A comprehensive, student-friendly guide to the IS 1893 Annex F simplified procedure for evaluating the liquefaction potential of soils during earthquakes โ covering SPT, CPT, and shear wave velocity methods.
Before diving into the procedure, understand the phenomenon you are evaluating.
Liquefaction occurs when saturated, loosely packed sandy or silty soil temporarily loses its shear strength and behaves like a liquid during earthquake shaking. Pore water pressure rises, effective stress drops to zero.
Susceptible conditions: loose saturated sands/silts, shallow water table (< 10 m), earthquake magnitude Mw โฅ 5.0, and peak ground acceleration sufficient to induce cyclic shear stress.
Liquefied ground can cause foundation failure, building settlement, lateral spreading, loss of bearing capacity, and pipeline damage. Bhuj (2001) earthquake in Gujarat caused widespread liquefaction damage.
Clause 6.3.5.3 of IS 1893 (Part 1) : 2016 mandates liquefaction assessment for sites in Zones III, IV, and V with saturated sandy soils. Annex F provides the simplified procedure.
Undisturbed sampling from liquefiable sites is nearly impossible โ the loose sandy fabric is destroyed during sampling. IS 1893 therefore mandates in-situ tests: SPT, CPT, or shear wave velocity measurements.
Clean sands and silty sands (SP, SM in IS classification), Fines Content (FC) typically < 35%, Plasticity Index (PI) < 7 for silts, Relative Density Dr < 60โ70%, depth โค 20 m below ground.
Each step from Annex F is explained with the original IS 1893 formula, plain-language explanation, and worked examples.
The IS 1893 Annex F procedure begins with assembling a complete picture of the subsurface. Without accurate field data, no analytical result is reliable.
Required Information (Clause F-1, Step 1):
At every depth z where a liquefiable layer exists, you must compute two stresses. These are the foundation of the CSR and CRR calculations.
Example: Soil profile: dry unit weight ฮณd = 18 kN/mยณ, saturated ฮณsat = 20 kN/mยณ, water table at 2 m depth.
At z = 5 m: ฯvโ = 18ร2 + 20ร3 = 96 kPa, u = 9.81ร3 = 29.4 kPa, ฯ'vโ = 96 โ 29.4 = 66.6 kPa
A rigid soil column would transmit shear stress uniformly with depth. In reality, soils are deformable โ shear stress reduces with depth. The factor rd (โค 1.0) accounts for this.
where z = depth below ground surface in metres
| Depth z (m) | rd value | Reduction from 1.0 |
|---|---|---|
| 0 m | 1.000 | 0% |
| 3 m | 0.977 | 2.3% |
| 6 m | 0.954 | 4.6% |
| 9.15 m | 0.930 | 7.0% |
| 12 m | 0.853 | 14.7% |
| 15 m | 0.773 | 22.7% |
| 20 m | 0.640 | 36.0% |
CSR represents the seismic demand โ how hard the earthquake is shaking the soil. It is compared against the soil's resistance (CRR) to find the Factor of Safety.
| Seismic Zone | Zone Factor Z | Use as amax/g |
|---|---|---|
| Zone II | 0.10 | Low seismicity |
| Zone III | 0.16 | Moderate |
| Zone IV | 0.24 | High |
| Zone V | 0.36 | Very High |
Worked Example: Zone IV site (Z = 0.24), z = 6 m, ฯvโ/ฯ'vโ = 1.44, rd = 0.954
CSR = 0.65 ร 0.24 ร 1.44 ร 0.954 = 0.216
The CRRโ.โ obtained from Step 6 is valid only for Mw = 7.5 earthquakes at shallow depth. Apply three corrections to get the actual CRR:
MSF Values by Magnitude:
| Mw | MSF | Effect on CRR |
|---|---|---|
| 5.0 | 2.20 | Doubles resistance |
| 6.0 | 1.52 | +52% resistance |
| 7.0 | 1.09 | +9% resistance |
| 7.5 | 1.00 | Reference (no change) |
| 8.0 | 0.73 | โ27% resistance |
Kฯ Exponent f by Relative Density:
| Relative Density Dr | f value |
|---|---|
| 40 โ 60% | 0.8 |
| 60 โ 80% | 0.7 |
| > 80% | 0.6 |
Sub-Step A: Standardize to Nโโ (60% efficiency)
Sub-Step B: Normalize to (Nโ)โโ at 100 kPa overburden
Sub-Step C: Correct for Fines Content to get (Nโ)โโcs
Sub-Step D: Calculate CRRโ.โ from (Nโ)โโcs
Example: (Nโ)โโcs = 15 โ CRRโ.โ = 15/(34โ15) + 15/135 + 50/[10ร15+45]ยฒ โ 1/200
= 0.789 + 0.111 + 50/[195]ยฒ โ 0.005 = 0.789 + 0.111 + 0.00131 โ 0.005 = 0.897 โ but check with chart
(Note: Use the equation or Fig. 8 from IS 1893 โ the Clean Sand Base Curve)
The CPT method normalizes cone tip resistance qc and corrects for soil behavior type using the Soil Behavior Type Index Ic.
