Checking a retaining wall after heavy rain in Auckland.

01 / The short answer

Look for change, not for perfection.

The useful question after heavy rain is not whether the wall is perfect. It is whether anything has changed since before the rain. New cracking, new lean, new water paths and new ground movement are the signals worth acting on.

This is much easier if you have photographs from before. Taking a set of dated photographs of your walls while conditions are dry is a small job that makes every future check meaningful.

02 / The check

What to check on a retaining wall after a storm.

Check the face for new cracking or bulging, the top for new lean, the ground behind for cracking or dropping, the ends for erosion, and the base for water discharging or ground washing out.

The face

New cracks, cracks that have widened, blocks that have stepped or moved relative to each other, timber that has bowed or split.

The top

A gap opening behind the top of the wall. A fence or planting above the wall going out of plumb is often the first visible sign.

The ground behind

Cracking in lawn, paving, paths or driveway behind the wall. A step or depression forming parallel to the wall.

The ends

Erosion or washout at either end, which usually means water is running around the wall rather than through the drainage.

The base

Water discharging, ground softening, or material washing out from the toe of the wall.

Water paths

Where water actually went during the rain. Downpipes discharging near the wall, paving directing flow toward it, or an overflow running across the retained ground.

03 / What to do

What to do if something has changed after heavy rain.

Keep people and vehicles clear of the top and base, stop loading the retained ground, photograph what you can see from a safe position, contact your insurer, and arrange a site assessment.

• Keep people, pets and vehicles away from the top and the base of the wall
• Stop parking, storing material or driving on the ground above it
• Photograph the change from a safe position, with dates, including a wide context shot
• Note when the change appeared relative to the rain
• Redirect obvious surface water away from the wall if it is safe to do so
• Contact your insurer, particularly following a declared weather event
• Arrange a site assessment rather than waiting to see whether it settles

04 / Why rain does it

Why heavy rain is what exposes a retaining wall problem.

Saturated soil pushes much harder against a wall than drained soil, so a wall with inadequate drainage sees loads during sustained rain that it never sees in normal conditions. Rain does not usually cause the underlying problem; it reveals it.

Auckland's clay soils hold water and drain slowly, so pressure can build and stay high for days after the rain stops. Walls sometimes move after the weather has cleared.

This is also why a wall can perform for many years and then move suddenly. The drainage may have been slowly failing for a decade before a wet enough period arrived to expose it.

05 / COMMON QUESTIONS

Common questions about after a storm in Auckland.