Reviewed by Er. Inderjit Sharma (Retd.), Punjab PWD
What is CBR? The California Bearing Ratio (CBR) tells you how strong a soil is compared to standard crushed stone. A 50 mm plunger is pushed into the soil, and the load needed is compared with the load needed for the same depth in crushed stone.
Why it matters: CBR is the main input for designing road pavements. A weak subgrade (low CBR) needs a thicker, costlier pavement; a strong subgrade (high CBR) needs a thinner one.
This guide follows IS 2720 (Part 16): Laboratory Determination of CBR.
The Idea in One Picture
Apparatus Required
Mould (2250 cc) with collar
150 mm diameter mould with base plate, collar and spacer disc.
Rammer (4.9 kg)
Compacts the soil in 5 layers, 55 blows each.
Loading machine
At least 5000 kg capacity, with proving ring; moves at 1.25 mm/min.
Plunger (50 mm)
Pushed into the soil to measure its resistance.
Dial gauges (0.01 mm)
Read the penetration and the swell.
Surcharge weights
2.5 kg annular weights; represent the pavement layers above.
Test Setup
Procedure
Step 1: Prepare and Compact the Specimen
- Use soil passing the 19 mm sieve (or 37.5 mm for coarse material). Replace oversize material with an equal weight of material passing that sieve and retained on 4.75 mm.
- Take about 6 kg of soil and mix it at its optimum moisture content (OMC), found from the Proctor test.
- Weigh the empty mould (m1). Fit the collar, put the spacer disc on the base plate, and a filter paper on top.
- Compact the soil in 5 equal layers, giving each layer 55 blows of the 4.9 kg rammer dropped from 450 mm, spread evenly.
- Remove the collar, trim the top level with a straight edge, turn the mould over, remove the spacer disc and weigh it (m2). Prepare three specimens in the same way.
Step 2: Soak and Measure Swell
- Put the perforated plate and surcharge weights on the soil (equal to the weight of the pavement layers above, to the nearest 2.5 kg).
- Note the specimen height (h) and set up the swell dial gauge on the tripod. Note the first reading (ds).
- Keep the mould under water for 96 hours (4 days) so water reaches the top and bottom. Read the dial every day; note the final reading (df).
- Take it out, tilt it for about 15 minutes to drain, and weigh it.
Step 3: Penetration Test
- Put the mould on the loading machine. Place the surcharge weights back on top (a 2.5 kg ring first, to stop the soil from heaving).
- Seat the plunger with a small load of 4 kg, then set both dial gauges to zero.
- Push the plunger in at 1.25 mm per minute. Note the load at 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, 5.0, 7.5, 10.0 and 12.5 mm.
- Take a water content sample from the top 30 mm of the soil. Check that no big stone was directly under the plunger.
Calculating CBR
| Penetration (mm) | Unit standard load (kg/cm²) | Total standard load (kg) |
|---|---|---|
| 2.5 | 70 | 1370 |
| 5.0 | 105 | 2055 |
| 7.5 | 134 | 2630 |
| 10.0 | 162 | 3180 |
| 12.5 | 183 | 3600 |
📈 Plot the curve
Plot load against penetration. If the start of the curve bends upward (because of an uneven surface), draw a tangent and shift the zero point to where it meets the x-axis.
✅ Which value to report?
Find CBR at 2.5 mm and 5.0 mm. Usually the 2.5 mm value is higher and is reported. If the 5.0 mm value is higher, repeat the test; if it is still higher, report the 5.0 mm value.
🔢 Final result
Report the average of the three specimens, to two decimal places.
Try It: CBR Calculator
Enter the proving ring factor and the dial readings (divisions) at each penetration. The values filled in are a worked example.
Precautions
🕳️ Clean the holes
Clean the holes in the base plate and perforated disc so water can enter freely during soaking.
🎯 Centre the plunger
Line up the surcharge weights so the plunger passes through the centre and penetrates freely.
⏱️ Steady speed
Keep the penetration rate at exactly 1.25 mm/min.
🪨 Watch for stones
If a large stone sits right under the plunger, the result will be too high. Check after the test and repeat if needed.
Practical Tips for Site Engineers
1. Typical Minimum Values
| Layer | Minimum soaked CBR |
|---|---|
| Subgrade (top 500 mm) | As per pavement design (often 8–10%) |
| Granular sub-base (GSB) | 30% |
Always follow the values in your design and contract specification.
2. What the Result Means
CBR below 5%
Poor subgrade. Usually needs improvement: replacing with better soil, lime/cement stabilisation, or a thicker pavement.
CBR 5–10%
Fair. Acceptable for many roads with proper pavement design; check against the design CBR.
CBR above 10%
Good subgrade; allows a thinner pavement and saves cost.
3. Good Site Habits
- Match the field to the lab: the lab CBR assumes the soil is compacted to the required density at OMC. If field compaction is poor, the real CBR will be lower. Check field density layer by layer.
- Soaked CBR is the safe value: roads get wet in the monsoon, so design is based on the 4-day soaked CBR, not the dry value.
- Test every borrow area: do CBR tests for each source and whenever the soil changes. Don’t rely on one result for the whole stretch.
- Watch the swell: high swell in the CBR test warns of expansive soil that can crack the pavement; check free swell index too.
- Quick field check: a Dynamic Cone Penetrometer (DCP) gives a fast estimate of in-place strength and helps spot weak patches between lab tests.
- Plan the time: soaking takes 4 days, plus Proctor before it. Sample early so results don’t hold up the work.
Key Takeaways
- CBR = test load ÷ standard load × 100; standard loads are 1370 kg at 2.5 mm and 2055 kg at 5.0 mm.
- Compact at OMC in 5 layers × 55 blows (4.9 kg rammer, 450 mm drop); soak 96 hours with surcharge.
- Penetrate at 1.25 mm/min; report the 2.5 mm value (repeat if 5.0 mm is higher); average of 3 specimens.
- Swell (%) = (df − ds) ÷ h × 100.
- On site: GSB needs at least 30%; subgrade as per design; make sure field compaction matches the lab density.
References: IS 2720 (Part 16): Methods of Test for Soils – Laboratory Determination of CBR; IS 9669: CBR Moulds and Accessories; IS 9189: Compaction Rammer; IRC:37 (Design of Flexible Pavements); MoRTH Specifications for Road and Bridge Works.