Step 1: Normalize qc to get qc1n
Step 2: Soil Behavior Type Index Ic
Step 3: Apply grain characteristic correction kc
Step 4: Find CRRโ.โ from (qc1n)cs
The shear wave velocity Vs measured in the field is first corrected for overburden stress, then used directly to estimate CRRโ.โ .
Step 1: Overburden-corrected shear wave velocity Vs1
Step 2: Calculate CRRโ.โ using Vs1
| FS Value | Interpretation | Action |
|---|---|---|
| FS < 1.0 | Liquefaction occurs | Ground improvement mandatory |
| 1.0 โค FS < 1.2 | Marginal โ some deformation | Detailed analysis recommended |
| FS โฅ 1.2 | Safe โ minimal permanent deformation | Generally acceptable |
IS 1893 Clause F-1, Step 8: If FS < 1.0, the soil is assumed to liquefy.
If liquefaction is predicted:
Post-liquefaction effects to check:
Enter your site and soil data to compute CSR, CRR, and Factor of Safety using the IS 1893 Annex F simplified procedure.
Fill in the inputs and click Calculate
to see results here
Fill in CPT inputs and click Calculate
Fill in Vs inputs and click Calculate
Reproduced from IS 1893 (Part 1) : 2016, Annex F (Clause F-1, Step 6a) for quick reference.
Clause F-1, Step 6(a)
| No. | Element | Standard Specification |
|---|---|---|
| i | Sampler | Standard split-spoon sampler: OD = 51 mm, ID = 35 mm (no room for liners) |
| ii | Drill Rods | A or AW type for depths < 15.2 m; N or NW type for greater depths |
| iii | Hammer | Standard (safety) hammer: weight = 63.5 kg, drop height = 762 mm (76.2 cm) |
| iv | Rope | Two wraps of rope around the pulley |
| v | Borehole | 100โ130 mm diameter rotary borehole with bentonite mud for stability (hollow stem augers where SPT taken through stem) |
| vi | Drill Bit | Upward deflection of drilling mud (tricone or baffled drag bit) |
| vii | Blow Count Rate | 30 to 40 blows per minute |
| viii | Penetration Resistance Count | Measured over range of 150 mm to 450 mm of penetration into the ground |
Clause F-1, Step 6(a) โ For computing Nโโ from measured N
| No. | Correction For | Correction Factor Values |
|---|---|---|
| i | Non-standard hammer (rope-and-pulley) | CE = 0.75 (Donut hammer with rope & pulley) CE = 1.35 (Donut hammer with trip auto) |
| ii | Non-standard hammer weight / drop height | CE = โ(W ร H / (63.5 ร 762)) where H = height of fall (mm), W = weight (kg) |
| iii | Non-standard sampler (with liners, but not used) | CS = 1.1 (loose sand), CS = 1.2 (dense sand) |
| iv | Non-standard sampler (with liners, liners used) | CS = 0.9 (loose sand), CS = 0.7 (dense sand) |
| v | Rod length correction CR | 0โ3 m: 0.75 | 3โ4 m: 0.85 | 4โ6 m: 0.95 | 6โ10 m: 1.00 | >10 m: 1.00 |
| vi | Borehole diameter CB | 65โ115 mm: 1.00 | 150 mm: 1.05 | 200 mm: 1.15 |
IS 1893 (Part 1) : 2016 was amended in September 2017. The key change affecting liquefaction calculation:
This significantly increases (Nโ)โโcs for silty soils with high fines content. Always use the amended value ฮฑ = 5 in all calculations.
Critical points from IS 1893 Annex F that every engineer must remember.
Only saturated soils can liquefy. Depth > water table is not analyzed. Effective stress ฯ'vโ must be positive for liquefaction to occur.
CSR comes from the earthquake (seismic input). CRR comes from the soil (field test data). FS = CRR/CSR โ same logic as a structural load/resistance ratio.
If the fines-corrected, overburden-normalized blow count exceeds 30, the soil is considered too dense to liquefy. No further calculation needed.
MSF > 1.0 for Mw < 7.5 (increases CRR โ more resistant). MSF < 1.0 for Mw > 7.5 (decreases CRR โ more vulnerable). Always check the magnitude.
Always use the September 2017 Amendment No. 1 value. The original IS 1893:2016 print contained a typographic error (ฮฑ = 0.5 instead of ฮฑ = 5).
IS 1893 mandates liquefaction assessment particularly for high-seismicity zones. Zone II sites with special conditions may also require assessment.
For depths โค 15 m with ฯ'vโ โค 100 kPa, Kฯ โ 1.0 and can be ignored. It only reduces CRR meaningfully for deep, dense layers under high overburden.
The analysis must be performed for each potentially liquefiable layer independently. Report the minimum FS across all layers โ that governs the design.
Capture all calculated KPIs and generate a print-ready project submission report based on your calculator inputs.
IS 1893 (Part 1) : 2016, Annex F โ Simplified Procedure
โก The report will pull values from the SPT Calculator. Run the SPT calculation first, then generate the report.