| S.No. | Field / Checkpoint | Reference | Status |
|---|---|---|---|
| A. SAMPLE | |||
| A1 | Sample location + depth + soil description Acceptance: Per source | Per bore log | OK NC NA |
| A2 | Sample preparation — natural / compacted / soaked Acceptance: Per method | Per IS 2720 Pt 16 | OK NC NA |
| B. TEST EXECUTION | |||
| B1 | Sample compaction — Proctor / Modified Proctor + MDD + OMC Acceptance: Per code | Per IS 2720 Pt 7 / 8 | OK NC NA |
| B2 | 4-day soaking with surcharge (for soaked CBR) Acceptance: Per code | Per IS 2720 Pt 16 | OK NC NA |
| B3 | Penetration test — 1.25 mm/min Acceptance: Per device | Per IS 2720 Pt 16 | OK NC NA |
| B4 | Load at 2.5mm + 5mm penetration recorded Acceptance: Logged | Per code | OK NC NA |
| C. CBR CALCULATION | |||
| C1 | CBR % = (Test load / Standard load) × 100 Acceptance: Per IS 2720 Pt 16 | Standard 1370 kg @ 2.5mm; 2055 kg @ 5mm | OK NC NA |
| C2 | CBR @ 2.5mm and 5mm — higher value reported Acceptance: Both reported | Per code | OK NC NA |
| C3 | Soaked CBR vs unsoaked CBR — significant difference for clayey soils Acceptance: Both reported if applicable | Per project | OK NC NA |
California Bearing Ratio (CBR) is the single most important geotechnical parameter for road / highway / airport pavement design. The Indian Roads Congress (IRC) entire flexible pavement design methodology (IRC 37:2018) is built around the 4-day soaked CBR of the subgrade — your CBR value determines pavement thickness, layer composition, and ultimately the cost of every kilometre.
A subgrade CBR of: - < 2% is unsuitable; subgrade improvement required (mechanical / chemical stabilisation) - 2-5% is poor; pavement is thick + expensive - 5-10% is good; standard pavement thickness - > 10% is excellent; pavement can be optimised; capping layer may be eliminated
For a 1-km 4-lane highway, a CBR rise from 4% to 7% can save ₹0.5-1 crore in pavement cost. Therefore the CBR test is treated as a critical project-economics input, not just a routine lab test.
The test is governed by IS 2720 Part 16:1987 (laboratory CBR) — and the result is the basis for IRC 37 pavement design, NHAI standards, and PWD specifications across India.
Standard procedure (IS 2720 Part 16):
Sample preparation: - Disturbed sample collected from intended subgrade depth (typically 300-500 mm below proposed top of subgrade) - Air-dried + sieved through 19 mm IS sieve; soil > 19 mm replaced with 4.75-19 mm fraction (same mass) - Two sample sets prepared: - Unsoaked (for stiff non-expansive soils, infrequently used) - Soaked (4 days, for all subgrade work — represents worst case)
Compaction: - Mould: 150 mm dia × 175 mm height (split for sample removal) - Compaction effort: typically Modified Proctor (10 lb rammer, 18 inch drop, 5 layers × 56 blows) OR Heavy Proctor per project spec - Sample compacted at OMC (Optimum Moisture Content) ± 0.5% - Bulk density must match 95-98% of MDD for it to represent field-rolled subgrade
Soaking (for soaked CBR): - Surcharge weight placed on top (2.5 kg minimum; simulates pavement layer above) - Sample immersed in water for 4 days (96 hours) at room temperature - Top of sample monitored — swell measurement (mm) recorded - Drained for 15 min before testing
Penetration test: - Sample placed on testing machine with surcharge - Standard piston (50 mm dia, 25 mm² area) - Penetration rate: 1.25 mm / minute (slow + steady) - Load + penetration plotted; readings at 0.5 / 1.0 / 1.5 / 2.0 / 2.5 / 3.0 / 4.0 / 5.0 / 7.5 / 10.0 / 12.5 mm
Calculation: - CBR = (Test Load / Standard Load) × 100 - Standard load at 2.5 mm = 1370 kg (13.44 kN); at 5.0 mm = 2055 kg (20.15 kN) - Report CBR @ 2.5 mm AND 5.0 mm - Higher of the two is reported as final CBR - If 5 mm value > 2.5 mm value, repeat test with proper compaction
1. Unsoaked vs soaked confusion — IRC 37 + NHAI specify soaked CBR; some labs report unsoaked (5-10× higher); pavement designed on wrong basis becomes under-designed.
2. Wrong compaction effort — Standard Proctor vs Modified Proctor gives different MDD + OMC + CBR. Specify in test request.
3. Sample not at OMC — sample compacted too dry or too wet; gives non-representative CBR.
4. Single sample tested — should be minimum 3 samples per stratum; CBR varies 30-50% naturally. Single value statistically meaningless.
5. Soaking time short — 4 days mandatory; some labs do 24-48 hrs; over-estimates CBR for expansive soils.
6. Surcharge weight wrong — should match pavement layer weight above (typically 2.5-4.5 kg); using lighter surcharge gives optimistic CBR.
7. Penetration rate too fast — speed > 1.25 mm/min gives higher CBR; calibrate testing machine.
8. Coarse soil with > 19 mm gravel — replacement procedure not followed; CBR not representative of in-situ soil.
9. Lab CBR vs Field CBR mismatch — lab compaction is uniform; field is not. Always design for 90th-percentile (lower) CBR, not average.
10. Swell measurement skipped — soils with > 2% swell during soaking indicate expansive nature; require subgrade treatment beyond CBR-based design.
Companion formats: - Bore Log Form — sample source documentation - Atterberg Limits Test — soil classification - Proctor Compaction Test — MDD + OMC determination - Sieve Analysis — gradation curve - Lab Test Request Form (FMT-GEO-005) - Plate Load Test (FMT-GEO-003)
Codes: - IS 2720 Part 16:1987 — Laboratory CBR test method - IS 2720 Part 7:1980 — Light Compaction Test (Standard Proctor) - IS 2720 Part 8:1983 — Heavy Compaction Test (Modified Proctor) - IRC 37:2018 — Guidelines for Design of Flexible Pavements - IRC SP 72:2015 — Guidelines for Design of Flexible Pavements for Low Volume Rural Roads - IRC SP 84:2019 — Manual of Specifications and Standards for Four-Laning of Highways - MORTH Specifications 2013 — Section 300 (Earthwork + Subgrade) - NHAI Standard Specifications — Project-specific CBR requirements