9. CALIFORNIA BEARING RATIO TEST
Aim:
To determine the unsoaked CBR value of a soil sample
Apparatus:
1. CBR mould of 150 mm internal diameter and total height = 175 mm, with detachable
extension collar, 50 mm high and detachable base plate, 10 mm thick
2. Spacer disc 148 mm diameter and 47.7 mm high
3. Hammer 2.5 kg weight with a free fall of 310 mm
4. Balance to weigh 10 kg
5. Penetration piston of 50 mm diameter and 100 mm long
6. Loading frame with accessories.
Theory:
The CBR test is conducted for evaluating the suitability of the subgrade and the
materials used in sub base and base of a flexible pavement. The plunger in the CBR test
penetrates the specimen in the mould at the rate of 1.25 mm per minute. The loads required
for a penetration of 2.5 mm and 5.0 mm are determined. The penetration load is expressed as
a percentage of standard loads at the respective penetration levels of 2.5 mm and 5.0 mm.
CBR value = 𝑃𝑒𝑛𝑒𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑙𝑜𝑎𝑑
𝑆𝑡𝑎𝑛𝑑𝑎𝑟𝑑 𝑙𝑜𝑎𝑑 ∗100
CBR value is determined corresponding to both the penetration levels. The greater of these
values is used for the design of pavement.
Procedure:
1. Arrange the mould on the base plate with the spacer disc and assemble the extension collar
at top.
2. Weigh about 6 kg of soil sample which is retained on 4.75 mm IS Sieve.
3. Mix the soil thoroughly with a required percentage of water (OMC) and compact the soil in
three layers, each layer being compacted by 56 blows by 2.50 kg rammer falling through 31
cm, the blows being distributed uniformly all over the surface.
4. Remove extension piece, trim the soil and remove the base plate and spacer disc and
reassemble the base plate.
5. Keep a minimum of 4.5 kg of surcharge on the sample in the shape of disc weights.
The surcharge load usually corresponds to the weight of the pavement above.
6. Keep the entire mould under a loading frame and seat the penetration piston on the soil and
