Concrete foundations

[Slideintime]

Hi, a relative is planning to build a brick outbuilding about 10' x 18' on clay soil. While helping to dig the stip foundations we dug out a great many boulders most can only just be lifted. The question is that if the stip foundations are 12" deep and 15" wide can those boulders be utilised back into the footings if they are surrounded by the concrete mix. My logic which may be wrong is that they are heavy and would make a good solid foundation if the concrete completly surrounded each boulder. I know builders use brocken bricks for hardcore with sand to fill in any gaps so boulders mixed in with the concrete mix seemed a possible option for them rather than have to remove them all from the garden. Many thanks

[thedoctor]

Strip foundations must be a uniform thickness throughout. Any interruptions could cause the different "sections" to move independently. The boulders may well turn out to be the most stable part of the foundation as they have been there so long but if the surrounding concrete moves, even a tiny amount as it surely will do, the wall will crack as the boulder may not move at all.

[Slideintime]

Hi and many thanks for the quick reply. So would it be ok to use the boulders as hardcore as in dry with sand and smaller aggregate, tamp that down and then pour the concrete over the top?