Retaining wall strategies for split-level Adelaide builds

A retaining wall is required wherever the design creates a level difference between two adjacent areas that cannot stand as an unsupported soil bank.

Standard practice: soil banks steeper than roughly 1 in 3 (rise to run) typically need retaining. On Adelaide clay soils, banks steeper than 1 in 4 often need retaining because clay lacks the cohesion of some other soil types.

Split-level designs by definition create level differences. Some are absorbed within the building structure (integrated retaining walls); others sit outside the building as freestanding retaining walls in the garden.

The cost of retaining scales with height and length. A wall retaining 1 metre of soil costs significantly less per linear metre than a wall retaining 3 metres. Design the split levels to minimise the total retaining requirement.

Retaining wall materials each have different cost, appearance and life expectancy.

Masonry (concrete block or brick) is durable, offers a variety of finishes, and can be reinforced with steel for higher walls. It is the mainstream choice for retaining walls that are visible from the home.

Concrete (either pre-cast panels or in-situ poured) is the strongest option and often used for high retaining walls. It can be exposed as a modern architectural finish or hidden behind cladding or planting.

Timber (typically treated hardwood posts or engineered timber sleepers) is the lowest cost but has a shorter life expectancy (20 to 40 years depending on treatment). It suits informal or garden-scale retaining.

Gabion (rock-filled steel cages) has a strong architectural presence and works well in landscape settings. It requires careful specification but is durable and permeable.

The choice depends on the wall visibility, height, budget and desired aesthetic.

Retaining walls above 1 metre high typically require engineering certification. Some councils apply lower thresholds (600mm to 900mm).

The engineer designs the wall for the specific soil conditions, the load above, any surcharge from vehicles or buildings, and drainage requirements. Certification is required for building consent.

For walls integrated with the building structure, the same engineer typically designs both. For freestanding walls, a specialist retaining wall engineer may be engaged.

Do not build over-height retaining walls without engineering. A retaining wall failure can be catastrophic and expensive to remediate.

The biggest cause of retaining wall failure is inadequate drainage behind the wall. Water builds up in the soil behind the wall, adds weight and pressure, and eventually causes movement or collapse.

Standard drainage detail: gravel backfill behind the wall (typically 300mm minimum), a slotted agricultural pipe at the base of the gravel, and an outlet to the stormwater system. Any weep holes in the wall face itself are secondary; the buried drainage does the work.

For high walls or walls in wet ground conditions, additional drainage layers may be required. A geotechnical engineer can advise on complex sites.

Do not cut corners on drainage. It is the cheapest part of the wall to install and the most expensive to remediate later.

Retaining walls on or near property boundaries can trigger the SA Fences Act 1975 cost-sharing provisions. If the wall retains soil that supports both properties, both owners may share responsibility for construction and maintenance.

Talk to the neighbour before starting boundary retaining wall work. Formal notice under the Fences Act may be required to establish cost sharing rights. A cooperative neighbour is worth much more than a dispute after the wall is built.

If the retaining is entirely for your own benefit (for example, you are cutting your ground down and the neighbour ground is unchanged), the cost is typically yours alone. If the retaining supports the neighbour ground (they are on the high side), cost sharing may apply.

A specialist property lawyer can advise on the specific circumstances if the boundary retaining is significant. Fees are modest relative to the potential dispute cost.