cbr grp 1&6

7/25/2019 CBR GRP 1&6

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1

OBJECTIVETo Determine the CBR value of soil in Remoulded condition.

2

APPARATUS

2.1 Loading Machine

The loading machine shall have a capacity of at least 5 000 kg and be equipped with a movable head or

base which enables the plunger to penetrate into the specimen at a deformation rate of 1.25 mm/min. The

machine shall be equipped with a load machine device that can read to suitable accuracy.

2.2 Mould

A cylindrical metal mould with a nominal capacity of 2125 cm3, internal diameter of 152.4 mm and

height of 116.3 mm is taken. The mould will be fitted with a detachable collar and base plate. The exact

volume of the mold without the collar is determined by weighing the amount of water required to fill it.

2.3 Base Plate

A suitable seating about 2 mm deep shall be provided on the top face for proper seating of the mould and

shall be of same material as mould.

2.4 Collar

The collar shall be made from same material as that of mould. Two similar ears as in the case of the

mould shall. be cast integral with the body or welded. It shall have suitable seatings at the lower end for

sitting flush with the mould.

2.5 Metal Rammer

A metal rammer having a 50.8 mm diameter circular face with a 4.53 kg weight which will drop freely fora distance of 457 mm.

2.6 Weights

2.7 Penetration Plunger

2.8 Dial Gauge

2.9 Sieves

47.5 mm IS Sieve and 19 mm IS Sieve

2.10 Miscellaneous Apparatus

Balance- A balance sensitive to 0.1 g.

Drying apparatus - Oven suitable for drying samples.

Mixing tools - mixing pan, trowel.

7/25/2019 CBR GRP 1&6


3

Test Specimen

A remoulded sample of black cotton has been taken with field density as 18.08KN/m3. The

material used in the remoulded specimen shall pass a 19-mm IS Sieve. Allowance for larger

material shall be made by replacing it by an equal amount of material which passes a 19-mm IS

Sieve but is retained on 4.75-mm IS Sieve. The soil is further dynamically compacted using 4.53

kg rammer dropped from a height of 457 mm.

4

Scope

The California bearing ratio (CBR) is a penetration test for evaluation of the mechanical strength

of road subgrades and base courses. It was developed by the California Department of

Transportation. The test is performed by measuring the pressure required to penetrate a soil

sample with a plunger of standard area. The measured pressure is then divided by the pressure

required to achieve an equal penetration on a standard crushed rock material.

5

Theory

It is the ratio of force per unit area required to penetrate a soil mass with standard circular piston at the

rate of 1.25 mm/min. to that required for the corresponding penetration of a standard material.

C.B.R. = Test load/Standard load x 100

The following table gives the standard loads adopted for different penetrations for the standard material

with a C.B.R. value of 100%

Penetration of plunger (mm) Standard load (kg)2.5

5.0

7.5

10.0

12.5

1370

2055

2630

3180

3600

The test may be performed on undisturbed specimens and on remoulded specimens which

may be compacted either statically or dynamically.

7/25/2019 CBR GRP 1&6


6 Procedure

6.1

Dynamic Compaction

1. Take about 5.5 kg of soil and mix thoroughly with water (16% optimum moisture content).

2. Fix the extension collar and the base plate to the mould.

3. Compact the mix soil in the mould using heavy compaction in 5 layers, 56 blows to each layer by the

4.89 kg rammer.

4. Remove the collar and trim off soil.

5. Turn the mould upside down and remove the base plate.

6.2 Penetration Test

1. Place the mould assembly with the surcharge weights on the penetration test machine.

2. Seat the penetration piston at the center of the specimen with the smallest possible load, but in no case

in excess of 4 kg so that full contact of the piston on the sample is established.

3. Set the stress and strain dial gauge to read zero. Apply the load on the piston so that the penetration rate

is about 1.25 mm/min.

4. Record the load readings at penetrations of 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, 5.0, 7.5, 10 and 12.5 mm.

Note the maximum load and corresponding penetration if it occurs for a penetration less than 12.5 mm.

7

Observation and Recording

7.1 For dynamic compaction

Optimum water content (%) 16%

Dry density g/cc 18.08KN/m3

7.2 For penetration Test

Calibration factor of the proving ring 91 div=2.5KN=254.82 kgf

Least count of penetration dial 1 Div. = 0.01 mm

If the initial portion of the curve is concave upwards, apply correction by drawing a tangent to the curve

at the point of greatest slope and shift the origin. Find and record the correct load reading corresponding

to each penetration.

C.B.R. = PT*100/PS

7/25/2019 CBR GRP 1&6


where PT = Corrected test load corresponding to the chosen penetration from the load penetration curve

PS = Standard load for the same penetration taken from the table I.

Penetration Data

Surcharge Weight Used = kg

S.No Penetration(mm)

Load MeasuringDevice Reading

CaliberationFactor

load(kgf)

1 0.5 12.0 2.80 33.6

2 1.0 25.0 2.80 70

3 1.5 37.0 2.80 103.6

4 2.0 45.0 2.80 126

5 2.5 52.0 2.80 145.6

6 3.0 57.0 2.80 159.6

7 3.5 61.0 2.80 170.8

8 4.0 64.0 2.80 179.2

9 4.5 66.0 2.80 184.810 5.0 69.0 2.80 193.2

11 5.5 71.0 2.80 198.8

12 6.0 72.0 2.80 201.6

13 6.5 73.0 2.80 204.4

14 7.0 74.5 2.80 208.6

15 7.5 75.5 2.80 211.4

16 8.0 77.0 2.80 215.6

17 8.5 77.5 2.80 217

18 9.0 78.5 2.80 219.8

19 9.5 79.0 2.80 221.2

20 10.0 80.0 2.80 224

21 10.5 80.5 2.80 225.4

22 11.0 81.0 2.80 226.8

23 11.5 82.0 2.80 229.6

24 12.0 82.0 2.80 229.6

25 12.5 82.5 2.80 231

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8 Interpretation and recording:

C.B.R. of specimen at 2.5 mm penetration

C.B.R. of specimen at 5.0 mm penetration

The C.B.R. values are usually calculated for penetration of 2.5 mm and 5 mm. Generally the

C.B.R. value at 2.5 mm will be greater that at 5 mm and in such a case/the former shall be taken

as C.B.R. for design purpose. If C.B.R. for 5 mm exceeds that for 2.5 mm, the test should be

repeated. If identical results follow, the C.B.R. corresponding to 5 mm penetration should betaken for design.

9

Results and significance:

The CBR value for given soil sample is 10.62 corresponding to 2.5 mm penetration.

The results obtained by this test are used with empirical curves to determine the thickness of

pavement. This is the most widely used method for the design of flexible pavement.

0

50

100

150

200

250

0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0

l o a d

i n n k

g f

penetration in mm

load vs penetration

cbr grp 1&6

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