ACI 318 Concrete Retaining Wall Design Calculator (Cantilever)
Preliminary design and stability checks for cantilever reinforced concrete retaining walls: active earth pressure, sliding, overturning, bearing, and stem moment.
Disclaimer: For educational and preliminary design only. Final designs must be prepared and checked by a licensed professional engineer using full ACI 318 and project-specific requirements.
1. Geometry & Loads
Wall type
Cast-in-place reinforced concrete cantilever wall with vertical stem and rectangular base slab. Backfill is level at top of wall.
Design method
Working stress stability checks with typical safety factors. Stem moment is reported for subsequent ACI 318 flexural design.
Geometry (user units: kN, m or kips, ft)
Soil & material properties
Safety factors (service load)
2. Stability & Stem Design Results
Earth pressure
Sliding & overturning
Bearing pressure at base
Stem moment for ACI 318 flexural design
Stem is treated as a vertical cantilever fixed at base.
How this retaining wall design calculator works
This tool focuses on a conventional cantilever reinforced concrete retaining wall with level backfill. It performs the key geotechnical and structural checks used in preliminary design:
- Active earth pressure using Rankine theory
- Sliding stability at the base
- Overturning stability about the toe
- Bearing pressure distribution under the base slab
- Stem bending moment at the base for ACI 318 flexural design
1. Active earth pressure (Rankine)
For level backfill and a vertical wall with no wall friction, the Rankine active earth pressure coefficient is:
\[ K_a = \tan^2\left(45^\circ - \frac{\varphi}{2}\right) \]
where \(\varphi\) is the soil friction angle.
The lateral pressure at depth \(z\) is:
\[ \sigma_h(z) = K_a \, \gamma_{soil} \, z + K_a \, q_s \]
where \(\gamma_{soil}\) is the soil unit weight and \(q_s\) is any uniform surcharge at the surface.
The resultant active force on the back of the wall is:
\[ P_a = \frac{1}{2} K_a \gamma_{soil} H^2 + K_a q_s H \]
acting at a height \(H/3\) above the base for the triangular component, plus \(H/2\) for the rectangular surcharge component. The calculator combines these into an equivalent resultant and lever arm.
2. Sliding check
Sliding is checked at the base interface. The driving force is the horizontal earth pressure \(P_a\). The resisting force is friction between the base and soil:
\[ R_{sliding} = \mu \, W \]
\[ FS_{sliding} = \frac{R_{sliding}}{P_a} \]
where \(W\) is the total vertical load (self-weight of concrete + soil over heel) and \(\mu\) is the base–soil friction coefficient.
3. Overturning check
Overturning is checked about the toe. The overturning moment is produced by the lateral earth pressure. Stabilizing moments come from the vertical loads acting at their centroids.
\[ FS_{OT} = \frac{\sum M_{stabilizing}}{\sum M_{overturning}} \]
Typical minimum target: \(FS_{OT} \ge 2.0\) under static loading.
4. Bearing pressure under the base
The resultant vertical load \(W\) and moment about the base centroid produce a linearly varying bearing pressure. For a base width \(B = L_{heel} + t_{stem} + L_{toe}\):
\[ q_{avg} = \frac{W}{B} \]
\[ e = \frac{M}{W} \]
\[ q_{max,min} = q_{avg} \left(1 \pm \frac{6e}{B}\right) \]
The calculator reports \(q_{max}\) and \(q_{min}\) and compares \(q_{max}\) to the allowable bearing pressure \(q_{allow}\). If \(e > B/6\), tension (uplift) occurs at the heel, which is flagged by a negative \(q_{min}\).
5. Stem bending moment
The stem is modeled as a vertical cantilever fixed at the base. The maximum bending moment at the base is computed from the lateral pressure distribution:
\[ M_{stem} = \int_0^H \sigma_h(z) \, z \, dz \]
This moment is reported as a service-level value. For ACI 318 design, you should apply appropriate load factors (e.g., 1.6 for lateral earth pressure in many combinations) and use φ-factors for flexure to size vertical reinforcement.
Design tips and limitations
- Drainage: Provide adequate drainage (weep holes, granular backfill, geocomposite drains) to avoid hydrostatic pressure, which is not included in this simple model.
- Seismic loads: Seismic earth pressure (e.g., Mononobe–Okabe) is not included. For seismic regions, additional checks are required.
- Soil parameters: Use site-specific geotechnical data for unit weight, friction angle, and allowable bearing pressure whenever possible.
- Reinforcement detailing: This tool gives you the governing stem moment and base reactions. Bar sizes, spacing, development length, shear design, and crack control must follow ACI 318 in detail.
- Wall types: Gravity block walls, MSE walls, and anchored walls require different design approaches and are not covered here.
Frequently asked questions
Can I use this for residential retaining walls?
Yes, many residential walls use cantilever concrete sections similar to what this calculator assumes. However, even for small walls, local building codes may require a design prepared or reviewed by a licensed engineer, especially when supporting structures, driveways, or slopes.
How do I choose heel and toe lengths?
Start with a heel length of about 0.5–0.7 H and a toe length of 0.3–0.5 H as a first guess, then iterate using the calculator until sliding, overturning, and bearing checks are satisfied with comfortable safety factors.
What if my backfill is sloping?
This version assumes level backfill. For sloping backfill, the active earth pressure coefficient changes and the resultant force increases. You should use more advanced earth pressure theory or dedicated software for that case.
How does this relate to ACI 318?
ACI 318 governs the concrete and reinforcement design (strength, detailing, serviceability). The calculator provides the key actions—moments, shears, and base reactions—that you then use in ACI 318 strength design checks. It does not replace the code.