Segmental Retaining Walls —
Proper Analysis Is Key to Design Success

By Gabriela Mariscal, NCMA Geotechnical Engineer

Segmental retaining walls create a stunning entrance to the 1600-acre Seven Bridges residential community in Platte City, Missouri. Using different size concrete block units gives the SRW bridge the appearance of a vintage stone structure at less cost and requiring less maintenance.
Architect/Engineer: Shafer, Kline, & Warren, Inc., Lenexa, Kansas

Design flexibility, aesthetics, performance, ease of installation and cost effectiveness make the segmental retaining wall (SRW) a valuable addition to any landscape architect’s design tool box.

The landscape design for this Delaware, Ohio residence turned a significant grade into multi-level spaces that add beauty and visual engagement. Steps and tiered walls tie the grade changes together to give the impression that the house is elevated high on a hill. The dramatic entrance to the house features curved retaining walls that double as seat walls surrounding planting beds.
Architect: Gary Farrell, Wilson Landscape Associates, Columbus, Ohio

The use of SRWs has expanded substantially over the past 15 years and continues to grow at a rate of about three percent a year. Chances are you will be involved in an SRW design project, if you have not already.

SRWs are modular block retaining walls used for vertical grade change applications. The walls are designed and constructed as gravity retaining walls and reinforced soil retaining walls. The system consists of dry-cast concrete units that rely on their unit to unit interface and mass to resist overturning and sliding. The systems may also employ soil reinforcement that extends into the backfill and allows for the construction of walls with significant heights (e.g. in excess of 50 feet) that could not be accomplished with the units alone.

As gravity retaining walls, SRWs rely primarily on their mass (weight) for stability. They use no mortar so they can be assembled quickly. Reduced labor time/costs, durability, and low or no maintenance requirements make SRWs cost effective options for addressing steep grades, preventing erosion and meeting other landscape design challenges.

To combat erosion from the Chesapeake Bay, a 5,500-square foot segmental retaining wall was designed for this shoreline residence in Onancock, Virginia. Concrete is not only aesthetic, but sufficiently rugged to combat the marine environment, and environmentally friendly so that it doesn’t add pollutants to adjoining waterways.
Architect: James Brawley, Landmark Design Group, Virginia Beach, Virginia

SRW units are “dry stacked,” relying on their weight and frictional, mechanical or shear interlock to create safe structures that can feature a range of patterns, textures, colors, and finishes.

The International Building Code requires a building permit for earth retaining structures over four feet high. In most states and cities, SRW structures less than six feet high (from leveling pad to top of wall) not subject to surcharges can be built following manufacturers design tables and don’t require a design provided by a registered professional. It is always recommended to become familiar with local requirements for SRWs before starting a job.

Key to safe SRW design and construction is proper stability analysis. Years ago, this involved tedious manual calculations, but software now makes the analysis fast, convenient and reliable.

Hardscaping can be elevated to towering dimensions. The segmental retaining walls for the Briarwood Community in Kansas City, Missouri, while visually striking, support pad site development and allow for the construction of a $30 million residental-commercial building complex on this Kansas City, Missouri hilltop.
Architect/Engineer: Shafer, Kline, Warren, Inc., Lenexa, Kansas

Using product data from a specific SRW-geosynthetic systems and geotechnical data, software can analyze the design to determine the margin of safety against a number of factors affecting stability.

The non-profit National Concrete Masonry Association (NCMA) developed its first SRW design software in 1993, updating the software and companion reference manuals over the years to incorporate the latest findings from SRW research. NCMA also assists design professionals and leading SRW system manufacturers with research, testing and publications.

Four Important Stability Analyses

Ideally, anyone designing a SRW should have a basic understanding of four important analyses - those of external stability, internal stability, facing stability, and internal compound stability.

External stability examines the reinforced soil zone and the facing column to determine the appropriate geosynthetic length to overcome base sliding, overturning about the toe, and foundation bearing capacity.

Internal stability analyses for geosynthetic reinforced soil walls determines the appropriate strength and distribution of the geosynthetic layers to ensure that the structural integrity of the reinforced zone is preserved with respect to reinforcement over-stressing, pullout of geosynthetic reinforcement layers from the anchorage zone, and internal sliding along the reinforcement layers.

Facing stability analysis helps ensure the facing column is stable at all elevations above the toe of the wall and connections between the facing units and reinforcement layers are not over-stressed.

The site for the Montao de El Dorado retail/restaurant complex near Sacramento, California suffered from a very steep grade. SRWs proved more cost effective than cast-in-place concrete to create a 20-foot high wall. The warm, sandstone color of the SRW units fit neatly into the Southwestern look of the complex’s architecture. The design also incorporates columns, decorative lighting and wrought iron fencing.
SRW Contractor: Retaining Walls Company, Tracy California

Internal compound stability analyses the coherence of the block-geosynthetic-soil system through potential compound slip circles that originate behind the soil-reinforced SRW and exit at the face of the wall.

The internal compound stability method, introduced in the Third Edition of the DMSRW and new SRWall 4.0 software, is a special case of the global stability analysis and does not replace it. The analysis considers the three parts in the system: the un-reinforced soil forces analyzed through slope stability methods; the reinforcement with resisting forces; and the facing contributing with resisting shear or connection forces.

Compound failure surfaces will not generally be critical for simple structures with rectangular geometry, relatively uniform reinforcement spacing, and a near vertical face. But if complex conditions exist (high surcharge loads, significant slopes at the toe or above the wall, or tiered structures), compound failures may be a design limit state.

Segmental retaining walls at this residence are used to create inviting elevated garden and patio spaces that connect the house to the outside environment.
Architect: R. Patrick Worzer, ASLA/RLA, Land Designs Services, Inc, St. Louis, Missouri

While future SRW systems will likely offer greater performance standards allowing for a broader range of applications, a basic understanding of the analyses required for safe SRW design will always serve the landscape architect well. Landscape architects wanting more information on proper SRW design are welcome to visit the National Concrete Masonry Association at www.ncma.org.

Note: NCMA makes a free 30-day trial of version 4.0 of NCMA’s Segmental Retaining Walls Design Software (SRW4.0) available at its web site.