Consolidation Settlement of Normally Consolidated Clay

Squeeze a clay and the water has to leave before the grains can move closer, so settlement is slow and its magnitude is set by how far the void ratio falls. On a plot of void ratio against the logarithm of effective stress a normally consolidated clay is a straight line whose slope is the compression index $C_c$, and integrating that line over the layer gives this expression. With $C_c$ = 0.32, H = 3 m, e₀ = 0.85, and stress rising from 100 to 200 kPa: S = (0.32 × 3 ÷ 1.85) × log₁₀(2) = 0.519 × 0.301 = 0.156 m, or 156 mm.

The trap is the word "normally consolidated". If the clay has been pre-loaded — by ice, by erosion, by an old building — the first part of the stress increase runs along the much flatter recompression line with slope $C_r \approx C_c/5$ to $C_c/10$, and using $C_c$ for the whole range can overpredict settlement fivefold. Check the preconsolidation pressure from an oedometer test before anything else. The second trap is time: this is the final settlement, reached after all the excess pore pressure has drained, which in a thick clay can mean decades. The Leaning Tower of Pisa began tilting during construction in 1178 because the Pancone clay beneath it is still consolidating differentially eight centuries later, and secondary creep runs on long after primary consolidation has finished.