California Bearing Ratio

Introduction

The California bearing ratio (CBR) is a penetration test for evaluation of the mechanical strength of

road subgrades and basecourses. It was developed by the California Department of Transportation

before World War II. In this test, a plunger of a satandard area is pushed into the soil at a fixed rate

of penetration, and the force requied to maintain thath force is measured. The CBR value is then

defined as the ratio of the measured force to that required for similar penetration into a standard

sample of crushed California limestone rock:

CBR=(F/Fs).100%

Where F is the measured force and Fs is the force required for similar penetration into a standard

sample. Higher values of CBR indicate harder surface of material. The standard crush rock from

California values are as follows:

Standard Crushed Rock From California

Load (kN) 13.24 19.96

Penetration (mm) 2.5 5.0

The test is performed by measuring the pressure required to penetrate a soil sample with a plunger

of standard area. The measured pressure is then divided by the pressure required to achieve an

equal penetration on a standard crushed rock material. The CBR test is described in ASTM Standards

D1883-05 (for laboratory-prepared samples) and D4429 (for soils in place in field), and AASHTO

T193. The CBR test is fully described in BS 1377 : Soils for civil engineering purposes : Part 4,

Compaction related tests.

The CBR rating was developed for measuring the load-bearing capacity of soils used for building

roads. The CBR can also be used for measuring the load-bearing capacity of unimproved airstrips or

for soils under paved airstrips. The harder the surface, the higher the CBR rating. A CBR of 3 equates

to tilled farmland, a CBR of 4.75 equates to turf or moist clay, while moist sand may have a CBR of

10. High quality crushed rock has a CBR over 80. The standard material for this test is crushed

California limestone which has a value of 100. The test remains in existence around the world due to

its low equipment requirements, easy of performance and history of use.

CBR FIELD TEST

Apparatus

1. Mechanical Jack – for continuously increasing the applied load

2. Ball seating with connection – the part which is in direct contact with the reaction load.

3. 50kN proving ring – for load measurement

4. CBR penetration piston – the part which penetrates into the ground when load is applied.

5. Extension rods

6. Datum bar assembly

7. Analogue dial gauge – used to measure depth of penetration.

8. Surcharge weights – used to simulate the actual load from the road surface.

Procedure

A circular area of about 30cm in diameter is trimmed and levelled. Particular care should be taken at

the center where the plunger is to be seated. The surcharge load will be placed on the surface and

the plunger is seated properly. Sufficient penetration surcharge should be tested to produce an

intensity of loading equal to the weight of the base material and pavement that overlie the soil being

test. A 4.54kg annular surcharge weight is placed on the soil surface prior to seating the piston to

prevent upheaval of soil into the hole of the surcharge weights. The dial gauge to measure

penetration is attached to the plunger along with the datum frame. A seating load of 4kg is applied

and the load and penetration dials are set to zero.

The load is applied to the plunger by means of the mechanical jack as such that the rate of

penetration is approximately 1.25mm/minute. The load readings are recorded for at penetrations at

0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, 5.0, 7.5, 10.0 and 12.5mm. After that, the load is then released and

moisture content specimen is taken from underneath the plunger.

CALIFORNIA BEARING RATIO (CBR) TEST

Project:

Date: Sample No.: Job No.:

Location: Station:

Foundation Material: Test By:

Jack No.: Check By:

Penetration

(mm)Reading

Load

(kg)Pressure

0.0

0.5

1.0

1.5

2.0

2.5

3.0

4.0

5.0

7.5

10.0

12.5