Construction

Construction

Common Materials Common retaining wall materials include wood, masonry (concrete blocks and bricks), natural stone (rubble), poured concrete, and precast (landscape, garden) blocks. Landscape block walls may not be reinforced or tied together (pinned), and sometimes do not have a footing. Their height may be limited by the manufacturer to around two or three feet. A gabion retaining wall is a wire basket or cage filled with natural stone or rubble. Gabion retaining walls are less common.

Wood retaining walls may be constructed using preservative treated landscape timbers or railroad ties that are treated with creosote (a black tar-like material) or a similar material. Landscape timbers are available in many sizes from 3×5 inches to 8×8 inches. Railroad ties are usually about 7×9 inches. Walls made from wood may be limited to four feet tall in some areas. Preservative treated wood should be a category that is rated for ground contact (UC4A or higher). Wood preservative treatment cannot be determined by looking at the wall. The only visual means to determine wood preservative treatment is by the tag attached to the wood. Wood retaining walls have a typical service life of between ten and twenty years depending on materials used, drainage, and soil conditions.

Masonry retaining walls are usually built using concrete masonry units (CMUs, also called concrete blocks). Walls built using bricks are possible, but are less common. CMU cells in taller walls should be grouted (filled with mortar) and if grouted should be reinforced with steel. CMU cells in shorter walls may be built with the cells left hollow. Walls built with bricks are usually not reinforced, so they should be shorter. Be careful not to describe CMU walls covered with brick veneer as brick walls.

Poured concrete retaining walls are built by erecting metal or wood forms and pouring concrete between the forms. Metal bars called ties are usually placed between the forms to hold the forms together. The forms are usually removed after the concrete has hardened. Reinforcing steel should be placed horizontally in taller walls. The ties sometimes project from the wall. This is mostly a cosmetic issue with retaining walls; however, the rust can cause the ties to expand and cause cracks. Reinforcing steel should also be placed in the footings and project vertically into the wall. This helps tie the wall to the footing and helps the wall resist horizontal movement (sliding) and to a lesser extent resist rotation and other wall failures.

Construction techniques Masonry and concrete retaining walls should be constructed on a poured concrete footing that is reinforced with steel as appropriate. Wood, garden block, and rubble walls may be constructed on compacted sand or gravel if adequate drainage is established so that water does not undercut the footing, and if the walls are not over approximately four feet tall. Remember that walls over four feet tall may require an engineered design. Even if an engineered design is not required, it is a good idea.

Retaining walls taller than about four feet should have structural components to help reduce wall failure caused by forces such as water pressure and frost heave. Wood retaining walls should have large nails (spikes) to secure each course to the next. Retaining walls may have vertical posts located on the outside of the wall (buttresses and shoring) to secure the wall. A more aesthetically pleasing means to secure the wall involves installing horizontal lumber buried in the retained soil. Horizontal lumber consists of two parts. The part that connects with the retaining wall is a tieback. The tieback runs several feet under the retained dirt and is connected to a deadman anchor that is installed perpendicular to the tieback.

Structural components that help reduce wall failure in masonry and concrete walls include vertical and horizontal reinforcing rods embedded in grout (masonry walls) or in the concrete. Reinforcing rods are not usually visible for inspection. Footings for concrete and masonry walls may be unusually wide to help provide additional failure resistance (known as cantilever retaining walls).

Drainage Most retaining walls should have a means to drain water that accumulates behind the wall. Lack of drainage can cause the wall to rotate, crack, bow, slide, or deteriorate because of pressure from retained wet soil, or from frost heave. Wood and rubble walls will usually drain water through the natural spaces between the members. Masonry and concrete walls should have openings, called weep (drain) holes, near the bottom of the wall. The quantity, size, and location of weep holes will vary based on site conditions.

Weep holes can be reduced in size or completely blocked by dense fill, such as clay, placed directly behind the wall. A layer of gravel covered by filter fabric should be placed directly behind the wall. Taller walls and walls where water may be a problem may need drainage pipe at the footing in addition to weep holes. Drainage components such as gravel and filter fabric are not usually visible for inspection.

Retaining walls may be drained using footing drains similar to those found around basement foundation walls. The footing drain terminations should be visible at the end of the wall. Retaining walls with footing drains are uncommon in residential construction.