IS 2720:1991 Part 10 is the Indian Standard (BIS) for methods of test for soils - determination of unconfined compressive strength. This standard prescribes the method for determining the unconfined compressive strength of undisturbed or remoulded cohesive soil specimens. The test applies an axial load at a constant rate of strain to a cylindrical soil sample to rapidly estimate its undrained shear strength.
Specifies the method for determining the unconfined compressive strength of cohesive soil specimens.
Specimen size, loading rate, failure criterion and consistency classification (very soft → hard) from unconfined compressive strength.
| Reference | Value | Clause |
|---|---|---|
| Test | Unconfined compressive strength (UCS) of cohesive soil | Cl. 1 |
| Specimen — minimum diameter | 38 mm | Cl. 4.1 |
| Specimen — typical diameter (lab) | 38 mm or 50 mm | Cl. 4.1 |
| Height-to-diameter ratio | 2:1 (min 1.8, max 2.5) | Cl. 4.1 |
| Loading rate (axial strain) | 0.5 – 2 % per minute (typical 1.25 mm/min) | Cl. 5.3 |
| Failure criterion — strain limit | 20 % axial strain (if no peak) | Cl. 5.4 |
| qu — UCS definition | Peak axial stress before failure | Cl. 5.5 |
| Undrained shear strength (cu) from UCS | cu = qu / 2 | Cl. 6 |
| Sensitivity (St) | qu(undisturbed) / qu(remoulded) | Cl. 6 |
| Consistency — very soft clay | qu < 25 kN/m² | Cl. 6 / IS 1498 |
| Consistency — soft | 25 – 50 kN/m² | Cl. 6 / IS 1498 |
| Consistency — firm / medium stiff | 50 – 100 kN/m² | Cl. 6 / IS 1498 |
| Consistency — stiff | 100 – 200 kN/m² | Cl. 6 / IS 1498 |
| Consistency — very stiff | 200 – 400 kN/m² | Cl. 6 / IS 1498 |
| Consistency — hard | >400 kN/m² | Cl. 6 / IS 1498 |
| Specimen trimming — undisturbed | Trim from Shelby tube; minimise disturbance | Cl. 4.2 |
| Area correction (for axial strain) | A = A₀ / (1 − ε) | Cl. 5.5 |
| Reporting — strength precision | 1 kN/m² | Cl. 7 |
IS 2720 (Part 10) specifies the method for determining unconfined compressive strength (UCS) of soil. UCS is the simplest measure of cohesive soil shear strength — a cylindrical soil sample is loaded axially with no lateral confinement until it fails. The peak axial stress is the unconfined compressive strength q_u; the undrained shear strength c_u = q_u / 2.
Use IS 2720 Part 10 for: - Cohesive soils (clays, silty clays, soft to stiff) — the test is meaningful only for soils that hold their shape unconfined - Quick site characterisation during boring (most labs run UCS in 1 hour vs days for triaxial) - Allowable bearing capacity estimates for shallow foundations on clay (IS 6403 uses c_u) - Pile capacity in cohesive soil — adhesion factor × c_u for shaft friction (IS 2911 Part 1) - Slope stability of cohesive embankments / cut slopes (short-term undrained analysis) - Evaluating remoulding sensitivity — UCS on undisturbed and remoulded specimens gives sensitivity ratio
Does NOT apply to: - Granular soils (sand, gravel) — they cannot stand unconfined; use direct shear (IS 2720 Part 13) or triaxial (Part 11) instead - Very stiff clay or shale — better characterised by triaxial UU/CD or in-situ pressuremeter
1. Sample: undisturbed cohesive soil, trimmed to a cylinder. Standard size: 38 mm diameter × 76 mm length (height-to-diameter ratio 2:1, slenderness limit). Larger 50 × 100 mm or 75 × 150 mm allowed. 2. Trimming: from a Shelby tube sample or block sample, using a soil lathe and wire saw. End faces must be plane and perpendicular to the axis. 3. Initial measurements: diameter, length, mass; compute initial bulk density. 4. Loading: place sample between platens of a hand-operated or motorised UCS frame; apply axial load at constant strain rate of 0.5-2.0 % strain per minute (typical 1.25 %/min — fast enough for undrained conditions, slow enough to measure). 5. Read: load (proving ring or load cell) and axial deformation (dial gauge) at regular intervals. 6. Failure: defined as the load corresponding to peak stress, OR the load at 20 % axial strain — whichever occurs first. 7. Calculations:
- Cross-sectional area at strain ε: `A = A₀ / (1 − ε)` (assuming constant volume) - Stress: `σ = P / A` - q_u = σ at failure (peak) - c_u = q_u / 2 (undrained shear strength, total stress) 8. Report: q_u in kPa, strain at failure (%), bulk density, moisture content, plot of stress vs strain.
