What is the Seed-Idriss simplified procedure?

Developed by H.B. Seed and I.M. Idriss in 1971, it is the most widely adopted empirical method for estimating liquefaction potential. It compares the earthquake-induced cyclic stress ratio (CSR) with the soil's cyclic resistance ratio (CRR) derived from SPT blow counts.

What does a Factor of Safety below 1.0 mean?

It means the cyclic stress imposed by the earthquake exceeds the soil's resistance to liquefaction, indicating that liquefaction is likely to occur at that depth. Mitigation such as densification or drainage may be required.

What is the rd (stress reduction coefficient)?

rd accounts for the flexibility of the soil column. It reduces from 1.0 at the surface to lower values at depth. This calculator uses the simplified depth-dependent expression from Liao & Whitman (1986).

How is fines content used?

High fines content (silts/clays mixed into sand) increases CRR. The calculator applies the Idriss-Boulanger (2008) fines correction to the clean-sand equivalent SPT value (N1,60,cs) before entering the CRR curve.

What magnitude scaling factor (MSF) is used?

The MSF normalizes CRR from the reference magnitude of 7.5 to the actual earthquake Mw, using the Idriss (1999) expression: MSF = 10^2.24 / (Mw^2.56).

Is this calculator suitable for design?

It is intended for screening and educational purposes. Final geotechnical design should use site-specific data, laboratory testing, and be reviewed by a licensed geotechnical engineer.

Soil Liquefaction Potential Calculator

Name: Soil Liquefaction Potential Calculator
Availability: InStock
Author: Nham Vu

Enter soil and earthquake parameters to compute CSR, CRR, and factor of safety against liquefaction using the Seed-Idriss simplified method.

Site & Earthquake Parameters

Earthquake

PGA (fraction of g)

Magnitude M_w

Vertical Stress at Depth

Total stress σ_v (kPa)

Pore pressure u (kPa)

Depth z (m)

SPT Data

N₆₀ (blows/ft)

Fines content FC (%)

Summary

Enter soil and earthquake parameters to compute CSR, CRR, and factor of safety against liquefaction using the Seed-Idriss simplified method.

How it works

Enter the peak ground acceleration (PGA) as a fraction of g (e.g. 0.30 for 0.30g).
Input total vertical stress and pore water pressure at the depth of interest.
Enter the SPT blow count (N-value) and the fines content of the soil.
Provide the earthquake moment magnitude (Mw) for the magnitude scaling factor.
The tool computes CSR, applies corrections to CRR, and returns the Factor of Safety.

Use cases

Screen shallow sandy layers for liquefaction risk during seismic hazard studies.
Support geotechnical reports for buildings in seismic zones.
Evaluate remediation needs when FS < 1.0 is returned.
Cross-check hand calculations from SPT borings and seismic design spectra.
Rapid classroom or study exercises on the Seed-Idriss procedure.
Compare liquefaction susceptibility across multiple depths on a borehole log.

Frequently Asked Questions

Last updated: 2026-07-01 · Reviewed by Nham Vu