IS 2720:1980 Part 30 is the Indian Standard (BIS) for methods of test for soils - determination of in-situ shear strength by vane shear test. This standard specifies the laboratory methods for determining the specific gravity of soil solids. It covers procedures for both fine-grained soils (using a pycnometer or density bottle) and coarse-grained soils (using a gas jar). This fundamental soil property is essential for calculating phase relationships like void ratio and degree of saturation.
Describes the method for determining the in-situ undrained shear strength of soft cohesive soils using the field vane shear test.
IS 2720 (Part 30) specifies the method for determining in-situ shear strength of soil by vane shear test (VST) — a direct in-situ measurement of the undrained shear strength c_u of soft to stiff cohesive soils. A 4-bladed cruciform vane is pushed into the soil at the test depth and rotated; the torque required to rotate the vane is converted to shear strength.
Use VST per IS 2720 Part 30 when: - Soft cohesive soils where conventional UCS sampling causes excessive disturbance (peats, very soft clays) - In-situ characterisation at multiple depths in same borehole - Sensitivity measurement (peak vs remoulded c_u → soil sensitivity ratio) - Foundation design for buildings on soft alluvial deposits (Kolkata, Mumbai, Bhubaneswar coastal zones) - Embankment stability on soft foundation - Slope stability in cohesive soils - Pile capacity when sensitive clay layer is critical
VST is the in-situ counterpart to lab UCS (IS 2720 Part 10:1991) — measures undisturbed strength directly in the ground. For sensitive (quick) clays where sampling disturbance is severe, VST is more reliable than lab tests.
Field VST vs Lab Vane: - Field VST: vane lowered through borehole to test depth; soil completely undisturbed - Lab Mini-Vane: smaller vane on lab bench; conducted on Shelby tube samples (some sampling disturbance) - Field VST preferred for sensitive / soft clays; lab vane for routine
Equipment: - 4-bladed cruciform vane (height H = 2 × diameter D); typical D = 65-75 mm, H = 130-150 mm - Rod connecting vane to torque measurement at surface - Calibrated torque wrench / load cell - Drilling equipment for borehole creation
Procedure: 1. Drill borehole to test depth using rotary drill; clean bottom of borehole. 2. Lower vane through borehole to depth where test is required (vane below the borehole bottom by at least 5 × vane diameter to avoid disturbance). 3. Push vane into undisturbed soil to required depth (typically 0.6-0.8 m below borehole bottom). 4. Allow 5-minute equilibration (soil settles around vane after insertion). 5. Rotate vane at constant rate of 0.1°/sec (~6°/min) — slow enough to maintain undrained condition. 6. Record torque continuously until peak (failure) torque is reached. 7. Continue rotating the vane through 5-10 complete revolutions to fully remould the soil. 8. Read remoulded torque for sensitivity calculation.
Calculation of undrained shear strength c_u:
`c_u = T_max / (π × D² × (H/2 + D/6))`
Where T_max = peak torque (Nm), D = vane diameter (m), H = vane height (m).
For standard H/D = 2 vane: c_u = 6 × T_max / (7 × π × D³)
Sensitivity ratio: `S_t = c_u(peak) / c_u(remoulded)`
Reporting: - c_u(peak) at depth — undisturbed strength - c_u(remoulded) at depth - Sensitivity ratio S_t - Test depth, soil description
Typical c_u values (from VST):
| Soil consistency | c_u (kPa) | |---|---| | Very soft | < 12 | | Soft | 12-25 | | Medium / firm | 25-50 | | Stiff | 50-100 | | Very stiff | 100-200 | | Hard | > 200 |
Sensitivity classification:
| Sensitivity ratio S_t | Description | |---|---| | < 2 | Insensitive | | 2-4 | Slightly sensitive | | 4-8 | Medium sensitive | | 8-16 | Sensitive | | > 16 | Quick (extra sensitive) |
Quick clays (S_t > 16) — found in some glacial / marine deposits, usually in coastal areas; lose strength dramatically when disturbed; design carefully.
