Over Consolidation Ratio
Casagrande (1936) proposed to determine the pre-consolidation stress from e – log σv' plot as shown in Figure 23.4. Locate the point O where there is smallest radius of curvature. Draw a line OA parallel to stress or x axis. Draw a tangent OB at point O. Draw the line OC such that it bisects the angle AOB. Extend the straight portion of the e – log σv' plot. The stress corresponding to the point of intersection of extended line and OC line is the pre-consolidation pressure the soil was subjected. If the present effective overburden pressure σ'v0 is equal to the preconsolidated pressure σc' (maximum past effective overburden pressure), the soil is normally consolidated soil. However, if σ'v0 < σc', the soil is overconsolidated. Thus, overconsolidation ratio (OCR) is defined as: OCR = σc'/σ'v0.
Fig. 23.4. Determination of preconsolidation stress.