The popularity of segmental retaining walls (SRWs) has exploded over the past 15 years. The SRW and paver markets have grown 3% annually. The general landscape market will continue to grow about 6% annually over the next four years, according to the National Concrete Masonry Association (NCMA) and Freedonia market research.
Segmental retaining walls have many advantages, including design flexibility, aesthetics, economics, ease of installation, structural performance, and durability. Applications include landscaping walls, structural walls for changes in grade, bridge abutments, stream channelization, waterfront structures, tunnel access walls, wing walls, and parking area supports. Like any other system to function as planned, SRWs must be properly designed and installed.
SRWs function as gravity structures by relying on self-weight to resist the external destabilizing forces of retained soil (backfill) and surcharge loads. The durable and long-lasting system consists of manufactured modular concrete units (typically dry-cast, machine-produced concrete) placed without mortar (dry stacked), which rely on a combination of mechanical interlock, unit to unit interface friction, or shear capacity and mass to prevent overturning and sliding.
The units may also be used in combination with horizontal layers of soil reinforcement that extend into the backfill to increase the effective width and weight of the gravity mass.
SRWs must be properly designed and installed. The "Design Manual for Segmental Retaining Walls 3rd Edition" (DMSRW) and the companion software, SRWall 4.0, which was recently published by NCMA, outline minimum design recommendations and introduce new evaluation procedures for compound stability failures in SRWs.
The following information from the DMSRW does not replace proper design practices. However, it does provide a basic outline for designers and inspectors.
Engineered versus non-engineered
The International Building Code, Section 105, requires a building permit for earth retaining structures over 4 feet in total height. Many local building codes or officials also require a design prepared by a design professional, although there are many locations without provisions for engineered design. Where there is no specific requirement, NCMA suggests the guidelines in the table below.
For tiered or terraced walls, or multiple walls to create a change in grade, the engineered design method is suggested, except when the total combined height is less than 6 feet and the horizontal spacing between walls is at least twice the height of the lower wall (i.e., H < 6 ft and D > 2h1).
Engineered design method
Stability analyses for geosynthetic reinforced SRWs under static and seismic loading conditions involve multiple calculations to establish factors of safety against external, internal, facing, and internal compound modes of failure. The factors of safety for the SRWs are calculated by dividing the resisting forces by the driving forces on the system.
External stability calculations consider the reinforced soil zone and the facing column as a monolithic gravity structure. The evaluation of factors of safety against base sliding, overturning about the toe, and foundation bearing capacity are similar to that used for conventional reinforced concrete masonry gravity structures.