What we need from you
For retaining wall designs, the following is required:
- Geotechnical reportSoil profile, lateral earth pressures, groundwater and slope stability at the wall location.
- Site topography (site survey)A contour plan showing existing and proposed levels either side of the wall.
- Site plan showing boundaries and wall locationsThe site boundary and the position, length and alignment of each wall.
- Loading plansAny structures, driveways or services above or below the wall that will surcharge it.
Why a geotechnical report should be your first step
Everything starts with the ground behind it
A retaining wall doesn’t just hold up soil — it holds back an entire landscape. The forces acting on it are invisible, relentless, and often underestimated. When a retaining wall fails, it fails spectacularly: the ground moves, the wall tilts, cracks propagate, and the cost to fix it can dwarf the original build.
A geotechnical report tells you exactly what that wall is up against — what the retained soil will do under pressure, where water will try to go, and what the ground beneath the wall can actually support.
What does a geotechnical report cover for a retaining wall?
A qualified geotechnical engineer visits your site, drills or excavates at the wall location, takes samples, and produces a report that covers:
- Soil type and profile — what material sits behind, beneath, and in front of the wall
- Lateral earth pressures — the horizontal force the retained soil will exert on the wall, which varies dramatically by soil type (clay pushes harder than gravel)
- Groundwater levels — where the water table sits and how hydrostatic pressure will build up behind the wall
- Slope stability — whether the broader slope is at risk of failure and how the wall interacts with it
- Bearing capacity — whether the ground beneath the wall can support its weight without settlement
- Sliding and overturning resistance — whether the foundation soil provides enough friction to prevent the wall from sliding forward or tipping over
- Surcharge loads — the effect of buildings, driveways, vehicles, or other loads on the retained side
- Wall type recommendations — whether your site suits a gravity wall, cantilever wall, anchored wall, or piled solution
Without this information, retaining wall design is gambling. And when retaining walls lose, the house loses too.
The Building Code requires it
New Zealand’s Building Code Clause B1 (Structure) applies to retaining walls just as it applies to building foundations. Building Consent Authorities will require a Producer Statement (PS1) for any retaining wall over 1.5 metres in height — and many councils require one for walls lower than that, particularly where a building or driveway sits within the zone of influence.
For timber pole retaining walls designed to NZS 3604, the standard provides limited prescriptive designs — but only for walls up to 1.5 metres on “good ground” with flat backfill and no surcharge. The moment your site deviates from these idealised conditions, a geotechnical report becomes mandatory. Most real-world sites deviate.
Two retaining wall risks that catch people out
Hydrostatic pressure
Water is the single biggest enemy of a retaining wall. When groundwater builds up behind a wall without adequate drainage, the pressure can double or triple the force the wall has to resist. Walls that were perfectly adequate on paper fail because water wasn’t accounted for.
A geotechnical report identifies groundwater conditions and informs the drainage design — weep holes, strip drains, filter material, and subsoil drainage — that keeps water from destroying your wall from behind.
Slope creep
On sloping sites, the ground itself can be in slow, persistent motion. A process called soil creep — barely perceptible but relentless — can transfer far more load onto a wall than simple earth pressure calculations suggest. This is particularly common on clay slopes in the Auckland region, where wet winters and dry summers drive repeated shrink-swell cycles.
A geotechnical report assesses whether the broader slope is stable and whether your wall needs to act as a retaining structure or a full slope-stabilisation measure. The difference in cost and complexity is significant, and finding out late is far worse than finding out early.
The cost of skipping it
Retaining wall failures are among the most expensive residential ground problems to remediate. Without a proper ground investigation:
- Walls can be under-designed for actual soil and water conditions
- Drainage failures can cause gradual wall rotation and eventual collapse
- Undetected slope instability can push walls beyond their structural capacity
- Building consent can be refused or delayed
- Remediation requires demolishing the wall, excavating the slope, and starting over
- Insurance may not cover failure arising from inadequate site investigation
- Damage to neighbouring properties from a failed retaining wall can create significant legal liability
A geotechnical report costs less than the investigation that follows a failure.
How Foenix Foundations can help
At Foenix Foundations, we design retaining wall foundations and structures based on real ground data. We assess the soil, the water, the slope, and the loads before a single post goes into the ground.
Whether you need a simple timber pole wall, a reinforced concrete cantilever, or a fully engineered anchored solution, we start with what’s underfoot — and behind.