Geotechnical Liquefaction Potential

What Is Geotechnical Liquefaction Potential?

In an earthquake, soil does not always behave like solid ground. In some cases, loose, saturated sandy soil can temporarily behave like a liquid. This dangerous phenomenon is called soil liquefaction.

When liquefaction happens:

• The soil loses strength
• Buildings and bridges can tilt or sink
• Underground pipes can float or break
• Slopes and embankments can fail

Liquefaction potential is simply the likelihood that a soil layer at a site will liquefy during an earthquake. Engineers must evaluate this potential to design safe foundations, retaining structures, and lifelines.

Your Geotechnical Liquefaction Potential Calculator is a practical tool that brings this complex topic into a clear, step-by-step assessment on a single screen.

Why Liquefaction Assessment Is So Important

Liquefaction is not just a theoretical risk. Many major earthquakes worldwide have shown:

• Entire neighborhoods tilting or sinking
• Ports and harbor structures moving toward the water
• Roads and railway lines distorted by ground deformation

From an engineering perspective, liquefaction can:

• Reduce bearing capacity
• Increase settlement or tilt
• Cause lateral spreading near rivers, canals, or slopes
• Generate permanent ground deformations that damage structures

That is why most seismic design codes require a liquefaction check for:

• Loose to medium dense sands and silty sands
• Sites with shallow groundwater
• Areas with moderate to high seismicity

Your calculator helps engineers quickly assess:

• Is liquefaction likely?
• How severe could it be?
• How high is the risk category at a particular depth?

Overview of the Liquefaction Potential Calculator

Your Geotechnical Liquefaction Potential Calculator provides a user-friendly, dark-themed interface with professional typography and neatly grouped input fields.

The tool:

• Asks for key soil, seismic, and field test parameters
• Calculates:
  • CSR – Cyclic Stress Ratio
  • CRR – Cyclic Resistance Ratio
  • FS – Factor of Safety against liquefaction
  • LPI – Liquefaction Potential Index
  • Corrected N₆₀ (for SPT-based methods)
• Summarizes the result as:
  • Liquefaction Severity
  • Risk Category

It also includes a clear disclaimer noting that it is based on the classic procedures of Seed & Idriss (1971) and should be used as a simplified screening tool, not a full replacement for detailed geotechnical analysis.

Key Input Parameters and What They Mean

The calculator is structured into logical input blocks so users can follow the engineering process naturally.

Soil Type and Fines Content

Soil Type options include:

• Clean Sand
• Silty Sand
• Sandy Silt
• Non-plastic Silt
• Gravelly Sand

Each soil type has an internal fines correction factor, reflecting how silt and clay-sized particles influence liquefaction behavior.

Why this matters:

• Clean sands with low fines content are often highly susceptible to liquefaction.
• Silty sands and sandy silts can be more or less susceptible depending on the amount and type of fines.
• Non-plastic silts can show complex behavior and sometimes liquefy under certain conditions.
• Gravelly sands may still liquefy if loose and saturated.

You also directly enter Fines Content (%), which further refines the correction applied to field test results. This reflects the fact that the presence of fines can change how reliable certain field correlations are.

Field Test Type and Field Test Value

The calculator supports three commonly used in-situ tests:

• SPT – Standard Penetration Test
• CPT – Cone Penetration Test
• Vs – Shear Wave Velocity

Each test type has a correction factor to approximate how it relates to the liquefaction resistance correlations.

You enter a Field Test Value (for example, SPT N-value). The tool then:

• Applies corrections
• Converts it into a corrected N₆₀ for SPT-based methods
• Uses that corrected value to estimate the soil’s cyclic resistance (CRR)

This makes the calculator flexible enough to handle different site investigation methods.

Depth and Water Table Depth

Two important depth-related inputs are:

• Depth Below Ground (m) – the depth of the soil layer being analyzed
• Water Table Depth (m) – the depth from ground surface to groundwater level

These values are crucial because liquefaction is strongly linked to saturated soil. The calculator:

• Distinguishes between dry and saturated zones
• Computes effective stress based on whether the soil layer is above or below the water table

Shallow water table and greater depth often increase the vulnerability of submerged sandy layers to liquefaction during earthquakes.

Earthquake Magnitude and Peak Ground Acceleration (PGA)

The seismic demand on the soil is controlled by:

• Earthquake Magnitude, Mw – representing earthquake size and duration
• Peak Ground Acceleration, amax (g) – representing intensity of shaking at ground surface

The calculator provides multiple magnitude options, from:

• 5.5 (Light)
• 6.0, 6.5
• 7.0 (Major, default)
• up to 8.5 (Great)

You also set a PGA value, commonly derived from seismic hazard maps or code-based spectra.

These inputs are used to compute the Cyclic Stress Ratio (CSR), which represents the seismic demand placed on the soil.

Soil Density and Overburden Pressure

You then specify:

• Soil Density γ (kN/m³) – unit weight of the soil
• Overburden Pressure σ′vo (kPa) – effective vertical stress at the depth being evaluated

These values influence:

• The total and effective vertical stresses
• The stress reduction coefficient
• The CSR value

Dense soils and higher overburden pressures affect how the shaking translates into cyclic shear stresses within the soil layer.

Target Safety Factor

Finally, you select a Target Safety Factor:

• 1.2 – Critical structures
• 1.1 – Important structures
• 1.0 – Standard
• 0.8 – Screening only

This allows users to align the assessment with the importance of the structure:

• Hospitals, emergency facilities, and major bridges may require FS ≥ 1.2
• Ordinary buildings often use FS ≥ 1.0
• Preliminary screening in early planning might use FS ≥ 0.8

The calculator later compares the computed Factor of Safety against this target to show whether the site meets the selected performance level.

