Liquefaction Analysis on the PE Geotechnical Exam: Step-by-Step Simplified Method
Six-step simplified Seed-Idriss liquefaction procedure for the PE Geotechnical exam, with an NCEES-style example problem and reference tables.
You learned the simplified Seed–Idriss procedure in a soil-dynamics course, and maybe ran one liquefaction analysis on a real project. Then a triggering question lands on your PE Geotechnical exam — saturated sand, an SPT log, a peak ground acceleration, an earthquake magnitude — and the question isn't whether you understand liquefaction. It's whether you can run the procedure in order, with the right factor at each step, under time pressure.
What the exam tests
At a high level, the exam tests whether you can carry a site through the simplified liquefaction-triggering procedure — weighing the demand an earthquake places on a soil against the resistance the soil can offer, and judging the resulting factor of safety. The individual steps are short; the test is assembling them in the right order, with the right corrected inputs and the right magnitude and depth adjustments.
Here's the kind of problem you'll face. The honest test: could you carry it all the way to a defensible answer, in about six minutes, on exam day?
Worked example. A site has clean fine-to-medium sand to depth, with the water table at 1.5 m. Soil unit weights: γmoist = 19.0 kN/m3 above the water table, γsat = 20.0 kN/m3 below. An SPT boring at depth 6.0 m gave N = 12 blows/ft using standardized equipment per NHI-11-032 Table 4-1 (safety hammer, rope and pulley, 4–5 in borehole, ~20-ft rod, no liners). Design earthquake: Mw = 7.5, amax = 0.20g. Fines content < 5%. Compute the FoS against liquefaction triggering.
Solution path: CSR: → CRR: → FoS: → Interpret:.
To see problems like this worked to the final answer on video, head to the PEwise PE Geotechnical course.
The full methods behind these — the relationships, the procedures, and the mistakes that quietly cost points — are taught step by step in PEwise's PE Geotechnical course, with animated worked problems rather than a wall of formulas.
See Liquefaction Triggering in Motion
PEwise's PE Geotechnical course visualizes pore-pressure rise, effective-stress collapse, and the SPT-CRR curve in animated lessons. When you can SEE the shaking drive pore pressure up and effective stress to zero, the formulas stop being abstract.
Connecting this to your overall PE exam strategy
Liquefaction triggering is one piece of NCEES Topic 4 (Earthquake Engineering and Dynamic Loads). Pseudo-static slope stability is another, and they often appear paired on the same form — a site that fails liquefaction triggering analysis is also a candidate for post-liquefaction lateral spreading and reduced-strength slope stability. Our slope stability problem-types post walks through the pseudo-static method that pairs naturally with the liquefaction analysis above.
For broader Topic 4 and Site Characterization context, our geotechnical PE exam study guide covers the full 24-module curriculum. And if you keep failing diagnostic exams on conceptual seismic-design questions (recognition of liquefaction triggers, identification of mitigation methods, pseudo-static kh selection), see our conceptual questions guide.
Master Liquefaction Analysis with PEwise
PEwise's Module 6: Liquefaction Analysis and Seismic Fundamentals covers the simplified Seed method, CPT-based methods, and post-liquefaction settlement with worked NCEES-style problems. Course author Mahdi Bahrampouri, Ph.D., is a Geotechnical Earthquake Engineer whose research specialty is exactly this topic — liquefaction, seismic slope stability, and dynamic foundation analysis.
Keep Reading
SPT and CPT Correlation Problems on the PE Geotechnical Exam
SPT N-value corrections (energy, overburden, rod-length) and CPT-to-SPT correlations (Robertson) for the PE Geotechnical exam — three worked NCEES-style problems plus the SPT correction-factor reference table.
PE Geotechnical Retaining Wall Design Problems: Cantilever, Anchored, MSE
Cantilever stability checks, anchored-bulkhead free-earth-support, and MSE wall external + internal stability for the PE Geotechnical exam — with three worked NCEES-style problems and the FoS reference table.
PE Geotechnical Lateral Earth Pressure: Coulomb vs Rankine vs At-Rest
Coulomb, Rankine, and at-rest earth-pressure theories for the PE Geotechnical exam — with three worked NCEES-style problems comparing horizontal forces, OC clay below the water table, and Coulomb passive caveats.