Generated by calculatgeek.com Liquefaction Calculator
In the 1964 Niigata earthquake, apartment buildings tipped over almost intact, like toys pushed by a child, because the sand beneath them briefly stopped being solid ground. Loose, saturated sand under rapid shaking wants to densify, but the water in its pores cannot escape fast enough; pore pressure climbs until it carries the entire load, effective stress hits zero, and the deposit behaves as a heavy fluid. That is liquefaction, and the screening method still used worldwide to predict it came from Seed and Idriss studying exactly those 1964 failures.
The simplified procedure is a stress comparison. The earthquake’s demand on a layer is the cyclic stress ratio:
CSR = 0.65 x (amax/g) x (sigmaV / sigmaV’) x rd
where rd is a depth reduction factor for the flexibility of the soil column. The soil’s capacity, the cyclic resistance ratio, comes from an empirical curve built from case histories: sites that did and did not liquefy, plotted against their corrected SPT blow count (N1)60cs. This calculator uses the clean-sand curve from the 2001 NCEER/Youd et al. consensus. Because the case-history database is anchored to magnitude 7.5 events, other magnitudes get a scaling factor, (Mw/7.5)^-2.56, before the factor of safety FS = CRR x MSF / CSR is formed.
A failing factor of safety answers only the triggering question: will pore pressure reach the critical level during this shaking? What happens next, settlement as the excess pressure drains, lateral spreading toward any free face, loss of pile support, sand boils, is a separate consequence assessment that builds on this result. The inputs also deserve their standard scrutiny: the blow count must be the fully corrected clean-sand value, which means energy, overburden, and fines-content corrections applied first, and a couple of extra fines percentage points can move a marginal layer across the line. Soils with meaningful plasticity generally fall outside this sand-based framework and get evaluated with different criteria. And a screening subtlety that surprises people: the water table used should be the highest credible during the design life, not the one on the day of drilling, because a seasonal rise of a meter can convert a safe site into a liquefiable one on paper and, occasionally, in reality.
The corrected blow count this needs comes from the SPT N-value corrections calculator, the looseness that makes sand susceptible is quantified in the relative density calculator, and foundation consequences connect to the bearing capacity calculator. The USGS overview of soil liquefaction is a solid public reference.