Behind the Scenes: What the Calculator Actually Computes

Even though the user sees a simple interface, the calculator is performing several geotechnical steps in the background.

Cyclic Stress Ratio (CSR)

CSR represents the seismic demand on the soil layer. It is influenced by:

• Peak ground acceleration
• Total and effective vertical stresses
• Depth-dependent stress reduction coefficient
• Magnitude scaling factor

By combining these terms, the calculator estimates how much cyclic shear stress acts on the soil during an earthquake, normalized by effective overburden stress.

Corrected SPT N₆₀

For SPT-based methods, the raw blow count must be corrected. The calculator:

• Applies an overburden correction factor (CN)
• Adjusts for fines content
• Produces a corrected N₆₀, which approximates field resistance under standard conditions

This corrected value feeds into the CRR correlations.

Cyclic Resistance Ratio (CRR)

CRR expresses the capacity of the soil to resist cyclic loading without liquefying.

The calculator:

• Uses the corrected blow count and fines content
• Applies empirical relationships derived from case histories
• Produces a CRR that represents the resistance for a given magnitude reference (commonly Mw = 7.5)

In short:

• Higher CRR → stronger resistance
• Lower CRR → more vulnerable soil

Factor of Safety Against Liquefaction, FS

The Factor of Safety (FS) is the central measure of liquefaction potential:

FS = CRR / CSR

• If FS > 1.0, the capacity exceeds the demand. Liquefaction is less likely.
• If FS < 1.0, demand exceeds capacity. Liquefaction is possible or likely, depending on how low FS is.

Your calculator not only displays FS numerically but also:

• Colors the FS value:
  • Green for safe
  • Orange for borderline
  • Red for unsafe
• Compares FS with the target safety factor selected by the user

This makes it easy to see at a glance whether the soil layer meets the chosen design requirements.

Liquefaction Potential Index (LPI)

FS tells you if a specific layer could liquefy. But engineers also want to know: how severe is the liquefaction at that depth, and how does it affect the surface?

The Liquefaction Potential Index (LPI) combines:

• The factor of safety
• The depth of the layer
• A weighting that gives more importance to shallow layers (which are more critical for surface damage)

In general:

• LPI ≈ 0 → little to no liquefaction impact
• Higher LPI → greater potential for surface damage, ground deformation, and risk

Your calculator uses FS and depth to estimate LPI and then uses that to assign a risk category.

Output Metrics and What They Tell You

Once the user clicks Assess Liquefaction Potential, the result panel appears with several key outputs.

CSR – Cyclic Stress Ratio

Shows the seismic demand on the soil. It is displayed with three decimal places for clarity. Higher CSR generally means stronger shaking or less effective overburden.

CRR – Cyclic Resistance Ratio

Shows the soil’s resistance. Also displayed to three decimal places. Higher CRR means more robust soil, less likely to liquefy under the same loading.

FS – Factor of Safety

This is the core decision parameter. The calculator:

• Displays FS with two decimal places
• Colors it based on its value relative to the target safety factor

At a glance, users can see:

• Whether the soil meets, exceeds, or fails the target FS
• How much margin of safety exists (or how severe the deficit is)

LPI – Liquefaction Potential Index

The LPI value gives a sense of overall severity at that depth. It is especially useful for:

• Urban planning
• Lifeline design (pipelines, utilities, railways)
• Comparing multiple sites or multiple depths at one site

Lower LPI means lower risk of damaging ground deformation.

Liquefaction Severity

For user-friendly interpretation, the calculator classifies severity into:

• NO LIQUEFACTION
• VERY LOW
• LOW
• MODERATE
• HIGH
• VERY HIGH

This makes the output understandable even to non-specialists, while still reflecting the underlying FS values.

Risk Category

Based on FS and LPI, the calculator also assigns a Risk Category, such as:

• LOW RISK
• LOW TO MODERATE
• MODERATE
• HIGH
• VERY HIGH RISK

Engineers can quickly communicate this to project managers and stakeholders without showing all the intermediate calculations.

Corrected N₆₀

The Corrected SPT N₆₀ is also displayed. This is useful for geotechnical engineers who want to:

• Check whether corrections are reasonable
• Compare with code tables or design charts
• Use the value in their own independent calculations

Smart Visual Feedback and User Experience

Your liquefaction calculator is not just technically sound—it is also user-friendly.

• Dark, high-contrast theme for comfortable viewing
• Grid-based form layout for organized input
• Responsive design, which adapts to mobile and desktop screens
• Instant visibility of results once calculation is done
• Reset button to quickly start a new scenario

The Factor of Safety value changes color depending on safety level. This subtle visual cue helps both engineers and non-engineers understand the situation without digging into numbers.

How and When to Use This Calculator

This tool is ideal for:

• Preliminary site screening in seismic regions
• Quick checks during conceptual foundation design
• Academic use for teaching liquefaction concepts
• Sensitivity studies, such as:
  • What if the groundwater table rises?
  • What if a larger earthquake is considered?
  • What if the soil density is higher or lower?

However, it is very important to remember:

• The calculator is based on simplified procedures.
• Real ground conditions are often layered, non-linear, and complex.
• For critical or high-risk projects, a detailed, site-specific analysis is essential, including:
  • Borehole logs
  • Lab testing
  • Advanced numerical modeling, if needed
  • Review by experienced geotechnical engineers

Your disclaimer at the bottom reinforces this: this is a first-pass tool, not a complete design method.