Construction method
Construction begins with setting out and drilling post holes at regular centres along the wall line, each hole deep enough to bury the base of the post securely below ground, generally at least one third of the wall’s exposed height, more on steeper or looser ground. Posts are typically treated timber, sized according to the wall height and the loads behind it, and are concreted into their holes to lock them in place.
Once the posts are set and the concrete has cured, a geotextile fabric is laid against the excavated bank behind the wall. This permeable membrane stops fine soil particles washing through into the drainage zone while still allowing water to pass freely, protecting the wall from long-term clogging and pressure buildup.
Behind that goes a drainage layer, usually free-draining gravel or scoria, running the full height and length of the wall, along with a perforated drainage pipe laid at the base to carry collected water away to a safe outlet point. This drainage system is arguably the most important part of the whole structure, the majority of retaining wall failures in New Zealand come down to hydrostatic pressure building up behind a wall with inadequate drainage, not from the timber or fixings themselves giving way.
With drainage in place, the facing boards or sleepers are fixed to the posts to form the visible wall face. Backfilling then proceeds in stages behind the drainage layer, compacted progressively to avoid excessive settlement later, with topsoil reserved for the final layer near the surface, ready for planting or lawn to go back in.
The finished wall not only holds back the bank behind it but, done properly, will stay dry, stable, and structurally sound for decades, since it’s the unseen drainage work behind the timber that does most of the heavy lifting over the wall’s lifetime.