Typical UCS ranges by clay consistency:
| Consistency | q_u (kPa) | c_u (kPa) | Field identification | |---|---|---|---| | Very soft | < 25 | < 12.5 | Easily penetrated several inches by fist | | Soft | 25-50 | 12-25 | Easily penetrated by thumb | | Medium / firm | 50-100 | 25-50 | Penetrated with effort by thumb | | Stiff | 100-200 | 50-100 | Indented by thumb only with great effort | | Very stiff | 200-400 | 100-200 | Indented by thumbnail | | Hard | > 400 | > 200 | Indented with difficulty by thumbnail |
Sensitivity ratio = q_u(undisturbed) / q_u(remoulded at same moisture): - Insensitive: < 2 - Slightly sensitive: 2-4 - Medium sensitive: 4-8 - Sensitive: 8-16 - Quick (extra sensitive): > 16
Allowable bearing pressure thumb-rule for shallow strip footings on cohesive soil (use only as preliminary; verify by IS 6403): - q_safe ≈ 2.5 × c_u (for FS 3, plane strain, square footing on deep clay, no submergence) - For c_u = 50 kPa medium clay: q_safe ≈ 125 kPa = 12.5 t/m² (approximate)
Strain at failure: - Brittle clays / cemented: 1-3 % - Plastic clays: 5-15 % - Very plastic / quick clays: 15-25 %
A brittle low-strain failure with sensitivity > 8 is a warning flag — soil disturbance during sampling or strain-softening behaviour; design should not use peak UCS alone.
1. UCS on disturbed samples. Disturbance during sampling or trimming reduces strength by 30-70 %. Always check sampling tube diameter ratio (≤ 8 % is good; > 12 % means significant disturbance). For critical projects, use thin-walled Shelby or piston samplers. 2. Moisture loss during preparation. Sample dries → strength rises artificially. Trim, weigh, and test within 30 minutes of extrusion. 3. Loading rate too fast. Above 2 %/min, the test stops being undrained and pore-pressure effects skew the result. Stick to 0.5-2 %/min. 4. Confusing q_u with c_u. q_u is the *peak axial stress*; c_u is *half* of that (undrained shear strength). Many reports interchange these — verify by units and context. 5. Reporting q_u from a granular soil. Doesn't make physical sense — granular soils have no cohesion. The test invalidates itself; the soil falls apart before any meaningful stress builds up. 6. Using UCS for long-term (drained) design. UCS gives undrained strength only — short-term (end-of-construction) cases. For long-term stability or settlement under sustained load, use effective stress parameters from triaxial CD/CU tests. 7. Single UCS test on a stratum. Cohesive strata have natural variability of ±30 % around mean. Run ≥ 3 tests per stratum; use lower 1/3 quartile for design, not the mean. 8. Ignoring the strain-at-failure. A soil with c_u = 80 kPa at ε = 2 % vs another at c_u = 80 kPa at ε = 15 % behave very differently under foundation settlement. Report and consider the strain. 9. Skipping bulk density in the report. Density affects the bearing capacity calculation (overburden contribution N_q × γ × Df). Always log mass and dimensions before testing.
Index test sequence (per stratum sampled): 1. Natural moisture content (IS 2720 Part 2) 2. Grain-size analysis Part 4 — confirms fines content > 50 % for UCS to be appropriate 3. Atterberg limits Part 5 — Liquidity Index, Plasticity Index 4. UCS Part 10 (this code) — c_u for short-term design 5. Specific gravity (Part 3) 6. Soil classification (IS 1498)
Strength test progression by project criticality: - Small residential / minor works: UCS only (Part 10) - General building: UCS + 1-2 triaxial UU per critical stratum - Bridge / industrial / multi-storey: UCS + triaxial UU + triaxial CU + consolidation - Major infrastructure / dam / tall building: above + in-situ vane shear, pressuremeter, full c'-φ' from CD triaxial
Use UCS in foundation design as: - c_u input to IS 6403:1981 Skempton / Meyerhof bearing capacity - c_u × adhesion factor (α typically 0.4-0.6) for pile shaft friction in clay (IS 2911 Part 1) - Short-term slope stability total-stress analysis - Liquidity-Index correlation check with field N-value via SPT IS 2131 — large mismatch flags sample disturbance