The Ontario Building Code | Foundation Capacity in Weaker Soil and Rock foundation Capacity in Weaker soil and Rock

(1) Where a soil or rockwithin a distance equal to twice the footing width below the bearing surface has a lower allowable bearing pressure than that at the bearing surfaceas shown in Article, the design capacity of the foundationshall not be greater than would cause the weakest soilor rock to be stressed beyond its allowable bearing pressure.

(2) In calculating subsurface pressures referred to in Sentence (1), the loads from the footings shall be assumed to be distributed uniformly over a horizontal plane within a frustum extending downward from the footing at an angle of 60° to the horizontal.