California Bearing Ra o (CBR) Test of Soil
Aim: To determine the california bearing ra o (CBR) of given soil sample in order to
evaluate its strength and suitability as subgreade.
Apparatus Required
CBR mould with a base plate and extension collar
Spacer disc
Loading machine (with a capacity of at least 50 kN)
Penetration plunger (50 mm diameter)
Dial gauges
Proving ring
Weighing balance
Oven
Soaking tank
Steel straight edge
Mixing tools
Stopwatch
Theory:
The California Bearing Ratio (CBR) test is used to evaluate the bearing capacity of subgrade soil, sub-
base, and base course materials for roads and pavements. It is a penetration test in which a plunger of
standard dimensions is forced into a compacted soil sample at a standard rate of loading. The pressure
required for penetration is recorded and compared to the standard load values. The result is
expressed as a percentage of the standard value and is called the CBR value.
Procedure:
1. The soil sample is irst air-dried, pulverized, and passed through a 4.75 mm sieve.
2. A required amount of water is added, and the soil is mixed uniformly. ( i.e OMC)
3. The moist soil is then compacted in the CBR mould in layers, each layer being compacted by a
standard number of blows ( 56) using a rammer.
4. A spacer disc is placed at the bottom to maintain uniformity.
5. After compaction, the extension collar and spacer disc are removed, and the surface is leveled using
a straight edge.
6. The mould is then weighed to determine the bulk density of the soil.
7. The specimen is either tested immediately (unsoaked) or soaked for 4 days before testing.
8. The CBR mould is then placed under the loading machine, and a plunger of 50 mm diameter is
brought into contact with the surface of the soil.
9. The load is applied at a uniform rate of 1.25 mm/min, and the load corresponding to 2.5 mm and 5.0
mm penetration is recorded.
10. The test is repeated for at least three identical specimens, and the average value is taken.
Observation and Calculation:
Penetration readings (mm) and corresponding loads (kg or N) are noted.
Penetration(mm) load
0.5 29
1 57
1.5 83
2 110
2.5 135
3 160
3.5 182
4 204
4.5 224
5 244
5.5 264
6 284
The CBR value is calculated using the formula:
𝐋𝐨𝐚𝐝 𝐜𝐚𝐫𝐫𝐢𝐞𝐝 𝐛𝐲 𝐭𝐞𝐬𝐭 𝐬𝐩𝐞𝐜𝐢𝐦𝐞𝐧 𝐚𝐭 𝟐.𝟓 𝐨𝐫 𝟓.𝟎 𝐦𝐦 𝐩𝐞𝐧𝐞𝐭𝐫𝐚𝐭𝐢𝐨𝐧
CBR (%) = X 100
𝐒𝐭𝐚𝐧𝐝𝐚𝐫𝐝 𝐥𝐨𝐚𝐝
Standard load at 2.5 mm = 1370 kg
Standard load at 5.0 mm = 2055 kg
From above table
At 2.5 mm =135 kg
At 5 mm = 244 kg
CBR at 2.5 mm= x100 =9.85
CBR at 5mm= x100=11.87
The greater value between 2.5 mm and 5.0 mm is taken as the CBR value, unless the 5.0 mm value is
consistently higher.
Result:
The CBR value of the given soil sample was found to be 11.87 % at 5 mm penetration.
According to standard classi ication, a CBR value between 10% and 20% indicates Good subgrade
strength.
Precautions:
The soil must be properly mixed and homogeneous before compaction.
Accurate compaction must be ensured for consistent results.
The loading rate must be maintained precisely at 1.25 mm/min.
The plunger and mould should be properly aligned to avoid eccentric loading.
The test should be conducted in a vibration-free environment.
