E6 Soil and Foundation Mechanics

Topic

Foundations are the elements that transfer a building's entire weight to the ground; designing them requires first understanding how the soil responds to its own weight: the pressure exerted by the ground increases with depth, in direct proportion to the soil's density.

When a footing rests on this ground, there is a maximum pressure the soil can withstand before failing due to sinking—its ultimate bearing capacity. This capacity depends on three combined factors: the soil's inherent cohesion (negligible in gravels, high in clays), the material's internal friction, and the width of the footing itself (as a wider footing mobilizes a larger volume of load-bearing soil). A safety factor is applied to this ultimate capacity to determine the allowable design pressure, ensuring a wide margin of safety before failure occurs.

Even without failing, the ground compresses under the load, causing the foundation to sink by a certain amount—known as elastic settlement. This depends on the magnitude of the load, the width of the footing, and the soil's deformability. Building codes are less concerned with this total settlement than with differential settlement—uneven sinking between different points of the same building. Uneven settlement creates additional stresses in the structure (the very structure whose equilibrium was established in the first module of this catalog), potentially causing cracks or serious damage, even if the settlement at each individual point remains within acceptable limits.

Thus, this module—which begins by discussing pressure increasing with soil depth—concludes by highlighting that the real danger for a foundation is not merely sinking, but settling unevenly: two footings that individually meet safety standards can, when combined, damage the structure they support.

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