What is CBR Test?
The California bearing ratio (CBR) test or CBR Test is a penetration test for evaluation the mechnical strength of natural ground or soil, sub-soil and basecourses beneath the new carriageway or road construction. The CBR test of soil was developed to determine the load bearing capacity of the soil for building roads.
A standard CBR test report of different type of natural ground is given below:
|S.NO.||Natural Ground Type/Material||CBR Test Value|
|2.||Turf or moist clay||4.75|
|4.||High quality crushed rock||80|
|5.||Crushed California limestone||100|
Crushed California limestone is considred to be the standard material. So according to the above table not unusual to see CBR values of over 100 in well compacted areas.
To Determine the California Bearing Ratio Value of the Sub-grade Soil by CBR Test.
- Loading Machine– Any compression machine can operate at constant rate of 1.25mm per minute can be used.
- Cylindrical Moulds– Moulds of 150mm diameter and 175mm height provided with a collar of about 50mm length and detachable perforated base.
- Compaction Rammer.
- Surcharge Weight– Annular weights each of 2.5kg and 147mm diameter.
- IS Sieve 20mm.
- Coarse Filter Paper.
The California Bearing Ratio(CBR) test was developed by the California Division of Highways as a method of classifying and evaluating soil- subgrade and base course materials for flexible pavements. CBR is a measure of resistance of a material to penetration of standard plunger under controlled density and moisture conditions. CBR test may be conducted in remoulded or undisturbed sample. In this test a cylindrical plunger of 50mm diameter causing to penetrate a pavement component material at 1.25mm/minute when gradual load is applied. The loads for 2.5mm and 5mm penetration during the penetration are recorded. This load is expressed as a percentage of standard load value at a respective deformation level to obtain CBR test value.
CBR = p/ps * 100
CBR = CBR in percentage ( % ).
p = Measured pressure for site soils [N/mm²].
ps = Pressure to achieve equal penetration on standard soil [N/mm²]
Procedure for CBR Test :
- Sieve the sample of sub-grade through 20mm IS sieve for CBR Test.
- Take 5kg of the sample of soil specimen. Add water to the soil in the quantity such that optimum moisture content or field moisture content is reached.
- Mixed the soil and water thoroughly. Place the spacer disc over the baseplate at the bottom of mould and a coarse filter paper over the spacer disc.
- The prepared a soil water mix and divided into five. Cleaned the mould and apply oil to it.
- After applying oil, fill one fifth of the mould with the prepared soil-water mix.
- Now compact the layer by giving 56 evenly distributed blows using a hammer of weight 4.89kg. The top layer of the compacted soil is scratched.
- Again second layer is filled and previous process is repeated.
- Repeat the process till the 3rd layer and after the third layer the collar is also attached to the mould and process is continued.
- Again continue the process till the fifth layer and after the fifth layer the collar is removed and excess soil is struck off. Remove base plate and invert the mould. Then it is clamped to baseplate.
- A surcharge weights of 2.5kg is placed on top surface of soil. Mould containing specimen is placed in position of the testing machine.
- Now bring the penetration plunger in contact with the soil and apply a load of 4kg(seating load) so that contact between soil and plunger is established.
- The dial readings are adjusted to zero. Apply the load in such way that the penetration rate is 1.25mm per minute.
- Note down the loads at penetration of 0.5,1,1.5,2,2.5,3,4,5,7.5,10 and 12.5mm.
Standard Load Values for CBR Test
|Penetration(mm)||Standard Load(kg)||Unit Standard Load(kg/cm2)|
- Weight of soil taken =
- Weight of surcharge =
- Area of plunger, A =
- Proving Ring Calibration Factor =
|Sl No.||Penetration(mm)||Proving dial reading||Load on plunger(kg)||Corrected load||Unit Load|
- California Bearing Ratio at 2.5mm penetration =
- California Bearing Ratio at 5.0mm penetration =
- California Bearing Ratio of subgrade soil =
The load bearing capacity of the sample is typically measured at the 2.5mm and 5mm penetration for design purposes. If the CBR value at 5mm exceed the value at 2.5mm then the test should be repeated.