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CBR test is done in construction materials laboratories to evaluate the strength of soil subgrades and base course materials.
The purpose of soil sample soaked in water for 7days is to find out the strength of the sample after soaking continuously in water. The effective CBR should be 75% .
Introduction :
The reason behind the soil sample soaked in water for 7 days in the CBR test is given below;
Hi,
Because to make a practical and environment situation for soil, it’s necessary to make the same conditions during laboratory testing. When the soil under the foundation and pavement. Alternate drying and wetting occur due to changes in climatic conditions and variation in the environment.
As you asked in the question, soil soaked for four days to make sure that after wetting and drying, bearing and resisting properties of the soil must be good enough to sustain and resist the applied load. If the engineer found a significant loss, then check for 5 and 7 days soaking of soil. It may depend upon the site engineer.
CBR (California bearing ratio)
= (Load sustained by the specimen at 2.5 to 5 mm penetration) / (load sustained by soil at corresponding penetration level). X 100
Hope you get the answer.
CBR test is carried out to find the strength of a material like aggregate and soil. But in general, we do it for soil. Therefore, to find the strength of soil, the California Bearing Ratio test is performed. The design of flexible pavement is based on the CBR value of soil.
It can be done for soaked soil samples or unsoaked soil samples.
But, we generally go for soaked (4 days) soil condition. The reason for it is the soil strength (shear) decreases as the moisture content increases. So, there might chance of percolation of water to soil subgrade from the road surface or by seepage, thereby increasing the moisture content of soil and decreasing the performance of the pavement.
So to simulate the field conditions for the worst situation, we soak the sample for four days and then do the test.
The CBR test is done on a soil sample soaked or submerged in water to simulate the worst condition the subgrade material will be in after the construction of the pavement.
First, we can’t guarantee that the water won’t penetrate thought the pavement down to the base and subgrade in the event of rains. Secondly, soil loses strength (especially shear strength) with increasing water content. So CBR value for a soaked sample is always less than that for the same dry sample. So, we design the pavement for a wet subgrade even though it may be dry at the time of construction. Otherwise, the soil will lose strength in the wet season, and the entire pavement will deteriorate with traffic load. So, we need to take the minimum CBR to design.
A step further, the soil for the CBR test is usually soaked for 96 hours (4 days), and if significant loss is detected, the engineers may further soak another sample for up to 5, 6, or 7 days as per their judgment to reach to their design CBR.
