A guide for residential segmental retaining wall systems
Segmental retaining walls (SRWs) can be used in any number of applications. Residential projects can range from a structural application, which may include extending a patio, or creating space for a pool, to a non-structural application, such as planters, fire pits, or seating areas throughout a yard. NCMA has developed a printable guide for SRW systems used in non-structural applications. This article summarizes the design guidelines, material lists, application steps, as well as tips for water management and inspection for installing an SRW. If you are looking for information regarding structural SRW applications, please reference the SRW Best Practices Guide.
Special Application—Tiered Walls
An SRW does not have to be a single, vertical wall. It can be broken into tiers that are offset from one another as shown in Figure 1. If a tiered wall is being considered, it is important to note that tiered walls can also fall under a structural or non-structural application, as described in the previous section.
Materials for SRWs
An SRW is constructed using both manufactured and naturally occurring materials. The appropriate use of each of these materials in a wall can ultimately determine whether a given wall is structurally sound or not, similarly to any other constructed walls. If questions or concerns arise about materials, consult with the certified professional installer for more information.
During the installation and at the wall’s completion, the homeowner and the contractor should follow the construction checklist, provided in NCMA TEK 18-11B Inspection Guide for Segmental Retaining Walls together, available in the NCMA Solutions Center, (www.ncma-br.org). Items covered in the checklist include, but are not limited to:
• Site Conditions
• Placement of Materials
• Finish Grading
NCMA-recommended Tolerances for SRW systems
NCMA recommends the owner requests a copy of the documentation the contractor kept during the construction process for their records.
One of the biggest potential impacts on any retaining wall structure is the infiltration and presence of water within the system. When water is not properly drained, the wall has the potential to retain water like a pool. If water is not able to disperse in a reasonable time frame, water pressure begins to build and push against the wall, which could result in failure. To prevent this from occurring, both granular, well-draining soil and the drainage pipe are used in the wall system to easily remove any presence of water that may occur.
Soil is an important component to ensuring successful wall performance. When used as backfill, soil must be granular and ha the ability for water to drain freely through it, such as gravel and sand. If soil is found to be organic (like top soil) or consisting mainly of clay, then outsourced granular, well-draining soil must be brought in. In the event the local soil is desired to be used but the type is unknown, consult a local design professional.
Gravel fill is a crushed, clean, free-draining material that is placed between and behind the SRW units. The gravel fill acts as a buffer between the soil and the SRW units.. Due to the lack of small shaped particles present in the gravel, incidental water is able to flow freely through the gravel fill and into the drainage pipe where it is drained away from the system. Despite its draining capabilities, the gravel fill is not meant to be used as a primary drainage method without the use of drainage pipe.
The most important consideration during the design and construction of a retaining wall is the drainage. To aid in the drainage and removal of incidental moisture around the wall, a drainage pipe (typically either a perforated corrugated plastic pipe or perforated PVC pipe) is typically placed along the bottom of the wall behind the first row of units. This provides a path for the water to travel away from the interior of the wall by either removing water through the facing or out from the side of the wall.
Geogrid is a synthetic material that in some applications is placed within the layers of the wall as it is being built, providing extra support to the system by unifying the soil and facing into one cohesive unit. It is important to refer to the manufacturer’s recommendations prior to the use of geogrid.
With the appropriate contractor, designer (if needed) and material selected, the installation of the SRW system can begin. NCMA recommends the homeowner review the NCMA SRW Installation Guide, as well as the manufacturer’s recommendations.
Step 1: Site Preparation and Excavation
• Removal of existing vegetation
• Excavation to desired elevation
Step 2: Install Leveling Pad
• Compact trench
• Place/compact gravel for trench
• Place drainage pipe per plans
Step 3: Place Base Course (First Row)
• Place units side by side individually
• Ensure each unit is level
Step 4: Place gravel and backfill materials
• Place gravel in cores, between the units and specified distance behind units
• Place soil behind gravel zone
• Compact at a maximum of 8 in. (203 mm) lifts
• Ensure proper compaction is achieved
Step 5: Install Additional Courses
• Repeat Steps 3 and 4 until the specified height of the wall is achieved
• If applicable, place geogrid per approved plans at required rows
Step 6: Finish Wall
• If desired, place the selected wall cap on top of the SRW with the use of concrete masonry adhesive
• Place, compact and grade soil around the system as needed
• Install landscape (if applicable)
Step 7: Clean the Site
Now that the required materials for an SRW have been identified, it is time to discuss the process of selecting the appropriate designer and contractor. Industry recommendations do not require a designer as long as the wall is a non-structural application as described in Table 1. However, it is important to always consult with your local building code and confirm this is the case.
In a previous article, the importance of hiring an NCMA Certified Contractor were discussed. Those details can be found here.
This guide is available for free download and printing here.