Retaining Walls: Brands, Styles and Options Add Up
By Erik Skindrud, regional editor
This view gives an idea of the versatility of modern retaining wall systems. The blocks seen here employ fiberglass pins for reinforcement, and can be used for retaining walls, planter beds, tree rings, steps, amphitheaters, etc. Small-block walls like these can contain significant slopes when built in stepped configurations, but many state and local ordinances regulate the size and height of un-engineered walls.Photo courtesy of Keystone Retaining Wall Systems, Inc.
The development of interlocking concrete retaining wall units has led to an expanded number of options for landscape architects and engineers. That’s the good news. On the other hand, more education is required to stay abreast of the options and reinforcement systems available.
Most retaining wall products use anchoring systems that differ considerably. Learning more about what’s out there is a necessary first step to choosing wisely when it comes to wall units for a specific project, budget and site.
The following introduction to retaining wall products and anchoring systems is based on text supplied by the National Concrete Masonry Association (NCMA).
With the photos that accompany it here, the article attempts to supply common information that applies to all retaining wall designs. More specific information and specifications, of course, can be found at manufacturer web sites or at the NCMA web site: www.ncma.org
This open-faced interlocking wall design is an economical choice often utilized by state departments of transportation. Groundcover and other plantings are sometimes grown between the units. These are manufactured by Criblock. (The wall at upper right is built with a different system by Lock + Load.)Photo by Erik Skindrud
Segmental Anchoring Systems
Segmental retaining walls (SRWs) are gravity retaining walls that rely primarily on their mass (weight) for strength and stability. The system consists of concrete masonry units which are placed without the use of mortar (dry stacked), and which rely on a combination of mechanical interlock, unit-to-unit 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 which extend into the backfill to increase the effective width and weight of the gravity mass.
Segmental retaining walls are flexible structures, so the footing does not need to be placed below the frost line, provided there is sufficient foundation-bearing capacity. SRW units are manufactured in conformance with standards and specifications to assure that units delivered to a project are uniform in weight, dimensional tolerances, strength, and durability—features not necessarily provided in site-cast materials.
This Minnesota wall is built of block units of varying sizes, which helps create a more organic, hand-built appearance. The blocks used here weigh 77, 45 and 34 lbs. and utilize a rear-lip system for alignment and stability.Photo courtesy of Anchor Retaining Wall Systems
Fitting It Together
The SRW system is composed of units whose size and weight makes it possible to construct walls in the most difficult of locations. Curves and other unique layouts can be easily accommodated. Segmental retaining walls have the ability to function equally well in large-scale applications (highway walls, bridge abutments, erosion control, parking area supports, etc.) as well as smaller residential landscape projects.
A close-up of the Lock + Load system shows that each “block” is really a relatively-thin face piece. Each unit is a two-part cantilevered system that is independently-anchored by soil. The system offers some of the characteristics of a wall that is reinforced by multiple geogrids.Photo by Erik Skindrud
Since SRW units are available in a variety of sizes, shapes, textures and colors, segmental retaining walls provide designers and owners with both an attractive and a structurally sound wall system.
Segmental retaining walls can be designed as either conventional or as reinforced soil. The structural capacity of the SRW system will vary with the SRW unit size, shape, batter, etc. Manufacturers recommendations should be followed regarding the capacity of their particular system for the soil loads under consideration.
This view shows a good mix of the practical and aesthetic. Terraces spread out the forces that can cause a vertical wall to fail, while providing opportunities for plantings. A variety of erosion-control and water feature hardware can be paired with most wall block products.Photo courtesy of Keystone Retaining Wall Systems, Inc.
Conventional SRWs are constructed with either single or multiple depths of units. For stability, the conventional SRW structure must have sufficient mass to prevent both sliding at the base and overturning about the toe of the structure. Since the system consists of individual units dry stacked one atop another, shear capacity is an important component to assure that the units act together as a coherent mass. Shear capacity provides a means of transferring lateral forces from each course to the succeeding course. This is provided by the frictional resistance between SRW units. Grip and strength are provided in the form of “keys” or leading/trailing lips which are an integral part of the units or by the use of clips, pins, or compacted columns of aggregate placed in the open cores.
Retaining wall blocks come with a number of shapes and anchoring systems that grows each year. Instructions and specifications differ for each, making careful research a must.Image courtesy of www.ncma.org
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Structural stability can be increased 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. Taller walls can also be achieved by using multiple depths of units. The multiple depths of units increase the weight of the wall system and provide a more stable base and greater resistance to soil pressures created by gravity.
