CALIFORNIA BEARING RATIO (CBR) TEST

Objective:

To determine the California Bearing Ratio (CBR) of the subgrade soil in the Lab.

Apparatus Required :

i. Cylindrical moulds

ii. Space disc

iii. Surcharge weights

iv. Compaction rammer

v. Loading Machine

vi. Adjustable stem, perforated plate, tripod and gauge

vii. Others: Is sieves, Mixing bowl, Straight edge, Dial gauges, Soaking tank, Drying

oven, Coarse filter paper, Dishes, Measuring jar, Balance, etc.

Theory :

California bearing ratio (CBR) test is developed by California Division of Highway for

evaluating the strength of subgrade soil and subgrade soil and base course materials for

flexible pavements. After Second World War, the U.S. crop of Engineers adopted CBR

test for use in the design of base course for the airfield pavements. The test is empirical

and the results of this test can’t be related accurately with the fundamental properties of

the material but useful in the design of flexible pavements. Unless the test procedure is

strictly followed, dependable results can’t be obtained. Based on the extensive CBR test

data collected, empirical design charts were developed by the California State Highway

Department, correlating the CBR value and flexible pavement thickness required. Indian

Road Congress (IRC) also uses this type of chart. As per IS: 2720 (part XIV)-1979, the

CBR is define as the ratio of ‘force per unit area’required to penetrate a soil mass with a

circular plunger of 50 mm diameter at a rate of 1.25 mm/min to ‘force per unit area’

required for corresponding penetration of a standard material.

Mathematically:

Hence, CBR is the relative measure of the strength of the soil or aggregate with respect

to crushing. Standard load is that load which has been obtained from tests on crushed

stone whose CBR value is taken to be 100% (table1.1). The ratio is usually determined

for penetration of 2.5 mm and 5.0 mm.

A graph (load Vs Penetration ) of Load as ordinate and Penetration as abscissa is

drawn and CBR value for 2.5 mm and 5.0 mm penetration are calculated for each

specimen from these graphs. Sometimes the initial portion of the curve is concave due to

surface irregularities. In such a case correction is applied by drawing a tangent at the

point of greatest slope. The point where this tangent meets the abscissa is the corrected

zero reading of penetration.

Generally, CBR at 2.5 mm is greater and this value is adopted. If CBR at 5.0 mm is

greater, the test is repeated. If again such condition arises, the higher value is adopted.

According to IRC, if the variation of results between three specimen is beyond the

maximum permissible variations (table 1.1) , the test is repeated for next three

specimens and average of these six specimen is adopted.

CBR test can be done in the filed or in the lab but the method is different. CBR test is

Lab may be performed on:

A. Undisturbed soil Specimen:

Due to high degree of disturbances in the soil sample, the test on undisturbed soil

specimen is not generally done.

B. Disturbed or Remoulded soil Specimen:

The test on demand or Remoulded soil specimen is generally done. Test specimens are

prepared by compacting soil either by static Compaction or Dynamic Compaction.

1. Statically Compacted Specimen:

When static compaction is adopted the specimen is called Statically Compacted

Specimen. For static compaction, the batch of soil is mixed with water to give the

required moisture content; the correct weight of moist soil to obtain the desired density is

placed in the mould and compaction is attained by pressing the spacer disc using a

compaction machine or jack. Generally, preparation of specimen by static compaction is

not adopted.

2. Dynamically Compacted Specimen:

When dynamic compaction is adopted the specimen is called Dynamically Compacted

Specimen.

Preparation of disturbed soil specimens by dynamic compaction is more commonly

adopted. Dynamic

compaction can be achieved either by IS Light Compaction or by IS Heavy Compaction.

a. IS Light Compaction (proctor test):

For this, soil is compacted in three layers each of compacted thickness about 44 mm by

applying 56 evenly distributed blows of the 2.6 kg Rammer from 31 cm height.

b. IS Heavy Compaction( Modified Proctor Test or Modified AASHO Compaction ):

For this, soil is compacted in five layers each of compacted thickness about 26.5 mm by

applying 56 evenly distributed blows of the 4.89 kg Rammer from 45 cm height.

PROCEDURE :

The following procedure was done:

Three specimens are prepared in the following manner and tested for better result.

A. Preparation of Disturbed (remoulded) Specimen by dynamic Compaction:

1. The oven dried material through 20 mm IS sieve was sieved and was let to retain on

the IS sieve of 4.75 mm.

2. About 6.5 kg of the mixture of course and fine grained granular soil in the mixing pan

was taken.

3. Water was added such that the moisture content of the specimen was equal to the

optimum moisture content of the specimen.

