Calculateur de Risque de Liquéfaction

Estimez la probabilité de liquéfaction du sol en fonction des paramètres sismiques et des conditions du terrain.

Assessment

Comprendre la liquéfaction des sols lors des séismes

La liquéfaction des sols se produit lorsqu'un sol granulaire meuble et saturé perd sa résistance et sa rigidité lors de secousses sismiques, le faisant se comporter comme un liquide plutôt que comme un solide. Lorsque les ondes sismiques traversent du sable ou du limon saturé d'eau, elles augmentent la pression de l'eau entre les grains du sol (pression interstitielle). Si cette pression s'élève suffisamment pour égaler le poids du sol sus-jacent, les grains perdent le contact entre eux et le sol s'écoule temporairement comme un fluide visqueux. Ce phénomène peut provoquer l'enfoncement, le basculement ou l'effondrement de bâtiments ; la remontée en surface de structures souterraines comme les canalisations et les réservoirs ; et la rupture de pentes par étalement latéral.

La valeur N de l'essai de pénétration standard (SPT) est une mesure de terrain clé utilisée pour évaluer la susceptibilité à la liquéfaction. Elle mesure le nombre de coups de marteau nécessaires pour enfoncer un tube d'échantillonnage de 30 cm dans le sol — des valeurs N plus élevées indiquent un sol plus dense et plus résistant. Les sols avec des valeurs SPT N inférieures à 15 en conditions saturées sont généralement considérés comme susceptibles de liquéfaction. La profondeur de la nappe phréatique est tout aussi importante : le sol doit être saturé pour que la liquéfaction se produise, les zones avec des nappes phréatiques peu profondes (moins de 5 mètres) présentant le risque le plus élevé. Les terrains gagnés sur l'eau et les remblais côtiers sont particulièrement vulnérables, comme l'ont démontré les séismes de Niigata en 1964 et de Christchurch en 2011.

Facteurs clés du risque de liquéfaction

Type de sol : les sables meubles et les sables limoneux sont les plus susceptibles. Les argiles et les graviers résistent généralement à la liquéfaction, bien que les limons non plastiques puissent se liquéfier sous de fortes secousses.
Profondeur de la nappe phréatique : les nappes phréatiques peu profondes (0-5 m) augmentent considérablement le risque. Les sites avec des nappes phréatiques plus profondes que 15 m sont généralement à l'abri de la liquéfaction.
Intensité et durée des secousses : la liquéfaction nécessite généralement des secousses soutenues (plusieurs secondes) à des accélérations supérieures à 0,1g. Les séismes de plus longue durée augmentent le risque.
Précédent historique : les zones qui ont subi une liquéfaction lors de séismes passés sont très susceptibles de se liquéfier à nouveau, car les conditions géologiques qui causent la liquéfaction persistent.

Utilisations courantes

Évaluation préliminaire du potentiel de liquéfaction pour l'évaluation immobilière dans les zones côtières sujettes aux séismes.
Comprendre quelles conditions de sol et quels paramètres sismiques contribuent à la liquéfaction.
Évaluer le risque pour les sites sur remblai, terrain gagné sur l'eau, ou à proximité de rivières et de côtes.
Soutenir les discussions éducatives sur l'ingénierie géotechnique parasismique.

How to Use

1
Enter Soil and Site Data
Input your site's soil type (sand, silt, clay, fill), water table depth, and layer thickness. Sandy soils saturated with water at depths less than 10 meters carry the highest liquefaction potential.
2
Specify the Earthquake Scenario
Enter the design earthquake magnitude (Mw) and peak ground acceleration (PGA) for your site. These can be obtained from the USGS seismic hazard map or from a site-specific hazard analysis at your return period of interest.
3
Interpret the Liquefaction Potential Index
Review your Liquefaction Potential Index (LPI) value and qualitative risk category. LPI < 5 is considered low risk; LPI 5–15 moderate; LPI > 15 high. The tool follows the Iwasaki (1982) framework widely used in Japanese and US engineering practice.

About

Soil liquefaction has been recognized as a major earthquake hazard since the 1964 Niigata and Alaska earthquakes, which produced dramatic documentary evidence of ground failure. The modern analytical framework for liquefaction assessment was developed largely by H. Bolton Seed and colleagues at UC Berkeley in the 1970s–80s, based on Standard Penetration Test (SPT) blow counts as a proxy for soil density and resistance. The Seed-Idriss Simplified Procedure, subsequently updated by Youd et al. (2001) and Idriss and Boulanger (2008), remains the standard for routine engineering practice and forms the basis of most liquefaction screening tools.

