Evaluate earthquake-induced liquefaction potential in real time. Enter SPT-N value, fines content, groundwater depth, and earthquake parameters to compute FL, PL index, and view the depth profile.
Earthquake & Soil Conditions
Magnitude M
Peak Ground Accel. α
g
Groundwater Depth Dw
m
SPT-N Value
Fines Content FC
%
Single Layer Results (5 m depth)
Calculating...
L = 0.65×(σv/σv')×α×rd
FL = RL/L
Liquefaction: FL < 1.0
What exactly is soil liquefaction? I've seen videos of ground turning to jelly during earthquakes, but what's happening underground?
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Basically, it's when strong shaking causes water-saturated, loose sandy soil to lose its strength and behave like a liquid. The water pressure between soil grains increases so much that the grains can't stay in contact. In this simulator, we use the FL method to predict if a specific soil layer will liquefy. Try moving the "SPT-N Value" slider down to a low number like 5 — you'll see the risk jump because loose soil is much more vulnerable.
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Wait, really? So the FL number is the key output here. What does it mean if my result is, say, 0.8?
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Good question. FL is a safety factor. If FL = 0.8, it's less than 1.0, meaning the earthquake's shaking demand exceeds the soil's resistance — so liquefaction is possible. In practice, engineers get worried when FL dips below 1.0. For instance, adjust the "Peak Ground Accel. α" to a high value like 0.4g. You'll watch FL drop because the seismic demand just went way up.
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Okay, I see the FL for one depth. But the FAQ mentions a "PL" value. How do we assess the risk for a whole site, not just one point?
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Exactly! A single layer might fail, but we need the big picture. PL is the "Liquefaction Potential Index." It integrates the FL deficiency over the top 20 meters of soil. Think of it as a weighted average of risk with depth. A common case is a building site with a shallow groundwater table. If you set "Groundwater Depth Dw" to just 1 meter in the simulator, you'll typically see the risk increase because more soil is saturated and susceptible.
Physical Model & Key Equations
The core of the FL method is comparing the seismic demand on the soil to the soil's inherent resistance. The demand is expressed as the Cyclic Stress Ratio (CSR), which represents the shear stress induced by the earthquake shaking, normalized for overburden pressure.
Where:
• $\alpha_{max}/g$ is the Peak Ground Acceleration (PGA) you input.
• $\sigma_v$ and $\sigma_v'$ are the total and effective vertical stresses (calculated from soil and water depth).
• $r_d$ is a stress reduction factor that decreases with depth.
The soil's capacity to resist liquefaction is the Cyclic Resistance Ratio (CRR). It's primarily determined from the Standard Penetration Test (SPT) blow count (N-value), corrected for overburden pressure and fines content.
Where $(N_1)_{60}$ is the SPT-N value, corrected for energy and overburden pressure. The "Fines Content FC" you input adjusts this value. The final Factor of Safety against liquefaction (FL) is the ratio:
$$ FL = \frac{CRR_{7.5}}{CSR} \cdot MSF $$
Here, MSF is a Magnitude Scaling Factor (based on your input Magnitude M) because longer-duration shaking from bigger quakes causes more damage.
Frequently Asked Questions
An FL value of less than 1.0 indicates a high possibility of liquefaction, but it does not guarantee occurrence. Actual liquefaction is influenced by ground heterogeneity and seismic motion characteristics. It is recommended to make a comprehensive judgment in conjunction with the PL value and adopt a conservative evaluation in design.
In principle, input the highest expected groundwater level (during normal conditions and during earthquakes) at the time of liquefaction evaluation. Since a shallower groundwater level increases liquefaction risk, it is common to use the historical highest water level or the planned groundwater level for a conservative evaluation.
If FC is unknown, estimate it from the soil classification, or conservatively calculate with FC=0%. A smaller FC results in a lower liquefaction resistance ratio RL, leading to a stricter judgment. In practice, referring to nearby soil data can also be effective.
At layer boundaries where N-values, FC, or groundwater levels change sharply, the FL value may become discontinuous. This is physically correct behavior. However, it may also be caused by input data errors or coarse sampling intervals, so please verify the original data.
