IS 4434 : 1978Code of practice for in-situ vane shear test for soils

IS 4434 is the code of practice for in-situ vane shear test (VST) for soils — the field methodology for determining undrained shear strength c_u of soft to stiff cohesive soils. It complements IS 2720 Part 30:1980 (which is the test method) by providing the operational framework + interpretation guidance for in-situ application.

VST per IS 4434 is widely used for: - Soft clay characterisation in coastal Bengal, Mumbai, Chennai sites - Embankment stability on soft subgrade - Foundation design for buildings on alluvial deposits - Slope stability in cohesive soils - Quick clay detection (sensitivity ratio > 8 = warning sign)

The vane is a 4-bladed cruciform device, 65-75 mm diameter × 130-150 mm height, lowered through a borehole to the test depth + pushed into undisturbed soil + rotated until failure torque is measured.

Vane size standards: - Standard: D = 65 mm, H = 130 mm (H/D = 2:1) - Larger: D = 75 mm, H = 150 mm (similar ratio) - Smaller (lab vane): D = 12-25 mm

c_u calculation: `c_u = T_max / (π × D² × (H/2 + D/6))`

For standard H/D = 2 vane, simplified: `c_u = 6 × T / (7 × π × D³)`

Where T = peak torque (Nm), D = vane diameter (m), result in kPa.

Rotation rate: - 0.1°/sec (~6°/min) — slow enough for undrained condition - Faster rates inflate c_u; slower allow drainage

Procedure summary: 1. Drill borehole to depth (under bentonite if loose / below water table) 2. Push vane ≥ 5 × D below borehole bottom (avoid disturbed soil) 3. Wait 5 min for soil-vane equilibration 4. Rotate at 0.1°/sec; record continuous torque 5. Note peak (failure) torque → c_u peak 6. Continue rotating to fully remould (5-10 turns) 7. Read remoulded torque → c_u remoulded

Sensitivity ratio: `S_t = c_u (peak) / c_u (remoulded)`

| S_t | Description | |---|---| | < 2 | Insensitive | | 2-4 | Slightly sensitive | | 4-8 | Medium sensitive | | 8-16 | Sensitive | | > 16 | Quick (extra sensitive) |

Bjerrum correction (for design): VST often over-predicts c_u for design. Apply correction μ: - PI < 20: μ = 1.0 - PI 20-40: μ = 0.85 - PI 40-60: μ = 0.75 - PI > 60: μ = 0.65

Design c_u = μ × VST c_u

Test cadence: - Every 1.5 m down borehole in cohesive layers - More frequent in critical strata - Always pair with IS 2131:1981 SPT for granular layers - Cross-check with lab UCS (IS 2720 Part 10) on Shelby tube samples

• IS 2720 Part 30:1980 — the test method (this is the operational code).
• IS 2720 Part 10:1991 — UCS (lab counterpart).
• IS 2131:1981 — SPT (granular soils).
• IS 1888:1982 — plate load test (in-situ for granular + shallow founding).
• IS 1892:1993 — site investigation procedure.
• IS 1498:1970 — soil classification.
• IS 6403:1981 — bearing capacity calculation.
• IS 8009 Part 1:1976 — settlement of shallow foundations.
• IS 1080:1985 — design of shallow foundations.
• IS 2911 Part 4:1985 — pile load test.
• ASTM D2573 — international counterpart for VST.

1. VST in non-cohesive soil. Granular soils don't have undrained state; VST result meaningless. Use direct shear / triaxial / SPT. 2. Rotation rate too fast. Higher c_u readings; design unconservative. Stick to 0.1°/sec. 3. No equilibration time. Insufficient soil-vane contact; reading low. Wait 5 min after vane insertion. 4. Vane near borehole bottom. Disturbed soil; reading low. Push ≥ 5 × D below borehole. 5. Bjerrum correction not applied for design. Over-design or unsafe design. Apply per PI. 6. No remoulded reading. Misses sensitivity assessment + quick-clay warning. 7. Rod friction not subtracted. Long rod has friction; over-estimates torque. Calibrate rod friction by spinning in air. 8. Calibration of torque measuring device stale. ±10 % uncertainty; recalibrate annually. 9. Single test depth. Soil heterogeneity ignored; design based on incomplete profile. 10. VST treated as definitive without lab cross-check. Always pair with UCS / triaxial on Shelby samples for important projects.

Investigation cascade for soft cohesive site:

1. Reconnaissance + boring plan 2. Boring with continuous SPT (IS 2131) 3. In-situ VST at 1.5 m intervals in cohesive layers (this code + IS 2720 Part 30) 4. Undisturbed sampling (Shelby tube) for lab tests 5. Lab testing — UCS, triaxial, consolidation 6. Synthesis: - c_u profile vs depth (VST + UCS cross-check) - Sensitivity profile - Bjerrum-corrected design c_u 7. Foundation design: - Allowable bearing per IS 6403 - Settlement per IS 8009 - Shallow vs pile decision

VST is the gold standard for soft-clay characterisation; for ports, marine works, large warehouses on coastal alluvium, VST + UCS + Bjerrum correction gives reliable c_u for safe design.