Soil Liquefaction Risk Estimator
Estimate soil liquefaction risk during earthquakes using SPT N-values, groundwater depth, and earthquake magnitude.
Calculate CSR, CRR, and factor of safety against liquefaction.
What Is Soil Liquefaction? Soil liquefaction occurs when saturated, loosely-packed soil temporarily loses its strength during an earthquake. The shaking increases pore water pressure in the soil, causing it to behave like a liquid rather than a solid. Buildings, roads, and infrastructure can sink, tilt, or collapse when the ground they stand on liquefies. Liquefaction is most common in young, saturated, loose, fine-grained sands and silts within the top 15–20 meters.
Historical Liquefaction Events The 1964 Niigata earthquake (Japan) caused widespread liquefaction that toppled concrete apartment buildings. The 1964 Great Alaska earthquake caused liquefaction in Anchorage. The 1995 Kobe earthquake (Japan) and the 2011 Christchurch earthquake (New Zealand) both produced severe liquefaction. In Christchurch, approximately 400,000 cubic meters of liquefied sand erupted to the surface.
The SPT N-Value The Standard Penetration Test (SPT) measures soil resistance by counting the blow count N needed to drive a sampler 30 cm (12 inches) into the ground. Low N (less than 10): very loose soil — high liquefaction risk. N 10–30: medium density. N > 30: dense soil — generally resistant to liquefaction. The measured N is corrected for overburden pressure and hammer efficiency: N₁(60) = N × CN × CE × CB × CR × CS.
Cyclic Stress Ratio (CSR) CSR represents the shear stress induced by the earthquake: CSR = 0.65 × (σ_v / σ’_v) × (a_max / g) × r_d Where σ_v = total vertical stress, σ’_v = effective vertical stress, a_max = peak ground acceleration, g = 9.81 m/s², r_d = depth reduction factor. r_d ≈ 1.0 − 0.00765 × depth (for depths up to 9.15 m).
Cyclic Resistance Ratio (CRR) CRR represents the soil’s capacity to resist liquefaction based on SPT N-values. Simplified formula by Youd et al. (2001): CRR₇.₅ = 1/(34 − N₁(60)) + N₁(60)/135 + 50/(10 × N₁(60) + 45)² − 1/200 Adjusted for earthquake magnitude M: CRR = CRR₇.₅ × MSF Magnitude Scaling Factor: MSF = 10^2.24 / M^2.56
Factor of Safety (FS) FS = CRR / CSR FS > 1.0: generally liquefaction resistant (FS > 1.3 is considered safe). FS = 1.0: threshold between liquefaction and non-liquefaction. FS < 1.0: liquefaction likely during the design earthquake. A factor of safety of 1.0 does not mean 50% probability — actual probability depends on variability in soil and earthquake parameters.
This Calculator’s Scope This calculator uses simplified methods (Seed & Idriss / Youd et al.) for preliminary assessments. It is appropriate only for educational use and initial screening. Final liquefaction assessments for building design require site-specific geotechnical investigation and professional engineering analysis per local codes (e.g., ASCE 7, Eurocode 8, NZS 1170.5).