National Concrete Masonry Association features new methodology in an updated software program and comprehensive design manual.

Research in the segmental retaining wall field is continually refining the design methodology and increasing the application possibilities to larger and more complex limits.

Segmental retaining walls (SRWs) were introduced in the United States about 25 years ago, first in landscape applications and later as structural retaining walls. These systems have great advantages compared with other wall systems, and SRW unit versatility has broadened their use and innovation. SRW units are dry stacked (without mortar) and 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.

The advantages of SRWs include the following:

Design flexibility—The size and weight of SRW units make it possible to construct walls on difficult topography or on limited-access sites. Curves and other unique layouts can be accommodated easily. SRWs can function equally well in large-scale applications (highway walls, bridge abutments, erosion control, parking area supports, et cetera) as well as in smaller residential landscape projects.

Aesthetics—Since SRW units are available in a variety of sizes, shapes, textures, and colors, SRWs provide designers and owners with both an attractive and a structurally sound wall system. Figure 1 shows some of the units available.

Economics—SRWs provide an attractive, cost-effective alternative to other retaining walls. Savings are gained because most on-site soils can usually be used, eliminating costs associated with importing fill and removing excavated materials. Additionally, there is no need for extensive formwork or heavy construction equipment.

Ease of installation—Most SRW units are small enough to allow placement by a single person. The dry-stack method of laying units without mortar allows erection of the wall to proceed rapidly.

Performance—Unlike rigid retaining wall structures, which may display cracks when subjected to movement, the flexible nature of SRWs allows the units to move and adjust relative to one another without visible signs of distress.

Durability—Segmental units are manufactured of high-compressive-strength, low-absorption concrete, which helps make them resistant to spalling, scour, abrasion, the effects of freeze-thaw cycles, rot, and insect damage.

Designs and specifications for SRWs should be prepared by a professional with technical knowledge of soil and structural mechanics. SRW manufacturers provide design information tailored to their products and will indicate the wall heights and design conditions when use of a qualified engineer is strongly encouraged. Unique design conditions that may warrant special consideration include the following:

• structures will be subject to surcharge loads;
• walls will be subjected to live loads;
• walls will be founded on poor foundations; or
• design conditions require special consideration.

The National Concrete Masonry Association (NCMA) has a strong history of providing technical support to the SRW design community through research, software, and publications. This summer, NCMA will release SRWall 4.0 software, which is accompanied by the Third Edition of the Design Manual for Segmental Retaining Walls (DMSRW). The First Edition of the DMSRW was published in 1993, and the Second Edition was published in 1997. In 1998, the Segmental Retaining Walls—Seismic Design Manual was published followed in 2002 by the Segmental Retaining Wall Drainage Manual. The Third Edition of the DMSRW incorporates the design recommendations of the previous DMSRW, SRW Drainage, and Seismic Design manuals into one document.

There are several changes that have been incorporated in the new DMSRW. These changes include the following:

Hinge height concept removed—Research during the last decade has demonstrated that the concept of hinge height does not realistically predict the vertical load on an SRW unit within a wall. Friction that develops between the reinforced soil and SRW units effectively transfers vertical load to the SRW units equivalent to the height of the wall above the unit regardless of hinge height.

Bulging calculation method updated—Bulging of a SRW in the vertical plane occurs when a SRW unit does not maintain its relative position with respect to the SRW units above and below it. The relative position of one course to the next is maintained by shear resistance. The Third Edition of the manual replaced the previous bulging calculation with the more rigorous compound stability analysis, which models the actual failure mechanism when bulging occurs.

Serviceability requirements for connection strength eliminated—When following the construction guidelines provided in this manual, the connection between the SRW unit and geosynthetic reinforcement is prestressed. This prestressing eliminates the serviceability issue for currently available combinations of SRW units and geosynthetic reinforcements. If, in the future, more flexible reinforcements are used to build SRWs, the serviceability requirement provided in the Second Edition of the DMSRW may be appropriate.

Offset surcharges added—Offset surcharges for segmental retaining walls have been added to analyze the effect of the setback in the facial stability calculations, specifically eliminating the surcharge from facial loads for SRW units above the influence zone of the surcharge.

Rectangular distribution of dynamic load added — Research has revealed that the inverted trapezoidal seismic load distribution is extremely conservative. Current research indicates that retaining walls with extensible reinforcement do not behave in that manor. Based on this research, a more realistic rectangular load distribution is used in the updated design manual.

Internal compound stability analysis added—The evaluation of potential compound slip circles that originate behind the soil-reinforced SRW and exit at the face of the wall is called internal compound stability (ICS) analysis (see Figure 1). The analysis is a special case of the global stability analysis and does not replace it. The method introduced in the Third Edition of the DMSRW considers the three parts in the system: the unreinforced soil forces analyzed through slope stability methods; the reinforcement with resisting forces; and the facing contributing with resisting shear or connection forces.

Figure 1: Evaluation of potential compound slip circles that originate behind the soil-reinforced SRW and exit at the face of the wall is called internal compound stability analysis.

