How high can a retaining wall be before you need an engineer to design it?

Question

in australia queensland

Answer 1

as long as its under 6 feet tall you can do it your self no permission is needed

Answer 2

The 1st fellow has a good answer. If you really want to know and you live in a city/town and have a code enforcment office or person it would be best to ask them. Someplaces Im betting no problem with height until 6 to 8 feet. Then like in someplaces that are really dry until it rains for a few days streight and mudslides are a problem 4 feet may be too tall. Or if you live where there is volcanic or movement IE quakes, the code may be all together differant for a differant reason. So you see the codes are not there for limitations sake but for safety. If your out in the country and no code is inforce Id think an average of the land fall/rise in your area devided by 2 would be the limit (or 1/4th of the maxium heigth differance). Remember this though, the higher you build the wider the base. After 10ft (maybe even 6 feet) the base may need to be widened by a factor of 2. So if 6" wide at top, the base needs to be 12". And I didnt know about the 5 degree from plumb the first fellow mentioned but that sounds reasonable. (FYI 5 degrees is equal to 1/35th of your verticle so its not much. Again the higher you go the less tollerance you have). If you double the height to 20feet the base needs to be 4 or 5 times the final width and all timbers or retainers have to be attached to the ground with stakes or inbedment into concreet to be sturdy and allow backfilling.

A book on yard improvment may be referenced at a local lumber yard. This should have sound practice methods.

Answer 3

I have built retaining walls here in England 10 ft high, and I did not need an engineer to design it. It all depends on what job the wall has to do, for example, if it is retaining heavy wet ground it will need to be able to withstand the lateral pressure, if it simply a retaining wall to support an undesturbed background, such as, a bank of earth which is the wall of an excavation  for a cellar, the background is not disturbed and therefore more stable and unlikely to move, this is a very wide question, because of all the variables, generally a retaining wall will be 4ft high or so, with a vertical lean of 5 degrees to the back, not plumb