The 'pre-consolidation pressure' σ'c of a soil sample is determined from the oedometer test by:
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Direct reading of water content
Casagrande construction on e-log σ' curve — bisection method to find point of maximum curvature
Triaxial test cell pressure only
SPT N-value at that depth
Correct Answer
B. Casagrande construction on e-log σ' curve — bisection method to find point of maximum curvature
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Casagrande construction (oedometer e-log σ' curve): find point of maximum curvature; draw horizontal and tangent lines; bisect angle; where bisector meets the extension of the straight-line portion of the compression curve = σ'c (pre-consolidation pressure). OCR = σ'c/σ'v0. If OCR>1: overconsolidated clay; OCR=1: normally consolidated. Critical for settlement prediction.
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