Retaining walls have been used for thousands of years to make use of land that would otherwise be unusable. The Incas used retaining walls to build terraces for farming and cities on the steep slopes of the Andes mountain range in South America. Romans and their successors used retaining walls for protection and to keep their enemies out of cities. Today, as land prices have increased, commercial and residential infill sites that were originally thought to be unbuildable are now viable options using retaining walls. To make these sites buildable and accessible by pedestrians and vehicular traffic, extensive grade modifications need to be made.
Most local building codes require slopes to be no steeper than 1 foot vertical to 3 feet horizontal, or 3/1, without approval from a geotechnical engineer. There are two reasons behind this requirement. The first is for ease of maintenance. A 3/1 slope is still able to be mowed with a standard commercial mower. Also, grass and other vegetation can be easily started on these slopes without expensive geotextile fabric to hold the soil in place until it takes root. The second reason is most soils are stable at a 3/1 slope, and compacting the soil in standard lifts during grading is acceptable. If the geography of the site cannot be graded to meet the local building code, the use of retaining walls will be required.

Retaining walls are designed to hold back the horizontal force of the dirt behind it and any vertical loads (surcharges), such as buildings, lakes, or parked cars that are in proximity. Retaining walls can be broken down into four different types depending on how they resist horizontal forces: gravity walls use their weight, piling walls use the embedment depth at the toe of the wall, cantilever walls use the footing and the weight of the backfill soil, and anchored walls use a mechanical fastener. The figure below shows examples of the different walls.

Most of the retaining walls in residential and commercial sites fall into the gravity, cantilever, or anchored types and can be constructed out of brick, concrete, stone, timber, and proprietary segmented retaining wall units. While the specifics of each retaining wall design will be different depending on material and use, the general construction guidelines are similar. The retaining wall must be designed to support the horizontal and vertical loads without overturning (leaning), sliding, settling, or heaving due to water pressure (walls adjacent to water features).

Most retaining wall failures are caused by excessive lateral earth pressure, hydrostatic pressure, or a combination of the two. A properly designed wall will resist the lateral earth pressure applied by the soil and be backfilled with material that is fast-draining to prevent the soils from becoming saturated or from building up hydrostatic pressure. Retaining walls are not typically designed to hold back saturated or liquefied soils which apply larger lateral forces due to the presence of moisture and the reduction of friction between the soil particles. Hydrostatic pressure causes the soil to act as a liquid because the soil particles are suspended in the water, further reducing the cohesion of the soil. The presence of these excess forces may cause the wall to fail. Proper design and maintenance of the integrity of the soil behind the wall will help ensure a retaining wall that performs for many years.