CBR Test Procedure – California Bearing Ratio Test

California Bearing Ratio Test (CBR)

California Bearing Ratio (CBR) test was developed by the California state highway department of USA for evaluation of subgrade strength of highway and air-filled pavement. The test is commonly known as the CBR test. CBR test is an empirical test used to determine the strength of the material for pavement design.

CBR test is a penetration test, wherein a standard piston is used to penetrate the soil at a standard rate of 1.25 mm/minute. California Bearing Ratio value is defined as the ratio of load per unit area required to penetrate a soil mass by a standard plunger at a specified rate to that corresponding load required to penetrate standard material. Here standard material is one having a CBR value of 100%. CBR value is usually determined at 2.5mm or 5mm penetration.

Apparatus Required

(As per IS: 2720 (part 16)-1987 (Re-affirmed 2002))

i) Mould with an internal diameter of 150mm and 175mm with a detachable collar, the detachable base plate having perforations at the bottom.

ii) A space disc of 148mm diameter and height of 47.7mm.

iii) Surcharge weights of mass 2.5kg each and having a central hole of 53mm diameter.

iv) Penetration plunger of diameter 50mm and 100mm height

v) A loading machine of capacity 5000kg and capable of traveling vertically at a 1.25mm/minute rate.

vi) Compaction rammers, proving ring, dial gauges, weight balance, filter papers, mixing tool, tray, and measuring cylinder.

Test Procedure

Test procedure consists of two parts

  1. Preparing test specimen
  2. Penetration test

Test specimens can be prepared either by static compaction or by dynamic compaction. A gradually increasing load is applied in static compaction, whereas in dynamic compaction, a specimen is compacted by a suitable rammer. Consider below table 

Mould Specification:

Diameter of the mould = 150mm

Height of the mould = 175mm

Height of the CBR soil specimen = 125mm.

Soil Specification:

Particle size = passing through 19mm sieve and retained in 4.75mm sieve

Penetration Test Procedure:

1) Determine the weight of empty mould.

2) Set the spacer disc on the base plate and a filter paper on the disc.

3) Fix the mould to the base plate so that the disc is inside the mould and then attach the collar over the mould.

4) Water is added to the specimen, and compaction is carried out in accordance with the Standard proctor test or Modified proctor test.

5) Once the compaction is completed, remove the collar and level the surface using the cutting edge.

6) Detach the base plate and remove the spacer disc.

7) Determine the weight of mould and compacted specimen and determine the bulk density of the specimen.

8) A sample is taken for the determination of moisture content, and hence the dry density is calculated.

9) Fix the filter paper on the perforated base plate.

10) Fix the mould upside down so that the surface of the specimen, which was downwards in contact with the spacer disc during compaction, is now turned upwards on which the penetration test shall be performed (unsoaked condition).

11) In case of soaked condition, fix a perforated plate and adjustable stem on the compacted soil specimen in the mould along with a surcharge load of 2.5kg.

12) Place the sample in the soaking tank for four days.

13)After completion of four days, measure the swell reading and determine the percentage of swelling with dial gauge reading.

14) Remove the mould from the soaking tank and allow the water to drain off.

15) Place the sample under the penetration piston and place a total surcharge load of 4kg (2.5kg during soaking + 1.5 kg during testing).

16) The load shall be applied to the plunger at the rate of 1.25 mm per minute.

17) Readings of the load shall be taken at penetrations of 0.5, 1.0, 1.5, 2.0, 2.5, 4.0, 5.0, 7.5, 10.0 and 12.5 mm 

18) With the available readings, determine the water content of the soil.

18) Plot load versus deformation curve.

Calculations:

California bearing Ratio (CBR):

                            CBR (%) = (Pt/Ps) x 100

Where,

Pt = corrected unit (or total) test load corresponding to the chosen penetration from the load penetration curve

Ps = unit (or total) standard load for the same depth of penetration as for PT taken from the table given below.

Generally, the CBR value at 2.5 mm penetration will be greater than that at 5 mm penetration and in such a case, the former shall be taken as the CBR value for design purposes.

If the CBR value corresponding to the penetration of 5 mm exceeds that for 2.5 mm, the test shall be repeated. If identical results follow, the CBR corresponding to 5 mm penetration shall be taken for design.

Corrections in Load vs. Deformation Curve:

The curve plotted may be convex upwards although the initial portion of the curve may be concave upwards due to surface irregularities. A correction shall then be applied by joining the tangent to the curve at the point of maximum slope.

The corrected curve shall be taken to be this tangent, together with the convex portion of the original curve, with the origin of strains shifted to the point where the tangent cuts the horizontal axis for penetration.

Observations:

Unsoaked Condition:

Maximum dry density of the 
specimen to be prepared
 g/cc
Optimum moisture 
content to be taken
 %
Weight of the empty mould g
Weight of the soil 
specimen and mould before soaking
 g
Volume of the soil 
specimen
 cm3
Bulk density of the 
specimen
 g/cc
Water content of the 
specimen
 %
Dry density of the 
specimen
 g/cc
Diameter of plunger cm
Area of plunger sq.cm
Deformation rate mm/minute
Proving ring constant kN/division
Standard pressure for 2.5mm penetration kg/cm2
Standard pressure for 5mm penetration kg/cm2
Dial 
gauge readig in divisions
Penetration in mmLoad in divisionLoad in kNPressure in kg/cm2CBR (after 
correction)
0     
50     
100     
150     
200     
250     
400     
500
750
1000
1250

Soaked Condition:

Maximum dry density of the 
specimen to be prepared
 g/cc
Optimum moisture 
content to be taken
 %
Weight of the empty mould g
Weight of the soil 
specimen and mould before soaking
 g
Volume of the soil 
specimen
 cm3
Bulk density of the 
specimen
 g/cc
Water content of the 
specimen
 %
Dry density of the 
specimen
 g/cc
Diameter of plunger cm
Area of plunger sq.cm
Deformation rate mm/minute
Proving ring constant kN/division
Standard pressure for 2.5mm penetration kg/cm2
Standard pressure for 5mm penetration kg/cm2
Dial 
gauge readig in divisions
Penetration in mmLoad in divisionLoad in kNPressure in kg/cm2CBR (after 
correction)
0     
50     
100     
150     
200     
250     
400     
500
750
1000
1250

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