Two requirements, not one

Every foundation has to satisfy two separate requirements.

It must not collapse. The ground beneath must be able to carry the load without a shear failure — soil squeezing out sideways and the foundation punching down. This is the ultimate limit state, and the property that governs it is the soil's strength.

It must not move too much. Even a foundation far from collapse settles, and the structure above has to tolerate it. This is the serviceability limit state, and the property that governs it is the soil's stiffness.

Strength and stiffness are different properties. A soil can be strong and soft, which is common in overconsolidated clay, and a foundation on it will pass the first check comfortably and fail the second.

It is worth being concrete about how lopsided this usually is. Take a 2.5 m square pad, founded 1.2 m down in a firm clay of undrained shear strength 75 kPa and unit weight 19 kN/m³.

Against collapse, that pad can carry a net ultimate pressure of 461 kPa. Divide by a traditional global factor of 3 and you have 154 kPa available. That sounds generous, and it is.

Now the other check. At a working net pressure of 150 kPa — just inside the allowable figure — that pad settles about 14 mm if the clay has a stiffness of 25 MPa. Fourteen millimetres is survivable for most structures, but it is no longer a comfortable margin, and it is arrived at with a strength check that said we had plenty of room.

Now make the foundation bigger. Replace the pad with a 6 m square raft carrying the same 150 kPa. The bearing capacity check becomes even more comfortable — a wider foundation mobilises more soil. But the settlement rises to about 34 mm, because a wider foundation stresses a deeper volume of ground, and there is more soil beneath it to compress.

This is the fact that catches people out. Making a foundation wider always helps against collapse. It does not always help against settlement, and on a deep compressible deposit it makes settlement worse.