Retaining walls are commonly used to level or retain slopes and give them a more vertical character. Generally, the more restricted or congested the site, the greater the need for retaining walls to provide usable space for landscape purposes.
Functional retaining walls are constructed for purely structural needs. For example, they can be used to level, retain, or terrace a sloping area; to maintain an existing grade around a tree or some other landscape feature you're trying to save; to allow a more abrupt change in grade than you can achieve with graded slopes; and to support a level area such as a patio or driveway.
Outdoor steps are modified human-scale retaining walls. Like outdoor steps, some retaining walls serve a purely functional purpose, and you may not be able to incorporate aesthetic considerations such as form, texture, color, shape, scale, and proportion.
Other retaining walls are used for visual effect in the landscape, and generally have several common characteristics. For example, they feature an informal shape and irregular placement of wall materials. They're less significant in size but more pronounced in visual character than functional retaining walls. They complement other landscape elements such as plants, paving and mulches. They're more compatible with human activity and can be adapted to many uses. They can be used as a bench wall, as a base for a fence, or an accent element in the landscape. And the materials used to build decorative retaining walls -- native stone, timbers and colored concrete bricks and blocks -- are chosen to provide the desired visual effect in the landscape.
The higher the retained slope, the more structural stability is required. Many times, critical structural requirements necessitate engineering a design that considers the length of slope, site-soil characteristics, the wall material used, construction space available and height of the wall. Complex structural walls require the design expertise of an experienced engineer. Many municipalities require that walls greater than four feet in height be designed by engineers. You'll need to verify this requirement with your municipality prior to planning construction of any major wall.
When designing a retaining wall, there are seven factors to consider: materials, type of wall, design or on-site placement, drainage, foundations, cant or batter, and anchors or "deadmen".
Installed wall costs generally range from US$20 to $35 per square foot (in 1998) of wall face for dry-laid materials. Cost will vary considerably depending upon materials used and overall wall height. Structural wall costs may range from two to ten times that of dry-laid walls.
Materials. When choosing materials, select the type that is best suited for the desired result. Many long-lasting materials are available for landscape use including flagstone rock, recycled concrete paving treated timbers, vertical poles, precast concrete modular units, poured concrete, and brick veneer.
Type of wall. The type of wall you choose should be determined by need. Decide if you need a poured-in-place concrete "structural" wall or a much less expensive "dry-laid wall" consisting of stacked, open-joint material.
Determine design or on-site placement. Planning ahead will help you avoid the expense and time it takes to relocate a wall or to modify retained areas once they're installed.
Drainage. Most retaining walls fail because of pressure against the wall caused by water or soil-moisture build up behind the wall. All walls should provide for the back-of-wall water to freely drain down and away from the wall. This is accomplished with gravel backfill, or manufactured drainage blankets and drain pipes. Structural walls require "weep" holes to allow water to drain from behind the wall.
Foundations. A wall is only as good as its foundation, and all retaining walls should be built on structurally sound, compacted foundation sub-base material. Leveled and compacted earth or gravel fill are acceptable. The foundation material should extend at least one foot beyond the front and back of the base width of the wall. When building dry-laid stone walls, place the largest, most stable stones on the bottom of the wall, and be aware that the base width may need to be as wide as the wall is high.
Cant or batter. Walls are more stable and structurally secure if they slope back or "lay back" into the retained slope. This amount of variance from true vertical is called "cant" or "batter".
Anchors or "deadmen." Timber walls and other walls of solid horizontal materials usually have "T" anchors or deadmen extending back into the slope into undisturbed earth. This helps walls resist pressures that force them forward, or cause them to pivot on footing material. A good rule of thumb is to provide at least one deadman per 16 square feet of exposed wall face.
Adapted from an article