IS 2720:1987 Part 16 is the Indian Standard (BIS) for methods of test for soils - laboratory determination of cbr. This standard specifies the laboratory method for determining the California Bearing Ratio (CBR) of undisturbed and remoulded soils. It details the preparation of soil specimens, soaking procedures to simulate worst-case moisture conditions, and penetration testing to evaluate subgrade strength for flexible pavement design.
Specifies the laboratory procedure for determining the California Bearing Ratio (CBR) of prepared soil samples.
Key apparatus dimensions, compaction parameters, test procedure limits, and standard load values for laboratory CBR testing of soils.
| Reference | Value | Clause |
|---|---|---|
| Mould Internal Diameter— Standard CBR mould internal dimension. | 150 mm | Cl. 4.1.1 |
| Mould Height— Standard CBR mould height. | 175 mm | Cl. 4.1.1 |
| Max. Particle Size in Specimen— Material for test specimen shall pass the 19 mm IS Sieve. | < 19 mm | Cl. 5.1 |
| Compaction Rammer (Light)— For compaction at standard Proctor density. | 2.6 kg mass, 310 mm drop | Cl. 6.1.1 (ref. IS 2720-7) |
| Compaction Rammer (Heavy)— For compaction at modified Proctor density. Some sources state 4.9 kg. | 4.89 kg mass, 450 mm drop | Cl. 6.1.1 (ref. IS 2720-8) |
| Penetration Plunger Diameter— Face area is approx. 19.6 cm². | 50 mm | Cl. 4.1.6 |
| Penetration Plunger Length | ≥ 100 mm | Cl. 4.1.6 |
| Surcharge Disc Mass— Minimum mass for both annular and slotted discs. | 2.5 kg | Cl. 4.1.4 |
| Surcharge during Soaking— Typically 2 discs of 2.5 kg each. | ≥ 5.0 kg | Cl. 6.2.2 |
| Surcharge during Penetration— Must be the same weight as used during soaking. | ≥ 5.0 kg | Cl. 7.1 |
| Soaking Period— For soaked CBR test. Maintain free water in mould. | 96 hours (4 days) | Cl. 6.2.2 |
| Swell Measurement Accuracy— Least count of dial gauge for measuring swell. | 0.01 mm | Cl. 4.1.9 |
| Rate of Penetration— Constant rate to be maintained throughout the test. | 1.25 mm/min | Cl. 7.1 |
| Load Measurement Penetrations— Load readings are taken at these specified penetration depths. | 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, 5.0 mm... | Cl. 7.1 |
| Standard Load for 100% CBR @ 2.5 mm— Corresponds to a pressure of 70 kg/cm². | 1370 kgf | Cl. 8.1 |
| Standard Load for 100% CBR @ 5.0 mm— Corresponds to a pressure of 105 kg/cm². | 2055 kgf | Cl. 8.1 |
| CBR Value Calculation— Calculated for both 2.5 mm and 5.0 mm penetrations. | (Test Load / Standard Load) x 100 | Cl. 8.1 |
| CBR Value to Report— This is the normally reported value. | CBR @ 2.5 mm penetration | Cl. 8.1 |
| High CBR @ 5.0 mm Rule— If CBR@5.0mm > CBR@2.5mm, re-test. If still higher, report 5.0mm value. | Report CBR @ 5.0 mm | Cl. 8.1 |
| Zero Correction— Adjust origin by projecting tangent from steepest part of curve. | Required for concave load-penetration curves | Cl. 7.2 |
IS 2720 (Part 16) specifies the method for laboratory determination of California Bearing Ratio (CBR) — the strength index used worldwide for pavement subgrade + sub-base + base material design. CBR is the key input to flexible pavement design (IRC:37:2018, IRC SP 72:2015) and is one of the most-run geotechnical tests for road / runway projects.
Use IS 2720 Part 16 CBR for: - Pavement subgrade characterisation (highway, runway, parking) - Sub-base + base course material acceptance - Borrow material qualification for embankment + sub-base - Soil stabilisation effectiveness measurement (cement / lime / fly ash treated soil) - Forensic investigation of pavement failure (subgrade weak?)
CBR is a comparative test: ratio of force required to penetrate a soil specimen vs force required to penetrate a standard crushed stone. Standard crushed stone CBR = 100; soil CBRs range from 1 % (very poor) to 80+ % (excellent granular).
Test variants: - Soaked CBR (4 days submerged) — simulates worst-case wet condition; primary value for design - Unsoaked CBR — for above-water-table / dry climate conditions; rarely used for design - Field CBR (per IS 2720 Part 31) — direct in-situ test; rarely used due to complexity
For pavement design, soaked CBR at 95-98 % MDD (Heavy Proctor IS 2720 Part 8:1983) is the standard input.
