Site investigation · In-situ testing
SPT corrections & correlations calculator
Raw N to N60 and (N1)60 with energy, rod, borehole and overburden corrections, and common correlations, plotted with depth.
Standard: AS 1289.6.3.1 · Youd et al. (2001) · Liao & Whitman CN
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Inputs in the worked example
The live calculator starts with these values; change any of them and every result, check and diagram updates instantly.
| Input | Value | Unit |
|---|---|---|
| Depth (m), field N | 1.5, 6 3.0, 9 4.5, 12 6.0, 15 7.5, 18 9.0, 24 10.5, 30 12.0, 34 | |
| Hammer energy ratio ER | 60 | % |
| Borehole diameter factor CB | 1 | |
| Sampler factor CS | 1 | |
| su/N60 factor (clays, Stroud) | 5 | kPa |
| γ above water table | 18 | kN/m³ |
| γsat below water table | 19.5 | kN/m³ |
| Water table depth | 1.5 | m |
Worked example — results
| Result | Value | Unit |
|---|---|---|
| z = 1.5 m: N → N60 → (N1)60 | 6 → 4.5 → 7.6 | Dr ≈ 41 %, φ′ ≈ 28° |
| z = 3 m: N → N60 → (N1)60 | 9 → 7.2 → 11.2 | Dr ≈ 49 %, φ′ ≈ 29° |
| z = 4.5 m: N → N60 → (N1)60 | 12 → 10.2 → 13.7 | Dr ≈ 55 %, φ′ ≈ 30° |
| z = 6 m: N → N60 → (N1)60 | 15 → 14.3 → 17.1 | Dr ≈ 61 %, φ′ ≈ 31° |
| z = 7.5 m: N → N60 → (N1)60 | 18 → 17.1 → 18.7 | Dr ≈ 64 %, φ′ ≈ 32° |
| z = 9 m: N → N60 → (N1)60 | 24 → 22.8 → 23.0 | Dr ≈ 71 %, φ′ ≈ 34° |
| z = 10.5 m: N → N60 → (N1)60 | 30 → 30.0 → 28.3 | Dr ≈ 78 %, φ′ ≈ 36° |
| z = 12 m: N → N60 → (N1)60 | 34 → 34.0 → 30.2 | Dr ≈ 81 %, φ′ ≈ 37° |
- ✓ CN capped at 1.7
Method, formulas and assumptions
- N60 = N · (ER/60) · CB · CS · CR, CR from rod length (0.75 < 3 m … 1.0 > 10 m)
- (N1)60 = CN · N60, CN = (Pa/σ′v)^0.5 ≤ 1.7
- Dr ≈ √((N1)60/46) (Idriss & Boulanger 2008)
- φ′ ≈ 27.1 + 0.3N60 − 0.00054N60² (Wolff 1989)
- su ≈ f1 · N60 for clays (Stroud 1974, f1 ≈ 4–6 kPa)
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For preliminary design and education. Check every result against the current standard and your own engineering judgement before use.