In short
- CBR is the load needed to push a 50 mm plunger 2.5 mm or 5 mm into a soil specimen, as a percentage of the load for the same penetration in a standard material (IS 2720 Part 16). Design uses the value after four days of soaking.
- IRC:37-2018 says the effective sub-grade CBR should be more than 5% for roads carrying over 450 commercial vehicles a day. The 8% figure belonged to the 2012 edition.
- A weaker sub-grade means thicker granular layers, and so more GSB and WMM to crush and haul for every kilometre.
- GSB has a CBR requirement of its own: a minimum of 30 at 98% dry density under MoRTH Table 400-2, unless the contract says otherwise.
Ask a pavement designer what the ground is like and the answer comes back as a CBR. The California Bearing Ratio is a penetration test: a 50 mm diameter plunger is pushed into a compacted soil specimen at 1.25 mm a minute, and the load it takes is expressed as a percentage of the load the same penetration takes in a standard material. A soil with a CBR of 6 resists the plunger 6% as well as the standard does.
The number matters because the pavement is sized on it. Under IRC:37-2018 the strength of the sub-grade, together with the traffic, sets how thick the layers above must be. Lower CBR, more stone.
How the test is done
The method is IS 2720 Part 16. Soil is compacted into the CBR mould at the density and moisture it will have in the road; for design, IRC:37-2018 Clause 6.2 asks for specimens remoulded at placement density and OMC. The specimen is soaked for 96 hours so that it is tested close to its worst condition. Then the plunger is driven in and the load is read at set penetrations from 0.5 mm to 12.5 mm.
Two readings give the result. The load at 2.5 mm is compared with a standard load of 1,370 kg, and the load at 5 mm with 2,055 kg. Usually the 2.5 mm value governs. If the 5 mm value comes out higher and stays higher on a repeat, the 5 mm value is taken.
What the number says about a soil
CBR is a strength index, not a fixed property of the soil like its grading. The same soil gives different values at different densities and moisture contents, so the test condition has to be stated with the result. An unsoaked CBR on a dry-season sample can look far better than the soil will perform in August.
Compaction moves the value too. A sub-grade rolled to 97% of MDD carries a higher CBR than the same soil left a few points short, one more reason sub-grade compaction is tested as hard as it is.
| Soaked CBR | What the soil is like | What it means for the pavement |
|---|---|---|
| Low: 5% or less | Plastic clays and silts that soften badly when wet | Short of what IRC:37-2018 wants as effective sub-grade CBR on roads over 450 CVPD. Improve it or replace it |
| Moderate: above 5% and up to 15% | Sandy and gravelly soils, good select borrow | The range the IRC:37-2018 design catalogues cover. Higher in the range means thinner granular layers |
| High: 30% and above | Well-graded crushed or natural granular material | Sub-base class. MoRTH Table 400-2 asks GSB for a minimum of 30 at 98% dry density |
The bands are a reading aid, not a classification from any code. Only the figures 5%, 15% and 30 come from the documents named.
How sub-grade CBR enters pavement design
IRC:37-2018 defines the sub-grade as the top 500 mm below the pavement, compacted to at least 97% of MDD on expressways, national and state highways and other heavily trafficked roads (Clause 6.1). For high-volume roads the design CBR is the 90th percentile of the soaked test values (Clause 6.2): a conservative figure, not the average.
The design does not use the sub-grade value alone. Clause 6.4 asks for an effective CBR: a single equivalent value for the 500 mm sub-grade together with the embankment soil beneath it, worked out through the IITPAVE software. A strong select sub-grade over a weak embankment is credited with less than its own test value. The effective CBR is then converted to a resilient modulus in MPa: 10 times CBR up to a CBR of 5%, and 17.6 times CBR to the power 0.64 above that, capped at 100 MPa.
Then the provision most often misquoted. Clause 6.4.3 says the effective sub-grade CBR should be more than 5% for roads carrying more than 450 commercial vehicles a day, two-way, in the year of construction. The 8% minimum still quoted in site offices was the wording of IRC:37-2012, which the 2018 edition superseded. Codes are revised and the contract governs, so check which edition your agreement names.
Why a weak sub-grade costs aggregate
Every layer of a flexible pavement exists to spread the wheel load until the soil at the bottom can carry it. The weaker the soil, the more spreading is needed, and the granular layers do most of that work cheaply. So for the same traffic, a design on a low effective CBR carries a thicker sub-base and base than a design on a high one.
A thicker granular layer across a four-lane carriageway, for the length of a package, is a large tonnage to quarry, crush and haul. How much GSB and WMM a kilometre needs shows how fast thickness turns into tonnes, and it is why the sub-grade CBR in the design report deserves a second look before the material requirement is frozen.
Improving a weak sub-grade
- Better borrow. Bring select soil for the top 500 mm from a source that tests higher. MoRTH Clause 903.2.2 asks for one CBR test, the average of three specimens, per 3,000 m3 of borrow from each source.
- Stabilisation. Mixing lime or cement into the soil raises its strength and cuts its sensitivity to water. The dosage is a laboratory design, not a site guess.
- Drainage. A sub-grade that stays dry performs above its soaked value. Side drains that work, and a sub-base that passes water out to the edge, protect the number the design assumed.
- Compaction to specification. The cheapest improvement is reaching the 97% the design already counted on.
The CBR requirement for GSB
CBR is not only a sub-grade test. MoRTH Table 400-2 requires granular sub-base material to have a CBR of at least 30 at 98% dry density (IS 2720 Part 8), unless the contract specifies otherwise. A grading that passes the sieves but carries too many plastic fines can fail it, which is why the same table limits the liquid limit to 25 and the plasticity index to 6. GSB gradings and tests under Clause 401 covers the rest.
For a crusher operator that puts the CBR on the commissioning sample alongside the grading. For the earthwork crew it means knowing the sub-grade's figure before the first layer goes over it, because after that the only way to change it is to dig.
Standards and sources
- IS 2720 (Part 16):1987, laboratory determination of CBR
- IRC:37-2018, Guidelines for the Design of Flexible Pavements (Fourth Revision), Clauses 6.1, 6.2 and 6.4
- IRC:37-2012 (superseded), for the earlier 8% sub-grade provision
- MoRTH Specifications for Road and Bridge Works, Fifth Revision (2013): Table 300-2, Table 400-2 and Clause 903.2.2
Your contract and its technical schedules override anything written here. Check the clause before you build to it.