CBR Test of Soil 【Definition】

The california bearing ratio test (CBR test) is defined as the ratio of the test load to the standard load, expressed as percentage for a given penetration of the plunger.

Theory of California Bearing Ratio ( CBR ) Test Procedure

In this method the combination of load penetration test performed in the laboratory or in-situ along with empirical design charts are analysed to determine the thickness of pavement and its constituent layers.

For designing the flexible pavement this is one of the most commonly used method. The thickness of the various elements comprising a pavement is determined by CBR values.

The CBR test is a small scale penetration test in which a cylindrical plunger of 5 cm in diameter is used. Cross-section is penetrated into sub-grade material, at the rate of 0.05 in per minute i.e 1.25 mm/min.

Observations are taken between the penetration resistance i.e. test load versus the penetration of plunger.

The penetration resistance of the plunger into a standard sample of crushed stone for the corresponding penetration is called the standard load.

Standard loads adopted for different penetrations for the standard material with a CBR value of 100% is given below in the table.

Penetration (mm)	Standard Load (kN)
2.5	13.44
5.0	20.16
7.5	25.80
10.0	31.20
12.5	35.32

The CBR test of compacted soil is then carried out. It is carried out in a CBR mould of 150 mm in diameter and 175 mm in height.

It is provided with a detachable collar of 50 mm in height along with a detachable perforated base plate. A displacer disc, 50 mm deep is also to be kept in the mould.

CBR Test Apparatus Diagram

Required Equipments for Test

1. CBR Mould	Inside diameter 150 mm
2. Total height of mould	175 mm with detachable extension collar 50 mm high and detachable base plate 10 mm thick
3. Spacer Disc	148 mm diameter and 47.7 mm height
4. Rammers	For light compaction – 2.6 kg @ 310 mm drop and For heavy compaction – 4.89 kg @ 450 mm drop
5. Slotted Masses	Annular 2.5 kg each, 147 mm diameter with hole, 53 mm diameter in the center
6. Penetration Piston	50 mm dia and 100 mm long
7. Loading Device	Capacity 50 kN, movable head at a uniform rate 1.25 mm/min
8. Two Dial Guage	Accuracy 0.01 mm
9. IS seives	4.75 mm and 20 mm size
10. Expansion measuring Appraratus	Perforated plate of 148 mm diameter

Also Read : Particle Size Distribution of Soil
Also Read : Aggregate | Classification & Test on Aggregate

CBR Test Procedure

• Take the soil sample which passes through 20 mm IS sieve, but it should retain on 4.75 mm IS sieve.
• Take about 4.5 to 5.5 kg of the material and mix it with water content. Taken water content should be equal to the optimum water content which is found by light compaction or heavy compaction test.
• Sample is to be compacted at optimum water content and corresponding dry density.
• Further, fixing of the extension collar to the top of the mould is done. Also fixing the base plate to the bottom is necessary.
• Spacer disc should be inserted over the base along with the central hole of the disc at the lower face. Place coarse filter paper disc on the top of the displacer disc.
• Take the soil sample in the mould. Further compact it either with the light compaction rammer or the heavy compaction rammer as favourable.
• For light compaction of the soil, it must be compacted in 3 equal layers, each layer is to be given 56 blows by 2.6 kg rammer with drop of 310 mm height.
• For heavy compaction, the soil must be compacted in 5 equal layers and each layer is should be given 56 blows by 4.89 kg rammer with drop of 450 mm height.
• Remove the extension collar. Trim excess compacted soil carefully with a straight edge with the top of the mould. Loosen the base plate and then remove the spacer disc and the base plate.
• Weight the mould with the compacted soil. After weighing the compacted soil, cover the filter paper disc on the base plate.
• Invert the mould with the compacted soil and clamp the base plate.
• Place a perforated disc fitted with an extension stem on the specimen top after placing a filter disc.
• Place 2.5 kg annular masses to produce a surcharge equal to the mass of the base material and wearing coat of the pavement expected. Minimum of two annular masses should be placed.
• Further the mould assembly should be immersed in a tank full of water and allow free access of water on the top and bottom of the specimen.
• Mount the tripod of the expansion measuring device on the edge of the mould and take the initial reading of the mould and dial gauge.
• Keep the mould in the tank undisturbed for 96 hrs. Take the readings of the dial gauge every 24 hrs and note the time of reading.
• Maintain water level constant in the tank. Take the final reading at the end of 96 hrs.
• Remove the tripod and take out the mould from the tank. Allow the water from the specimen to drain off for atleast 15 minutes.
• Free water from the mould should be completely removed without disturbing the specimen surface. Weight the mould with the soaked specimen.
• The mould containing the specimen along with the base plate should be placed in position where the top face is exposed on the lower plate of the loading machine.
• Place the required surcharge masses on the top of the soaked specimen. To prevent upheaval of soil into the hole of the surcharge mass, one 2.5 kg annular mass shall be placed on the soil surface prior to seating the penetration plunger.
• Further adding to it, the remaining masses are also placed.
• Seal the penetration plunger at the center of the specimen to establish full contact between the plunger and the specimen. The seating load should be about 40 N.
• The displacement dial gauge and the load dial guage should be set to zero.
• Already applied initial load to the plunger should be considered as zero.
• Apply the load on the plunger and keep the rate of penetration as 1.25 mm/min.
• At the end of the test, the plunger and the mould are raised and removed from the loading machine.
• Take about 20 to 50 gm soil specimen from the top 30 mm layer for the water content determination. If the water content of the whole specimen is required, take soil specimens from the entire depth.

Data Sheet for CBR Test

optimum water content                         =    ……………………..
Empty Mould Mass                                 =    ……………………..
Mass of mould + compacted soil         =    ……………………..
Mass of compacted soil                         =    ……………………..
Bulk density                                            =    ……………………..
Dry density                                              =    ……………………..

Soaking and Swelling :

Dry density before soaking                  =    ……………………..
Bulk density before soaking                =    ……………………..
Bulk density after soaking                   =    ……………………..
Surcharge mass after soaking             =    ……………………..

Date & Time
Dial Guage Reading
Total Expansion

Final expansion :

Surcharge Test :

Surcharge mass used                             =    ……………………..
Water content after penetration test   =    ……………………..

Plotting of load- penetration curve should be done along with the readings. Find the corrected loads, after zero correction, corresponding to penetration of 2.5 mm and 5.0 mm

CBR Test Result :

CBR (2.5 mm) =

CBR (5.0 mm) =