I need to prepare our carpark: one edge of it has a low wall which supports part of the parking area. The stone (local stone) wall forms the border to a footpath.    I would like to calculate the minimum depth of the foundation.   I need  to take the wall down, dig out the back of the wall to make space for the foundation [ and the shingle drainage and french drain] , then put a block wall up, and face that block wall with the stone I took down.    The wall height v