How to use the liquefaction screening calculator
Liquefaction happens when earthquake shaking raises pore water pressure in saturated sand until the soil briefly behaves like a fluid. This calculator applies the widely used simplified procedure (Seed and Idriss family) to compare the cyclic stress ratio the earthquake imposes with the cyclic resistance ratio the soil provides, and reports the ratio as a factor of safety.
CSR is computed as 0.65 · amax · (σv/σ'v) · rd, and CRR comes from the corrected blow count (N1)60 at a magnitude 7.5 reference, scaled by a magnitude scaling factor. A shallow water table and a low blow count push CSR up and CRR down at the same time, so risk climbs quickly.
No fines content correction is applied, which makes results conservative for silty sand. A corrected blow count of 30 or more is normally treated as non-liquefiable, and that case is flagged separately. Even with a factor of safety above 1.0, post-shaking settlement and lateral spreading remain possible, so the number alone is not reassurance.
This is a single-depth screening tool and is not a design basis. A real evaluation covers the full soil profile and must be confirmed by a licensed geotechnical engineer.
Frequently asked questions
In simplified screening, a factor below 1.0 signals likely triggering, 1.0 to 1.25 is marginal, and above 1.25 is considered low risk. Settlement and lateral spreading can still occur just above 1.0, so critical structures use a higher target.
Loose, saturated, sandy soils below the water table are the main concern. Unsaturated soil above the water table, free-draining gravel, and clay with high plasticity are normally screened out of the evaluation.
No. This is a single-depth screening calculation. A real evaluation needs fines content corrections, layer-by-layer profiles, and post-liquefaction settlement and lateral spread checks, all reviewed by a licensed geotechnical engineer.