I'm not really clear what you are asking. Assuming the walls will rest on the mat, and transfer loads to the mat, what you describe is the common practice. The image was not explanatory - it looks like a plan view with CMU walls at the perimeter. There are always issues with construction tolerances, but unless small variations are truly an issue, I would not concern myself with minor irregularities in how the walls and foundation align at the face of the wall.
yes, cmu edge exactly at the mat footing perimeter edge.
i once put a pedestal at the perimeter edge of a footing and a senior drafter said it's hard in construction and should give an "allowance" for formworks. i didnt argue with him and just put the pedestal at 6" away from the perimeter of footing.
delagina...no problem with doing as you propose. If the masonry walls were to be covered with a veneer of some sort, they should be inset, otherwise no reason that flush won't work.
Flush is fine in theory, but when the mat gets cast incorrectly, you can be left with walls overhanging the edge. So it is just a constructability issue, not a technical one.
Ignoring the tolerance issue, if there is one. I think your senior drafter was trying to explain that the formwork for the cast in place pier would ideally have a ledge to sit on, like the top of the mat in your previous case. In this case, the CMU wall isn't built with formwork, so a ledge might not be required.
Of course, the first course of the block should be laid in a full mortar bed and not just typical face shell bedding like the rest of the wall. It may not be in your code, but it recommended and is good practice.
Dick
Engineer and international traveler interested in construction techniques, problems and proper design.
If I understand your question correctly you want to start the CMU walls up off of the mat foundation. Some questions to consider: 1. Are these walls gravity load bearing, shear walls, both gravity load bearing and shear walls or just partition walls? Can the mat support these loads? 2. All masonry walls have minimum lateral support requirements. How are these walls being connected to the mat and does this methodology provide adequate lateral support? What is the support detail on the top of the wall? 3. Is this reinforced CMU or unreinforced CMU? If reinforced the mat construction must coordinate the placement of any vertical bars into the wall.
