液状化リスク計算機

地震パラメータと地盤条件に基づいて、土壌液状化の確率を推定します。

Assessment

地震における液状化現象の理解

液状化とは、飽和した緩い粒状土壌が地震の揺れにより強度と剛性を失い、固体ではなく液体のように振る舞う現象です。地震波が水で飽和した砂やシルトを通過すると、土粒子間の水圧（間隙水圧）が上昇します。この圧力が上載土の重量と等しくなるまで上昇すると、土粒子は互いの接触を失い、土壌は一時的に粘性流体のように流動します。この現象により、建物が沈下、傾斜、または倒壊し、パイプやタンクなどの地下構造物が地表に浮上し、斜面が側方流動で崩壊する可能性があります。

標準貫入試験（SPT）N値は液状化の感受性を評価するために使用される重要な現場測定値です。サンプリング管を地中に30 cm打ち込むのに必要なハンマー打撃回数を測定し、N値が高いほど密で抵抗力のある地盤を示します。飽和条件下でSPT N値が15未満の地盤は一般的に液状化の可能性があるとされます。地下水位も同様に重要です：液状化が起こるには地盤が飽和している必要があるため、地下水位が浅い地域（5メートル未満）が最もリスクが高いです。埋立地や沿岸の盛土は特に脆弱であり、1964年の新潟地震と2011年のクライストチャーチ地震で実証されました。

液状化リスクの重要な要因

土質：緩い砂とシルト質砂が最も影響を受けやすいです。粘土と礫は一般的に液状化に抵抗しますが、非塑性のシルトは強い揺れの下で液状化する可能性があります。
地下水位：浅い地下水位（0〜5 m）はリスクを劇的に増加させます。地下水位が15 mより深い場所は一般的に液状化から安全です。
揺れの強度と継続時間：液状化には通常、0.1gを超える加速度での持続的な揺れ（数秒間）が必要です。長時間の地震はリスクを増加させます。
過去の事例：過去の地震で液状化した地域は、液状化を引き起こす地質条件が持続するため、再び液状化する可能性が非常に高いです。

主な用途

地震が多い沿岸地域での不動産評価のための液状化の可能性の予備評価。
どの地盤条件と地震パラメータが液状化に寄与するかの理解。
盛土、埋立地、河川や海岸線付近のサイトのリスク評価。
地盤地震工学に関する教育的議論の支援。

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.

液状化を起こしやすい地盤はどのような種類ですか？

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.