What Decides Whether a Retaining Wall Lasts
Six things that separate a wall still standing in twenty years from one that starts leaning in five.
We Check the Surcharge Before the Height
The first question on a retaining wall is not how high it is. It is what sits above it. A 1.2 metre wall under a driveway is a bigger engineering problem than a 1.6 metre wall holding a lawn, and we establish that before we talk about price.
Drainage Designed With the Wall
Water is what pushes walls over, not soil. We build the free draining layer, the wrap and the outlet as part of the wall rather than as a finishing item, because once the backfill is in there is no way to add it later without taking the wall apart.
Footings Sized to the Retained Height
A wall fails by rotating at the base far more often than it fails by breaking. The footing is what resists that, so it gets sized off the retained height and the ground it bears on, and it goes in before anyone thinks about the face of the wall.
Seventy Five Millimetres of Cover Against Ground
Where concrete is cast against the ground the steel needs 75 mm of cover. On a retaining wall that face is permanently wet and permanently in contact with soil, so cover is the difference between reinforcement that lasts and reinforcement that rusts and splits the wall.
Walls Past the Exemption Go to an Engineer
A wall retaining more than 1.5 metres, or any wall carrying a surcharge, needs a building consent. We do not build past the exemption on judgement. It goes to an engineer for a design and the consent goes in with it.
We Say When a Wall Is the Wrong Answer
Some banks want a batter and planting, not a wall. Some want two low walls stepped rather than one tall one, which can drop the whole job under the exemption. We will tell you when the cheaper answer is the better one.