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    Geotechnical / Liquefaction

    Liquefaction Assessment

    The simplified Seed–Idriss procedure: the cyclic stress ratio the earthquake imposes against the cyclic resistance ratio the soil can mobilise. A factor of safety below 1 means the soil is expected to liquefy.

    Factor of safety0.53CSR0.272CRR0.145
    Factor of safety
    0.53
    Liquefaction expected
    CSR
    0.272
    Demand
    CRR
    0.145
    (N1)60cs 14
    Effective stress
    66 kPa
    at 5.0 m
    Factor of safety by depth
    CSR = 0.65·(σv/σ′v)·(amax/g)·rd; CRR from (N1)60.
    Factor of safety by depth
    Depth (m)CSRFS
    20.1920.76
    40.2550.57
    60.2840.51
    80.2990.49
    100.3010.48
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    Safety factor against (N1)60
    Denser soil is dramatically more resistant.
    Safety factor against (N1)600.0000.5171.031.552.072.59551010151520203030Liquefaction Assessment
    Canonical equations
    The exact relationships implemented by this engine.

    0.65·(σv/σ′v)·(amax/g)·rd · Seed–Idriss simplified.

    1/(34−N) + N/135 + 50/(10N+45)² − 1/200 · N = (N1)60cs.

    FS = CRR/CSR · Below 1 means liquefaction.

    engineering methodology

    A clear path from inputs to results.

    The simplified Seed–Idriss procedure: the cyclic stress ratio the earthquake imposes against the cyclic resistance ratio the soil can mobilise. A factor of safety below 1 means the soil is expected to liquefy.

    Step-by-step methodWorked exampleAssumptions explained

    How to use

    1. Set depth (m).
    2. Set total unit weight (kn/m³).
    3. Set water table depth (m).
    4. Set peak acceleration (g).
    5. Read the headline metrics, then check the table for the detail.

    The formula

    0.65⋅(σv/σ′v)⋅(amax/g)⋅rd0.65·(σv/σ′v)·(amax/g)·rd

    Reference: Seed–Idriss simplified CSR with the CRR curve for (N1)60cs.

    Worked example

    At the default inputs

    Inputs
    • Depth (m): 5; Total unit weight (kN/m³): 19; Water table depth (m): 2; Peak acceleration (g): 0.3; (N1)60: 12; Fines content (%): 15
    Calculation path
    1. 1

      Factor of safety: 0.53 · CSR: 0.272 · CRR: 0.145 · Effective stress: 66 kPa

    Result

    Factor of safety: 0.53 · CSR: 0.272 · CRR: 0.145

    Interpreting the result

    At 5.0 m the safety factor is 0.53.

    Assumptions & limitations

    • Level ground, no lateral spreading.
    • No ageing or overconsolidation correction.

    Frequently asked questions

    What does a factor of safety below 1 mean?

    The shaking is expected to exceed the soil’s resistance — liquefaction is expected.

    Is the fines correction included?

    Yes, as a capped increment to (N1)60.

    Methodology last reviewed 2026-10-22. Results are informational and not financial or engineering advice — see our disclaimer.