A lump of clay is said to be normally consolidated if the present effective overburden pressure P0 is the maximum pressure to which the layer has ever been subjected at any time in its history.
That is, if the clay has not been consolidated with stress greater than the present stress, it is called normally consolidated clay. Natural moisture content Wn, is commonly close to the liquid limit, Wl, for normally consolidated clay.
A clay layer is said to be overconsolidated if the layer was subjected at one time in its history to a greater effective overburden pressure(pc), than the present pressure (po).
That is if the clay has already been subjected to a stress greater than the present stress, it is called overconsolidated clay, natural content Wn is close to plastic limit Wp.