Overconsolidated soil may be defined as
“If the initial vertical effective stress of the soil sample is less than the preconsolidation stress, then the vertical effective stress in the field was once higher than its current magnitude, and the soil is called overconsolidated.”
The soil whose present effective overburden pressure is less than that which the soil experienced in the past is called overconsolidated soil.
The maximum effective past pressure is called preconsolidation pressure.
Why soil becomes overconsolidated ?
There are many processes that causes the soil to become over-consolidated.
- Extensive erosion or excavation which results in the decrease of ground surface elevation.
- Extra loading from a glacier, which has since melted.
- Extra loading from a structure, such as storage tank, which has since been removed.
- By rising of ground water table, which increases the pore water pressure.
- Drying of the pore water due to evaporation, absorption by plant roots which produces negative pore water pressure in the soil ( Stark and Duncan, 1991 ).
- Chemical changes in the soil.
- Aging effects.
From the term over consolidation it seems that there is an excessive consolidation in the soil. But that is not the case actually. Overconsolidation is almost always a good thing. Except in Cut slopes where overconsolidated soils are less desirable.