Calculus Designs

What is CBR Test and Why It’s Important?

1. Full Form of CBR:

CBR = California Bearing Ratio

It is a pe*******on test used to evaluate the strength of subgrade soil, sub-base, and base coursematerials for road and pavement design.

2. Purpose of the CBR Test:

The main aim of the CBR test is to determine how well the soil can resist load from wheel traffic.

It helps in designing the thickness of pavement layers.

3. Principle of the Test:

The test compares the bearing strength of a soil sample with that of a standard crushed stoneunder the same load conditions.
CBR (%) = (Test load / Standard load) × 100
Standard loads:

At 2.5 mm pe*******on → 1370 kg

At 5.0 mm pe*******on → 2055 kg

4. Types of CBR Test:

Laboratory CBR Test: Conducted on prepared soil samples in controlled conditions.

Field CBR Test: Done directly on the site to determine in-situ soil strength.

5. Test Procedure (Lab CBR):

1. Collect soil sample and sieve through a 20 mm sieve.

2. Add optimum moisture and compact it in layers inside a mold.

3. Soak the sample for 4 days.

4. Place a plunger and apply load at 1.25 mm/min.

5. Record load values at 2.5 mm and 5.0 mm pe*******on.

6. Calculate CBR (%) using formula above.

6. Typical CBR Values:

Type of Soil CBR Value (%)

Clayey soil --- 2 – 5

Sandy soil ----- 5 – 10

Gravelly soil ----- 20 – 30

WMM or Crushed Rock ----- 80 – 100

7. Importance of CBR Test:

Determines thickness of pavement layers.

Helps in road design for various traffic intensities.

Evaluates soil suitability for subgrade.

Ensures cost-effective and safe construction.

Used as a quality control test during road projects.

8. Conclusion:

The CBR test is one of the most essential soil strength tests in highway and pavement
engineering.

A higher CBR value indicates stronger soil, leading to thinner pavement layers and reducedconstruction cost.

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