Drainage turns total stress into effective stress
A sudden load on a saturated column is carried first by the pore water. Terzaghi’s one-dimensional theory is a heat equation for that excess pressure: the top drains, the base does not, and settlement follows the average degree of consolidation. Gibson’s self-weight problem starts from a triangular pressure, not a uniform one. In two or three dimensions the total stress is free to redistribute, and Cryer’s sphere shows the Mandel–Cryer rise: the centre pressure climbs above its undrained value before it falls.
Excess pressure
Degree of consolidation
∂u/∂t = cv ∂²u/∂z²
Colour is excess pore pressure. Orange on the time plot is uncoupled Terzaghi (monotonic). Navy is the coupled Cryer centre pressure, which rises before it falls.