A retaining wall is a vertical structure designed to contain soil and prevent erosion. The most common retaining walls used in residential construction are Gravity and Anchored walls. Gravity walls use the weight of the wall to hold back the soil. They are best suited to small slopes requiring walls of less than 3-4 feet in height. Anchored walls use tension supports, that are anchored to the soil or rock to support the wall and resist pressure.
A retaining wall is constantly fighting gravity to prevent erosion. To be successful, the wall requires careful planning and sound construction practices. Factors such as soil type, slope angle, height, and drainage impact the stability of a retaining wall. Local building codes and ordinances vary between communities. Check with your local building authority for requirements and necessary zoning or building permits.
Best Practices in Retaining Wall Construction
For walls 4 feet or less, a compacted gravel footer is usually sufficient. Taller walls require a poured footer below the frost line (check with the local authority for requirements).
The space directly behind a retaining wall should be filled with clean ¾” gravel, not dirt. Dirt absorbs water and swells, which puts additional pressure on the back of the wall. Gravel will allow water to drain which reduces the pressure. Backfill a minimum of 12”-18” immediately behind the wall with clean ¾” gravel. Any additional backfill should be clean native soil. Add the backfill after each course of block is laid and compact using a vibrating plate compactor. Make a minimum of 3-5 passes. Never try to compact more than one course of backfill at a time.
Shorter walls using precast retaining wall blocks will usually drain fine with just the gravel backfill. Larger walls and solid walls need proper drain pipes installed behind the wall along the footer. This will help carry water safely away from the wall and home.
(Always follow the block manufacturer’s and/or engineer’s instructions.)
Most gravity and anchored retaining walls are constructed so that they lean slightly toward the soil they’re containing. This design, known as “step-back construction,” creates a sturdy wall structure that pushes back against the lateral pressure of the soil behind it. Solid precast retaining wall blocks are designed to lock mechanically. Hollow blocks must be filled with compacted gravel after each course is laid to lock them in place. The first course of block must be installed below grade. For walls over 4 feet, the base
of the wall should be 2” below grade for every foot of
Many precast systems require reinforcement such as geotextile mesh layered between courses at certain intervals, based on the overall height. When building a terraced retaining wall, specific spacing and reinforcement is required based on the height of the walls and the soil type. Refer to the manufacturer or engineer design for these requirements.