The 'cone factor' Nkt for estimating undrained shear strength from CPT is applied as:
Choose the correct answer
Su = qc/Nkt (without overburden correction)
Su = (qt - σv0)/Nkt — corrected cone resistance minus overburden; Nkt empirically calibrated 10–18
Su = Nkt × Cu (not related to cone pressure)
Su = u2/Nkt (pore pressure only)
Correct Answer
B. Su = (qt - σv0)/Nkt — corrected cone resistance minus overburden; Nkt empirically calibrated 10–18
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Su from CPT: Su = (qt − σv0)/Nkt where qt = cone tip resistance (corrected for pore pressure: qt = qc + u2(1-a)), σv0 = total overburden stress, Nkt = cone factor (empirical, 10–18 depending on soil type, typically 12–15 for soft NC clay). Lower Nkt for more sensitive clays. u2 = pore pressure behind cone. Important to use qt not qc for soft soils with high pore pressure.
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