액상화 위험 계산기

지진 매개변수 및 지반 조건을 바탕으로 토양 액상화 확률을 추정합니다.

Assessment

지진 시 토양 액상화 이해

토양 액상화는 포화된 느슨한 입상 토양이 지진 진동 중에 강도와 강성을 잃어 고체가 아닌 액체처럼 작용하는 현상입니다. 지진파가 물로 포화된 모래나 미사를 통과하면 토양 입자 사이의 수압(간극 수압)이 증가합니다. 이 압력이 상부 토양의 무게와 같아질 만큼 충분히 높아지면, 입자들은 서로 접촉을 잃고 토양은 일시적으로 점성 유체처럼 흐릅니다. 이 현상은 건물이 가라앉거나 기울거나 붕괴하고, 배관과 탱크 같은 지하 구조물이 지표면으로 부상하며, 경사면이 측방 유동으로 파괴될 수 있습니다.

표준 관입 시험(SPT) N값은 액상화 감수성을 평가하는 데 사용되는 핵심 현장 측정값입니다. 이는 시료 채취관을 지반에 30 cm 관입하는 데 필요한 해머 타격 횟수를 측정합니다. N값이 높을수록 더 조밀하고 저항력이 높은 토양을 나타냅니다. 포화 조건에서 SPT N값이 15 미만인 토양은 일반적으로 액상화에 취약한 것으로 간주됩니다. 지하수위 깊이도 동등하게 중요합니다: 액상화가 발생하려면 토양이 포화되어야 하므로, 얕은 지하수위(5미터 미만)를 가진 지역이 가장 위험합니다. 매립지와 해안 성토지는 특히 취약하며, 이는 1964년 니이가타 지진과 2011년 크라이스트처치 지진에서 입증되었습니다.

액상화 위험의 핵심 요소

토양 유형: 느슨한 모래와 실트질 모래가 가장 취약합니다. 점토와 자갈은 일반적으로 액상화에 저항하지만, 비소성 실트는 강한 진동 하에서 액상화될 수 있습니다.
지하수위 깊이: 얕은 지하수위(0~5 m)는 위험을 크게 증가시킵니다. 지하수위가 15 m보다 깊은 부지는 일반적으로 액상화로부터 안전합니다.
진동 강도 및 지속 시간: 액상화는 일반적으로 0.1g 이상의 가속도에서 수 초간의 지속적인 진동이 필요합니다. 지속 시간이 긴 지진일수록 위험이 증가합니다.
역사적 선례: 과거 지진에서 액상화가 발생한 지역은 액상화를 유발하는 지질학적 조건이 지속되므로 다시 액상화될 가능성이 매우 높습니다.

일반적인 용도

지진 빈발 해안 지역의 부동산 평가를 위한 액상화 가능성 예비 평가.
어떤 토양 조건과 지진 매개변수가 액상화에 기여하는지 이해.
매립지, 간척지 또는 하천과 해안선 인근 부지의 위험 평가.
지반 지진 공학에 대한 교육적 논의 지원.

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.

어떤 토양 유형이 액상화에 가장 취약한가요?

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.