Ground engineering · Earthquake geotechnics
Liquefaction triggering (SPT) calculator
Layer-by-layer CSR, CRR, factor of safety, LPI and post-liquefaction volumetric settlement from SPT blow counts and fines content.
Standard: Boulanger & Idriss (2014) · Iwasaki LPI · Idriss & Boulanger (2008) settlement
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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, fines % | 1.5, 7, 8 3.0, 6, 12 4.5, 9, 10 6.0, 11, 6 7.5, 14, 5 9.0, 18, 15 10.5, 25, 5 12.0, 32, 4 | |
| Earthquake magnitude Mw | 6.5 | |
| Peak ground acceleration amax | 0.15 | g |
| Hammer energy ratio | 60 | % |
| Borehole factor CB | 1 | |
| PI of fines (screen out if FC > 35 % and PI > 7) | 0 | % |
| 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 | 1.21 | |
| Liquefaction potential index LPI | 0.0 | very low |
| Post-liquefaction settlement (1D) | 23 | mm |
| z = 1.5 m: CSR, CRR, FS | 0.096, 0.151, above GWL | |
| z = 3 m: CSR, CRR, FS | 0.128, 0.154, 1.21 | εv 0.35 % |
| z = 4.5 m: CSR, CRR, FS | 0.140, 0.169, 1.21 | εv 0.44 % |
| z = 6 m: CSR, CRR, FS | 0.145, 0.177, 1.22 | εv 0.45 % |
| z = 7.5 m: CSR, CRR, FS | 0.146, 0.196, 1.35 | εv 0.33 % |
| z = 9 m: CSR, CRR, FS | 0.144, 0.289, 2.00 | εv 0.00 % |
| z = 10.5 m: CSR, CRR, FS | 0.142, 0.369, 2.60 | |
| z = 12 m: CSR, CRR, FS | 0.138, 0.656, 4.75 |
- ✓ No layer with FS < 1
- ✓ LPI ≤ 5 (low)
Method, formulas and assumptions
- CSR = 0.65 · (σv/σ′v) · amax · rd, rd = exp[α(z) + β(z)·M]
- (N1)60cs = CN·N60 + Δ(N1)60(FC), CN = (Pa/σ′v)^m iterated
- CRR7.5 = exp[N/14.1 + (N/126)² − (N/23.6)³ + (N/25.4)⁴ − 2.67]
- MSF = 1 + (MSFmax − 1)(8.64e^(−M/4) − 1.325); Kσ = 1 − Cσ ln(σ′v/Pa) ≤ 1.1
- FS = CRR7.5 · MSF · Kσ / CSR
- LPI = Σ F(z)·(10 − 0.5z)·Δz over 20 m, F = 1 − FS
- εv = 1.5·exp(−0.369√(N1)60cs)·min(0.08, γmax)
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For preliminary design and education. Check every result against the current standard and your own engineering judgement before use.