Stiffness, and the piled raft

Raft stiffness decides how much of that conversion actually happens.

A genuinely rigid raft settles as a plane. Wherever the ground is soft it sheds pressure to stiffer areas, and the differential settlement across the building approaches zero — at the cost of large bending moments in the raft itself, since it is now spanning between hard spots.

A flexible raft follows the ground. It attracts small moments and behaves as a collection of independent footings that happen to be joined, so it inherits the ground's variability more or less unchanged.

Real rafts sit between the two, and where they sit depends on the raft's thickness against the plan dimension and on the ground's stiffness. The important consequence is that raft moments and differential settlement trade against each other: you cannot reduce both at once by adjusting the raft alone.

The piled raft is the case where neither a raft nor a pile group is quite right.

The conventional approach is either/or: if the raft settles too much, replace it with piles designed to carry the whole load. A piled raft instead keeps the raft carrying most of the load and adds a smaller number of piles specifically as settlement reducers, placed where the settlement profile is worst — usually beneath the more heavily loaded central area.

The piles are deliberately loaded close to their capacity, which is unnerving the first time you meet it, but it is the point: a pile that is not fully mobilised is not doing the job it was added for. What the arrangement controls is the shape of the settlement, which is what the structure cares about.