SPT Overburden Correction (Liao–Whitman)

The standard penetration test counts the hammer blows needed to drive a split-spoon sampler 300 mm, and the same sand gives a higher count deeper down simply because it is confined more tightly. To compare one borehole with another — or with the liquefaction databases — the count must be normalised to a reference overburden of one atmosphere, and Liao and Whitman's 1986 factor C_N = √(p_a/σ′v) is the form the codes adopted. A field N₆₀ of 20 at an effective overburden of 150 kPa becomes (N₁)₆₀ = 20 × √(100/150) = 20 × 0.816 = 16.3.

The trap is doing this before the energy correction. Raw field N must first be corrected to 60% of the theoretical hammer energy — a safety hammer on a rope-and-cathead is around 60%, an old pinweight hammer nearer 45%, a modern automatic trip hammer 80–90% — plus rod length, borehole diameter and liner corrections. Skipping that and applying C_N to a raw N is the single most common misuse of the SPT. The second trap is shallow depth: at σ′v below about 25 kPa the square root inflates C_N past 1.7, and most standards cap it at about 2. Note too that σ′v is the effective stress, so below the water table you use the buoyant unit weight.