Structures
Design a cantilever retaining wall stem
Learn how earth pressure loads a cantilever retaining wall, then model the stem as a plate, analyse it and design its reinforcement in Struxt3D step by step.
A cantilever retaining wall holds back soil where the ground level changes — basements, terraces, road cuttings. The vertical part, the stem, cantilevers up from a base slab and is bent by the pressure of the soil behind it. The Retaining wall template in Struxt3D models the stem as a concrete plate fixed along its base, loaded by at-rest earth pressure, with a capping beam along the top.
How earth pressure loads the stem
Soil pushes sideways on a wall in proportion to its depth, like water but less: the horizontal pressure is a fraction K of the vertical stress γz. A wall that cannot move — propped, or very stiff — feels the at-rest pressure, with the coefficient K0. Struxt3D uses Jáky's formula for K0:
The pressure grows linearly from zero at the top to K0γH at the base. Its resultant is a force at one third of the height above the base, and the stem works as a vertical cantilever:
Because the soil is on the back, the stem bends with tension on the soil face, so the main vertical bars go on that side. Modelling the stem as a plate (rather than a one-metre strip) also shows what happens at the ends of the wall and how the capping beam spreads load along it.
Build the model
- Pick the template: Foundations › Retaining wall.
- Set the size and soil with the parameters below.
- Choose the materials. Concrete; choose the code and grade, and a section for the Capping beam. The stem thickness comes from the size step.
- Supports and loads. The stem is fixed along its whole base line by the template; there are no point supports to choose. Turn Wind off for a retaining wall — the soil pressure is the load that matters, and wind on the exposed face is small. Look at the load case Earth under Show on the model to see the pressure.
- Run the analysis (F5).
The template's parameters
| Parameter | Default | Range | What it does |
|---|---|---|---|
| Wall length | 12 m | 4–40 m | Length of the wall along its base. |
| Retained height | 4 m | 1.5–10 m | Height of soil behind the stem (also the stem height). |
| Stem thickness | 0.35 m | 0.2–1 m | Thickness of the concrete stem. |
| Soil unit weight | 18 kN/m³ | 15–22 | γ of the retained soil. |
| Soil friction angle | 30° | 20°–40° | φ, used for K0 = 1 − sin φ. |
With the defaults: K0 = 1 − sin 30° = 0.5, pressure at the base 0.5 × 18 × 4 = 36 kPa, force 72 kN/m and base moment 0.5 × 18 × 4³ / 6 = 96 kN·m/m. Compare with the slab moment the app shows at the middle of the wall's base.
Load combinations with earth pressure
The earth pressure is its own load case, Earth. Struxt3D factors it with the dead-load factor of each combination (for example 1.35 in Eurocode's 1.35D + 1.5L), except in NSCP 2015, which gives it its own factor H = 1.6 (1.4 in the 1.4D combination). The service combination D + L used for deflections includes the earth pressure at 1.0. See Loads and load combinations.
What to check in the results
- Deflection. Show Displacement: the top of the stem leans away from the soil, most in the middle of the wall. The ends lean less only if something holds them; in this model both ends are free, so the movement is nearly even along the length.
- Stem moment. Show Slab moment. The vertical moment peaks at the base; check it against the hand value above.
- Capping beam. Show Moment on the beam. It is lightly loaded in a uniform wall; it matters when the pressure varies along the length.
- Reactions along the base give the shear and moment the base slab must take.
Design the stem
Click Run design (F6). The stem is designed as a concrete plate: bar meshes on both faces in the vertical and horizontal directions from the plate moments (Wood–Armer), extra bars where needed, and shear. The capping beam is designed as an RC beam. Show Slab reinforcement for the steel area needed per metre, or Reinforcement bars to see the bars in 3D. The design is explained in Slab design.
The wall stands on a line support, so no pad footings are designed under it.
What you still have to check
Struxt3D designs the stem only. A complete retaining wall design also needs:
- the base slab (toe and heel) and its reinforcement,
- stability: sliding, overturning and bearing pressure under the base,
- surcharge from traffic or buildings behind the wall, water pressure if drainage can fail, and compaction pressure,
- active pressure (Ka) instead of at-rest, if the wall is free to move and your code allows it — K0 is the safer choice.
Common problems and fixes
| What you see | Why | Fix |
|---|---|---|
| Stem shear fails at the base | Tall wall, thin stem. | Thicker stem; shear reinforcement is rarely used in walls. |
| Heavy vertical bars at the base | Moment grows with H³. | Thicker stem at the base (a tapered stem in reality). |
| Large top movement | Slender cantilever. | Thicker stem, or a propped wall if the height is large. |
Rules of thumb
Starting points only.
- Stem thickness at the base about H/10 to H/12, at least 200–250 mm.
- Base slab width about 0.5H to 0.7H.
- Cantilever walls are economical up to about 6–8 m; taller walls are usually counterforted or anchored.