What to do about a failing retaining wall in Auckland.

01 / The short answer

Report what you can see, from a safe position, and get the site looked at.

You do not need to diagnose a failing retaining wall. Record what is visible from a safe position, keep people and vehicles away from the top and the base, and arrange a site assessment. Cause is determined on site, not from a photograph.

The most useful information you can provide is what has changed and when. A wall that moved five years ago and has been stable since is a different project from a wall that moved last month after a storm.

02 / Signs

The visible signs an Auckland retaining wall is failing.

The common signs are leaning or rotation at the top, bulging in the middle of the face, cracking or stepping in block walls, timber splitting or rot at ground level, water coming through the face, and ground dropping or cracking behind the wall.

Leaning or rotation

The top of the wall has moved outward relative to the base. Often visible as a gap opening behind the top of the wall, or a fence above the wall going out of plumb.

Bulging

The middle of the wall face has pushed outward while the top and bottom stay put. Common in timber infill walls where individual boards or rails deflect.

Cracking or stepping

In block and concrete walls, cracks that step diagonally, open progressively, or appear at one end usually indicate differential movement rather than shrinkage.

Timber deterioration

Splitting, softness or rot, most often at or just below ground level where moisture and oxygen are both present. Fixings corroding is a related sign.

Water through the face

Water seeping or running through the wall face, or ponding behind it, usually points to a drainage system that is blocked, undersized or absent.

Ground movement behind

Cracking in paving, paths or driveways behind the wall, or a step forming in lawn. This can indicate the retained ground is moving, which is a wider question than the wall.

Erosion at the ends

Ground washing out around either end of the wall, which often means water is running around the structure rather than through the drainage.

03 / Causes

What usually causes a retaining wall to fail in Auckland.

Water is the most common underlying cause. Drainage that was never installed, has blocked, or discharges nowhere lets pressure build behind the wall. The others seen regularly are surcharge added after construction, embedment that was too shallow, poles set too far apart, timber reaching the end of its service life, corroded fixings, and movement in the wider slope. Beyond water being first, these are not in order: no published New Zealand dataset ranks them by frequency.

Water pressure

Without free-draining backfill, geotextile separation and a working subsoil drain, water builds up behind the wall and adds substantial load the wall may never have been designed for.

Added surcharge

A driveway, vehicle access, pool, shed or imported fill added behind a wall after it was built. The wall was adequate for the original load and is not adequate for the new one.

Insufficient embedment

The wall does not extend deep enough below finished ground on the low side to resist overturning. Often the case with walls built without design input.

Poles set too far apart

Spacing is a design output, not a convenience. Widen it and every pole carries more of the retained ground, while the timber spanning between them has further to go. A survey of walls damaged in the Canterbury earthquakes found the damaged pole walls were all spaced wider than a metre, and concluded closer spacing would have prevented the damage in every case.

Timber service life

Treated timber has a finite life in ground contact. Walls built decades ago reach the end of it, particularly where drainage has kept the timber wet.

Corroded fixings

Bolts, brackets and connections corroding faster than the timber degrades, which can fail a wall whose members still look sound.

Wider slope movement

The wall is a symptom rather than the problem. Where the slope itself is moving, a bigger wall alone does not solve it and a geotechnical view is needed.

04 / What to do first

The first steps when you notice a retaining wall moving.

Keep people and vehicles clear of the top and the base, stop adding any load behind it, photograph the condition from a safe position, and note when the movement started and whether it followed rain.

• Keep people, pets and vehicles away from both the top and the base of the wall
• Stop parking, storing material or driving on the retained ground above it
• Take dated photographs from a safe position, including a wide shot showing context
• Note whether the movement followed heavy rain, earthworks nearby, or a change of use
• Check whether water is discharging near the wall from a downpipe, tank overflow or paving
• Contact your insurer, particularly where the movement followed a storm event
• Do not excavate at the base of the wall to investigate

05 / Repair or replace

Whether a failing retaining wall gets repaired or replaced is a site decision.

Some walls can be remediated by fixing the drainage, reducing the surcharge or reinforcing the existing structure. Others are better replaced, particularly where timber has reached the end of its service life or the original design was never adequate for the load.

The honest position is that this cannot be settled remotely. It depends on what the wall is made of, how it was built, what is loading it, what the ground is doing and what the property needs the wall to do going forward.

Where the wall is consented or the replacement will be, the design comes from an engineer. SRW engages and briefs that engineer, works the design through with them, prepares and lodges the consent, and builds the wall.

06 / COMMON QUESTIONS

Common questions about failing and leaning walls in Auckland.