Very interesting question. I was thinking if I was a little bit of concrete some where in the foundation block, what would cause me to try to move sideways away from the house? Needs a sideways force and I can't see where its coming from. If the foundation was laid as a wedge, then there is more weight of of it over the "thick" end then the thin end, so its like a see-saw that is overbalanced and its trying to rotate its balance point over the central point of the pivot in the the ground. I reckon foundations are stepped so each step has its bit of weight balanced over the pivot point so each step wants to stay horizontal. Imagine a plank laying on your lawn with a pile of bricks on one end of it. eventually the heavy end will sink, because its balance point will be way off the central support point, so there will be a large rotational force on it. So the real question is "how out of balance are the forces on your foundation block?". No one seems to worry about the weight of the back wall of an extension (doubling or more) the ground pressure under the foundations under the back wall. I suspect that the bits do move around (settlement) but only by fractions of a millimetre until the forces balance out again. If you had a precision measuring device you could actually measure this. On very old castles etc. you can see walls that are out of plumb, yet have stood for hundreds of years.
I would make a decision about what the minimum thickness the foundation should be, lay the block to this and step the top so make your block laying easier.
Frank
