tar epoxy per City standard Specifications from the top of the wall to a minimum of 2 feet below the bottom of the lowest lagging.
Stairways: Stairways in public right-of-way shall be designed according to Seattle Standard Plans 440a and 440b Pedestrian lighting shall be provided for stairways.
Handrails and pedestrian rails: Railings shall be designated as "handrails" or "pedestrian rails" and their usage shall be as determined by Figure 4-24: Determination of Hand Rails vs. Pedestrian Rails.
Handrails shall be designed in accordance with Standard Plan 442 or 443, as appropriate.
Pedestrian rails shall be designed in accordance with criteria established by the SDOT Director, in compliance with guard requirement of the International Building Code (IBC), meaning that they shall have a maximum spacing of 4” for vertical elements of the railing.
Traffic barriers: Vehicular railings on bridges shall be designed in accordance with AASHTO standards. Vehicular railings on retaining walls shall be designed in accordance with AASHTO standards unless otherwise approved by the SDOT Director. Vehicular guardrails on approaches to structures shall be designed according to Washington State Department of Transportation standards. For guardrails not on structures, the SDOT Director will determine the type of guardrail required.
Pedestrian overpasses / underpasses and skybridges: Pedestrian overpasses and underpasses typically span a transportation right-of-way and provide a connection between destinations that have a high volume of pedestrian use. Pedestrian overpasses shall be designed in accordance with criteria established by the SDOT Director. Skybridge permit policies are defined in the SMC section 15.64 Skybridge Permits. SDOT requires AASHTO LRFD standard designs over the street provided it does not conflict with the applicable code to the building where the bridge is bearing on.
Areaways: Use the IBC for structural design with 250 lbs/sf live load for sidewalk top or HS20 whichever governs. Include curb ramps in the surface of the areaway to mimic Standard Plan 422 at intersections (including “T” intersections).
Bridges: Bridges shall be designed in accordance with AASHTO LRFD Bridge Design Specifications.
Other structures: Sign support structures and streetlight poles shall be designed in accordance with AASHTO Standard Specifications for Structural Supports for Highway Signs, Luminaires, and Traffic Signals.
Slope stability shall be maintained at all times during the constructions of the wall system. Common methods used include temporary cut slopes, temporary shoring or temporary shotcrete.
Submittal of the construction specifications are required for the mechanically stabilized earth (MSE) wall.
Permission: During construction access rights from adjacent property owners may be needed. A negotiation for these rights by the developer needs to begin as soon as the need is discovered. SDOT will provide the needed document for signature by the affected property owner giving the right to enter private property for the scope of the construction and maintenance of the project.
Ownership/Maintenance Acceptance: Acceptance of privately built retaining walls that will be maintained or owned permanently by SDOT require the following prior to formal acceptance:
Reduced size as-built final design calculations with the Structural Engineer’s stamp and signature as well as the geotechnical report submitted to Roadway Structure’s staff and all punch list items are completed to the satisfaction of the inspector.
- As-built shop drawings and plans in Mylar and are filed in the City’s Record Center, (SPU’s Record Vault).