Ref: IS 2720 (Part XVI) 1979Purpose: To determine the CBR Value of soil sample..

Theory:

CBR is ratio of load carried out by soil sample at defined penetration level to load carried out by standard crushed stone at the above penetration level. The CBR is a measure of sharing resistance of the material under controlled density and moisture conditions considering limitations of CBR test, it is stated the test procedure should be strictly adhered if high degree of reproducibility is desired. The CBR test is conducted the moulded specimen in the laboratory. Many methods exist to day which utilize mainly CBR strength value for designing pavement structure. The test is simple and extensively investigated for field correlations.

The test consists of causing a cylindrical plunger of 50mm diameter to penetrate a pavement component material at 1.25mm / minute. The loads for 2.5mm and 5mm are recorded. The load is expressed as a percentage of standard load value at a respective deformation level to obtain CBR value. The standard load values were obtained from the average of a large number of tests on different crushed stones.

Standard load values on crushed stones for different penetration value:

Penetration Standard Load Unit standard load Kg/Cm2

2.5 1370 705.0 2055 1057.57.5 2630 13410.0 3180 16212.5 3600 183

Apparatus:

a. Loading machineb. Cylindrical Mouldsc. Compaction rammer d. Adjustable stem perforated platee. Tripped dial gauge f. Annual weight

Test Procedure:

1. 45 Kgs. Of material is dried and sieved through 20mm sieve.2. Determine the optimum moisture content and maximum dry density of the given soil

sample.3. Batch of soils of (at least 5.5 kg. For granular soil and 4.5 to 5 Kg. For grained soil) is

mixed with water upto the optimum content.4. The spacer disc is placed at the bottom of the mould and a filter paper over it, the

processed soil sample is placed over this.5. The soil is compacted by dynamic compaction with 56 blows per layer in three layers

with compacted thickness about 50mm each.

6. The collar is removed and the excess soil at the top of the third layer is struck off using a straight edge.

7. Such three CBR test specimens are prepared from each mould soil samples are collected for moisture content determination.

8. The filter paper is now placed on the base plate and the mould is turned upside down.9. The filter paper is placed over the sample top along with the performed plate with

adjustable stem over this surcharge weight 5 or 75 kg is placed. Soaking is done for 4 days.

10. The sample is allowed to drain off water in a vertical position for 15 minutes. The sample along with mould is again weighed to calculate the percentage of water absorbed.

11. The surcharge weight is again provided and the assembly with the base plate is placed in the compression machine. The plunger is brought in contact with the top surface of the sample. A seating load of 4 kg is applied. The dial is attached for measuring the penetration and is set to zero.

12. The load is applied smoothly at the rate of 1.25mm/minute. Load readings are recorded at penetration 0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, 5.0, 7.5, 10.0 and 12.5. The load is released and the mould is removed from the loading machine.

13. A soil sample from the top 3cm layer is collected and weighed for moisture content determination.

Calculations:

CBR Penetration = Load carried by soil sample at defined penetration level x 100 Load carried by standard crushed stones at the above penetration

Results:

The CBR value at 2.5mm and 5.0mm penetration are found. Generally the value at 2.5mm is higher and this is adopted. However, CBR value determined at 5mm is higher than the test repeated to verify this.

Observation Sheet

California Bearing Ratio Test

Compaction moisture content

Dry density

Condition of test specimen: Soaked / Un-soaked

Moisture content:

a. At top 3cm layer after soakingb. Average after soaking:

Proving Ring calibration factor:

Surcharge weight:

Period of soaking expansion ratio:

Sample Number

Penetration mm

Proving Dial reading

Load on plunger kg

Corrected load kg.

Unit / Kg2

Cm2

1.0 0.00.51.01.52.02.53.04.05.07.510.012.5

CBR at 2.5mm

CBR at 5.0 mm

1 2 3 4 5 6

CBR at 2.5mm

CBR at 5.0 mm

Average CBR value at penetration mm: -%