For simple structures with rectangular geometry, relatively uniform reinforcement spacing, and a near vertical face, compound failure surfaces will not generally be critical. However, if complex conditions exist—such as high surcharge loads, significant slopes at the toe or above the wall, or tiered structures—compound failures may be a design limit state. The Third Edition of the DMSRW recommends that the responsibility for ensuring adequate compound stability rests with the retaining wall designer.

Bearing capacity, excessive settlement, and global/overall slope stability are also highlighted as geotechnical concerns that could compromise the performance of the system. The Third Edition of the DMSRW places particular emphasis on the use of the geotechnical tools when weak retained or foundation soils, high surcharges, or slopes could influence the system. The Third Edition of the DMSRW recommends that the responsibility for ensuring adequate bearing capacity and global/overall slope stability be coordinated between the retaining wall designer, project civil engineer, and project geotechnical engineer. (See Table 1 for the recommended minimum safety factors to design SRWs)

Research in the SRW field is continually refining the design methodology and increasing the application possibilities to larger and more complex limits. NCMA is providing state-of-the-practice engineering approaches for the analysis and design of segmental retaining walls by releasing the SRWall 4.0 software and the Third Edition of the Design Manual for Segmental Retaining Walls. The new methodology from NCMA provides practical and trustworthy tools to design challenging projects in a broad spectrum of applications from beautiful landscape projects to hard-working, sloped retaining walls subject to large and dynamic loading conditions.


Gabriela Mariscal is a geotechnical engineer with the National Concrete Masonry Association. She can be contacted at gmariscal@ncma.org.


SIDEBAR: Segmental retaining walls 101

System components
The basic elements of each segmental retaining wall (SRW) system are the foundation soil, leveling pad, SRW units, retained soil, gravel fill, and for reinforced-soil SRWs, the soil reinforcement.

Foundation soil—The foundation soil supports the leveling pad and the reinforced soil zone of a soil-reinforced SRW system.

Leveling pad—The leveling pad is a level surface consisting of crushed stone or unreinforced concrete that distributes the weight of the SRW units over a wider area and provides a working surface during construction. The leveling pad typically extends 6 inches from the toe and heel of the lowermost SRW unit and is at least 6 inches thick.

SRW units: SRW units are concrete masonry units that are used to create the mass necessary for structural stability and to provide stability, durability, and visual enhancement at the face of the wall.

Retained soil—Retained soil is the undisturbed soil for cut walls or the common backfill soil compacted behind infill soils.

Gravel fill—Gravel fill is free-draining granular material placed behind the facing units to facilitate the removal of incidental groundwater and minimize buildup of hydrostatic pressure on the wall. It is sometimes also used to fill the cores of the units to increase the weight and shear capacity. In some cases, a geotextile filter is installed between the gravel fill and the infill to protect the gravel from clogging.

Reinforced soil—Reinforced soil is compacted structural fill used behind soil-reinforced SRW units that contains horizontal soil reinforcement. A variety of geosynthetic soil-reinforcement systems are available.

Wall types
SRWs can be designed as either conventional or reinforced soil (see Figure 2). The structural capacity of the SRW system will vary with the SRW unit size, shape, batter, et cetera. Manufacturers’ recommendations should be followed regarding the capacity of their particular systems for the soil loads under consideration.

Figure 2: Segmental retaining walls can be designed as either (a) conventional or (b) reinforced soil.

Conventional SRWs are constructed with either single or multiple depths of units. For stability, the conventional SRW structure must have sufficient mass to prevent sliding at the base and overturning about the toe of the structure. Since the system consists of individual units dry stacked, shear capacity is an important component to ensure that the units act together as a coherent mass.

Shear capacity provides a means of transferring lateral forces from each course to the succeeding one. This is provided by the frictional resistance between SRW units and in the form of "keys," leading/trailing lips, clips, pins, or compacted columns of aggregate placed in the open cores (see Figure 3).

Figure 3: Keys, leading/trailing lips, clips, and pins can provide frictional resistance between segmental retaining wall units.

Structural stability of the SRW also can be improved by increasing the wall batter. Batter is achieved through the setback between SRW units from one course to the next. In most cases, the batter is controlled by the location of shear pins or leading/trailing lips (Figure 3); however, some systems allow some adjustment to the batter.

Taller walls also can be achieved by using multiple depths of units, as shown in Figure 2a. The multiple depths of units increase the weight of the wall system and provide a more stable base and greater resistance to soil pressures.

Reinforced-soil walls should be specified when the maximum height for conventional gravity walls is exceeded or when lower structures are surcharged by sloping backfills, live loads, or have poor foundations. A reinforced-soil SRW is designed and constructed with multiple layers of soil reinforcement placed between the SRW courses and extending back into the soil behind the wall at designated heights and lengths as shown in Figure 2b. The geosynthetic reinforcement and the soil in the reinforced zone act as a composite material, effectively increasing the size and weight of the wall system.