Determination of CBR Value Objectives [PDF]

Zitiervorschau

DETERMINATION OF CBR VALUE OBJECTIVES:  

To determine the California Bearing Ratio (CBR) of the sub-grade soil. To determine the moisture content of sub-grade soil.

APPRATUS REQUIRED: 1. 2. 3. 4.

Mould Steel cutting collar Space disc Surcharge weight

5. Dial gauge 6. IS sieve of size 47.5mm and 20mm 7. Penetration plunger 8. Loading machine capacity up to 5000kg

THEORY California Bearing Ratio (CBR) test was originally developed by California Division of highways (USA) is one of the most commonly used method to evaluate the strength of sub-grade soil for design of pavement thikness.CBR value as defined by IS: 2720 (PART XVI)-1979 is the ratio of the force per unit area required to penetrate a soil mass with a circular plunger of 50mm diameter at the rate of 1.25mm/minute, to that required for corresponding penetration of a standard material. Standard load is that load which has been obtained from tests on a crushed stone whose CBR value is taken to be 100 percent. The ratio is usually determined for penetration of 2.5mm and 5.0mm. The result of this test cannot be related accurately with fundamental properties of the material but are useful in design of flexible pavement. Corresponding load for reading at proving ring Table 1 Load Applied in KN 0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 3.0

Average deflection in Division, mm 93.5 187.5 280.3 374.6 467.8 561.1 651.6 748.1 842.6 935.4

Standard load for Different Penetration Values 1

Table2 Penetration mm 2.5 5.0 7.5 10.0 12.5 Compute CBR value as follows;

CBR value =

Unit standard load, kgf/cm2 70 105 134 162 183

Total standard kgf 1370 2055 2630 3180 3600

Test load corresponding ¿ ¿ chosen penetration(PT ) *100 Standard load for the same penetration(PS)

Usually the CBR is calculated for 2.5mm and 5.0mm penetration .Generally the CBR value at 2.5mm penetration will be greater than that of 5.0mm and in such a case the former is taken for design purpose .If the 5mm value is greater the test is repeated, if the same result follows, the CBR value corresponding to 5mm penetration is adopted for design purpose.

Figure 1 showing the apparatus required for CBR test.

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PROCEDURE: A. Making of dynamically compacted specimen:         

The material was sieved through 20mm IS sieve. 5 kg of the sample was taken. Water was added to the soil in the quantity such that the moisture content of the specimen was either equal to field moisture content or OMC as desired. The soil and water was mixed uniformly. The mould was clamped along with the extension collar to the base plate. The coarse filter paper was placed on the top of the spacer disc. The soil sample was placed in the mould in three layers giving 56 blows with the rammer weighing 2.5 kg for light compaction. The extension collar was removed and the excess soil was trimmed by a straight edge. The base plate, spacer disc and the filter paper was removed and the weight of mould and compacted specimen was noted.

B .Testing of specimen:       



The mould containing the specimen, with base place plate in position was placed on the testing machine. The annular weight of 2.5kg was placed on the top surface of soil. The penetration plunger was brought in contact with the soil surface and the load was applied. The surcharge load was placed on the top of the specimen. The reading of dial gauge was made zero. The load at penetration of 0.5, 1.0, 1.5, 2.0, 2.5, 3, 4.0, 5.0, 7.5, 10.0 and 12.5mm was recorded. Plot the load penetration curve with load as ordinate and penetration as abscissa. Sometimes the initial portion of the curve is concave upward due to surface irregularities .In such a case apply a correction .Draw tangent at the point of greatest slope. The point where this tangent meets the abscissa is the correct zero reading of penetration. From the curve, determine the load value corresponding to the penetration value at which the CBR is desired.

OBSERVATION AND CALCULATION: 3

Here, 935.4 mm dial reading=3KN 1mm dial reading=0.3269 kg Penetration plunger mm 0 0.5 1 1.5 2 2.5 3 4 5 7.5 10 12.5

of Dial Gauge Load dial Reading, Reading Division, mm 0 19.2 35.2 47.2 58.2 67.6 78.2 101.8 119.6 142.8 158.4 175.2

0 96 176 236 291 338 391 509 598 714 792 876

Load(KG) 0.00 31.38 57.53 77.15 95.13 110.49 127.82 166.39 195.48 233.41 258.90 286.36

Sample calculation For penetration of 0.5, Division =19*5+2/2 Dial gauge reading =96 Load =96*0.3269 =31.38kg

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CBR curve

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Load at 2.5mm penetration=110.49kg Load at 5mm penetration=195.48kg load at 2.5 mm penetration ∗100 CBR value at 2.5mm penetration= standard load at 2.5 mm 110.49 ∗100 = 1370 = 8.06% load at 5.0 mm penetration ∗100 CBR value at 5mm penetration= standard load at 5.0 mm =

195.48 ∗100 2055

= 9.51% Since, CBR (2.5mm) < CBR (5mm) Therefore, the CBR value of sub-grade soil is 9.51%

For moisture content; Weight of wet soil (W1) =36gm Weight of soil after oven drying (W2) =34gm weight of water ∗100 Moisture content = weight of dry soil

=

W 1−W 2 ∗100 W2

36−34 ∗100 = 34

=5.88%

RESULT:

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The CBR value of the sub grade soil is found to be 8.06% and 9.51% at 2.5mm and 5mm of the penetration value of first set of observation. As the CBR value at 5 mm penetration is greater than CBR value at 2.5 mm penetration. Hence the CBR value to be adopted is 9.51%. The moisture content is was found to be 5.88%

DISCUSSION AND CONCLUSION: In our experiment, the CBR value in 2.5mm penetration is smaller than at 5mm. So the experiment should be repeated. But due to lack of time. We could not perform the experiment well. California bearing Ratio is an empirical value and useful in design of flexible pavement. Depending upon the expected traffic volume, the thickness of the base course and sub base course can be determined by CBR value, the suitability of soil tested for use of sub grade material in the road construction works. The practical done with the sample made in the practical lab is not same as the soil of sub-grade in actual field. For the design of new road Pavements, the sub grade soil sample should be compacted to OMC to proctor density whenever suitable compaction equipment is available to achieve this density in the field otherwise the soil may be compacted to the dry density expected to be achieved in the field. Since it is performed in the lab, so the value obtained in the lab may not be the actual CBR value of the sub-grade soil. This is only in order to be familiar with the practical method .The CBR value obtained is very low for the road and hence we have to increase the strength of sub-grade soil by various methods. On the basis of above experiment, we can conclude that the CBR value of sub-grade and moisture content of sub-grade soil can be determined. PRECAUTION:   

The mould should be compacted in three layers and each 56 number of blows. The holes of the base plate of the mould should not be blocked. The surcharge weight should be aligned with the plunger so that the plunger penetrates freely into the soil.

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