A practitioner-oriented study guide covering all three field test methods for evaluating earthquake-induced liquefaction potential — with interactive calculators, fully worked examples, and step-by-step formula breakdowns based on the Indian seismic design standard.
Liquefaction is a process where loose, saturated, cohesionless soils lose their shear strength and stiffness during earthquake shaking. The cyclic loading from seismic waves causes excess pore water pressure to build up. When this excess pressure equals the effective confining stress, the soil grains lose contact with each other — and the soil behaves like a liquid.
This phenomenon was dramatically observed in the 1964 Alaska and Niigata earthquakes, where buildings literally sank into the ground, and bridges collapsed due to lateral spreading of liquefied soil.
Liquefaction occurs when: Δu = σ'v₀ — excess pore pressure equals initial effective vertical stress.
IS 1893:2016 Annex F adopts the internationally accepted simplified stress-based procedure of Seed & Idriss (1971) as updated by Youd et al. (2001). The core concept is simple: compare the seismic demand on the soil (CSR) with the soil's capacity to resist liquefaction (CRR).
What the earthquake DEMANDS from the soil.
Represents the shear stress induced by the earthquake as a fraction of effective overburden pressure. Higher PGA or shallower depth → higher CSR.
What the soil CAN RESIST before liquefying.
Derived from in-situ test results (SPT, CPT or Vs). Denser soils → higher CRR → more resistant to liquefaction.
| σᵥ | Total vertical stress at depth z (kPa) |
| σ'ᵥ | Effective vertical stress at depth z (kPa) |
| aₘₐₓ | Peak ground surface acceleration (g) |
| rᵈ | Stress reduction coefficient (dimensionless) |
| 0.65 | Factor to convert peak to equivalent uniform cycles |
rᵈ accounts for flexibility of the soil column. Decreases with depth.
CRR charts are calibrated for Mw = 7.5. MSF adjusts for other magnitudes.
| Mw | MSF | Meaning |
|---|---|---|
| 5.5 | 2.20 | Smaller quakes less damaging |
| 6.0 | 1.76 | CRR effectively higher |
| 6.5 | 1.44 | – |
| 7.5 | 1.00 | Reference magnitude |
| 8.5 | 0.64 | Large quakes more damaging |
Standard Penetration Test blow counts N are corrected to N₁(60) — the blow count normalized to 60% hammer energy efficiency and 1 atm overburden. Most widely used in India.
Cone Penetration Test tip resistance qc and sleeve friction fs are normalized. The soil behavior index Ic identifies soil type and fines content without sampling.
Vs measured by MASW, SASW, or seismic CPT. Useful for soils with gravels where SPT/CPT are unreliable. Non-invasive option.
The SPT method follows the Seed-Idriss simplified procedure as modified by Youd et al. (2001) and codified in IS 1893 Part 1: 2016, Annex F. The SPT blow count is measured in a borehole and corrected for equipment and overburden effects before comparing with the CRR boundary curve.
At each depth z, compute σᵥ (total) and σ'ᵥ (effective). Above the water table: σ'ᵥ = σᵥ. Below water table: σ'ᵥ = σᵥ − u, where u = γw × (z − zwt).
The measured SPT blow count N must be corrected for: energy ratio, borehole diameter, rod length, sampler liner, and overburden stress.
Where: Cₙ = overburden correction, Cₑ = energy, C_B = borehole dia, C_R = rod length, Cₛ = sampler liner
| Factor | Equipment Variable | Value |
|---|---|---|
| Cₑ — Energy | Donut Hammer (trip/rope) | 0.75 |
| Safety Hammer (rope) | 1.00 | |
| Auto-trip Donut | 1.33 | |
| Auto-trip Safety | 1.33 | |
| C_B — Borehole dia | 65–115 mm | 1.00 |
| 150 mm | 1.05 | |
| 200 mm | 1.15 | |
| C_R — Rod length | < 3 m | 0.75 |
| 3 – 4 m | 0.80 | |
| 4 – 6 m | 0.85 | |
| 6 – 10 m | 0.95 | |
| > 10 m | 1.00 | |
| Cₛ — Liner | With liner (standard) | 1.00 |
| Without liner | 1.1 – 1.3 |
The CRR boundary is defined for clean sand. If fines content FC > 5%, apply correction factors α and β to get the clean-sand equivalent N₁(60)cs:
| Fines Content | α | β |
|---|---|---|
| FC < 5% | 0 | 1.00 |
| 5% ≤ FC ≤ 35% | exp(1.76 − 190/FC²) | 0.99 + FC1.5/1000 |
| FC > 35% | 5.0 | 1.20 |
Using the analytical expression from the clean-sand CRR boundary curve (Youd et al., 2001):
Magnitude Scaling Factor
Overburden stress correction
| Layer # | Depth to Mid (m) | N (blows) | Hammer Type | Borehole Dia (mm) | Rod Length (m) | Liner? | Fines FC (%) | Action |
|---|
| Layer | Depth (m) | N | Cₙ | N₁(60) | N₁(60)cs | σᵥ (kPa) | σ'ᵥ (kPa) | rᵈ | CSR | CRR₇.₅ | MSF | Kσ | CRR_M | FS | Status |
|---|
Calculations are per IS 1893 (Part 1):2016 Annex F, using the Seed-Idriss simplified procedure (Youd et al., 2001). CRR₇.₅ uses the analytical expression for the clean-sand boundary curve. MSF = 10²·²⁴/Mw²·⁵⁶. Kσ computed per Robertson & Wride (1998). A factor of safety FS = 1.0 is used as the liquefaction criterion.
