CBR from Plate Bearing Test - UK Design Manual for Roads and Bridges

[Mudman57]

There is a formula for obtaining the CBR value from plate bearing tests in the UK DMRB, IAN 73/06. It is

CBR % = 6.1 x 10^-8 x (k762)^1.733

There is no worked example in the current version of this document. An example in an older version (DMRB Volume 7 Section 2 Part 2) suggests that the formula should actually read

6.1 x (10x10^-8) ...... (or, in scientific format) 6.1 x 10E-8

which gives an answer an order of magnitude higher than some results I've had from a testing lab - i.e., they've used

6.1 x 10^-8.... (or) 6.1E-8

This version looks like the published formula, but doesn't give the answer in the example (using their value for k (24,100), the example gives a CBR of 24%, whereas the formula as published gives 2.4%, at least by my reckoning).

Results I've seen elsewhere agree with the published example! As a result I'm at a bit of a loss to know which version to use. Does anyone know definitively which version is correct, or, the provenance of the formula? I suppose it is possible that the published example had an error in it, but you'd like to think not.

The only reference to this I could find on here gave an example, which agreed with the second version above....

 

[Ron]

Look at the material to determine the reasonableness of your answer. A CBR of 2.4 would be a structurally unstable material, such as a soft silt or clay. A CBR of 24 would represent a moderately stable material such as a clean fine sand, slightly silty fine sand or similar.

What is the validity range of your formula?

 

[Mudman57]

Ah, that's where it gets confusing.

The results I've had from a lab were for tests through large cored holes, on a slab sub-base of granular material. They quoted CBRs of 12-40 (so were considered reasonable; some areas had failed). These results indicated that the calculation had been done using the 'lower magnitude' version of the formula.

I have also seen results on compacted crushed concrete, so also a competent material, which had been calculated using the 'higher magnitude' version.

Hence the confusion.

I'm not sure about the validity range; the manual doesn't state this and doesn't state where the formula comes from.

 

[Ron]

I would expect compacted crushed concrete to have a CBR value greater than 150 or 200. In my area, the state department of transportation uses a bastardized version of the CBR called an LBR (Limerock Bearing Ratio). It is almost identical to the CBR, just with a slightly different scaling. The CBR is typically 80 percent of the LBR in value, so a material with a CBR of 80 would have an LBR of 100.

We routinely see values (equivalent CBR) in the range of 130 to 200 for crushed limerock base and crushed concrete base.

I can't imagine that you could even do a plate load test on a material with a CBR of 2.4!