I wondered what the guidelines are for retaining walls - at what point must they become double skinned? It is a patio with foot traffic only.
Impossible to answer generically. It depends on how close it is to a permanent structure (your house, for example), what type of soil you have (clay is stable, sand is not) how much ground water pressure may be involved, etc etc. If you google 'retaining walls building regulations' or something similar there are various .gov links and reference docs
I saw some collapsing walls this morning, raised beds. Some were only 2' tall and leaning out. Some parts of a 3' wall had been buttressed, those parts that hadn't looked really dodgy. Soil sinks and pushes out
rasingirl is is totally corrects. It depends on a lot of factors based on soil you require to retaining to distance from existing house. To a patio to create 2 levels therefore retaining soil on the lower half. I would suggest a 2 brick wide wall consisting of concrete blocks to the retained side with a engineering brick to the outer skin. Again it all depends what soil your are retaining and the water levels that will impact on the retained wall
Also look to get some professional advice from a structural engineer
how high do you need to have the wall and how long is it, you can have piers running on the outside of the wall which helps a little, the piers could be built into the soil side but I would run them like groynes into the retained side and tie them into the wall well. something else to consider is drainage holes (40mm pipe) at say 1m ctrs at base of wall to drain water from behind wall, its the hydrostatic pressure that typically makes retaining walls fail. personally at a guess I would put a singleskin retaining wall at max 450mm, 300mm ideally