Specifying a Segmental Retaining Wall

By Robert Bowers, P. Eng., licensee support manager at Risi Stone Systems

A grading plan with the proper specifications is a must if you want accurate, comparable quotes on your segmental retaining wall. The plan should include a designation of the top and bottom of the wall, the grades in front and behind the wall, and a general idea of the usage of the wall. Because SRWs are tailored specifically for the site, the requirements must be accurate for each bidder, otherwise price can fluctuate wildly and what is built might not satisfy the end user’s requirements.

The following is a list of elements you should address when developing a design that you want to have different installers competitively bid on.

When specifying a segmental retaining wall, or SRW, it is important that your design package includes enough information, so that each installer understands the requirements of what is to be constructed and the limitations that they will have to work within.

This will help ensure that all bids you receive are accurate, complete, and comparable to each other. It is possible for you as the specifier to approach a segmental retaining wall supplier and have them develop the structural drawings for the SRW on your specific project. You should be aware that the structural design they develop will only be applicable to the SRW system they supply. If your intention is to receive competitive bids based on different SRW systems, it is probably better to provide only the design requirements and leave the structural details up to the installer. Typically, the installer will provide your specification package to their SRW supplier and have them develop a structural design for the SRW. It is this design that the installer will base their estimate on.

Grading plans provide the most significant amount of information necessary for the structural design of the SRW and they should indicate:

• Top of Wall (TW) and Bottom of Wall (BW) elevations along the entire length of the wall and specifically where changes in the slope along the top or bottom of the wall occur;
• Precise location of the wall relative to other structures (catch basins, buildings, parking areas, etc);
• Alignment of the wall including specific angles for corners, start and end points for curves, and the radius of curves;
• Slopes above and below the wall;
• Drainage swales and other surface water management details;
• Details of any structures, above or below ground, within a distance from the wall equal to three times the height of the wall, even if these structures are on the adjacent property;
• Structures that are to be installed in conjunction with the SRW (e.g. handrails, guardrails, fences, and curbs);
• If site soils are to be used as the replaced soil in the reinforced soil zone, reusing site soils will typically require larger SRW units or stronger and longer geogrid lengths, as well as additional drainage considerations; and
• The allowable values for effective friction angle and in-place density for the retained, foundation, and infill zones; the use of different values can significantly affect the contractor’s structural design of the wall. If these values are not available, we recommend that an assumed set of parameters be provided so that competing designs use the same information.

When determining the location for the wall, you must consider the following factors:

• The horizontal space required for the SRW facing will be the sum of the SRW unit’s depth and setback on each course. Different SRW systems will have different horizontal space requirements. Make sure you define your allowable range.
• The complete SRW structure will typically require excavation from the face of the wall back into the embankment a minimum of 60 percent of the total height, and possibly up to as much as 100 percent of the wall height. Building regulations will typically require you to keep the entire structure and excavation limits contained on the property under development. You will also want to keep utility lines and subsurface structures out of the reinforced soil zone. Occupational Health and Safety guidelines for excavations must also be considered as temporary shoring may be required if space constraints exist.

This sidewalk is actually sitting on a 16-foot high retaining wall that is underneath the water. The man-made lake was drained to allow for construction. The designers wanted no on-site cutting, which proved extremely difficult for the SRW manufacturer because the entire project consisted of nearly 6,000 different pieces of concrete. SRW technology dictates that some site modification is necessary. It is important to remember that the more modification in the design, the more costly the project will be.

To achieve the curved effect for this retaining wall that doubles as a staircase, standard coping units had to be saw-cut on site. The pie-shaped edge gives a rotation to each tread, allowing the staircase to wrap around as it descends to the lower courtyard. Geogrid reinforcement was specified for this project due to the height that was necessary for the wall.

It is important to identify the standards to which the wall must be constructed. It is recommended that SRWs be designed in accordance with the National Concrete Masonry Association’s Design Manual for Segmental Retaining Walls, Second Edition and incorporate the requirements of the local building code. If two terraces are separated by twice the height of the lower terrace, they may be considered as separate structures and designed independently of each other. If the separation is less than twice the height of the lower wall, both terraces must be designed together since the upper wall will apply a load to the lower wall. Under this condition, for preliminary design purposes it would be appropriate to consider the height of the wall equal to the height from the bottom of the lowest terrace to the top of the upper terrace. Installers will typically bid on an SRW project based on a unit rate for the face area of the wall. The face area will be the sum of the area exposed above grade and the portion embedded below grade. They may additionally provide a unit rate for the placement of fill between the reinforced soil and the retained soil if required.

If you have considered all of the elements and provided the appropriate level of detail to your installers quoting on your job, you will be well along the path to completing a successful segmental retaining wall project.