Equipment: - CBR mould: 150 mm diameter × 175 mm height (volume ~2250 mL) - Surcharge weights (5-10 kg) - Compaction hammer (per IS 2720 Part 7 for Standard Proctor; Part 8 for Heavy) - Penetration plunger (50 mm diameter) - CBR loading frame (capacity 50 kN) - Dial gauges for penetration measurement
Procedure: 1. Prepare specimen: compact soil at OMC to 95-98 % MDD (matching field compaction target). 2. Surcharge: apply 5-10 kg surcharge weight on top (simulates pavement weight). 3. Soak (optional but standard): submerge mould in water for 4 days; measure swell. 4. Test: apply load via penetration plunger at 1.25 mm/min penetration rate. 5. Record load at penetration intervals: 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, 5.0, 7.5, 10.0, 12.5 mm. 6. Plot load vs penetration curve. 7. Compute CBR:
`CBR (%) = (Test load at 2.5 mm penetration / Standard load at 2.5 mm) × 100`
Where standard load at 2.5 mm = 13.24 kN (1370 lbf for crushed stone reference).
8. Repeat at 5.0 mm penetration. 9. Report higher of CBR at 2.5 mm and CBR at 5.0 mm as result.
Reporting: - Specimen identification, density, moisture - Soaked / unsoaked - Swell (if soaked) - Load-penetration curve - CBR (%) - Mean of 3 specimens
Test cadence: - Source qualification: every borrow / quarry source - Periodic re-test: per source change OR per 10,000 m³ - For field / quality control: per 1000 m² of subgrade or sub-base
Typical CBR values for Indian soils (soaked):
| Soil type | Soaked CBR (%) | |---|---| | Black cotton soil (PI > 30) | 2-5 | | Sandy clay (PI 15-25) | 4-8 | | Silty soil (PI 10-15) | 5-10 | | Lateritic gravel | 8-25 | | Murrum / weathered rock | 10-30 | | Granular soil (sandy, gravel) | 15-30 | | Crushed stone aggregate | 80-100+ |
Pavement design CBR requirements (per IRC:37:2018, IRC SP 72:2015):
| Layer | CBR requirement | |---|---| | Subgrade (top 500 mm) | ≥ 8 % preferred; minimum 5 % | | Granular Sub-base (GSB) | ≥ 30 % at MDD | | Wet-Mix Macadam (WMM) | ≥ 80 % | | Aggregate base | ≥ 100 % |
Improving subgrade CBR: - Subgrade < 5 %: stabilise with lime (cohesive) or cement (sandy) per IRC SP 70:2005 - Or capping layer (selected granular fill 200-500 mm thick) - Or geosynthetic separator + reinforcement per IRC SP 59:2019
For PMGSY rural roads (IRC SP 72:2015): - Subgrade CBR 5 %: typical sub-base + base course thickness 350-380 mm - Subgrade CBR 10 %: thinner pavement (250-300 mm) - Subgrade CBR 2-3 %: stabilise / cap; otherwise pavement very thick + costly
Statistical considerations: - 3 specimens per test; mean = result - Variability ±15-20 % typical (CBR is low-precision test) - Use 'characteristic CBR' (e.g., 33rd percentile) for design, not mean
Test cost: - Soaked CBR (3 specimens): ₹4000-8000 per sample at NABL lab - Unsoaked: ₹2000-4000 - Source qualification batch: ₹15000-30000 - Cost per result: significant — invest in quality investigation
1. Test on partially-cured specimen. Soaked test requires 4 days; some labs cut short. Stick to standard. 2. Compaction not at design density. Specimen at 92 % MDD reads much lower CBR than at 98 %. Compact to specified density. 3. Swell not measured. Black cotton + high-PI soils swell substantially; design must account. Always measure + report. 4. Single test result extrapolated. CBR has high variability; multiple tests + statistical analysis essential. 5. Wrong density / moisture for project context. Test at compaction conditions matching field. If field at 95 % MDD, test at 95 % (not 98 %). 6. CBR at 5 mm reported when 2.5 mm gives lower value. Convention is to take whichever is HIGHER (not lower); follow IS 2720 Part 16 reporting protocol. 7. No load-penetration curve in report. Just CBR number; can't verify or detect anomalies. Always plot. 8. Test in laboratory without field correlation. Laboratory CBR may not match field; periodic field CBR (Part 31) for verification. 9. CBR confused with strength. CBR is an empirical index, not engineering strength. Don't confuse with bearing capacity. 10. Aggregate gradation not preserved. Coarse particles excluded; result skewed. Test full-gradation sample. 11. Penetration rate wrong. 1.25 mm/min standard; faster gives higher CBR. Use motorised loading frame. 12. No design CBR characteristic value used. Mean used; design unconservative. Use 33rd percentile or per IRC:37 statistical methodology.
Pavement design cascade:
1. Geotechnical investigation (IS 1892:1993): - Boreholes per project - Soil sampling per stratum 2. Material classification + strength tests: - Gradation, Atterberg, classification - CBR (soaked) per IS 2720 Part 16 — this code - Heavy Proctor MDD (Part 8) 3. Synthesis: - Design CBR = characteristic value (33rd percentile or weakest expected) - Stabilisation strategy if CBR < 5 % 4. Pavement design (IRC:37:2018 for highway; IRC SP 72:2015 for rural): - Subgrade CBR + traffic + climate → catalogue layer thicknesses 5. Construction: - Subgrade compaction to 98 % MDD - Sub-base, base, surface courses per design 6. Quality control: - Field density tests - Periodic CBR field verification 7. Performance monitoring: - IRI, distress, structural condition
CBR is one of the most-used Indian geotechnical tests. Despite known variability + simplicity (CBR doesn't capture all soil behaviour), it remains the design currency for flexible pavement in India + globally.