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Why do foundations on clay soil settle over time?

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Figure 1: The position vector x and the velocity vector v of a material point, the body force fdV acting on an element dV of volume, and the surface force TdS acting on an element dS of surface in a Cartesian coordinate system 1, 2, 3 (see text).
mechanics of solids Figure 1: The position vector x and the velocity vector v of a material point, the body force fdV acting on an element dV of volume, and the surface force TdS acting on an element dS of surface in a Cartesian coordinate system 1, 2, 3 (see text).

Foundations on clay soil settle over time because of the unique way clay interacts with water and structural loads. Clay particles form an open, compressible structure containing numerous voids filled with water.1

When a structure is built, the initial load is primarily supported by the water within these voids, which is relatively incompressible. However, because the openings between the clay particles are extremely small, the flow of water out of these voids is highly restricted.1

Settling occurs as a slow, time-dependent process where the load stress is gradually transferred from the pore water to the clay mineral structure itself. As the water is squeezed out, the clay structure compresses, causing the foundation above to settle. This process can continue for several years after construction is completed. The extent of this behavior is influenced by the clay-water relations, as the mineral's plasticity and water-retention capacity dictate how the soil responds to stress.12