Simplified liquefaction assessment per EN 1998-5 §F + EN 1997-1 annex using the Seed & Idriss (1971) procedure as updated by Youd et al. (2001) and Idriss & Boulanger (2008). Calculates CSR, CRR, and Factor of Safety for each soil layer to 20 m depth, estimates post-liquefaction settlement (Ishihara & Yoshimine), and provides mitigation recommendations.
Standards compliance
EN 1998-5:2004 §F — Informative Annex: Simplified Liquefaction Assessment Method
EN 1997-1:2004 — Geotechnical Design (informative reference)
Seed & Idriss (1971) — Original simplified procedure
Youd et al. (2001) — NCEER corrections and CRR correlations
Idriss & Boulanger (2008) — Fines content correction and Kσ factor
EN 1998-5 §F provides an informative (non-mandatory) simplified procedure for seismic liquefaction assessment of level ground. It is based on the Seed & Idriss (1971) simplified method, using cyclic stress ratio (CSR) from the earthquake demand and cyclic resistance ratio (CRR) from the soil corrected SPT blow count (N₁)₆₀,cs. The method also incorporates the magnitude scaling factor (MSF) and overburden correction (Kσ).
CSR = 0.65 × (a_max/g) × (σ_v/σ′_v) × r_d, where a_max is the design peak ground acceleration, σ_v and σ′_v are the total and effective vertical stresses at the depth of interest, and r_d is the stress reduction coefficient from Liao & Whitman (1986). CSR represents the normalized cyclic shear stress demand imposed by the earthquake.
CRR is the cyclic resistance of the soil, expressed as CRR = CRR₇.₅ × MSF × Kσ. CRR₇.₅ is the clean-sand base resistance for M=7.5 derived from (N₁)₆₀,cs via Youd et al. (2001) correlations. MSF scales this to the design magnitude. Kσ corrects for effective overburden stress. A Factor of Safety FoS = CRR / CSR > 1 indicates the soil is stable; FoS < 1 indicates liquefaction.
(N₁)₆₀,cs is the SPT blow count normalized to an energy ratio of 60%, corrected for overburden pressure (CN, where CN = √(101/σ′_v) and capped at 2.0), and further adjusted for fines content (FC) using Idriss & Boulanger (2008): for FC ≥ 35%, a fines correction ΔN = 5.6×exp(0.06·FC − 0.22) is added to the blow count before computing the clean-sand equivalent. This allows CPT q_c data to also be used after converting to equivalent N₆₀.
Using Ishihara & Yoshimine (1992), the estimated volumetric strain ε_v is derived from the Factor of Safety and the relative density Dr (inferred from (N₁)₆₀,cs). Layers with FoS < 1.0 contribute proportionally; layers with FoS < 0.6 contribute fully. The settlement estimate is conservative and should be used only as a preliminary guide — detailed assessment should be carried out by a qualified geotechnical engineer.
Ground improvement is recommended when site susceptibility is high (FoS < 1.0 in critical layers) or moderate (FoS < 1.25), particularly for important structures (Importance Class III/IV). Common methods include stone columns or vibro-compaction for loose sandy soils, jet grouting for silty sands and clays, driven piles or caissons to bypass liquefiable layers, or excavation and replacement. The choice depends on soil type, depth, accessibility, and project budget.
Methodology
1. Cyclic Stress Ratio (CSR): CSR = 0.65 × (a_max/g) × (σ_v/σ′_v) × r_d. The 0.65 factor accounts for the number of equivalent uniform cycles. r_d is the stress reduction coefficient from Liao & Whitman (1986).
2. Corrected SPT Blow Count (N₁)₆₀,cs: The SPT N-value is first corrected to 60% energy ratio (N₆₀), then normalized for overburden (CN), and finally adjusted for fines content (FC) using Idriss & Boulanger (2008): for FC ≥ 35%, a fines correction ΔN = 5.6×exp(0.06·FC − 0.22) is added.
3. Cyclic Resistance Ratio (CRR): CRR₇.₅ is derived from (N₁)₆₀,cs using the Youd et al. (2001) clean-sand correlation. MSF scales CRR to the design magnitude (Seed & Idriss 1979, Idriss 1995). Kσ corrects for effective overburden stress (Harder & Boulanger 2006).
5. Post-Liquefaction Settlement: Volumetric strain ε_v is estimated from Ishihara & Yoshimine (1992) using the FoS and relative density Dr (inferred from (N₁)₆₀,cs). The settlement is S = ε_v × H_layer.