Segmental Retaining Walls: Where Form and Function Meet
Enjoying your backyard to its fullest often involves adding amenities like built-in barbecues, fireplaces, ponds, expanded patios or elegant walkways. To achieve such designs, homeowners often have to cut into or fill in the existing soil to create a suitable foundation for the project. Many times, a retaining wall is needed to create these features. A hilly backyard or steep slope may also call for a retaining wall. Whether creating a low wall for bed borders, walls for sitting in the garden, or a tall reinforced wall to tame a sloping bank, homeowners have many options for creating beautiful, functional walls.
One of the most popular options today that offers both affordability and aesthetic value is the use of segmental retaining wall blocks. Segmental blocks come in a variety of textures and colors, and can be used for projects large and small. Because they are mortarless, lightweight, and easy to stack and install, they have become a very popular alternative. The options are nearly limitless.
Building Your Wall
If you are considering a retaining wall project using segmental blocks, you’ll first have to determine how tall the wall will be; this will help you decide what size blocks to choose. If your wall will be less than 4 feet tall, a 3- to 4-inch-tall product makes the most sense. Walls taller than 4 feet will require some reinforcement and engineering, and should incorporate blocks in the 6- to 8-inch range. Picking the right product for your application is important. Though smaller projects can often be done in a “do-it-yourself” fashion, a qualified, experienced contractor can also help you select the appropriate wall system. It’s especially important to involve the pros when your wall is greater than 4 feet tall.
The next step is to consider the design of your wall. A professional installer can help lay out the location of the wall to determine exactly how tall and long it will be. The installer will consider if there are any slopes in front or behind the wall, and will consider how the wall relates to nearby structures. If the wall will be tiered, located near water, or in poorly drained areas, the installer may involve an engineer to properly design the wall. In most cases, your contractor will need a permit to construct a wall over 4 feet, and will need to submit engineering plans for approval by your county, town or city building office. The expense for engineering and permits should be included in your bid.
Check Before You Dig
Before constructing a retaining wall, you or your contractor are required by law to contact your local utility notification service, known as “Miss Utility,” 48 hours in advance of any digging. To do this, dial 811, a national “one call” service that is free and easy. Homeowners often make risky assumptions about whether or not they should get their utility lines marked, but every digging job requires a call—even small projects like planting trees and shrubs. Digging without calling can disrupt service to an entire neighborhood, harm you and those around you, and potentially result in fines and repair costs.
Whether you are building your own wall or hiring a contractor to install it, you should be aware of the minimum industry construction guidelines, as established by the National Concrete Masonry Association (NCMA).
For starters, it is important to have a firm and level ground surface to begin construction of a segmental retaining wall. The contractor will need to excavate a small trench below grade to create a leveling pad for the wall.
The leveling pad is a surface that is a minimum of 6 inches thick and about 24 inches wide, constructed with good road base material. This pad must be well compacted and should result in at least one block below grade. The minimum industry standard is to always bury at least 6 inches of the wall below grade, or 10 percent of the total wall height.
NCMA also recommends that segmental retaining walls have a drainage zone behind the wall block that is a minimum of 12 inches wide. In most cases, a 4-inch perforated drain pipe is also positioned in the drainage stone to collect any water. This pipe should drain from the wall at the face, or at the side of the wall if possible. Soil behind the wall must be well compacted and free of debris.
Manufacturers make durable concrete blocks to industry standards, addressing strength and dimensional tolerances, and help recommend the appropriate block size, shape and color. Most units also come with a solid capping block that is typically 3 or 4 inches thick. The caps are attached to the top course of the wall using concrete adhesive.
Reinforcing the Soil
Typically, walls 4 feet or taller need soil reinforcement such as geogrid, a polyester yarn that is woven or knitted into a stable structure with a PVC or polymeric coating. The requirements for the type, length and spacing of geogrid should always be done by a licensed professional engineer. Your contractor should always have an engineer he relies on to prepare plans and wall design calculations.
Segmental retaining walls are a durable and economical solution to incorporate elevation changes into a landscape design. Since they are manufactured to perform over the life of the structure, little or no maintenance is required. Because of the small unit sizes and variety of colors, textures and shapes, they provide incredible design options to fit almost any backyard scenario.
Kevin Earley is director of commercial sales for Nicolock, a manufacturer of unit concrete products. Robert Powers is owner of P&W Architectural Stone & Landscaping LLC, a firm dedicated to the sale and installation of man-made and natural stone, hardscape systems and retaining walls.