Manufacturers of retaining wall units incorporate different features to increase shear strength and slip resistance. Slips, pins and interlocking units are all used to increase wall strength.Image courtesy of www.ncma.org
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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, and/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. The geosynthetic (fiber mat) reinforcement and the soil in the reinforced zone acts as a composite material, effectively increasing the size and weight of the gravity wall system.
Wall block manufacturers produce products in a variety of textures and colors. This example by Versa-Lok employs a top-pinning system. The slots where the securing pins fit can be seen on the top of the blocks shown here.Photo courtesy of Versa-Lok Retaining Walls
The basic elements of each segmental retaining wall system are the foundation soil, leveling pad, segmental retaining wall units, retained soil and drainage fill. (For reinforced soil SRWs, we can include the mass of soil reinforcement.)
Foundation soil: The foundation soil is the soil that supports the leveling pad and the soil zone of a soil-reinforced SRW system.
This big-block product is heavy enough to hold back slopes up to eight feet high without special excavation and reinforcement. Don’t strain your back, though—these blocks weigh 240 lbs. each and are installed with a custom clamp attached to a backhoe. This Versa-Lok product is called Brute.Photo courtesy of Versa-Lok Retaining Walls
Leveling pad: The leveling pad is a level surface, consisting of crushed stone or unreinforced concrete, which distributes the weight of the SRW units over a wider area and provides a working surface during construction. The leveling pad typically extends typically 6 in. (152 mm) from the toe and heel of the lowermost SRW unit and is at least 6 in. (152 mm) thick.
Segmental retaining wall units: Segmental retaining wall 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.
Drainage fill: Drainage fill is free-draining granular material placed behind the wall to facilitate the removal of 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. The dry stacked method of construction used for segmental retaining walls permits water to drain through the face of the wall, aiding in the removal of groundwater. In some cases, a geotextile filter is installed between the drainage fill and the infill to protect the drainage fill from clogging.
Reinforced soil: Reinforced soil is compacted structural fill used behind soil-reinforced SRW units which contains horizontal soil reinforcement. A variety of geosynthetic soil reinforcement systems are available.
This is not a segmental retaining wall, but a 7-foot-high wall built of local stone and two-foot-thick reinforced concrete. This wall dates to the 1940s and holds back scree slopes to create parking at Mount Baldy ski resort above Los Angeles, Calif. If the soil here were not highly permeable, hydraulic loading could create problems.Photo by Erik Skindrud
Typical designs and specifications for segmental retaining walls should be prepared by an engineer or landscape architect who has technical knowledge of soil and structural mechanics. Each SRW unit manufacturer can provide design information tailored to that product, which will indicate the wall heights and design conditions when an SRW should be designed by a qualified engineer.
In addition, SRW systems should be designed by a qualified engineer when:
- structures will be surcharged
- walls will be subjected to live loads
- walls will be founded on poor foundations
- the nature of the design conditions requires special consideration.
The following general site information should be provided:
- a wall profile, including the grade at the top and bottom of the wall, the physical elevation of the top and bottom of the structure to be retained, and the variation of the design section along the height of the wall,
- a description of the infill, foundation, and retained soils,
- a wall plan, which should include geometry for curved wall lengths and the proximity to any existing or proposed surcharges, structures, or utilities that may affect wall construction or performance. Ends of the wall should be designed with consideration of how surface water flow is directed around the wall ends to prevent erosion. This data should be sufficiently accurate to develop an efficient, safe, and cost-effective structural design.
Guide specifications for segmental retaining walls are available in standard Construction Specifications Institute (CSI) format in the Design Manual for Segmental Retaining Walls. Two guides are presented: a materials specification (product/method) and an end-result specification (design/build). The traditional product/method specification, designating materials and installation requirements, stipulates that a site-specific design be performed by the engineer.
Designs should be such that specified SRW and soil reinforcement properties can be met by a number of manufacturers, and should include properties of the on-site soil. SRW and soil reinforcement properties are then specified as the minimum properties that must be met. The end-result specification can be used to solicit proposals from various segmental retaining wall suppliers. Each supplier is then required to furnish a project-specific engineering design for that supplier’s particular system.
This type of specification requires one source, experienced in the design and construction of SRWs, to be responsible for the wall and assures it will be built economically due to competition.
Build It Right
The success of any segmental retaining wall installation depends on complete and accurate field information, careful planning and scheduling, the use of specified materials, proper construction procedures, and inspection. It is good practice to have the retaining wall location verified by the owner’s representative. Existing and proposed finish grades shown on the drawings should be verified to ensure the planned design heights are in agreement with the topographic information from the project grading plan. The contractor should coordinate the delivery and storage of materials at the site to ensure unobstructed access to the work area and availability of materials. Materials delivered to the site should be accompanied by the manufacturer’s certification that the materials meet or exceed the specified minimum requirements.