Bjerrum correction factor (for VST results used in design): - VST often over-predicts c_u for design; apply correction factor μ: - μ = 1.0 for low-PI clays (PI < 20) - μ = 0.85 for medium-PI (PI 20-40) - μ = 0.75 for high-PI (PI 40-60) - μ = 0.65 for very-high-PI (PI > 60) - Design c_u = μ × VST c_u
Test cadence: - VST every 1.5 m down the borehole in cohesive layer - More frequent tests in critical strata - Run as part of standard geotechnical investigation in soft-soil sites
When to choose VST over UCS: - Very soft / quick clays where sampling disturbance is severe - In-situ verification of UCS results - Dispute resolution (lab vs field) - Soft soils below water table where sampling is difficult - Multi-depth strength profile in same borehole (faster than multiple UCS samples)
When to prefer UCS over VST: - Stratigraphy includes both cohesive and granular layers - Routine investigation where SPT + UCS combination is sufficient - Lower budget (UCS is cheaper than VST equipment + personnel) - Strain-controlled stress-strain behaviour needed (UCS provides this; VST gives only peak + remoulded)
1. VST in non-cohesive soil. VST measures undrained shear strength; granular soils don't have meaningful 'undrained' state. Use direct shear / triaxial / SPT for granular. 2. Vane too close to borehole bottom. Disturbed soil near borehole bottom; reading low. Push vane ≥ 5 × diameter below borehole. 3. Rotation too fast. Reading time-rate-dependent; fast rotation reads higher than slow. Stick to 0.1°/sec (6°/min). 4. No equilibration time after vane insertion. Soil hasn't settled around vane; reading wrong. Wait 5 min after insertion. 5. Rod friction not accounted. Long rod has friction in borehole; subtract from torque reading. Calibrate rod friction by spinning vane in air. 6. Calibration of torque measuring device stale. ±10 % uncertainty in c_u. Calibrate annually. 7. No remoulded reading. Sensitivity ratio not calculated; misses quick-clay warning. Always continue rotation to fully remould. 8. Bjerrum correction not applied for design. Over-predicts c_u; foundation under-designed. Apply correction factor based on PI. 9. Reading depth confused (rod length vs actual vane depth). Vane is at the bottom of the rod; depth = rod insertion + vane height. Verify carefully. 10. Vane geometry non-standard. Different H/D ratio gives different reading; conversion formula different. Use standard H/D = 2 vane for routine. 11. No pause between repeat tests at same depth. Soil disturbed by previous test; fresh undisturbed needed for repeat. Wait or move to new depth. 12. Treating peak c_u as design value without sensitivity check. Sensitive soils (S_t > 4) lose strength when disturbed; design must account for actual loaded state.
Investigation cascade for soft-clay site (e.g., Kolkata / Mumbai coastal zone):
1. Reconnaissance — desk study, geological maps. 2. Boring — typically 2-3 boreholes per building footprint. 3. Index tests (IS 2720 Part 4, Part 5) — gradation, Atterberg limits, classification. 4. In-situ testing: - SPT (IS 2131:1981) at 1.5 m intervals - VST (this code, IS 2720 Part 30) at 1.5 m intervals in cohesive layers - Pressuremeter (advanced; less common) 5. Lab testing on Shelby samples: - UCS (IS 2720 Part 10:1991) - Triaxial UU / CU - Consolidation 6. Cross-check between in-situ + lab: - VST (in-situ) vs UCS (lab) — typically VST > UCS due to sampling disturbance - Sensitivity from VST guides confidence in lab results - Bjerrum correction reconciles 7. Synthesis: - c_u profile vs depth - Sensitivity vs depth - Allowable bearing pressure (IS 6403) - Settlement estimate (IS 8009) 8. Foundation design: - Shallow if c_u sufficient at shallow depth - Deep (piles per IS 2911 Part 1 Sec 2:2010) if soft layer extends deeper 9. Construction monitoring: - Settlement plates - Inclinometers - VST at intermediate depths during embankment construction
VST is the gold standard for soft-clay characterisation in India. For sensitive sites, multiple VST readings + Bjerrum correction + careful design distinguishes safe foundation from settlement / failure.