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Liquefaction Screening

Liquefaction triggering of a layered soil profile by the simplified stress-based procedure, from SPT blow counts or CPT tip and sleeve resistances: the earthquake-induced cyclic stress ratio against the cyclic resistance ratio layer by layer, by the NCEER workshop procedure (Youd and Idriss), Idriss and Boulanger (2008) or Boulanger and Idriss (2014), with the factor of safety, post-liquefaction settlement and the liquefaction potential index.

  • CSR = 0.65 (amax/g)(σv/σ′v) rd with rd by the Liao–Whitman lines, the Blake polynomial or Idriss (1999); separate water tables at exploration and for design

  • SPT: energy, borehole, rod-length and sampler corrections, CN, fines correction and the clean-sand curve; CPT: Robertson–Wride normalisation with the Ic sequence and Kc, or the Boulanger–Idriss 2014 correlation

  • Magnitude scaling factor and overburden factor of each procedure; factor of safety per layer against the entered criterion and the acceleration that triggers each layer

  • Reconsolidation settlement by the Ishihara–Yoshimine strains, liquefaction potential index with its severity class, and plasticity screening by the Bray–Sancio or Chinese criteria

Liquefaction Screening

Unit System: /

Earthquake and Procedure








Water Table




Test Data and Corrections











Soil Layers, top to bottom (one row per test interval; the first row starts at the ground surface)
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Plasticity Screening of Fine-Grained Layers (optional)









Methods, Relations and Sources Used | Method page
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Liquefaction Screening

Liquefaction Screening

What the tool does

The tool applies the simplified stress-based procedure of Seed and Idriss to a layered soil profile. At the mid-depth of every saturated sand-like layer it compares the cyclic stress ratio the design earthquake imposes (CSR) with the cyclic resistance ratio of the soil (CRR), which is read from a standard penetration test blow count or from cone penetration test tip and sleeve resistances. The ratio FS = CRR / CSR is the profession's measure; the tool compares it with the criterion entered (1.0 in the Caltrans manual, above 1.1 in FHWA-NHI-11-032, up to 1.3 for critical structures) and reports the consequences where it falls below 1.0: reconsolidation settlement and the liquefaction potential index. Three published procedures are offered: the NCEER workshop consensus (Youd and Idriss 1997; Youd et al. 2001), Idriss and Boulanger (2008, 2010) and Boulanger and Idriss (2014). The result is a screening, not a design: it holds for level or gently sloping ground and depths to about 15 m, where the field cases behind every curve lie.

Conventions and units

Depths are positive downward from the ground surface. Layers are contiguous; each row of the layer table is one test interval and is analysed at its mid-depth. A layer crossed by the design water table is split there, the part above being unsaturated and not analysed. Two water tables are entered: the one met during the exploration, with which the penetration resistance is normalised, and the design one, with which the earthquake stresses are computed (FHWA-NHI-11-032 p. 6-31: the normalised resistance is then held constant). Pa is one atmosphere, 101.325 kPa. Lengths, unit weights and stresses go through the unit dictionary (SI by default; cone tip resistance in kip/ft² in USCS, 1 ton/ft² being 2 kip/ft²); the magnitude, the acceleration in g, blow counts, percentages and factors are plain numbers.

Cyclic stress ratio

CSR = 0.65 (amax/g)(σv/σ′v) rd (NCEER-97-0022 eq. 1), where amax is the peak horizontal acceleration at the ground surface, σv and σ′v the total and effective vertical stresses and rd the stress reduction coefficient of the flexible soil column. Three forms of rd are offered (z in metres):

  • the Liao–Whitman lines of the NCEER report (eqs 2a to 2d): 1.0 − 0.00765 z to 9.15 m, 1.174 − 0.0267 z to 23 m, 0.744 − 0.008 z to 30 m, 0.50 below;
  • Blake's polynomial of the mean Seed–Idriss curve (eq. 3): (1 − 0.4113 z0.5 + 0.04052 z + 0.001753 z1.5) / (1 − 0.4177 z0.5 + 0.05729 z − 0.006205 z1.5 + 0.001210 z²), which runs up to 0.03 below the lines between 15 and 22 m;
  • Idriss (1999), which depends on the magnitude: rd = exp[α(z) + β(z) M], α = −1.012 − 1.126 sin(z/11.73 + 5.133), β = 0.106 + 0.118 sin(z/11.28 + 5.142), angles in radians (UCD/CGM-14/01 eq. 2.14).

