Under consolidated soil

It can be defined as

“If the initial vertical effective stress is greater than the pre consolidatedstress then the soil will be under consolidated soil.”

It means that the soil is still in the process of consolidating under a previously applied load.

Over consolidation margin

It can be defined as

“The difference between pre consolidation stress and initial vertical effective stress is known as Over consolidation margin.”

It should be constant throughout the stratum of the same geologic origin.

The pre consolidation stress from the laboratory only represent at the sample depth. To find the this stress at any other depth of the strata of the same geological origin, initial vertical effective stress at desired depth is found.

Overconsolidation margin = preconsolidation stress – Initial vertical effective stress

Over consolidation margin should be constant throughout in a stratum with common geologic origins. Therefore, we can compute the pre consolidation stress at other depths in that stratum by using above equation with initial vertical effective stress at desired depth. Normally consolidated soils have OCM is o kPa. Slightly over consolidated soils have 0-100 kPa. Moderately overconsolidated have 100-400 kPa. Heavily overconsolidated have more than 400 kPa.

Over consolidation ratio

It can be defined as

The ratio of pre consolidation stress to the initial vertical effective stress is known as over consolidation ratio.

OCM remains same throughout the data but OCR varies as a function of depth. It cannot be used to compute pre consolidation stress at other depths in a stratum.

Casagrande suggested a simple graphic construction to determine the PC stress.

For normally consolidated soils OCR = 1

Reference

Geotechnical engineering principal and practices by Donald P Coduto