Roadbase Behind a Retaining Wall

Drainage Backfill for Retaining Walls

Proper drainage behind a retaining wall is essential for the wall's longevity and structural integrity. Water pressure building up behind a retaining wall can cause failure even in well-built structures. The free-draining backfill zone, typically roadbase or gravel, allows water to drain down to a drainage pipe at the base rather than building pressure against the wall. For a 6 metre retaining wall with a 500mm wide drainage zone extending 300mm up the wall height, you'll need approximately 0.9 cubic metres of DGB20 roadbase, weighing around 1.6 tonnes.

The dimensions used here represent the drainage zone only, not the full backfill behind the wall. Above the drainage layer, regular fill material or garden soil typically makes up the remainder of the backfill. The drainage zone width and height vary based on wall height, soil conditions, and engineering requirements.

Drainage System Components

The roadbase backfill works as part of a complete drainage system. At the base of the wall, a slotted agricultural drain or slotted PVC pipe collects water that filters down through the roadbase. This pipe should fall to an outlet point or soak-away. Geotextile fabric wrapped around the pipe and sometimes lining the excavation prevents fine soil particles from clogging the drainage system over time. The roadbase provides both the drainage medium and structural backfill support.

Height Considerations

Taller retaining walls require more extensive drainage systems and correspondingly more backfill material. As a rough guide, the drainage zone should extend at least one-third of the wall height, with wider drainage zones for taller walls. Walls over one metre high often require engineering design, which will specify exact backfill requirements. For DIY garden walls under one metre, a 300mm wide drainage zone with roadbase from the footing to at least 300mm below the top typically provides adequate drainage.

For French drain calculations, see our French drain calculator. Use our main roadbase calculator for different wall dimensions.