cbr testing 2

[rangers1]

How does one correlate the results of insitu cbr testing to lab testing. ie the bs 1377 part 4 indicates that one has to measure the cbr at the optimum moisture content of the material whereas to do an insitu cbr test there is no control over the moisture content of the soil tested

 

[iandig]

The original CBR test is based upon the sample being confined and tested within a mould. You make reference to the testing of the CBR at it's OMC from BS 1377-4:1990, this is a target value in order to assess the material at what has been deemed to be it's optimum (based on a specified compaction effort). You do not have to always test a sample at it's optimum, you should select the criteria required for the test, including type of compaction, target moisture content, target density, specified compactive effort etc...
When testing the CBR in-situ, the material is not confined, and it is normal to expect a slightly lower value because of this. This can be improved by using a mould, pushed into and then extracted from the ground and submitted for a lab test. However the big problem with this is the damage it causes to the ground. Another option is if the material is being compacted on site, to compact a portion of the fill into a mould and then submit this for a lab test. This idea works very well for stabilised (stabilized) soils.
If you are undertaking a ground investigation and need to identify the CBR of the ground, the CBR will be dependant upon the prevailing conditions, including the moisture at the time of the assessment. By measuring the moisture and comparing it against the OMC then you can assess whether the soil is wetter/drier/similar to the OMC and also what the effect would be to dry/wet the soil up to the OMC.
The highest CBR achievable with a soil is unlikely to occur at it's OMC, as the test measures the force requried to penetrate a soil with a plunger. The drier/stiffer a soil is then the higher the CBR can become to a point where it is too dry and crumbles, and this may well be dry of the OMC.
I hope this answers your question, in summary the lab and in-situ CBR are likely to be very close, with the lab slightly higher, for the same soil at the same density and moisture.