1. Lateral Earth Pressures (Rankine vs Coulomb)
Retaining wall stability depends primarily on the lateral earth pressure exerted by retained soil. Under active conditions where the wall yields sufficiently away from the soil backfill, the active earth pressure coefficient Ka according to Rankine theory for horizontal backfill is:
The total thrust per unit length Pa acting at a height H/3 from the base of the stem is:
Where q represents surface surcharge load (e.g., traffic or storage loads).
2. Eurocode 7 Limit State Verifications
EN 1997-1 requires retaining walls to be checked against three primary failure conditions:
| Limit State | Failure Mechanism | Verification Criterion |
|---|---|---|
| EQU (Equilibrium) | Loss of static equilibrium by overturning about toe | Mdst,d ≤ Mstb,d |
| GEO (Geotechnical) | Basal sliding along footing base or bearing failure | HEd ≤ Rh,d & VEd ≤ Rd |
| STR (Structural) | Bending and shear failure of concrete stem or toe/heel slabs | MEd ≤ MRd & VEd ≤ VRd,c |
3. Overturning & Sliding Resistance
For EQU verification, destabilizing moment Mdst,d from lateral soil and surcharge pressure about the toe must be balanced by stabilizing moment Mstb,d from wall self-weight and soil weight on the heel slab:
To improve sliding resistance where base friction is insufficient, a shear key below the base slab can be provided to mobilize passive earth pressure Kp ahead of the key.
4. Stem Bending & Drainage Detailing
The vertical stem acts as a cantilever fixed at the top of the base slab. Maximum bending moment occurs at the base section:
Key Design Recommendations
- 1
Subsurface Drainage
Install weep holes (100 mm @ 1.5 m spacing) and granular filter backfill to prevent hydrostatic water build-up.
- 2
Base Proportions
Base width B should typically equal 0.5 to 0.7 times total wall height H for economic stability.
- 3
Movement Joints
Provide vertical expansion/contraction joints at 6 to 9 m intervals to accommodate concrete shrinkage and thermal strain.