The Automatic choice pairs each procedure with the rd it was derived with: a triggering correlation is meant to be used with the relations that located its curve among the case histories.

SPT: the NCEER procedure

(N1)60 = N CN CE CB CR CS, with CN = (Pa/σ′v)0.5 (Liao and Whitman) limited at shallow depth, CE = ER/60 for the hammer energy ratio, and the borehole, rod-length and sampler factors of NCEER-97-0022 Table 2. The rod-length factor is taken by the rod length (test depth plus stick-up): 0.75 below 3 m, 0.80 for 3 to 4 m, 0.85 for 4 to 6 m, 0.95 for 6 to 10 m, 1.0 beyond. Fines raise the resistance: (N1)60cs = α + β (N1)60 with α = 0, β = 1 up to 5 percent fines; α = exp(1.76 − 190/FC²), β = 0.99 + FC1.5/1000 between 5 and 35 percent; α = 5, β = 1.2 above (eqs 5 to 7). The resistance for magnitude 7.5 is the clean-sand base curve, in either of two fits: CRR7.5 = 1/(34 − N) + N/135 + 50/(10 N + 45)² − 1/200 (Youd et al. 2001 eq. 4, as printed in the Caltrans manual) or Blake's rational polynomial (NCEER-97-0022 eq. 4), both for N = (N1)60cs below 30; a soil at 30 or more is too dense to liquefy. The two fits agree within 0.01 up to 25 blows and part by 0.03 near 30.

SPT: Idriss and Boulanger

The overburden factor depends on the density: CN = (Pa/σ′v)m ≤ 1.7 with m = 0.784 − 0.0768 √(N1)60cs, solved by iteration. The fines adjustment is additive: Δ(N1)60 = exp[1.63 + 9.7/(FC + 0.01) − (15.7/(FC + 0.01))²], which is zero for clean sand and levels off near 5.5 above 35 percent fines. CRR7.5 = exp[N/14.1 + (N/126)² − (N/23.6)³ + (N/25.4)⁴ − 2.8] with N = (N1)60cs (UCD/CGM-14/01 eqs 2.15, 2.23, 2.25). The curve turns vertical near 37 blows; the tool treats a value of 2.0 or more as too dense to liquefy.

CPT: Robertson and Wride (NCEER)

The friction ratio is F = 100 fs/(qc − σv) percent and the soil behaviour type index Ic = [(3.47 − log Q)² + (1.22 + log F)²]0.5. Q is first computed as for a clay, Q = (qc − σv)/σ′v (exponent n = 1.0); an Ic above 2.6 classes the layer as clay-like and it is not analysed. Otherwise Q = qc1N = CQ qc/Pa with CQ = (Pa/σ′v)n, n = 0.5; if that Ic exceeds 2.6 the soil is very silty, n = 0.7 is used and the tool prints the report's request for a sample. The clean-sand value is (qc1N)cs = Kc qc1N, Kc = 1 up to Ic = 1.64 and −0.403 Ic⁴ + 5.581 Ic³ − 21.63 Ic² + 33.75 Ic − 17.88 above, and CRR7.5 = 0.833 (q/1000) + 0.05 below 50, 93 (q/1000)³ + 0.08 from 50 to 160 (NCEER-97-0022 eqs 11 to 19); 160 and above is too dense to liquefy. The Bureau of Reclamation notes that this curve corresponds to a probability of liquefaction of about 30 percent, and treats Ic of 2.5 to 2.7 as a transition; the cut-off is an input.

