Retaining walls

A retaining wall holds back soil where the ground changes level, as at road cuttings, basements, bridge abutments and hillside terraces. Gravity walls of masonry or mass concrete resist earth pressure by their weight. Cantilever walls of reinforced concrete use a thin stem fixed to a base slab whose heel carries soil weight, making them economical up to about 6 to 8 m. Counterfort walls add ribs for taller heights. Reinforced soil walls, gabions, crib walls and anchored or nailed walls are alternatives suited to particular sites.

Design checks external stability first: overturning about the toe, sliding along the base, bearing pressure under the base (with the resultant in the middle third), and overall slope stability of the wall and soil together. A shear key under the base can improve sliding resistance. Earth pressures come from Rankine or Coulomb theory, with surcharge from traffic or buildings and with seismic increments in earthquake zones. Then the stem, toe and heel are designed as reinforced concrete cantilevers for bending and shear.

Drainage is critical. Weep holes, a graded filter or geocomposite behind the stem and a drain at its base stop water pressure building up. Many wall failures in monsoon follow blocked drains. Backfill should be granular and compacted in thin layers with light plant near the wall.