Site investigation · In-situ testing
CPT interpretation (Robertson SBTn) calculator
Normalised cone resistance and friction ratio, Ic, soil behaviour type plotted on the Robertson chart and as a profile.
Standard: Robertson (2009) Qtn–Fr chart · Ic
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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), qc (MPa), fs (kPa), u2 (kPa) | 1, 3.5, 25, 0 2, 1.2, 35, 20 3, 0.6, 22, 60 4, 0.7, 25, 80 5, 4.0, 30, 40 6, 9.0, 55, 45 7, 12.0, 70, 50 8, 2.0, 60, 150 9, 1.6, 48, 180 10, 15.0, 90, 70 | |
| Net area ratio a | 0.8 | |
| Cone factor Nkt | 14 | |
| 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 m: Qtn, Fr, Ic | 58, 0.72 %, 2.01 | zone 6: Sands: clean to silty sand |
| z = 2 m: Qtn, Fr, Ic | 20, 3.00 %, 2.76 | zone 4: Silt mixtures: clayey silt to silty clay · su ≈ 83 kPa |
| z = 3 m: Qtn, Fr, Ic | 9, 3.96 %, 3.09 | zone 3: Clays: silty clay to clay · su ≈ 40 kPa |
| z = 4 m: Qtn, Fr, Ic | 11, 3.90 %, 3.04 | zone 3: Clays: silty clay to clay · su ≈ 46 kPa |
| z = 5 m: Qtn, Fr, Ic | 54, 0.77 %, 2.06 | zone 5: Sand mixtures: silty sand to sandy silt |
| z = 6 m: Qtn, Fr, Ic | 107, 0.62 %, 1.76 | zone 6: Sands: clean to silty sand |
| z = 7 m: Qtn, Fr, Ic | 132, 0.59 %, 1.67 | zone 6: Sands: clean to silty sand |
| z = 8 m: Qtn, Fr, Ic | 21, 3.20 %, 2.76 | zone 4: Silt mixtures: clayey silt to silty clay · su ≈ 134 kPa |
| z = 9 m: Qtn, Fr, Ic | 15, 3.28 %, 2.88 | zone 4: Silt mixtures: clayey silt to silty clay · su ≈ 104 kPa |
| z = 10 m: Qtn, Fr, Ic | 140, 0.61 %, 1.66 | zone 6: Sands: clean to silty sand |
- ✓ Normalisation converged (n between 0.5 and 1)
Method, formulas and assumptions
- qt = qc + u2(1 − a)
- Fr = fs / (qt − σv) × 100
- Qtn = [(qt − σv)/Pa]·(Pa/σ′v)^n, n = 0.381Ic + 0.05σ′v/Pa − 0.15 ≤ 1 (iterated)
- Ic = √[(3.47 − log Qtn)² + (log Fr + 1.22)²]
- su = (qt − σv)/Nkt for Ic > 2.6
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For preliminary design and education. Check every result against the current standard and your own engineering judgement before use.