Drainage Behind Retaining Walls

A retaining wall designed for the soil behind it and built without working drainage is a wall designed for a load case that does not occur.

Water is the dominant factor in retaining wall failure, and the drainage detail is a small fraction of the wall cost.

Why water is the problem

Soil behind a wall exerts a lateral pressure that increases with depth. Saturate that soil and two things happen at once.

Hydrostatic pressure is added. Water pressure increases with depth at a steeper rate than the effective soil pressure, and it acts in addition to it. A fully saturated backfill can roughly double the total lateral load on a wall, sometimes more.

The soil's own strength drops, because the effective stress between particles falls as pore pressure rises. In a clay this can be a large reduction.

Which means a wall designed for drained conditions and allowed to become undrained is not slightly overloaded. It is loaded well beyond what it was ever checked for.

On Sydney's reactive clay sites there is a third effect: the clay swells as it wets, and swelling pressure on the back of a wall is additional again, which connects to [reactive clay sites and footing design](/blog/reactive-clay-sites-footing-design-nsw).

What the drainage has to do

Three jobs, and all three have to work.

Collect the water that reaches the back of the wall.

Carry it to an outlet without the path clogging.

Discharge it somewhere legitimate, which is a stormwater question and often a council approval one, covered under [stormwater management](/blog/stormwater-management-urban-developments).

A system that collects and cannot discharge is a tank.

The components

A free-draining layer against the wall, either a granular backfill zone or a prefabricated drainage composite. Its job is to let water reach the drain faster than it can build pressure.

A filter, meaning a geotextile or a graded filter between the drainage layer and the retained soil. Without it, fine particles migrate into the drainage layer and it stops draining. This is the component most often omitted and the most common cause of a system that worked for five years and then stopped.

A perforated subsoil drain at the base, laid to a fall, with the perforations oriented correctly and surrounded by filter material.

Weepholes, on a masonry wall, as a secondary path. Useful and not a substitute for a drain.

An outlet, at a known location, connected, and able to be inspected and cleaned.

Surface treatment at the top, so surface water is directed away rather than being allowed to run into the backfill. A wall with perfect subsurface drainage and a paved area draining straight into the top of the backfill is still going to have a water problem.

Where it goes wrong

No filter, so the drain silts up.

The drain laid flat or backfalling, which holds water along its length.

The outlet never connected, which is more common than it sounds on sites where the wall went in long before the drainage.

Fine backfill used because it was on site. The excavated clay going straight back behind the wall is cheap, convenient, and removes the drainage path entirely.

Compaction plant working too close to the wall, which imposes a temporary load higher than the design and can push a wall out before the backfill is even finished.

No provision for inspection, so a blocked system cannot be found until the wall moves.

Drainage designed and then built by a different trade with no inspection of the filter and the drain before the backfill covered it, which is the only moment either can be seen.

How much it changes the design

A wall that can be designed for drained conditions is substantially lighter than the same wall designed for a saturated backfill.

Which means the drainage is not an accessory to the wall. It is the reason the wall can be the size it is. Designing a wall for full hydrostatic pressure is the conservative alternative, and on a tall wall it is a large cost increase, so the usual answer is to design for drained conditions and detail the drainage properly.

That logic applies across the [types of retaining wall](/blog/retaining-walls-types-design-applications), from a masonry garden wall to an [anchored or soil-nailed](/blog/soil-nailing-and-anchored-retaining-walls) face, and it is part of why a [basement needs tanking](/blog/basement-tanking-and-hydrostatic-uplift) rather than drainage where the water table sits above the floor.

Assessing an existing wall

A wall that is leaning, cracking in a pattern that indicates rotation, showing efflorescence, or wet in dry weather is telling you about its drainage.

The investigation is to find the outlet, check whether it flows, and establish whether a drain exists at all. On older walls the answer to the last question is frequently no, and the remedy is a retrofitted drain in front of or through the wall rather than a thicker wall.

ACSES provides geotechnical engineering and retaining wall design across Sydney. Talk to us about a project.