I often see inadequately built retaining walls which should have been designed by an engineer and I see these walls fail.
It isn’t just how high a wall is which dictates the walls size, but also slope of the ground, where the wall is located, if extra loads are on the wall such as cars, buildings, fences and what the ground supporting the wall is like, these are all important considerations.
Retaining walls do not require a building consent when they retain not more than 1.5 metres depth of ground; and do not support any surcharge or any load additional to the load of that ground (foample, the load of vehicles.
But even if a wall is less than 1.5m high, and does not have an obvious surcharge, there are other considerations.
If the wall is located along a property boundary, this wall should consider possible surcharges which may arise from changes in land use bordering the wall.
If the wall is bordering a boundary, if the boundary is not clearly defined, it is also recommended that a boundary survey is carried out so as not to build the wall crossing your boundary.
Being a geotechnical engineer, another consideration is the ground. If the ground is soft or the wall is located within fill/manmade ground, the wall may not have a suitable foundation for support.
Drainage behind the wall is also key for a durable wall, I would recommend even if the wall is for landscape purposes only that free draining backfill is used to prevent water pressure build up behind the wall.
If you are building a wall and are unsure what is required, I would recommend seeking a registered engineer for advice.
When a building consent is required, the wall must be specifically designed by a registered engineer.