Retaining Walls: What Fails Them (and How to Build Them Right)
Retaining walls don’t fail because the wall material is “weak”. They fail because the wall wasn’t designed or built as a system: soil + water + structure.
1. The Two Enemies: Soil Pressure and Water Pressure
Soil exerts lateral pressure. Water trapped behind the wall adds hydrostatic pressure, which can exceed soil pressure and dramatically increase loads.
Rule: A retaining wall without drainage is not a retaining wall. It’s a future insurance claim.
2. Wall Types and When They Make Sense
- Gravity walls: rely on self-weight; common for small heights and modular blocks.
- Cantilever RC walls: efficient for moderate heights; require good footing design and reinforcement detailing.
- Anchored walls: used where space is limited; loads are shared with anchors/tiebacks.
- Segmental block walls: performance depends on geogrid, backfill quality, and compaction control.
3. Drainage Detailing (Non-Negotiable)
- Free-draining backfill: clean aggregate zone behind wall.
- Separator geotextile: stops fines clogging the aggregate and drain.
- Subsoil drain: perforated pipe to lawful discharge, with fall and cleanouts where practical.
- Weep holes: only useful if the drainage layer and outlet path are correctly formed.
4. Surcharge Loads: The Hidden Multiplier
Surcharge is any load near the top of the wall: vehicles, driveways, buildings, stockpiles, even heavy landscaping. Surcharge increases pressure and can push a “fine” wall into failure.
5. Construction Quality: Where Walls Live or Die
- Backfill compaction: poor compaction leads to settlement; over-compaction too close to the wall can push it forward.
- Footing preparation: walls on uncontrolled fill behave unpredictably.
- Waterproofing: required where walls form part of habitable spaces or basements.
6. Practical Site Checklist
- Confirm finished levels and falls direct surface water away from the wall.
- Confirm drainage line discharge point and keep it clear during works.
- Keep heavy loads away from the wall crest unless designed for it.
- Photograph geotextile, aggregate, and drain before covering.