California Bearing Ratio: CBR Test, Values and Uses

The California bearing ratio, almost always shortened to CBR, is one of the oldest and most widely used strength tests for the soil and stone under a pavement. It answers a simple question: compared with a good crushed stone, how well does this material resist a piston being pushed into it? The answer feeds pavement thickness design, subgrade decisions and base material specs.

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The short version

Origin: created at the California Division of Highways about 1930, then picked up by agencies across the U.S. and overseas.
Lab standards: ASTM D1883 and AASHTO T 193. ASTM D4429 covers testing in place.
Result: a percentage of the load a reference crushed stone carries at the same penetration. A high-quality crushed stone sits around 100.
Main use: rating subgrade and base support for flexible pavement design.

How the lab CBR test works

Compact the specimen. Soil is compacted in a 6-inch diameter mold, usually near the optimum moisture found by a Proctor test. Many labs compact several specimens with different effort so they can read CBR at a target density.
Soak it. The standard soak is 96 hours with a surcharge weight on top, at least 10 pounds, meant to stand in for the pavement that will sit on the soil. A dial gauge tracks swell during the soak.
Push the piston. A piston about 1.95 inches across, roughly 3 square inches in area, is pushed into the soil at 0.05 inch per minute while load is recorded at set depths.
Plot and correct. Load is plotted against penetration. If the start of the curve bends upward because of surface irregularities, the curve is corrected before reading values.
Compute the ratio. The stress at 0.1 inch is divided by 1,000 psi and the stress at 0.2 inch by 1,500 psi, each times 100.

The value at 0.1 inch is normally reported. If the 0.2 inch value comes out higher, the test is rerun, and if the check gives the same pattern the 0.2 inch value is used.

A worked example

Example (made-up readings): the piston needs 300 pounds of force at 0.1 inch and 420 pounds at 0.2 inch.

Stress at 0.1 inch: 300 ÷ 3 = 100 psi. CBR = 100 ÷ 1,000 × 100 = 10.
Stress at 0.2 inch: 420 ÷ 3 = 140 psi. CBR = 140 ÷ 1,500 × 100 ≈ 9.3.
The 0.1 inch value is higher, so the reported CBR is 10.

Typical CBR values by soil type

Pavement Interactive publishes broad CBR ranges by Unified Soil Classification group. They are useful for a gut check, not as a design value for a real site:

Soil group	Typical CBR range
Well-graded gravel (GW)	40 to 80
Poorly graded gravel (GP)	30 to 60
Silty gravel (GM)	20 to 60
Clayey gravel (GC), well-graded sand (SW)	20 to 40
Poorly graded or silty sand (SP, SM)	10 to 40
Clayey sand (SC)	5 to 20
Silts and clays (ML, CL, MH, CH)	15 or less, often far less
Organic soils (OL, OH)	5 or less

What CBR is used for

Pavement thickness design. Many older and local design methods use CBR directly to size the base and asphalt. Newer methods use resilient modulus, and a rough conversion often quoted from the 1993 AASHTO guide era is modulus in psi ≈ 1,500 × CBR for fine-grained soils with a soaked CBR of 10 or less. Treat that as a ballpark only.
Base and subbase specs. Some agencies set a minimum CBR for aggregate base so weak or overly fine stone is rejected. Our road base guide covers what those materials look like.
Subgrade decisions. A low design CBR is a signal to thicken the base, stabilize the soil or undercut and replace it. Paving on clay soil walks through that choice for driveways.
Swell check. High swell during the soak warns of expansive clay that may heave once paved.

Limits of the test

CBR is empirical. It does not model how soil behaves under repeated traffic, and the soaked lab value can be very different from what a field test shows on a dry summer day. The test is intended mainly for material with a maximum particle size under about three quarters of an inch, so coarse base stone needs care in sample prep. And a good lab CBR does not help if the fill is placed wet or poorly compacted, which is why field checks such as density tests and a proof roll still matter.

From CBR to tons of base

Once the designer settles on a base thickness, the rest is volume and weight. Multiply area by compacted base depth, convert to cubic yards and apply a density for the stone you are buying. The asphalt base calculator does that math, and our structural fill guide explains why compacted and loose densities should not be mixed up.

Questions people ask

What does the California bearing ratio measure?

It compares how much pressure it takes to push a standard piston into a compacted soil sample with the pressure needed for a reference crushed stone. The result is a percentage, so a CBR of 10 means the soil carried about a tenth of the reference load at the same penetration.

What are the standard loads in a CBR test?

ASTM D1883 uses standard stresses of 1,000 psi at 0.1 inch of penetration and 1,500 psi at 0.2 inch. The measured stress at each depth is divided by the standard stress and multiplied by 100.

Why are CBR samples soaked?

Soaking for 96 hours under a surcharge weight puts the soil in a near-worst wet condition, the state a subgrade may reach in a wet season. Swell is measured during the soak, which also flags expansive soils.

What is a good CBR value for a subgrade?

There is no single passing number. Clean gravels often test far higher than silts and clays, and the pavement designer sets layer thicknesses around the design CBR. Low values usually lead to a thicker base, stabilization or replacing the soil.

Is CBR the same as a Proctor test?

No. The Proctor test finds the maximum dry density and optimum moisture. CBR measures strength, usually on specimens compacted near those Proctor targets.

Can CBR be measured in the field?

Yes. ASTM D4429 covers in-place CBR, and many crews estimate field CBR with a dynamic cone penetrometer. Field values reflect current moisture and density and will not always match a soaked lab result.

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