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**Over Consolidation Ratio**

Casagrande (1936) proposed to determine the pre-consolidation stress from e – log σ_{v}' 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 σ_{v}' 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 σ_{c}' (maximum past effective overburden pressure), the soil is normally consolidated soil. However, if σ'_{v0} < σ_{c}', the soil is overconsolidated. Thus, overconsolidation ratio (OCR) is defined as: OCR = σ_{c}'/σ'_{v0}.

**Fig. 23.4. Determination of preconsolidation stress.**

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