Liquefaction assessment methods fall into two categories. Deterministic methods compute a factor of safety for each soil layer using measured in-situ properties (SPT N-value, CPT qc, shear wave velocity Vs) and the earthquake loading expressed as the Cyclic Stress Ratio. Probabilistic methods (Cetin et al., 2004; Boulanger and Idriss, 2014) incorporate uncertainty in both the demand and capacity parameters to produce probability-of-liquefaction values rather than binary pass/fail outcomes. Probabilistic approaches are increasingly favored in performance-based earthquake engineering (PBEE) frameworks, where risk is expressed as expected losses over the lifetime of a facility.

The 2010–2011 Canterbury earthquake sequence in New Zealand produced an unprecedented dataset for liquefaction research. The earthquakes affected Christchurch—a city founded on Holocene fluvial sediments—causing liquefaction across 80% of the residential Red Zone, the area ultimately purchased by the government for permanent managed retreat. GNS Science and the University of Canterbury deployed a multi-technique investigation including aerial photography, LiDAR surveys, and thousands of CPT soundings to map spatial variability in liquefaction manifestation at block-by-block resolution. The resulting Christchurch Liquefaction Vulnerability dataset is now one of the most comprehensive post-earthquake geotechnical records ever assembled and has driven significant advances in simplified liquefaction assessment procedures.

FAQ

What is soil liquefaction?

Soil liquefaction is a phenomenon in which saturated, loosely packed granular sediment (typically sand or silt) temporarily loses its shear strength and behaves as a fluid when subjected to rapid cyclic loading such as earthquake shaking. The mechanism involves the buildup of excess pore water pressure: when the soil grains are rapidly jostled, the water between them cannot escape fast enough, and the pore pressure increases until it equals the confining stress, at which point the soil loses all effective stress and grain-to-grain contact. The soil-water mixture flows laterally under gravity, causing ground failure. Structures founded on liquefied soil can sink, tilt, or experience foundation failure even if they themselves are seismically well designed.

Quels types de sols sont les plus susceptibles à la liquéfaction ?

Liquefaction susceptibility is highest in saturated, loose to medium-dense clean sands and non-plastic silts with low plasticity (PI < 12), at depths less than 20 meters, with water table within 3 meters of the surface. The Chinese Criteria (Seed and Idriss, 1982) identified fine sands with fines content less than 15% as most vulnerable. Recent research has extended liquefaction risk to low-plasticity silts and some sensitive clays (the 'cyclic softening' mechanism). Poorly compacted fills—including hydraulic fills used to reclaim land—are particularly susceptible and are responsible for much liquefaction damage in New Zealand (2010–2011 Canterbury earthquakes), Japan (Tokyo Bay waterfront, 2011), and San Francisco (Marina District, 1989 Loma Prieta).

How does liquefaction damage buildings?

Liquefaction causes damage through several mechanisms. Sand boils (or sand volcanoes) form when pressurized pore water and sand erupt to the surface through cracks or utility penetrations. Differential settlement occurs when liquefied zones compact unevenly after pore pressure dissipates, tilting or cracking structures. Lateral spreading is perhaps the most destructive mechanism: when liquefied ground near a slope or riverbank flows laterally by meters to tens of meters, it ruptures pipelines, buckles roads, and moves building foundations. The 1964 Niigata earthquake caused entire apartment blocks to tilt 15–40 degrees through liquefaction-induced settlement; the 2011 Christchurch earthquake sequence caused NZ$1.5 billion in liquefaction-related building losses in residential areas.

Can liquefaction risk be reduced?

Liquefaction risk can be significantly reduced through ground improvement and structural mitigation techniques. Vibro-compaction and vibro-replacement (stone columns) densify loose granular soils in place or replace susceptible soils with dense aggregate columns. Dynamic compaction uses heavy tamper drops to densify surface layers. Deep soil mixing and jet grouting inject binders (cement or lime) to create strengthened columns or grids within susceptible zones. For new construction, mat foundations that 'float' on the soil, deep pile foundations extending through susceptible layers to bearing strata, and perimeter sheet pile walls to contain lateral spreading all reduce liquefaction vulnerability. For existing structures, perimeter soil treatment and underpinning with micropiles can retrofit foundation performance.

What is the Liquefaction Potential Index?

The Liquefaction Potential Index (LPI), proposed by Iwasaki et al. (1982) and widely adopted in Japanese seismic design practice, is a single-value parameter that integrates the factor of safety against liquefaction over the full soil profile to a depth of 20 meters. The factor of safety at each depth (FS = CRR/CSR) is computed from the Cyclic Resistance Ratio (CRR—the soil's capacity to resist liquefaction) and Cyclic Stress Ratio (CSR—the seismic demand). FS values less than 1.0 indicate liquefaction; these layers contribute most to the LPI with depth-weighting that emphasizes shallow soils. LPI ≤ 2 is considered very low risk; 2–5 low; 5–15 high; > 15 very high. The index correlates reasonably well with observed surface manifestations of liquefaction from earthquakes in Japan, New Zealand, and Taiwan.