Guidelines and design considerations, retaining walls

General guidelines before checking stability

1. Choose the wall type from height: gravity wall for about 3-4 m, cantilever for 4-8 m, counterfort/buttressed for more than 8 m or with large surcharge.
2. Assume trial dimensions from the usual proportions (base width 0.5-0.7 H, stem and slab thickness H/12-H/10, toe about B/3).
3. Find the soil data: unit weight, $\phi$, $c$ of backfill and foundation soil, water table, bearing capacity.
4. List the loads: self-weight, soil weight over heel, active thrust, surcharge, water pressure, seismic force.
5. Select the earth pressure theory (Rankine for vertical back with smooth wall assumption; Coulomb for rough and inclined walls) and compute the active pressure; ignore passive pressure in the top 0.5-1 m, or take only a reduced part.
6. Check overturning, sliding, bearing, tension at base and overall stability.

Design considerations

• Backfill: preferably free-draining granular material (sand, gravel); avoid clay, which gives high and variable pressure.
• Drainage: weep holes (100 mm at 2-3 m spacing), a filter or geotextile and a longitudinal drain at the heel to remove water pressure.
• Stability requirements: FS against overturning $\ge 2$, sliding $\ge 1.5$, bearing $\ge 3$; resultant within the middle third.
• Foundation depth: below frost, scour and softened soil; use a shear key if sliding is a problem.
• Structural design: stem as a cantilever, heel and toe slabs in bending and shear; reinforcement on tension faces.
• Joints: expansion joints about 20-30 m and contraction joints about 10 m in concrete walls.
• Seismic effect and surcharge where relevant; allow for the effect of tension cracks in cohesive backfill.