Retaining walls do serious work. They hold back tons of soil, manage water drainage, and resist lateral pressure that never stops pushing. Whether you're building a garden wall, a driveway cut wall, or a structural retaining system, the concrete needs to be strong, well-reinforced, and properly calculated.

What PSI for Retaining Walls?

Retaining walls are structural elements. Use a minimum of 3,000 PSI for residential walls under 4 feet. For taller walls, walls with surcharge loads (driveways, buildings above), or engineered designs, 3,500–4,000 PSI is appropriate. Always follow your engineer's specifications if you have a stamped design.

• 3,000 PSI — residential walls under 4 ft, garden walls
• 3,500 PSI — walls 4–6 ft, walls with soil surcharge
• 4,000 PSI — engineered walls, walls with vehicle or building loads above

Anatomy of a Concrete Retaining Wall

A cast-in-place concrete retaining wall has two main components: the footing (which spreads the load into the ground) and the wall stem (the vertical section that holds back the soil). Both are poured with concrete, often in separate pours.

The Footing

The footing is typically 2–3 times the width of the wall stem and extends below the frost line. It's the anchor that prevents the wall from sliding or overturning. Calculate footing volume separately: length × width × depth ÷ 27.

The Wall Stem

The stem is the vertical wall. Thickness varies by height and design — typically 8–12 inches for residential walls. Calculate: length × thickness × height ÷ 27.

Calculating Volume — and Why to Add Extra

Forms for retaining walls are rarely perfectly uniform. Plywood bows, stakes shift, and the actual poured dimensions often exceed the design dimensions by an inch or two. On a long wall, this adds up quickly. Always add 10–15% overage to your calculated volume.

Running short on a wall pour is a serious problem. A cold joint in a retaining wall — where fresh concrete meets partially set concrete — is a structural weak point that can crack under lateral pressure. It's far better to have a small amount left over than to stop mid-wall.

Rebar is Non-Negotiable

Concrete is strong in compression but weak in tension. Retaining walls experience significant bending forces — the soil pushes horizontally against the wall, putting the back face in tension. Rebar resists this tension. Typical residential retaining walls use #4 or #5 rebar, vertically and horizontally, per engineering specifications.

Drainage Behind the Wall

Water pressure (hydrostatic pressure) is one of the leading causes of retaining wall failure. Install weep holes through the wall base, and place a drainage aggregate (clean gravel) and perforated drain pipe behind the wall before backfilling. This keeps water from building up and pushing against the wall.

Placement Tips for Wall Pours

• Pour in lifts of 18–24 inches and vibrate each lift before adding the next
• Don't pour faster than you can vibrate — trapped air voids weaken the wall
• Brace forms adequately — fresh concrete exerts significant lateral pressure on forms
• Cure the wall for at least 7 days before removing forms or backfilling

UNO Concrete delivers retaining wall mixes to contractors and homeowners across the Charlotte metro. Tell us your wall dimensions, footing size, and pour date — we'll calculate the right volume and get the right mix on site when you need it.