CBR from Plate Bearing Test - UK Design Manual for Roads and Bridges
CBR from Plate Bearing Test - UK Design Manual for Roads and Bridges
(OP)
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....
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....





RE: CBR from Plate Bearing Test - UK Design Manual for Roads and Bridges
What is the validity range of your formula?
RE: CBR from Plate Bearing Test - UK Design Manual for Roads and Bridges
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.
RE: CBR from Plate Bearing Test - UK Design Manual for Roads and Bridges
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!