Because there are different soil types in different areas and the test is a "standard". Some soils take longer to saturate than others, so in a 4-day span, most soils will saturate.
The reason for having a standardized test method is so that the test is repeatable from lab to lab.
VIRANGA, sorry for hijacking your thread but I have a question for Ron related to this:
Ron, have you had the experience to run CBR tests in non-soaked samples? Sometimes I get these requests, but I refused to do it because this will defer from the "standards".
Okiryu.....yes, I have run those for "grins"! Values are tremendously higher as you might expect and the failure point is abrupt. When we have run "field" CBR's, we have gotten similar results, since you can't saturate the soil in the field. The results are obviously not conservative!
I would not run those for record purposes. Just for interesting information.
Ron - on some jobs, wouldn't it be possible to develop a relationship based on a sufficient number of tests on the same soil type to determine a correlation between soaked CBR and unsoaked CBR. On many construction jobs where there is quite a bit of fill being placed, you don't have time, necessarily, to run a full "soaked" CBR. Had this in Laos - I wanted to determine correlations but we ended up doing the soak anyway . . . Of course, if the soil type changes, then the correlations wouldn't be valid.
When we run simple swell tests (ASTM D 4546, Method B), we run them for 96 hours as well. If you plot the time rate of swell, it often takes 2 to 3 days to get past the initial and primary swell for really active soils. The last day of testing will define the secondary swell portion of the curve for really active soils.
Although many swell tests are essentially complete with 36 to 48 hours, some do need to go 4 full days...which is why, in my opinion, the CBR test also runs for 4 days.
