The 'standard penetration resistance' (N) is corrected for overburden pressure using the 'overburden correction factor' (CN) because:
Choose the correct answer
Correcting for borehole diameter only
N values vary with confining pressure (depth) — CN normalises to reference effective stress for fair comparison and design
Correcting for water table location
The hammer always delivers different energy
Correct Answer
B. N values vary with confining pressure (depth) — CN normalises to reference effective stress for fair comparison and design
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At shallow depths, low confining pressure gives artificially low N values; at deep depths, high confinement gives high N. CN corrects N to a reference effective stress of 100 kPa: N1,60 = CN × N60. Liao-Whitman: CN = (Pa/σv')^0.5. Also corrected for energy ratio (60%), rod length, sampler, borehole diameter. N1,60 = corrected SPT used for liquefaction assessment.
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