GEO-34

SPT N60 energy correction

N60 = Nm (ER/60) CB CS CR CN. CN = min(1.7, √(Pa/σ′v)).

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In situASTM D1586Skempton N60

Governing equation

N_{60}=N_m\dfrac{E_R}{60}C_B C_S C_R C_N,\quad C_N=\min\bigl(1.7,\sqrt{p_a/\sigma'_v}\bigr)

where

N_m: Measured N (—)
E_R: Energy ratio (%)
C_B: Borehole CB (—)
C_S: Sampler CS (—)
C_R: Rod CR (—)
\sigma'_v: Effective overburden (kPa)
C_N: Overburden CN (—)
N_{60}: N60 (—)

Lecture brief

Historical brief

Soil mechanics became a quantitative laboratory after Karl von Terzaghi’s 1925–1943 work on effective stress, consolidation and bearing. These sheets still size shallow foundations, retaining walls, piles and drainage in SI. This sheet (GEO-34 — SPT N60 energy correction) is the form associated with ASTM D1586 · Skempton N60. Working symbols: $N_m$ , $E_R$ , $C_B$ , $C_S$ , $C_R$ , $\sigma'_v$ $\rightarrow$ $C_N$ , $N_{60}$ . N is hammer-energy dependent. 60 % of the theoretical 475 J is the reference. Overburden CN after Liao–Whitman.

Purpose

Purpose: compute

C_N

N_{60}

from

N_m

E_R

C_B

C_S

C_R

\sigma'_v

in Geotechnical engineering via

N_{60}=N_m\dfrac{E_R}{60}C_B C_S C_R C_N,\quad C_N=\min\bigl(1.7,\sqrt{p_a/\sigma'_v}\bigr)

N60 = Nm (ER/60) CB CS CR CN. CN = min(1.7, √(Pa/σ′v)). Use it when a real geotechnical engineering question must be answered in SI before a code check.

Live realistic example

In symbols

Live case. Given

N_m = 18.000\,\mathrm{—}

E_R = 55.000\,\mathrm{%}

C_B = 1.000\,\mathrm{—}

C_S = 1.000\,\mathrm{—}

C_R = 0.950\,\mathrm{—}

\sigma'_v = 80.000\,\mathrm{kPa}

, the governing relation

N_{60}=N_m\dfrac{E_R}{60}C_B C_S C_R C_N,\quad C_N=\min\bigl(1.7,\sqrt{p_a/\sigma'_v}\bigr)

yields

C_N = 1.118\,\mathrm{—}

N_{60} = 17.5\,\mathrm{—}

. A split spoon, a hammer, a corrected blow count. Move a slider: the numbers are this situation, not a canned story.

Calculator

Inputs

Measured N N_m18 —Energy ratio E_R55 %Borehole CB C_B1 —Sampler CS C_S1 —Rod CR C_R0.95 —Effective overburden \sigma'_v80 kPa

Outputs

Overburden CN C_N1.118 —
N60 N_{60}17.5 —

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Narration of this film

A split spoon, a hammer, a corrected blow count.

N is hammer-energy dependent. 60 % of the theoretical 475 J is the reference. Overburden CN after Liao–Whitman.

Reading speed

Watch on YouTube