Consolidation

When a saturated clay is loaded, it cannot compress immediately, because water fills the pores and water is nearly incompressible. At first the load is carried by excess pore water pressure. Water then slowly squeezes out, the pressure dissipates, the load transfers to the soil skeleton and the clay compresses. This time-dependent compression is consolidation. In sands it happens almost at once; in thick clays it can take years or decades, which is why buildings and embankments on soft clay keep settling long after construction.

Two questions matter: how much, and how fast. The magnitude comes from the oedometer test, which gives the relationship between void ratio and the log of effective stress. A normally consolidated clay, never loaded beyond its present stress, follows the virgin line with compression index Cc. An overconsolidated clay, once loaded more heavily, is much stiffer until the preconsolidation pressure is exceeded. Settlement of a layer is its thickness times the change in void ratio over one plus the initial void ratio.

Rate comes from Terzaghi's one-dimensional theory: degree of consolidation depends on the time factor Tv = cv t / H², where H is the longest drainage path. Doubling the drainage path quadruples the time. That is why vertical drains are installed in soft ground to shorten drainage paths, often with a surcharge to speed settlement before construction. Secondary compression, a slow creep after pore pressure dissipates, matters in organic clays and peat.