Real-World Applications
Seismic Safety for Critical Infrastructure: Before building a new hospital, power plant, or bridge, engineers use this FL method to assess the foundation soil. They drill boreholes to get SPT-N values and fines content, then run this exact analysis for the design earthquake. If PL is high, they might need deep foundations or ground improvement.
Post-Earthquake Damage Investigation: After an earthquake like the 2011 Christchurch (New Zealand) event, geotechnical engineers use liquefaction assessment tools to understand why certain neighborhoods sank or tilted while others did not. Correlating FL/PL with observed damage helps calibrate models for future predictions.
Land Use Planning and Zoning: City planners use liquefaction potential maps, generated using these principles, to restrict high-density construction in high-risk zones (PL > 15). This guides development towards safer areas and can dictate where parks or low-rise buildings are placed instead of high-rises.
Retrofit Design for Existing Buildings: For older structures in seismically active areas, a liquefaction assessment might reveal newfound risk due to updated seismic data. Engineers then design mitigation measures, such as installing stone columns or drainage systems to strengthen the ground, based on the depth and severity of layers where FL < 1.0.
Common Misunderstandings and Points to Note
When you start using this tool, there are a few key points to keep in mind. First, while it's tempting to think "just input the SPT N-value and you're good," the groundwater level (Dw) setting significantly influences the results. For instance, for the same sand layer with N=10, the FL value can nearly double between a groundwater level at GL-0.5m (almost ground surface) and GL-3.0m. Since water level data is often missing from field boring logs, the practical rule of thumb is to check seasonal variations and nearby well data and set it on the shallower side for a conservative (safer) estimate.
Next, avoid the simplistic understanding that "a high fine content (FC) = safe." While liquefaction resistance does increase when FC exceeds 35%, this applies specifically to clean sand mixed with silt or clay. On some sites, the soil may contain significant organic material or very soft silt. Such ground is often outside the applicable scope of the FL method (you can't even measure a reliable N-value!). So, don't blindly trust the tool's output; always cross-check it with the soil classification results (the soil profile).
Finally, regarding the interpretation of the PL value (Liquefaction Potential Index) the tool provides. Don't think of it as a binary threshold where PL=15 means "it will liquefy" and PL=14 means "it won't." This is a continuous risk indicator. Consider PL values exceeding 10 as a zone where some countermeasures should be considered. For example, building a lightweight warehouse in an area with PL=12 might be acceptable, but constructing a heavy electrical substation in the same area would likely require ground improvement.
Enter SPT-N value (blow count from Standard Penetration Test, typically 0–50 blows/300mm) in the sMNum field
Input fines content percentage (0–100%) representing silt and clay passing #200 sieve in sM
Specify groundwater depth in meters (sDwNum) — liquefaction risk increases as water table rises
Enter peak ground acceleration (PGA) in g-units (saNum) from seismic hazard maps; typical values range 0.2–0.5g for moderate earthquakes
Click Calculate to generate FL factor and liquefaction potential index (LPI) classification
Worked Example
Sandy silt deposit at 6m depth in Tokyo Bay region. SPT-N = 12 blows/300mm, fines content = 18%, groundwater table at 2.5m, seismic design PGA = 0.35g. The FL Method computes: correction factor for overburden (CN ≈ 1.15), cyclic stress ratio (CSR = 0.35 × 9.81 × 2.5/6 ≈ 0.144), cyclic resistance ratio from N-value correlation (CRR ≈ 0.18), FL = CRR/CSR ≈ 1.25 (safe), LPI = 0. With the same soil but PGA increased to 0.45g and N reduced to 8, FL drops to 0.82 (triggering liquefaction; LPI = 18–22, moderate hazard requiring mitigation).
Practical Notes
SPT-N values below 15 in clean saturated sand within 20m of surface demand rigorous liquefaction screening, especially near dams and nuclear facilities
Fines content exceeding 35% often inhibits liquefaction; conversely, 5–15% fines in sand creates maximum susceptibility
Groundwater depth corrections are critical: depths greater than 10m substantially reduce CSR and lower LPI risk classification
PGA inputs should match seismic codes (Eurocode 8, ASCE 7, or local standards) to ensure regulatory compliance in design reports
FL Method assumes cohesionless or very weakly cohesive soils; clay-rich layers (>50% fines) typically bypass liquefaction assessment