N-SPT Correction

About NSPT Correction

N-SPT values observed during testing are not utilized directly in the assessment of soil properties. These values need to be corrected to account for hammer efficiency,borehole diameter, sampling method, rod length and the overburden pressure. Current tool sets the rules to allow the calculations for the correct SPT numbers, \([N]_{60}\)and \([N_1]_{60}\). You can activate the tool and then go back to project page to plot corrected SPT numbers with depth.

The corrected number N60is calculated as follows:

Figure: Equipment used to perform the SPT

\(N_{60} = \text{CER} \cdot \text{CRL} \cdot \text{CS} \cdot \text{CB} \cdot \)measured N

The corrected number\([N_1]_{60}\)is calculated as follows:

\([N_1]_{60} = C_N - N_{60}\)

Where:

\(C_{ER}\)= actual energy ration (compared to 60%) = ER/60

\(C_{RL}\)= correction to account for length of rod

\(C_S\) = correction due to liner used

\(C_B\) = correction due to borehole diameter

\(C_N\) = overburden pressure correction factor

Overburden pressure correction factor, CNmethods are defined below:

1. Peck, Hanson and Thornburn [1974]

\(C_{N} = 0.77 \cdot \log \left( \frac{20}{\sigma''_{v0} (\text{TSF})} \right)\)

2. Seed [1976]

\(C_{N} = 1 - 0.25 \cdot \log \sigma''_{v0} (\text{TSF})\)

3.Liao & Whitman (1986)

\(C_{N} = \sqrt {\frac{1}{\sigma''_{v0}(TSF \text{ or } kg/\text{cm}^3)}} \)

4. Skempton [1986]

\(C_{N} = \frac{2}{1+\sigma''_{v0}(TSF)} \)

Table 1: Summary of typical rod energy ratios

Country Hammer Type Hammer Release Em(%) CER
Japan Donut Free fall 78 1.30
Donut Rope and pulley 67 1.12
United States Safety Rope and pulley 60 1.00
Donut Rope and pulley 45 0.75
Argentina Donut Rope and pulley 45 0.75
Donut Free fall 60 1.00
China Donut Rope and pulley 50 0.83
UAE Automatic Free fall 75 1.25

Table 2: Rod Length Correction, CRL

Rod Length CR
(m) (ft)
3 - 4 10 - 13 0.75
4 - 6 13 - 20 0.85
6 - 10 20 - 30 0.95
> 10 > 30 1.00

Table 3: Sampling Method Correction, Cs

Method Cs
Sampler without liner(most common) 1.2
Standard Sampler 1.0
With liner for loose sand 0.9
With liner for dense sand and clay 0.8

Table 4: Correction for Borehole Diameter, CB

Diameter CB
(mm) (in)
65 - 115 2.4 - 4.7 1
150 6 1.05
200 8 1.15

References

  1. Geotechnical investigation and testing – Field testing – Part 3: Standard penetration test (ISO 22476-3:2005).
  2. ASTM D 1586-99 Standard Test Method for Penetration Test and Split-Barrel Sampling of Soils.
  3. Kovacs, W. D. Salomone, L. A., and Yokal, F. Y., “Comparison of Energy Measurements in the Standard Penetration Test Using the Cathead and Rope Method,” National Bureau of Standards Report to the US Nuclear Regulatory Commision, 1983.
  4. Schmertman, J. H., “Use the SPT to Measure Dynamic Soil Properties ?-Yes, But..!”, Dynamic Geotechnical Testing, ASTM STP 654, American Society for Testing and Materials, 1978.
  5. Seed, H. B., Tokimastu, K., Harder, L. F. and Chung, R. F., “Influence of SPT Procedures in Soil Liquefaction Resistance Evaluations,” Journal of Geotechnical Engineering, Vol. 111, No. 12, December 1985.
  6. Skempton, A. W., “Standard Penetration Test Procedures and the Effects in Sands of Overburden Pressure, Relative Density, Particle Size, Ageing and Over Consolidation”, Geotechnique 36: 3, September, 1986.
Activated ags files
# Project Ags Name Efficiency Input CS Method CN Method Correct NSPT Maximum N Actions
error_outline
error_outline
SN Depth N SPTfield Bulk Density Total Normal Stress Pore Water Pressure Effective Normal Stress CER CR CS CB CN N60 (N1)60
m - kN/m3 kN/m2 kN/m2 kN/m2 - - - - - - -
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