We are doing some small treated pine retaining walls only 600mm high at most. How much is the steel retaining post supposed to go in the ground. This is for resale so I don't want to overengineer it. I have heard you only need a third of the height of the post in the ground, i.e. =600H buy 900H post and 300 in ground. Does anyone know the general rule for this?
300mm in the ground is going to do nothing. Mitre 10's guide How to build a timber retaining wall suggests as much below as above.
I would be 600m below ground and 600mm above as a min into concrete. Depends on the load If its backfilled and wet ground with poor drainage (consider ag drainage and backfill recycled concrete agg) perhaps 900mm. And the timber strength eg 25mm thin boards vs 150mm round logs or thicker wing splits etc. Your council may have a guide. Many do have minimum specs to guide street facing retaining walls. eg page 9 and page 35 here : https://www.blacktown.nsw.gov.au/fi...-development-within-the-residential-areas.pdf Its vague and requires suitably engineered walls. The distance between each support is also a factor. The landscape material provider is probably suitably experienced to guide you. The depth wont influence cost much. Just longer posts and more concrete. Bunnings sell prefab kits that may assist to guide depth ??
A decent guide is located here : http://www.hyne.com.au/documents/Feature Documents/TQ_09_Timber-Retaining-Walls_final.pdf I would usually use steel posts for concrete sleepers …. Don't forget to get clear your behind wall and top of wall drainage design before starting to build.
That is for wood posts - which do come a lot cheaper than steel posts. They do support wood sleepers. They mix concrete or have a concrete truck - I've used quick set concrete effectively, it's very easy to use. No mixing and it sets solid.
The original post mentioned treated pine, the posts are using the same. The treatment is to make them termite resistant.
In most cases if you are going 600mm above, then 600mm below ground in concrete footings would be sufficient. In some cases like sandy soil etc, you may consider 800mm below ground. It is also going to be dependent on spacing of your posts. In addition to footings, appropriate drainage and back fill materials should also be considered.
The design is dependent upon the material being retained, porosity & the angle of repose of that material