4. Soil and water was mixed up uniformly.

5. The coarse filter paper was placed on the mould clamped with collar.

6. Soil water mix was poured in the mould in such a quantity that after compaction, about

1/5th of the mould was filled for IS heavy compaction.

7. 56 Blows was given for each of 5 layers of sample by 4.89 KG hammer dropping

through 450 mm evenly spread surface.

8. After the 5 layers compaction, the extension collar was removed and the excess soil

sample was trimmed. A little sample was taken fro the determination of moisture content.

9. Then the weight of the sample with the method without the collar was taken.

10. Then a course filter paper was placed on the perforated base plate.

11. The mould containing the compaced soil was then inverted and was clamped to the

base plate.

B. Preparation of soaked specimen.

1. Another filterpaper was placed on the top surface of the sample and the perforated

plate was placed with the adjustable stem.

2. Surcharge weight of 2.5 KG was placed over the perforated plate and the whole

mould with the weights was placed in the water tank for soaking.

3. Then the swell measuring device consisting of the tripod and dial gauge was kept one

the mould.

4. Initial gauge readily was set to zero and the sample was let to soak for 96 hrs.

5. After 4 days the final reading of the dial gauge was noted.

6. The swell measuring device was then removed and was let to consolidate for 15 min.

7. The surcharge weight was removed.

C. Testing the specimen

1. Again the surcharge weight was placed over the base plate centrally.

2. The mould with the base plate was then placed under the plunger of loading machine

and the penetration plunger was kept in contact with soil surface. THE RABIND

Transportation Engineering-II Practical Rabindra Subedi 4

3. Then the dial gauges were set into position to fix to zero then, the load was applied

through the plunger at the uniform rate of 1.25 mm/min.

4.Correspoinding reading in dial gauge reading were recorded at 0,

0.5,1.0,2.0,2.5,3.0,4.0,5.0,1.5,10.0and 12.5 mm reading of penetration reading dial

gauge.

5. Then the load was released and mould was removed from the loading machine.

OBSERVATION AND CALCULATIONS:

Following was observed:

Sample Type: Disturbed

Compaction of Specimen: Dynamic with IS heavy compaction

Weight of Rammer: 4.89 kg

No.of blows: 56 nos.

Layers of compaction: 5 layers

Surcharge wt. placed: 2.9 kg

Condition of specimen: soaked

Soaking period: 4 days

Compacting Moisture Content: Optimum Moisture content dry density at OMC

Proving Ring Conversion Factor: no. of smallest div. of dial gauge ×1.282 kgf

Diameter of plunger: 5 mm

Area of plunger: π×502/4 = 1963.5 mm2

RESULT

From the moisture content determination graph the optimum moisture content of the soil

sample was found to be 11.32 %. And from the load penetration graph, the value of load

from graph corresponding to penetration of 2.5mm and 5.0 mm are calculated and

obtained as 117.944kgf and 166.66kgf respectively. The CBR value of the soil is found to

be 8.6%, i.e. higher of these two values.

CONCLUSION

Normally the CBR value at 2.5 mm penetration which is higher than at 5.0 mm

penetration which is

reported as the CBR value of the material. However, here we obtained that the CBR

value at 2.5 mm penetration is higher than that obtained at 5.0mm penetration. The

general procedure is if this criterion does not fulfill by the test the test should be again

repeated and if the value of CBR at 5.0 mm again comes more than 2.5 mm penetration

then the higher value of the three test specimen is reported as the CBR value. There

may be certain errors for the initial concavity of the load-penetration curve are:

a. The bottom surface of the plunger or the top surface of the soil specimen not being

truly horizontal, with the result the plunger surface not being in full contact with the top of

the specimen initially and,

b. The top layer of the specimen being too soft or irregular.

c. There may be error in loading at a rate of 1.25mm/minute.A correction is applied for

curves curving concavity upwards. A tangent is drawn from the steepest portion of curve

to intersect the x-axis as which is the origin for this specimen. The penetration values are

measured from this corrected origin.

Minimum two sample should be made and test for CBR, but we test only one sample

which is not the basic rule we followed. So we have to test at least for two sample

specimen and the result should be taken as mean of two which is repoted as CBR value

of this sample after correcting if error in curve occurs.

According to Indian standard soil code the CBR value between 8 to 20 falls in the range

of well graded sand,our result within this range.For the optimum moisture content at

least three samples should be tested but we test only one sample which is not correct

omc value of this subgrade soil.Hence I conclude that the soil sample is well graded

sand and the CBR test should be again done for the correct result and conformation of

single sample observation data.This CBR value is more useful for the flexible pavement

design of roads.