How deep is the base layer under a sidewalk, a retiaing wall with mortar and one without ?

I seem to get different answers - in Onatrio, Canada what is it ?How deep is the base layer under a sidewalk, a retiaing wall with mortar and one without ?
The trench should be 6'; to 12'; deep and 5'; wider than the blocks/stones you're using for the wall. Add 3'; to 6'; of gravel, with the spaces between the gravel filled with limestone powder/screenings.