The CPT method uses cone tip resistance qc and sleeve friction fs to evaluate liquefaction potential. The Robertson & Wride (1998) procedure, referenced by IS 1893:2016 Annex F, uses the Soil Behaviour Type Index Ic to identify susceptible soils and correct the tip resistance for fines content.
| Ic Range | Soil Behaviour Type | Susceptible? |
|---|---|---|
| Ic > 3.6 | Sensitive fine-grained soils | Generally No |
| 2.95 – 3.6 | Clay to silty clay | No |
| 2.60 – 2.95 | Silty clay to clayey silt | Check PI & LL |
| 2.05 – 2.60 | Clayey silt to silty sand | Possible |
| 1.31 – 2.05 | Sandy materials | Yes |
| Ic ≤ 1.31 | Clean sand to gravel | Yes (if saturated) |
Soils with Ic ≤ 2.6 are treated as susceptible to liquefaction. Soils with Ic > 2.6 are considered non-liquefiable (clay-like behaviour).
| Layer # | Depth to Mid (m) | qc (MPa) | fs (kPa) | FC (%) | Action |
|---|
| Layer | Depth (m) | qc1N | Q | F (%) | Ic | Kc | qc1Ncs | CSR | CRR₇.₅ | MSF | CRR_M | FS | Status |
|---|
The Vs method (Andrus & Stokoe, 2000), referenced in IS 1893:2016, uses in-situ shear wave velocity to evaluate liquefaction potential. It is particularly useful for gravelly soils and sites where SPT/CPT are difficult, and for post-earthquake reconnaissance where only geophysical data is available.
Unlike SPT and CPT, shear wave velocity measurements are less sensitive to loose sand layers (a small volume of loose sand has little effect on the average Vs). The method is best used as a screening tool or in combination with SPT/CPT. Also, Vs does not directly account for soil fabric, cementation, or aging effects.
| Layer # | Depth to Mid (m) | Vs (m/s) | Fines FC (%) | Action |
|---|
| Layer | Depth (m) | Vs (m/s) | σ'ᵥ (kPa) | Vs₁ (m/s) | Vs₁* | CSR | CRR₇.₅ | MSF | CRR_M | FS | Status |
|---|
| Aspect | SPT Method | CPT Method | Vs Method |
|---|---|---|---|
| Maturity of database | ●●● Extensive worldwide | ●●● Extensive, growing | ●●○ Limited case history |
| Applicability in gravels | ●○○ Poor (cobbles jam) | ●○○ Poor (refusal) | ●●● Excellent |
| Continuous profile | ●○○ Discrete intervals | ●●● Continuous | ●●○ Layer average |
| Detection of thin layers | ●●○ May miss thin layers | ●●● Detects 10–15 cm | ●○○ Poor (averages out) |
| Soil sample available | ●●● Yes, with SPT | ●○○ No (unless CPTu+sampler) | ●○○ No |
| Fines content from test | ●●● Direct measurement | ●●○ Indirect via Ic | ●○○ Must be known |
| Cost (relative) | $$ | $$–$$$ | $ |
| Repeatability | ●●○ Operator-dependent | ●●● Highly reproducible | ●●○ Good for geophysics |
| Use in IS 1893:2016 | Primary method | Recognised | Recognised |
| Zone | Z (Zone Factor) | Design PGA = Z/2 (g) | Example Regions |
|---|---|---|---|
| II | 0.10 | 0.05g | Parts of South India, Stable continental regions |
| III | 0.16 | 0.08g | Deccan Plateau, Andaman & Nicobar, parts of Gujarat |
| IV | 0.24 | 0.12g | Delhi, Jammu, parts of UP, Bihar, West Bengal |
| V | 0.36 | 0.18g | Kashmir, Himachal, Northeast India, Kutch (Gujarat) |