Assuming good soil i.e. ballast type not clay, and double brick thick, what depth foundation would I need for a wall some 20m+ long upto a) 1m and b) 2m in height? Also at what distances would brick columns be needed?
What purpose does the wall serve, is it absolutely straight with no changes of direction? For a wall one brick thick (215mm) you may not need any attached piers but they would certainly aid stability and you will also need a couple of vertical expansion joints if no change of direction, so having piers at those positions would be good idea. There is no specific foundation depth required for a garden wall but depending on where you live you may want to go deeper if you get severe frosts. For garden walls the following may be useful to you http://www.pavingexpert.com/featur03.htm
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