1. SPT N value correction
Raw SPT N values are corrected using the following equation:
where:
N raw = Raw SPT N values
CE = energy correction factor
CR = rod length correction factor
CB = borehole diameter correction factor
CS = liner correction factor
CE energy correction factor
CE = CH / 60, where CH is the hammer efficiency in percentage (%). CH can be adjusted in the Data Select module and has a default value of 60%.
CR rod length correction factor
The rod length correction factor depends on the total length of the drill rod. The length of the drill rod is approximately equal to the depth of the SPT test + 1 m. Once the length of the drill rod is determined for each SPT value, the CR value is determined automatically using the table below:
| Rod length (m) | Rod length correction factor, CR |
| rod length >= 10 | 1.0 |
| 6 <= rod length < 10 | 0.95 |
| 4 <= rod length < 6 | 0.85 |
| rod length < 4 | 0.75 |
CB borehole diameter correction factor
CB is the borehole diameter correction factor. This is set according to the diameter of the borehole. If the borehole diameter of the investigation is known, it will be automatically populated in the SPT Transform module using the table below:
| Diameter (mm) | Borehole diameter correction factor, CB |
| 60 - 120 | 1.0 |
| 150 | 1.05 |
| 200 | 1.15 |
If the borehole diameter is unknown, the user can manually specify CB. The default is 1.0.
CS liner correction factor
The liner correction factor depends on the sampler used to perform the test. The split spoon sampler may contain linear or not. The CS value for the sampler with liners is typically 1.0, while for samplers without liners, the Cs value ranges from 1.1 to 1.3. The default value is 1.0.
2. Assumptions
2.1 General design assumptions
1. In the SPT Transform module, the mid-depth of the SPT test is used for stress calculations and presentation of the graphs. The depth displayed on the output graphs is rounded to 2 dp.
e.g. if an SPT test is carried out between 3.0 - 3.45 m, the stress calculation uses 3.225 m to calculate the total stress and effective stress. This depth is also presented on the graphs.
2. When using Boulanger and Idriss (2014) to calculate N160cs, Eqn 2.15c is only applicable for N160cs values less than 46. This implies that the minimum value of m is capped at ~0.263. Instead of capping N160cs, the cap is applied to the stress exponent m, such that m cannot be less than ~0.263. This ensures that the calculated N160cs value is always equal to or greater than N160.
3. When estimating the friction angle using the Japanese Road Association (1996) method, if N160 is less than or equal to 5, the friction angle is not calculated.
4. When stress normalisation is needed, the overburden correction factor CN is calculated using Boulanger and Idriss (2014).
5. Equation 4-11 in Kulhawy and Mayne (1990) estimates the friction angle using SPT N values.
Infinity assumes the N60 value is used in Kulhawy and Mayne (1990), as page 2-20 in Kulhawy and Mayne (1990) mentions that the average energy ratio is about 55 - 60 % in the U.S.A, and Equation 4-11 comes with it's own stress normalisation calculations.
Extract of page 2-20 from Kulhawy and Mayne (1990)
Equation 4-11 from Kulhawy and Mayne (1990)
Equation 4-11 raises tan-1(...) to the power of 0.34. There appear to be some inconsistencies between Equation 4-11 and Figure 4-13. Equation 4-11 is an approximation of Fig 4-13. The inconsistencies are demonstrated in the two versions below.
Version 1:
If tan-1(...) is evaluated first, before being raised to the power of 0.34, the result produced by Equation 4-11 does not match that in Fig 4-13.
Version 1: Equation 4-11 as interpreted in Kulhawy and Mayne (1990), which does not match Fig 4-13
Version 2:
If the expression is first raised to the power of 0.34 before tan-1(...), the results match Fig 4-13 closely.
Version 2: Equation 4-11 in Kulhawy and Mayne (1990), which better matches Fig 4-13
Apeiron adopts Version 2, as it produces results matching Fig 4-13.
Two example calculations are shown below to demonstrate the difference between Version 1 and Version 2.
Fig 4-13 from Kulhawy and Mayne (1990) with two examples
Example 1:
N = 20
Vertical Stress ratio = 1
Friction angle using Version 1:
Friction angle using Version 2:
Reading the results directly from Fig 4-13, the friction angle should be approximately 40°.
Example 2:
N = 30
Vertical Stress ratio = 2
Friction angle using Version 1:
Friction angle using Version 2:
Reading the results directly from Fig 4-13, the friction angle should be approximately 40°.
In both examples, Version 2 produces results closer to the original graph that the equations are based on, and the results are more conservative. Apeiron adopts Version 2 in all calculations.
2.2 SPT N values used in different paper methods
Friction angle correlations:
| Paper | Paper method | Apeiron |
| Hatanaka and Uchida (1996) |
Adopts N1,78 Equation 8 in the original paper uses sqrt(20 * N1,78) + 20 |
Uses N1,60 Apeiron uses 15.4 * N1,60 + 20, which is the equivalent version for N1,60. |
| Wolff 1989 (functionalised Peck 1974) |
A graph from Fig 19.5, uses N1 of unknown energy efficiency | Uses N60 or N1,60 |
| Kulhawy and Mayne (1990) (Schmertmann (1975)) | Adopts N of unknown energy efficiency | Uses N60 |
| Kumar et al (2016) (functionalised Terzaghi and Peck (1967)) |
No mention of correction factors | Uses raw N, N1 or N1,60 |
| Japanese Road Association (1996) | N/a | Uses N60 or N1,60 |
Elastic modulus correlations:
| Paper | Paper method | Apeiron |
| Kulhawy and Mayne (1990) | Uses N60 | Uses N60 |
| Bowles (1997) | Uses N55 | Uses N55 |
3. References
- Boulanger, R. W., & Idriss, I. M. (2014). CPT and SPT based liquefaction triggering procedures. Report No. UCD/CGM.-14, 1.
- Kulhawy, F.H. & Mayne, Paul. (1990). Manual on Estimating Soil Properties for Foundation Design.