Tools › Civil and Geotechnical › Soils › Liquefaction Screening › Example
15 m sand profile with SPT blow counts, water table at 2 m, magnitude 7.5 and 0.25 g: cyclic stress ratio, NCEER resistance, factor of safety by layer, settlement and liquefaction potential index
| Tool | Liquefaction Screening |
| Source | Youd, T. L., and Idriss, I. M. (eds.) (1997). Proceedings of the NCEER Workshop on Evaluation of Liquefaction Resistance of Soils. Technical Report NCEER-97-0022, National Center for Earthquake Engineering Research, Buffalo, NY, equations (1), (2), (5) to (7), (9), (25) and Table 2; California Department of Transportation (2020). Caltrans Geotechnical Manual: Liquefaction Evaluation, Appendix B (the clean-sand curve and the overburden factor of Youd et al. 2001). |
| Level | Professional |
Problem
A level site is underlain by 15 m of sand with a clay-like layer from 7.5 to 9.0 m. Ten SPT tests at 1.5 m intervals give blow counts of 9, 8, 7, 10, 13, 4, 16, 21, 28 and 33 with fines contents of 5, 5, 15, 10, 5, 80, 20, 8, 5 and 5 percent (hammer energy ratio 60 percent). The unit weight is 18 kN/m³ above the water table and 19.5 kN/m³ below it; the water table is 2 m deep. For a magnitude 7.5 earthquake with a peak ground acceleration of 0.25 g, find the factor of safety against liquefaction of each layer by the NCEER procedure, the layers below a criterion of 1.1, the post-liquefaction settlement and the liquefaction potential index.
Inputs to Enter
- Magnitude 7.5, PGA 0.25 g, NCEER procedure with its own r_d (Liao–Whitman lines) and magnitude scaling factor (Idriss), clean-sand curve of Youd et al. (2001), required FS 1.1
- Water table 2 m at exploration and for design, water 9.81 kN/m³
- SPT: energy ratio 60 %, borehole and sampler factors 1.0, rod factor by rod length with 1.5 m stick-up, overburden cap 1.7, exponent f 0.7
- Ten layers of 1.5 m, 18 / 19.5 kN/m³; N = 9, 8, 7, 10, 13, 4, 16, 21, 28, 33; fines 5, 5, 15, 10, 5, 80, 20, 8, 5, 5 %; layer 6 clay-like; no plasticity screening
Answers
- Governing layer 3 at 3.75 m: σ_v = 70.1 kPa, σ′_v = 53.0 kPa, r_d = 0.9713, CSR = 0.65 · 0.25 · (70.1/53.0) · 0.9713 = 0.2090
- N_60 = 7 · 1.0 · 1.0 · 0.85 · 1.0 = 5.95 (rod length 5.25 m), C_N = 1.383, (N_1)_60 = 8.23; fines 15 %: α = 2.498, β = 1.048, (N_1)_60cs = 11.12
- CRR_7.5 = 0.1232, MSF = 0.9996, K_σ = 1.0: FS = 0.1231 / 0.2090 = 0.589; the layer reaches FS = 1 at 0.147 g
- FS by layer below the water table: 0.65, 0.59, 0.60, 0.61, (clay-like), 0.84, 0.80, 1.03, 1.27; 8.5 m of soil below FS 1.0 and 10.0 m below the 1.1 criterion: requirement not satisfied
- Reconsolidation settlement 261 mm (Ishihara–Yoshimine strains of 3.7 to 0.4 percent); liquefaction potential index 18.86, very high
- Same profile by Idriss and Boulanger (2008): lowest FS 0.622, settlement 259 mm, LPI 17.03
Notes
The tool's default inputs are this profile; press Submit. The profile is illustrative, not a printed example. Checked by test_liquefaction_screening.php (1,961 checks): the NCEER-97-0022 equations and Table 3, the two worked examples of the Caltrans Geotechnical Manual (FS 0.42 by CPT and 0.66 by SPT), 229 SPT and 251 CPT case histories and 283 sample computations tabulated in the UC Davis reports UCD/CGM-10/02 and UCD/CGM-14/01, and twenty seeded profiles computed independently with numpy and scipy; the default itself agrees with the numpy implementation to seven figures.