CPT: Boulanger and Idriss (2014)

qc1N = CN qc/Pa with CN = (Pa/σ′v)m ≤ 1.7, m = 1.338 − 0.249 (qc1Ncs)0.264 (qc1Ncs held between 21 and 254), iterated; Δqc1N = (11.9 + qc1N/14.6) exp[1.63 − 9.7/(FC + 2) − (15.7/(FC + 2))²]; CRR7.5 = exp[q/113 + (q/1000)² − (q/140)³ + (q/137)⁴ − 2.80] with q = qc1Ncs (eqs 2.15, 2.22, 2.24). The fines content is the entered one; where it is blank it is estimated as FC = 80 (Ic + CFC) − 137 percent, between 0 and 100 (eq. 2.30), with Ic from the sequence above. The scatter of that estimate is wide (CFC of ±0.29 is one standard deviation), so a few samples pay for themselves. The 2008 CPT curve of Idriss and Boulanger is not printed in the open reports; CPT layers use the 2014 correlation under either Idriss–Boulanger choice.

Magnitude and overburden

The resistance for the design magnitude and stress is CRR = CRR7.5 · MSF · Kσ. Magnitude scaling factors: Idriss, 102.24/M2.56 (NCEER-97-0022 eq. 25), the lower bound of the range the workshop recommended below magnitude 7.5 and its recommendation above; Andrus and Stokoe, (M/7.5)−3.3 (eq. 26), the upper bound of that range below 7.5; Idriss (1999), 6.9 exp(−M/4) − 0.058 ≤ 1.8; and Boulanger and Idriss (2014), MSF = 1 + (MSFmax − 1)(8.64 exp(−M/4) − 1.325) with MSFmax = 1.09 + ((N1)60cs/31.5)² or 1.09 + (qc1Ncs/180)³, not above 2.2, so that dense soil gains more from a short earthquake than loose soil. Overburden factors: for the NCEER procedure Kσ = (σ′v/Pa)f − 1, taken as 1.0 below one atmosphere, with f = 0.7 to 0.8 for relative densities of 40 to 60 percent and 0.6 to 0.7 for 60 to 80 percent; for Idriss–Boulanger Kσ = 1 − Cσ ln(σ′v/Pa) ≤ 1.1 with Cσ = 1/(18.9 − 2.55 √(N1)60cs) or 1/(37.3 − 8.27 (qc1Ncs)0.264), not above 0.3. Sloping ground (Kα) is outside the tool.

Factor of safety and the triggering acceleration

FS = CRR7.5 · MSF · Kσ / CSR for each layer. Because CSR is proportional to amax and nothing else in the ratio depends on it, the acceleration that brings a layer to FS = 1.0 at the same magnitude is acrit = amax · FS; the table lists it and a chart shows the governing layer against acceleration.

Settlement

The settlement of level ground after liquefaction is the sum of the reconsolidation strains over the layers, S = Σ εv H. The strain is that of Ishihara and Yoshimine (1992) in the closed form of Yoshimine et al. (2006), as adopted by Idriss and Boulanger and printed in the South Carolina DOT Geotechnical Design Manual (2022, eqs 13-75 to 13-85): εv = 1.5 exp(−0.369 √(N1)60cs) min(0.08, γmax), or 1.5 exp(2.551 − 1.147 (qc1Ncs)0.264) min(0.08, γmax) for the cone. The maximum shear strain γmax is zero for FS of 2 or more; equal to the limiting strain γlim = 1.859 (1.1 − √((N1)60cs/46))³ (or 1.859 (2.163 − 0.478 (qc1Ncs)0.264)³) for FS not above Fα; and min[γlim, 0.035 (2 − FS)(1 − Fα)/(FS − Fα)] between, with Fα = 0.032 + 0.69 √(N1)60cs − 0.13 (N1)60cs (blow count not below 7) or −11.74 + 8.34 q0.264 − 1.371 q0.528 (q not below 69). Layers with FS between 1 and 2 therefore settle a little too. The relations were derived with Idriss and Boulanger's clean-sand resistances; with the NCEER procedure the tool feeds them the NCEER clean-sand value, an approximation. The Tokimatsu and Seed (1987) charts and the cone-based curves of Zhang, Robertson and Brachman (2002) are alternatives the tool does not carry.

