hi if a wall is around 3ft 6 and double skinned, what should the footing be? the ground has clay and rubble in it?? thanks

a structure of any kind is only as good as it,s foundation dig down to a suitable strarter or natural ground on site visual and experiance will tell you weather it is suitable it may even need fabric added and battenry peirs if the horizontal fource is higher than the vertical waight and you must then add weep holes to release water presure from rain 225mmx450mm concrete strip foundation should be fine under normal conditions if you need any free advise or surveying please do not hesitate to contact me Dave 07834406291 a job worth doing is worth doing right dabconstruction

4 - 6 inchs

The footing should be 400mm to 450mm wide and 450 to 500mm deep

Regardless of the length the footing SHOULD be minimum 450mm x 450mm, Deeper if there are trees close by...but if the wall is 3' 6" high why bother to build 2 skins instead of a 9" wall?...if it's a retaining wall then the foundation should be Wider(possibly up to 3'!)...and is it a filled cavity?