Hello everyone
I know that there are many posts relating to the use of the CBR as well as the subgrade stiffness. I have a background in construction materials testing (20 years) and have spent the last 20 years as a Geotechnical Designer/Civil Engineer specializing in earthworks and geotechnics, so I am coming at the problem with over 40 years knowledge, but would welcome all thoughts from those willing and able to provide their thoughts.
In the UK highway pavement foundation design has always been based on a CBR value, although since 1984 there had been a push to use the subgrade stiffness modulus. CBRs were and are still widely used in the UK, and tend to be the default request, whilst an accurate understanding of what this actually means can be somewhat lacking.
With a recent (2019 ~ 2020) update to design standards, the methods of testing the CBR of the subgrade for highway design was removed, with designs now to be based on the subgrade stiffness (E), albeit the latest version of the design manual still retained a pre-existing relationship between E and the CBR, where E = 17.6 x(CBR)^0.64.
I am very comfortable with using E as part of the geotechnical design process, but as I am sure everyone reading this will appreciate, there are different methods for measuring E. To this end, I drafted a technical note for myself, which compared the different methods, and the E each one derived. I have appended a copy of this paper with this email
The initial conclusion from this was that in nearly every instance the E derived from static plate load testing, whether this was done to UK (BS 1997) or other standards, returned a very similar value, where the same input parameters were used. The only difference was where the plate test data was used to calculate the equivalent CBR, and then the CBR was converted to E using the equation above.
The discrepancy between a directly measured E and one from conversion of CBR to E was around 2.5. This could be explained by the 'type' of E the design standard uses (DMRB CD 225), which should be dynamic and not static.
However, since drafting the TDN in 2020, I have had the opportunity to gather data from a number of sites, where both static and dynamics tests were carried out on the same fill at the same location at the same time. The discrepancy between a directly measured E (for static & dynamic testing) compared to a converted CBR top E using the published relationship in the DMRB also gives a discrepancy typically between 2.3 and 2.5.
So my question is, has anyone else recorded similar discrepancy between the correlation between CBR and E.
I have tried to go back to the original derivation of E to CBR, and have also appended a 1990 paper on this. You may note that the relationships in the paper did not (in my opinion) support a single relationship between E and CBR, and the actual measured values were significantly different.
Thanks in anticipation
I know that there are many posts relating to the use of the CBR as well as the subgrade stiffness. I have a background in construction materials testing (20 years) and have spent the last 20 years as a Geotechnical Designer/Civil Engineer specializing in earthworks and geotechnics, so I am coming at the problem with over 40 years knowledge, but would welcome all thoughts from those willing and able to provide their thoughts.
In the UK highway pavement foundation design has always been based on a CBR value, although since 1984 there had been a push to use the subgrade stiffness modulus. CBRs were and are still widely used in the UK, and tend to be the default request, whilst an accurate understanding of what this actually means can be somewhat lacking.
With a recent (2019 ~ 2020) update to design standards, the methods of testing the CBR of the subgrade for highway design was removed, with designs now to be based on the subgrade stiffness (E), albeit the latest version of the design manual still retained a pre-existing relationship between E and the CBR, where E = 17.6 x(CBR)^0.64.
I am very comfortable with using E as part of the geotechnical design process, but as I am sure everyone reading this will appreciate, there are different methods for measuring E. To this end, I drafted a technical note for myself, which compared the different methods, and the E each one derived. I have appended a copy of this paper with this email
The initial conclusion from this was that in nearly every instance the E derived from static plate load testing, whether this was done to UK (BS 1997) or other standards, returned a very similar value, where the same input parameters were used. The only difference was where the plate test data was used to calculate the equivalent CBR, and then the CBR was converted to E using the equation above.
The discrepancy between a directly measured E and one from conversion of CBR to E was around 2.5. This could be explained by the 'type' of E the design standard uses (DMRB CD 225), which should be dynamic and not static.
However, since drafting the TDN in 2020, I have had the opportunity to gather data from a number of sites, where both static and dynamics tests were carried out on the same fill at the same location at the same time. The discrepancy between a directly measured E (for static & dynamic testing) compared to a converted CBR top E using the published relationship in the DMRB also gives a discrepancy typically between 2.3 and 2.5.
So my question is, has anyone else recorded similar discrepancy between the correlation between CBR and E.
I have tried to go back to the original derivation of E to CBR, and have also appended a 1990 paper on this. You may note that the relationships in the paper did not (in my opinion) support a single relationship between E and CBR, and the actual measured values were significantly different.
Thanks in anticipation
