🕳️ Calculators

Overburden-Corrected SPT (N1)60 — Liao and Whitman

Corrects the SPT blow count already normalized to 60% energy for the effect of vertical effective stress, producing the (N1)60 required by liquefaction and relative density correlations. The overburden factor adopted is that of Liao and Whitman (1986), CN = square root of (100 ÷ vertical effective stress) with stress in kilopascal, capped at 1.7; the result is (N1)60 = N60 × CN. The correction exists because the same soil, at the same density, resists penetration more when it is deeper: without it, a loose sand at 20 metres would look denser than the same loose sand at 3 metres, and the liquefaction potential would be underestimated. The reference pressure of 100 kPa (one atmosphere) and the cap of 1.7 recommended by the 1997 NCEER report were adopted, because without a cap the correction blows up at shallow depths; part of the literature uses 95.76 kPa, which is 1 tsf, and a cap of 2.0, changing the result by a few percent. Enter the measured N60 and the vertical effective stress at the test depth.

Measured N60 (blows/30 cm) Vertical effective stress (kPa)

Result

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SPT (N1)60: why the same sand fights harder deeper down

A borehole log hands over raw blow counts, and those will not compare across depths. The same sand, at the same relative density, resists the sampler more at 20 metres than at 3 metres, purely on confinement. Anyone sizing a footing on sand, checking settlement or running a liquefaction assessment — a topic that gained weight in Brazil once the tailings dam safety reviews began — needs the normalized value. Skip the overburden correction and a loose deep layer reads as dense, so liquefaction potential ends up underestimated exactly where it counts.

The Liao and Whitman correction fits on one line: CN = √(100 ÷ σ'v), vertical effective stress in kilopascal, and (N1)60 = N60 × CN. Above 100 kPa the factor shrinks the measured N; below 100 kPa it lifts the value. With the on-screen defaults, 18 blows at 65 kPa, CN works out at 1.2403 and the answer is 22.33 blows per 30 cm. The reference pressure of 100 kPa, one atmosphere, and the 1.7 cap from the 1997 NCEER report were adopted, because an uncapped root runs away near the surface and would return absurd corrections in the first metre. Part of the American literature works from 95.76 kPa, which is 1 tsf, with a cap of 2.0: those same 18 blows would land at 21.85, roughly 2% lower.

Two conditions have to hold before anything gets typed. N60 must already carry the energy correction; a pinweight-and-rope hammer in Brazil tends to deliver 70 to 80% of theoretical energy, so the field N gets multiplied by something near 1.2 to become N60. And the stress asked for is the effective one: below the water table, subtract pore pressure or the correction comes out too small. The fit covers clean sands, never clay or gravel, where SPT itself is already shaky. Watch the unit — typing 0.065 while thinking in MPa returns 30.60 blows with no error message, since the 1.7 cap hides the absurdity.

Frequently asked questions

How does 22.33 come out of the default values?

CN is the square root of (100 ÷ 65), that is, the square root of 1.5385, which gives 1.2403. Since 1.2403 falls below the 1.7 cap, it passes through whole: 18 × 1.2403 = 22.33 blows per 30 cm, shown to two decimals. Those 65 kPa correspond roughly to 4 metres of depth in a sand above the water table, or nearer 7 metres below it. Because the stress sits under the 100 kPa reference, the correction raises the measured value — at exactly 100 kPa, CN equals 1 and (N1)60 matches N60.

At what depth does the 1.7 cap start to bite?

The cap engages once 100 ÷ σ'v exceeds 2.89, meaning vertical effective stress below 34.6 kPa — something like the top two metres of a dry sandy profile. Over that range the square root climbs fast, and an uncapped correction would inflate blow counts that were poor by nature anyway, in a stretch where the sampler still struggles with the weight of the rod string. Enter 20 kPa, for instance, and the raw CN is 2.236; the tool applies 1.7 instead, giving 30.60 rather than 40.25.

Is the stress asked for total or effective?

Effective: total vertical stress minus pore pressure at the test depth. In a sand of unit weight 18 kN/m³ with the water table at 2 metres, total stress at 6 metres is 108 kPa and pore pressure is 39.2 kPa, so the field takes 68.8 kPa. Feeding total stress below the water table shrinks CN and returns an (N1)60 lower than the real one, making the layer look looser than it is: conservative for liquefaction work, expensive in foundation design, where it turns into extra piles.

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The results provided by this tool are for general informational and educational purposes only and do not constitute professional, financial, medical, legal, tax or accounting advice. Always confirm important decisions with a qualified professional and official sources.