Eurocode Design Guide · Part 9 of 9
Retaining Wall Design — EC2, EC7 & EC8
Cantilever retaining wall design per Eurocode: EC7 Design Approaches (DA1/DA2/DA3), Rankine and Coulomb earth pressure, stability verification (overturning, sliding, bearing), seismic increment per EC8-5 Mononobe-Okabe, and structural design per EC2.
1. Wall Types & Pre-Sizing
Gravity Wall
H ≤ 1.5 m
Mass concrete or masonry. Stability by self-weight alone.
Cantilever Wall
H = 1.5–7.5 m
Most common RC type. L-shape or T-shape base. Heel carries retained soil.
Counterfort Wall
H > 7.5 m
Stem tied to base slab by counterforts every 3–6 m. Economical for tall walls.
Sheet Pile / MSE
Specialist
Driven steel or RC sheet piles; mechanically stabilised earth. Specialist design.
Cantilever Wall Pre-Sizing
- Base width: B ≈ 0.4–0.6·H (SI: m; US equiv: 0.4–0.6·H ft)
- Stem thickness at base: tstem,base ≈ H/12 – H/10
- Base slab thickness: tbase ≈ H/12 – H/10
- Toe length: Btoe ≈ 0.1–0.2·B; heel = B − Btoe − tstem
- All dimensions to be verified by design calculation; these are starting points only
2. Geotechnical Design Approaches (EC7 §2.4.7)
EN 1997-1 defines three Design Approaches (DAs). The National Annex specifies which DA applies in a given country.
| Design Approach | Action factors | Material factors | Resistance factors | Countries |
|---|
| DA1 – C1 |
γG=1.35, γQ=1.50 |
γφ'=1.0, γcu=1.0 |
γR=1.0 |
UK, Sweden, Ireland |
| DA1 – C2 |
γG=1.0, γQ=1.30 |
γφ'=1.25, γcu=1.40 |
γR=1.0 |
| DA2 |
γG=1.35, γQ=1.50 |
γφ'=1.0, γcu=1.0 |
γR=1.1–1.4 |
Germany, Czech Republic |
| DA3 |
γG=1.35 (structural) |
γφ'=1.25, γcu=1.40 |
γR=1.0 |
Netherlands, France |
Design to both DA1 combinations: When using DA1, the wall must satisfy both C1 and C2. C1 typically governs structural design (stem, base); C2 typically governs geotechnical capacity (sliding, bearing). Both must be checked.
3. Earth Pressure (EC7 §9 + Annex C)
Rankine Coefficients
Design Friction Angle (DA1-C2 / DA3)
| φ'k (°) | Ka (characteristic) | Ka (DA1-C2, φ'd with γ=1.25) |
|---|
| 25° | 0.406 | 0.455 |
| 28° | 0.361 | 0.408 |
| 30° | 0.333 | 0.379 |
| 32° | 0.307 | 0.352 |
| 35° | 0.271 | 0.311 |
4. Stability Verification (GEO / STR / EQU)
Overturning — EQU Limit State
Sliding — GEO Limit State
Bearing Capacity — GEO Limit State (EC7 Annex D)
Eccentricity limit: EC7 recommends e ≤ B/3 for drained conditions (resultant within middle third of base). For undrained / uplift: e ≤ B/2. Check at both DA1-C1 and DA1-C2.
5. Seismic Design (EC8-5 §7.3)
Seismic Earth Pressure Increment — Mononobe-Okabe
Point of Application
The seismic increment ΔPAE is applied at 2H/3 from the base (EC8-5 recommendation). The static component KA·γ·H²/2 acts at H/3 from the base. Combine for total moment about toe.
| kh | φ' = 30° | φ' = 32° | φ' = 35° | Notes |
|---|
| 0 (static) | KA = 0.333 | KA = 0.307 | KA = 0.271 | Rankine (δ=0) |
| 0.10 | KAE ≈ 0.42 | KAE ≈ 0.39 | KAE ≈ 0.35 | M-O; δ=φ/2 |
| 0.20 | KAE ≈ 0.55 | KAE ≈ 0.51 | KAE ≈ 0.46 | M-O; δ=φ/2 |
| 0.30 | KAE ≈ 0.73 | KAE ≈ 0.68 | KAE ≈ 0.61 | Site-specific study may be needed |
Liquefaction check: EC8-5 §4.1.3 requires checking for liquefaction potential if the site has liquefiable soils (loose saturated sands, ag·S > 0.15g). Ground investigation must confirm vs,30 and penetration resistance before relying on passive resistance.
6. Structural Design (EC2)
Once geotechnical stability is confirmed, the structural elements are designed as cantilever RC members under factored earth and water pressures.
6.1 Stem Design (DA1-C1: γG = 1.35)
6.2 Base Slab — Heel & Toe
- Heel: Net upward soil reaction below backfill minus self-weight of heel slab and soil above. Moment at back of stem. Tension on top face.
- Toe: Net bearing pressure under toe minus self-weight. Moment at front of stem. Tension on bottom face.
- Shear at base of stem: VEd = γG·½·Ka·γsoil·Hstem² — verify VEd ≤ VRd,c (no shear links required in slabs)
6.3 Shear Key
A shear key below the base slab increases passive resistance when sliding is critical:
7. Drainage
Water pressure behind a retaining wall dramatically increases lateral load. Proper drainage eliminates or greatly reduces hydrostatic pressure:
- Weep holes: ≥ 100 mm diameter, at ≤ 3 m horizontal spacing, ≥ 150 mm from base slab; cover with filter fabric
- Granular filter layer: 300–500 mm gravel drain immediately behind stem; geotextile separator between backfill and filter
- Perforated collector pipe: At the heel, connected to outlet; prevents back-flooding
- Without drainage: Full hydrostatic pressure adds γw·Hw²/2 to lateral load (γw = 9.81 kN/m³); simultaneously reduces effective stress and passive resistance
EC7 note on water: EN 1997-1 §2.4.6.1(6) requires that the most unfavourable possible groundwater conditions be considered. Where reliable drainage is not guaranteed, design for full water table at top of backfill.
Educational use only. This article uses EN 1997-1:2004, EN 1992-1-1:2004, and EN 1998-5:2004 recommended values. The applicable Design Approach (DA1/DA2/DA3) and all partial factors must be confirmed from the National Annex for your project's jurisdiction. Geotechnical design always requires site-specific investigation by a qualified geotechnical engineer.