Build a BETTER SEGMENTAL RETAINING WALL

By Brad Stowe Grotto Hardscapes/ Chandler Concrete

A Brief History

The basic principles and methods being used today to build a drystacked segmental retaining wall (SRW) have been around for thousands of years. Early notable examples of such walls include ziggurats, found in the Middle East, and the Great Wall of China. Constructed millennia ago, these structures basically consist of drystacked stones backfilled with compacted soil that has been reinforced with woven reed mats or tree branches.

The first geosynthetic reinforced retaining walls in the United States were constructed in the mid-1970s. Then, in 1975, a Canadian concrete block producer began to market a dry-stacked block for retaining walls, and U.S. block producers followed suit in the 1980s, producing small SRW blocks. By the mid-1980s, geosynthetics were combined with SRWs to build larger structures. Today, SRW units are used widely, both in simple residential applications and in large commercial projects.

Historically, the most common method for building a retaining wall involved using natural stone. However, this method has a big drawback, in that the taller the wall being built, the more massive the structure needs to be. Another common practice was to build with railroad ties and timbers, but these materials have a very low life expectancy, and many of the preservatives used on them have since been banned by the U.S. Environmental Protection Agency. Other methods include mortared concrete masonry units and cast-inplace concrete walls—both of which have high installation costs. SRWs, on the other hand, have become a popular alternative, and offer a high level of design flexibility.

Planning and Designing a Retaining Wall

A successful SRW project begins in the design phase. Knowing the soil types, drainage and water flow on the build site is crucial. In general, walls are best constructed when backfilled with gravels and sandy soils, due to better drainage and greater ease of compaction. Unfortunately, in our region of the country, the soil types we encounter most often are clays, which are difficult to compact and very sensitive to changes in water content. When building a gravity wall—a type of wall that only uses the mass of the SRW units to hold back the forces applied to it—the soil type also affects the maximum height of such a wall. Soil types will also dictate how much additional soil reinforcement is required on taller walls, as well as which types of compaction equipment will be needed. For example, gravels and sands can be vibrated into place with a vibratory tamper, whereas clay soils need to be pounded into place with a jumping jack tamper or sheepsfoot compactor.

Additionally, different types of SRW units have different maximum heights, ranging from 1.5 feet to 4 feet for most types of wall units, when used in constructing a gravity wall. These heights assume that the wall is being constructed with good soils and proper soil compaction, and that no slopes or surcharges are present (a surcharge is any added weight above a retaining wall, such as a driveway, patio or swimming pool). Slopes or surcharges located within a distance of twice the height of the wall will affect how the wall should be constructed. If a wall needs to be taller, or has a slope or surcharge, geogrid should be used to reinforce the soil behind the wall.

Given the complexities of some wall designs, consider whether or not you should involve a licensed geotechnical engineer on your project. Typically walls 4 feet or less in height do not require an engineerapproved wall design. However, it’s always best to consult with your local building inspector prior to construction—an engineered design will give you peace of mind that your wall will last over time.

Retaining Wall Construction Best Practices

The first step in construction is to excavate for the wall leveling pad. It’s always best to start at the lowest point of the wall, since it’s much easier to step up your block than to attempt to step down (a similar idea applies if your wall will have a corner, since it’s easier to work out from a corner than to work toward a corner).

The leveling pad needs to consist of 6 inches of road base aggregate (crusher run) or clean stone. In some cases, a lean, very weak concrete mix can be used. As a rule of thumb, embedment should be equal to at least 10% of the exposed wall height, but all walls need to have at least 6 inches of the wall embedded—and embedment will need to be increased for taller walls and those built on slopes. Ensuring that the leveling pad is thoroughly compacted and leveled, before placing any block, is crucial.

Next you can begin placing the base course of block. If you’re using a string line, place this adjacent to the smooth back edge of the block. A 4-foot level should be used to check the block level from side to side; a torpedo level can be used to check level front to back. A dead-blow hammer can be useful for adjusting the block.

The below-grade courses should be backfilled with compacted soil, and exposed courses should be backfilled with clean stone (to facilitate drainage). The drainage stone should be 12 inches behind the wall for solid SRWs and 6 inches behind the wall for hollow-core SRWs. Once a block is laid, it should be pulled forward to engage whatever type of connection the particular type of block features. A 4-inch perforated drain pipe should be placed behind the first exposed course and should be vented through the block every 50 feet.

If hollow-core units are used for every course, wall units should be backfilled and core-filled. Compaction should begin at the wall and move away from the wall, performed in small lifts and never to exceed 6-8 inches. If geogrid is being used to reinforce soil, it should be placed perpendicular to the wall and not overlap, and should be pulled tight and staked down to avoid wrinkles. Make sure there are at least 6 inches of soil cover before operating machinery over geo grid. Continue this process for the remainder of the wall.

Caps should be placed above the top course of block and secured with concrete adhesive. Many installers like to allow caps to overhang, which adds a bit of flair to the wall and also helps to hide any imperfections that may be present. One idea to consider is using a contrasting color on the cap to add to the wall’s visual beauty.

Following the guidelines outlined above should help you achieve success in your next wall project.

Jan. 16-20, 2023 | Greensboro, NC