AIM: To determine the California Bearing Ratio(CBR) value of the subgrade soil.

APPARATUS:-

Ø  Loading machine- any compression machine can operate at constant rate of 1.25mm per minute.

Ø  Cylindrical mould of 150mm diameter and 175mm height provided with the collar about 50mm length and detachable perforated base.

Ø  Compaction rammer of rate 2.5kg.

Ø  Coarse filter paper.

Ø  Balance

THEORY:-

The CBR test was developed by the California division of highways as a method of classifying and evaluating soil subgrade and base course materials for flexible pavements. CBR is the measure of resistance of a material to penetration of a standard plunger under controlled density and moisture conditions. CBR test may be conducted in a remoulded or a undisturbed sample. The test consists of causing a cylindrical plunger of 50mm diameter to penetrate of pavement component material at 1.25mm per 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.

PROCEDURE:-

Sieving the sample through 20mm IS-sieve. Taking 5kg of the sample of specimen. Adding water to the soil in the quantity that optimum moisture content or field moisture content is reached. Then soil and water is mixed thoroughly. Spacer disc is placed over the base plate at the bottom of mould and a coarse filter paper is placed over the spacer disc. The prepared soil water mix is divided into fire. The mould is cleaned and oil is applied. Then filling one fifth of the mould with prepared soil. That layer is compacted by giving 56 blows with a rammer of weigh 2.5 kg. The top layer is compacted and soil is stretched. Again the process is repeated. After the fifth layer collar is removed and excess soil is struck off, removing the base plate and invert the mould. Surcharge weight of 2.5kg is placed on top surface of soil. Mould containing specimen is placed in position on testing machine. The penetration plunger is brought in contact with the soil and a load of 4kg is applied so that contact between the soil and plunger is established. Then dial gauge readings are adjusted to zero. Load is applied such that penetration rate is 1.25 mm per minute. Load at penetration of 0.5, 1.0,1.5,2.0,2.5,3,4,5,7.5,10 and 12.5 are noted

OBSERVATION AND CALCULATION:

1.       CBR TEST WITHOUT ADMIXTURE:

PENETRATION	PROVING (PR) LOAD	LOAD	CORRECTED LOAD	STANDARD LOAD	CBR PERCENTAGE %
0	0	0
0.5	0	0
1	6	0.348
1.5	16	0.928
2	22	1.276
2.5	26	1.508		1730
3	30	1.74
4	34	1.972
5	40	2.32		2055
7.5	52	3.016
10	64	3.712
12.5	72	4.176

CALCULATIONS:

CBR at 2.5mm penetration =  ×100

                                                 = ×100

                                                 =0.11×100%

                                                 =11%

CBR at 5 mm penetration = ×100

                                               =× 100%

values changes .this is an example of CBR Test