CBR vs plate bearing test

[Okiryu]

I did some CBR and 30 inches plate bearing tests in lean clay soils and got some interesting results. CBRs in average are 2. The plate tests results are 125 and 450 pci. I am planning to use the 125 pci result and neglect the 450 pci value as it is not in line with the type of tested soils and CBR results. I observed the plate tests and did not see anything that could affect the test results too drastically. Do you think that neglecting the highest value makes sense? Also, I still want to know what caused this high value...

 

[Okiryu]

BY THE WAY, the CBRs tests were laboratory tests. Samples were soaked for 4 days.

 

[Ron]

After doing the plate bearing test, did you run an in-place density test of the soil below the plate? Did you check the moisture content of the in-place soil at each location? Did you do a manual auger boring to a depth of 4 to 6 feet below the plate after testing? These would likely tell you why the plate bearing values were so disparate.

In the type of soil you described, the moisture content makes a tremendous difference in its apparent stability strength. This soil is prone to "bridging" when the moisture content is low, resulting in a falsely high apparent bearing or stability.

Yes...neglect the 450 pci.

 

[Okiryu]

Ron , thanks for the reply. Water contents were around 22% to 24% in the area, however I did not check in-situ densities. I do have SPT data. These are medium stiff clays (N value around 6 to 8). Soils were consistent/uniform to a depth of 4 to 6 feet below. Perhaps field densities were different. Sorry, BTW, I am not familiar with the term "bridging" in this type of situations, could you expand on this? Thanks again!