Liquefaction potential index

Iwasaki's index weights the deficit of the factor of safety by depth over the top 20 m: LPI = ∫ F (10 − 0.5 z) dz, F = 1 − FS where FS is below 1 and zero elsewhere (U.S. Geological Survey Open-File Report 2008-1270, eqs 1 and 2). It runs from 0 to 100. Iwasaki's classes are very low at 0, low to 5, high from 5 to 15 and very high above 15; the Geological Survey's field comparison found median values of 5 where sand boils appeared and 12 where the ground spread laterally.

Plasticity screening

Fine-grained soil is screened by index tests before the triggering analysis. Bray and Sancio (2006), as stated in Reclamation's Design Standards No. 13: a soil with a plasticity index below 12 and a water content above 0.85 of the liquid limit is susceptible; with PI below 18 and water content above 0.80 LL it is moderately susceptible and is analysed here with a flag; otherwise it is not susceptible. The older Chinese criteria (liquefaction possible only with less than 15 percent finer than 0.005 mm, a liquid limit below 35 and a water content of at least 0.9 LL) are offered for comparison with older reports; Reclamation and Caltrans no longer use them because they have proved unconservative. All limits are inputs. A layer without the index values keeps the Sand-like or Clay-like entry of the table. Clay-like soil does not liquefy in this sense but may soften under cyclic loading, which is a strength question the tool does not address.

What is not covered

Sloping ground and static shear stress, lateral spreading and flow slides, the settlement of dry sand above the water table, gravels, aged or cemented deposits, shear-wave-velocity correlations, thin-layer corrections of the cone, probabilistic curves, and site response analysis in place of rd. Below about 15 m the simplified procedure is extrapolated and the tool says so.

Checks

The library is checked by test_liquefaction_screening.php against the equations and the anchor values of NCEER-97-0022 (Table 3, the rd lines, the fines coefficients); the two worked examples of the Caltrans Geotechnical Manual (a cone layer with FS 0.42 and a blow-count layer with FS 0.66); 229 SPT and 251 CPT case histories and 283 per-sample computations tabulated in the University of California, Davis reports (CN, clean-sand resistance, rd, Kσ, MSF and CSR, to the printed figures); the asymptotes of the strain curves of the South Carolina manual's figure; closed forms of the index; and twenty seeded profiles computed independently with numpy and scipy. The default profile is illustrative.



References

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, State University of New York at Buffalo, Buffalo, NY.

Youd, T. L., Idriss, I. M., Andrus, R. D., Arango, I., Castro, G., Christian, J. T., Dobry, R., Finn, W. D. L., Harder, L. F., Hynes, M. E., Ishihara, K., Koester, J. P., Liao, S. S. C., Marcuson, W. F., Martin, G. R., Mitchell, J. K., Moriwaki, Y., Power, M. S., Robertson, P. K., Seed, R. B., and Stokoe, K. H. (2001). "Liquefaction Resistance of Soils: Summary Report from the 1996 NCEER and 1998 NCEER/NSF Workshops on Evaluation of Liquefaction Resistance of Soils." Journal of Geotechnical and Geoenvironmental Engineering, Vol. 127, No. 10, pp. 817–833.

Idriss, I. M., and Boulanger, R. W. (2010). SPT-Based Liquefaction Triggering Procedures. Report No. UCD/CGM-10/02, Center for Geotechnical Modeling, University of California, Davis, CA.

Boulanger, R. W., and Idriss, I. M. (2014). CPT and SPT Based Liquefaction Triggering Procedures. Report No. UCD/CGM-14/01, Center for Geotechnical Modeling, University of California, Davis, CA.

