A poured concrete retaining wall offers a durable and permanent solution for managing significant changes in ground elevation. This structure is prized for its strength and ability to withstand the immense lateral pressure exerted by retained soil. Constructing a concrete wall requires careful planning and execution, resulting in a robust barrier that can be customized in texture and color. This guide provides a step-by-step overview of the process, focusing on the technical requirements for a successful, long-lasting installation.
Planning and Site Preparation
The initial phase involves thorough planning to ensure structural integrity and compliance with local regulations. Walls exceeding four feet in height (measured from the top of the footing to the top of the wall) typically require a building permit and a structural design sealed by a professional engineer. Before excavation begins, contact the national Call 811 service to have all underground utility lines marked, preventing accidental damage.
Excavation must be precise, starting with the trench for the wall’s footing, which must be wider than the wall itself to distribute the load effectively. In colder climates, the footing must extend below the local frost line to prevent movement and damage from freeze-thaw cycles. The depth of the trench is determined by this frost depth, which varies by region.
Once the trench is dug, the base must be leveled and compacted to provide a stable foundation. Place a layer of crushed stone or gravel, typically six inches deep, at the bottom of the trench. This layer provides a firm, well-draining base, which is essential for the long-term stability of the structure.
Building the Formwork and Reinforcement
The formwork acts as the mold for the wet concrete and must withstand the significant hydrostatic pressure of the pour. Forms are typically built using plywood sheeting braced by dimensional lumber, such as two-by-fours, secured with stakes and snap ties that run through the wall. The forms must be plumb and level, and the bracing must be robust enough to prevent bulging or shifting during concrete placement.
Steel reinforcement, or rebar, is incorporated into the formwork to provide the tensile strength that concrete lacks, allowing the wall to resist the lateral forces of the retained soil. A grid pattern of vertical and horizontal rebar is created, with spacing often specified at 12 inches on center, depending on the wall’s design. The rebar intersections are secured with wire ties to maintain the cage’s position during the pour.
A specific distance, known as “concrete cover,” must be maintained between the rebar and the interior face of the formwork to protect the steel from corrosion. For concrete cast against the earth, the clear cover should be at least three inches. Using small concrete or plastic spacers, called “chairs” or “dobies,” ensures the rebar cage remains correctly positioned and does not shift against the forms.
Mixing and Pouring the Concrete
The selection of the concrete mix is a structural decision, typically requiring a strength between 3000 and 4000 pounds per square inch (psi) for adequate durability. For larger projects, ordering ready-mix concrete is the most practical option. The water-to-cement ratio, often described by the concrete’s slump, must be carefully controlled; a mix that is too wet will be weak, while one that is too stiff will be difficult to consolidate around the rebar.
Concrete should be placed into the forms in horizontal layers, or “lifts,” rather than pouring the entire height at once. Pouring in lifts, typically six to eight inches deep, allows the concrete to settle and reduces the risk of form failure. This sequential placement also helps prevent the segregation of the aggregate, where the heavier stone separates from the cement paste.
Consolidation is necessary to remove trapped air pockets and ensure the concrete flows completely around the rebar and into all corners of the formwork. This is achieved using a concrete vibrator, which is plunged into the wet mix to temporarily liquefy it, allowing air bubbles to rise. Avoid over-vibrating, which can cause the aggregate to settle, weakening the upper portion of the wall. After the forms are filled, the top surface is leveled and smoothed using a screed board or float.
Curing, Stripping, and Drainage
Curing is the process where concrete gains strength through hydration, a chemical reaction requiring moisture and time. The wall must be kept moist for the first three to seven days, often by covering it with plastic sheeting or periodically misting the surface. This prevents the concrete from drying out too quickly, which can lead to surface cracking and reduced compressive strength.
Formwork can typically be removed, or “stripped,” after 24 to 72 hours, depending on the ambient temperature and the specific concrete mix used. The concrete must have achieved sufficient strength to support its own weight before the forms are taken down. Once removed, the wall should be handled with care, as it continues to gain strength for several weeks.
The most significant threat to a retaining wall is hydrostatic pressure, the force exerted by water trapped in the soil behind the structure. Proper drainage is non-negotiable for wall longevity. This system involves placing a layer of clean, crushed stone (typically three-quarters to one inch in size) immediately behind the wall, extending at least 12 inches back and running the full height.
A perforated drain pipe, often called a weeping tile, is installed at the base of the wall within the crushed stone layer. The pipe must be sloped toward an outlet to carry water away. A filter fabric is placed between the drainage stone and the native backfill soil to prevent fine particles from clogging the stone and the pipe. This drainage system ensures that water pressure does not build up, protecting the wall.