This value serves as a recommendation for the usage of the engineering soil in subgrades and base courses in pavements. The CBR value is a Rating or percentage which is calculated by comparing the measured pressure with the standard pressure. Then compared with the recommendation given by different engineering practicing bodies and authorities. The test is carried according to BS 1377: Part 4: 1990.
As the procedure is already discussed in detail in the Laboratory Manual and the BSI Standard. Whereas the brief outline is that the first step is to prepare the sample to be tested, then the soil sample is compacted using the standard method. Then the sample is placed in the compression machine and after applying the seating load, the test is initiated and reading is taken for every 0.25 mm interval. Lastly the value of 2.5 mm and 5.0 mm penetration is recorded, and finally, the CBR value is calculated.
But the results our test show that the curve for the bottom is above the curve of the standard this is not correct. Hence there is some source of error. These errors can be due to any reason; this could be due to the faulty apparatus, and there might be a permanent error in the dial gauges. There might be some initial reading that could have been considered and not eliminated later. There is a large error seen in the readings of the bottom of the CBR test, the result shows that the CBR value is more than 100% that is not possible. So these reading are not considered and discarded. Therefore the CBR value of 5% is found as an appropriate value.
Then there are certain considerations to be taken while the experiment to have accurate readings. Firstly the face of the plunger must be leveled onto the sample face and no kern should be there. Then the surcharge plate must also be placed which will restrict from the heaving of the soil.