Idriss, I. M., and Boulanger, R. W. (2008). Soil Liquefaction During Earthquakes. Monograph MNO-12, Earthquake Engineering Research Institute, Oakland, CA.

Kavazanjian, E., Wang, J.-N. J., Martin, G. R., Shamsabadi, A., Lam, I., Dickenson, S. E., and Hung, C. J. (2011). LRFD Seismic Analysis and Design of Transportation Geotechnical Features and Structural Foundations. Geotechnical Engineering Circular No. 3, FHWA-NHI-11-032, Federal Highway Administration, Washington, DC.

California Department of Transportation (2020). Caltrans Geotechnical Manual: Liquefaction Evaluation. Division of Engineering Services, Geotechnical Services, Sacramento, CA, January 2020.

Bureau of Reclamation (2015). Design Standards No. 13: Embankment Dams, Chapter 13: Seismic Analysis and Design. DS-13(13)-8, U.S. Department of the Interior, Denver, CO, May 2015.

South Carolina Department of Transportation (2022). Geotechnical Design Manual, Chapter 13: Geotechnical Seismic Hazards. Columbia, SC, January 2022.

Holzer, T. L., Noce, T. E., and Bennett, M. J. (2008). Liquefaction Hazard Maps for Three Earthquake Scenarios for the Communities of San Jose, Campbell, Cupertino, Los Altos, Los Gatos, Milpitas, Mountain View, Palo Alto, Santa Clara, Saratoga, and Sunnyvale, Northern Santa Clara County, California. Open-File Report 2008-1270, U.S. Geological Survey, Reston, VA.

Seed, H. B., and Idriss, I. M. (1971). "Simplified Procedure for Evaluating Soil Liquefaction Potential." Journal of the Soil Mechanics and Foundations Division, Vol. 97, No. SM9, pp. 1249–1273.

Liao, S. S. C., and Whitman, R. V. (1986). "Overburden Correction Factors for SPT in Sand." Journal of Geotechnical Engineering, Vol. 112, No. 3, pp. 373–377.

Robertson, P. K., and Wride, C. E. (1998). "Evaluating Cyclic Liquefaction Potential Using the Cone Penetration Test." Canadian Geotechnical Journal, Vol. 35, No. 3, pp. 442–459.

Ishihara, K., and Yoshimine, M. (1992). "Evaluation of Settlements in Sand Deposits Following Liquefaction During Earthquakes." Soils and Foundations, Vol. 32, No. 1, pp. 173–188.

Yoshimine, M., Nishizaki, H., Amano, K., and Hosono, Y. (2006). "Flow Deformation of Liquefied Sand Under Constant Shear Load and Its Application to Analysis of Flow Slide of Infinite Slope." Soil Dynamics and Earthquake Engineering, Vol. 26, Nos. 2–4, pp. 253–264.

Iwasaki, T., Tatsuoka, F., Tokida, K., and Yasuda, S. (1978). "A Practical Method for Assessing Soil Liquefaction Potential Based on Case Studies at Various Sites in Japan." In Proceedings of the 2nd International Conference on Microzonation, San Francisco, CA, pp. 885–896.

Toprak, S., and Holzer, T. L. (2003). "Liquefaction Potential Index: Field Assessment." Journal of Geotechnical and Geoenvironmental Engineering, Vol. 129, No. 4, pp. 315–322.

Bray, J. D., and Sancio, R. B. (2006). "Assessment of the Liquefaction Susceptibility of Fine-Grained Soils." Journal of Geotechnical and Geoenvironmental Engineering, Vol. 132, No. 9, pp. 1165–1177.

Wang, W. (1979). Some Findings in Soil Liquefaction. Water Conservancy and Hydroelectric Power Scientific Research Institute, Beijing, China.

Worked Examples

Published examples this tool reproduces; each page lists the inputs to enter and the printed answers.

All worked examples

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