Ground engineering · Earthquake geotechnics
Liquefaction triggering (CPT) calculator
CPT-based triggering with Ic screening, clean-sand equivalent Qtn,cs, FS profile, LPI and indicative settlement.
Standard: Robertson & Wride (1998) / Robertson (2009) · NCEER rd & MSF
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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, 3.0, 20, 0 2, 2.5, 18, 5 3, 3.5, 22, 10 4, 4.5, 30, 20 5, 6.0, 38, 30 6, 1.0, 40, 120 7, 7.5, 45, 50 8, 10.0, 55, 60 9, 14.0, 70, 70 10, 16.0, 80, 80 | |
| Magnitude Mw | 6.5 | |
| amax | 0.15 | g |
| Net area ratio | 0.8 | |
| 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 |
|---|---|---|
| Minimum FS | 1.32 | |
| Liquefaction potential index LPI | 0.0 | very low |
| Post-liquefaction settlement (1D) | 15 | mm |
| z = 1 m: CSR, CRR, FS | 0.097, 0.111, above GWL | |
| z = 2 m: CSR, CRR, FS | 0.111, 0.104, 1.36 | εv 0.28 % |
| z = 3 m: CSR, CRR, FS | 0.129, 0.119, 1.32 | εv 0.36 % |
| z = 4 m: CSR, CRR, FS | 0.140, 0.133, 1.37 | εv 0.33 % |
| z = 5 m: CSR, CRR, FS | 0.147, 0.153, 1.51 | εv 0.23 % |
| z = 6 m: CSR, CRR, FS | 0.151, 0.143, clay-like | |
| z = 7 m: CSR, CRR, FS | 0.154, 0.161, 1.50 | εv 0.24 % |
| z = 8 m: CSR, CRR, FS | 0.156, 0.207, 1.91 | εv 0.03 % |
| z = 9 m: CSR, CRR, FS | 0.158, 0.323, 2.95 | |
| z = 10 m: CSR, CRR, FS | 0.156, 0.395, 3.66 |
- ✓ No layer with FS < 1
- ✓ LPI ≤ 5 (low)
Method, formulas and assumptions
- Qtn and Ic as in Robertson (2009); Ic > 2.6 treated as clay-like (not liquefiable, check cyclic softening)
- Kc = 1 (Ic ≤ 1.64), else 5.581Ic³ − 0.403Ic⁴ − 21.63Ic² + 33.75Ic − 17.88; Qtn,cs = Kc·Qtn
- CRR7.5 = 93(Qtn,cs/1000)³ + 0.08 (50 ≤ Qtn,cs < 160) or 0.833(Qtn,cs/1000) + 0.05
- rd (NCEER) = 1 − 0.00765z (z ≤ 9.15 m), 1.174 − 0.0267z (to 23 m); MSF = 10^2.24 / M^2.56
- Settlement uses an approximate (N1)60cs ≈ Qtn,cs/5 with Idriss & Boulanger strains: indicative only
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For preliminary design and education. Check every result against the current standard and your own engineering judgement before use.