Liquefaction Potential Calculator – Seed-Idriss & Boulanger (CPT/SPT) | Earthquake Engineering

Liquefaction Potential Calculator

Seed‑Idriss (simplified) • Boulanger (2008/2012) • CSR & CRR • Factor of Safety • CPT/SPT input

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📐 1. Liquefaction Assessment – Seed‑Idriss & Boulanger Methods

Liquefaction occurs when saturated granular soils lose strength during earthquakes. This calculator implements the two most widely used simplified procedures:

Seed‑Idriss (1971, updated 1985): CSR = τ_av/σ'_v = 0.65·(a_max/g)·(σ_v/σ'_v)·r_d. CRR from SPT (N₁)₆₀ or CPT q_c1N.
Boulanger & Idriss (2012, 2014): Updated correlations, magnitude scaling factor, overburden correction, and fines content adjustment.

Factor of Safety (FoS) = CRR / CSR
FoS < 1 → liquefaction likely. This tool calculates FoS at specified depth and plots profile vs depth.

🚀 2. Liquefaction Calculator (Single Depth)

Test method

Depth z (m)

Peak ground acceleration a_max (g)

Earthquake magnitude M_w

Total vertical stress σ_v (kPa)

Effective vertical stress σ'_v (kPa)

For CPT: q_c (MPa) or SPT: N₆₀

Fines content FC (%) – for correction

Batch CSV (depth, qc/N, σv, σv_eff)

Get PDF Report via Email

CSV Template

📊 3. Factor of Safety vs Depth Profile

📈 4. Calculation History

📘 5. How to Use the Calculator

Select test method (CPT or SPT).
Enter depth, peak ground acceleration (a_max), earthquake magnitude.
Provide total and effective vertical stresses at that depth.
Enter CPT tip resistance q_c (MPa) or SPT N₆₀ blow count.
Specify fines content (%) for correction.
Click "Compute FoS" – CSR, CRR, and factor of safety are calculated (Seed‑Idriss).
The depth profile shows FoS vs depth for a user-defined range (use the "Refresh Profile" button to regenerate after changing inputs).
Batch CSV: upload multiple depths to compute FoS profile at once.
PDF report includes all parameters and the FoS‑depth chart.

❓ Frequently Asked Questions

❓ What is a safe FoS against liquefaction?

FoS ≥ 1.2 is generally considered safe. FoS < 1.0 indicates liquefaction likely. This tool marks FoS < 1.0 as "Liquefaction likely".

❓ How do I get σ_v and σ'_v?

Total stress = γ_total × depth. Effective stress = total stress – pore water pressure. For water table at surface, σ'_v = (γ_sat–γ_w)×depth.

❓ Why use both CPT and SPT?

CPT provides continuous profile and is more repeatable; SPT is traditional and widely used. The tool supports both with appropriate correlations.

🌐 References

Seed & Idriss (1971) – Simplified procedure | Boulanger & Idriss (2012, 2014) | NCEER Workshop (1997)

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