Seed-Idriss simplified method: compute CSR, CRR from SPT N-values, factor of safety FS, MSF, and liquefaction potential index IL with real-time depth profiles.
Parameters
Presets
Seismic / Soil Parameters
Peak ground accel. amax
g
Earthquake magnitude Mw
Mw
Water table depth dw
m
Unit weight γ (above WT)
kN/m³
Saturated unit weight γsat
kN/m³
SPT at depth z = 3 m
SPT N-value
Fines content FC
%
While paused, move the sliders to update the result instantly.
What exactly is "liquefaction potential"? I've heard of solid ground turning to liquid during earthquakes, but how do engineers predict it?
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Basically, it's the risk that saturated, loose sandy soil will lose its strength and behave like a fluid when shaken. In practice, we predict it by comparing the earthquake's shaking demand to the soil's inherent resistance. In this simulator, you control the shaking demand with the "Peak ground accel." and "Earthquake magnitude" sliders, and the soil's resistance with the "SPT N-value" and "Fines content" inputs.
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Wait, really? So the SPT N-value from a field test is that important? What are CSR and CRR that the calculator shows?
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Exactly! The Standard Penetration Test (SPT) N-value is a key in-situ measurement of soil density and strength. CSR (Cyclic Stress Ratio) is the earthquake's "demand" – the shear stress it induces. CRR (Cyclic Resistance Ratio) is the soil's "capacity" – the stress level it can resist before liquefying. Try lowering the SPT N-value in the simulator; you'll see the CRR line drop, making liquefaction more likely.
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So the Factor of Safety (FS) is just CRR divided by CSR? What does a value like 0.8 actually mean for a building on that soil?
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That's right, FS = CRR / CSR. An FS of 0.8 means the earthquake demand is 25% greater than the soil's resistance, so liquefaction is expected. For instance, a shallow foundation on that layer could sink or tilt dramatically. That's why design codes often require FS ≥ 1.2–1.5 for safety. Adjust the water table depth higher and see how the FS improves because the effective stress increases.
Physical Model & Key Equations
The core of the Seed-Idriss method is calculating the Cyclic Stress Ratio (CSR), which represents the seismic demand on the soil layer. It scales the peak surface acceleration down to a cyclic shear stress at depth, accounting for overburden pressure and a depth reduction factor.
Where: $\sigma_v$ = total vertical stress, $\sigma'_v$ = effective vertical stress (controlling soil strength), $a_{max}$ = peak ground acceleration, $g$ = gravity, $r_d$ = stress reduction factor (≈1 at surface, decreases with depth). The ratio $\sigma_v / \sigma'_v$ shows why a high water table (which reduces $\sigma'_v$) increases liquefaction risk.
The Cyclic Resistance Ratio (CRR) quantifies the soil's capacity to resist liquefaction. It is empirically derived from the corrected SPT blow count, $(N_1)_{60}$, which normalizes the field N-value to an equivalent energy and overburden pressure.
Where: $(N_1)_{60}$ is the SPT N-value corrected for overburden pressure and hammer efficiency. This equation defines a "clean sand" curve. The final Factor of Safety is $\mathrm{FS}= (\mathrm{CRR}_{7.5}\times \mathrm{MSF}) / \mathrm{CSR}$, where MSF is a Magnitude Scaling Factor to adjust for earthquake duration.
Real-World Applications
Seismic Building Code & Foundation Design: Geotechnical engineers use this analysis to determine the required depth of pile foundations or the need for ground improvement. For a high-rise in a seismic zone, calculating FS profiles with depth dictates whether costly soil densification is needed before construction.
Critical Infrastructure Assessment: The stability of embankment dams, bridge abutments, and port facilities during earthquakes hinges on liquefaction potential. A common case is evaluating existing older infrastructure to plan retrofits, like injecting grout to stabilize soils around bridge piers.
Post-Earthquake Forensic Analysis: After events like the 2011 Great East Japan Earthquake, which caused widespread liquefaction in the Kanto Plain, this method is used back-calculate ground motions and understand why certain areas (often reclaimed land or river deposits) failed while others did not.
Land Use Planning and Zoning: Municipalities use regional liquefaction potential maps, created using these principles, to restrict high-density development in high-hazard zones or mandate specific engineering controls. The integrated Liquefaction Potential Index (IL) from this simulator helps classify hazard as low, moderate, or high across a site.
Common Misunderstandings and Points to Note
While this simulator is useful, there are several key points you need to understand to avoid a false sense of understanding and potential misinterpretation. First, do not assume that "entering just the SPT N-value is enough." In actual site investigations, N-values are obtained at various depths, but their meaning changes depending on whether the soil contains gravel or silt. The tool assumes a "homogeneous sand layer." Therefore, for instance, a soil with an N-value of 20 but high clay content (a "silty sand") is less prone to liquefaction. Conversely, very loose sand with an N-value around 5 presents bearing capacity issues even before liquefaction calculations are considered.
Next, the idea that "a Factor of Safety (FS) above 1.0 guarantees absolute safety" is a dangerous misconception. This calculation is merely an assessment of "potential." An FS of 1.05 represents a very unstable state, "barely avoiding liquefaction." In practice, safety factors from 1.2 to over 1.5 are required depending on the importance of the structure. For example, FS > 1.2 is often targeted for general residential foundations, while FS > 1.5 is common for critical facilities like hospitals or power plants.
Finally, the selection of the input parameter "Peak Ground Surface Acceleration (amax)". This represents "the maximum shaking anticipated at that location." It should be determined based on historical seismic records or hazard maps; it is incorrect to arbitrarily set a high value simply because "a larger value is safer." Overestimation can lead to unnecessarily costly countermeasure proposals. Start by referring to values indicated in the "Seismic Hazard Map" for your area (e.g., around 400 Gal for lowland areas in Tokyo).
Enter peak ground acceleration (PGA) in g-units (e.g., 0.35g for moderate seismic zone) and select earthquake magnitude (Mw 6.5–8.0) based on local hazard maps.
Input SPT N-values at multiple depths (1m to 20m) and effective overburden stress (σ'v) for each layer; the calculator corrects N-values to N₁₆₀ using depth and energy correction factors.
Review CSR (cyclic stress ratio) and CRR (cyclic resistance ratio) at your critical depth; Factor of Safety FS = CRR/CSR indicates liquefaction onset when FS < 1.0.
Worked Example
Sandy soil deposit at z = 3m with SPT N = 8 blows, σ'v = 30 kPa, Mw = 7.2, PGA = 0.42g. Calculation yields CSR = 0.36 (from rd·amax/σ'v with rd = 0.91), CRR = 0.18 (Seed-Idriss correlation for N₁₆₀ = 12 after corrections), yielding FS = 0.50 and Liquefaction Potential IL = 3.2, indicating high liquefaction hazard requiring mitigation (ground improvement, drainage, or structural isolation).
Practical Notes
SPT N-values below 10 in clean sands at depths 5–15m are red flags; pair with piezocone (CPTu) for cone tip resistance (qc) cross-check in marginal cases.
Magnitude scaling factor (MSF) reduces CRR for earthquakes smaller than Mw 7.5; M 6.0 events use MSF ≈ 1.32, raising FS and reducing overestimation of risk in low-magnitude zones.
Fines content (FC) above 15% reduces liquefaction susceptibility; adjust CRR curves if silt or clay exceeds thresholds per NCEER guidelines.