Gtting CBR of over 150%

[avirut]

We have got a confusing with results from CBR unsoaked.
It yields a CBR of over 150%. A materail is sandy silt compacted with modified Proctor. Even after repeating the CBR test, the results are the more or less the same. Could anybody give us any comments?

 

[avirut]

Hi, Just to correct the upper data.
The soil type is silty sand having fine grain soil of 15%.

 

[BigH]

Does seem a bit high for the soil type you describe. I wouldn't be surprised if it was a well graded crushed stone base course. I wouldn't call a soil with 15% fines a silty sand (usually "silty" means between 20 and 35% silt (e.g., in your case fines). What was the gradation of the sand? What was the uniformity coefficient? Has your firm or others (talk to your peers) ever experienced such high CBRs from the same source - DOT have any data? I'd check this out. What was the relative compaction of the CBR in the mould compared to your modified Proctor MDD? Give these considerations some thought.

 

[fattdad]

something's not right.

¡papá gordo ain’t no madre flaca!

 

[avirut]

Many thanks, Big H for the guide. Here is some information according to your post. From the PSD curve, the soil composes of coarse, medium, and fine grains of 5%, 60%, and 20%, respectively. The uniformity coefficient is 6.88 and the coefficient of curvature is 1.64. The degree of compaction in mold is 100% compared to the modified Proctor MDD. I also surprised when getting the result, thus I ask a laboratory boy to repeat it. However, as I post before, the repeat result is similar to the previous one.

Thanks also fattdad.

 

[fattdad]

Please relate the compaction moisture content and the optimum moisture content. I just now noticed that this was an unsoaked CBR - maybe I'll rethink my original post. Just out of curiosity, what led you to do an unsoaked CBR?

f-d

¡papá gordo ain’t no madre flaca!

 

[avirut]

Fattdat, many thanks for your opinions. The reason of doing unsoaked CBR is that this road is just for temporary purpose but it must be able to carry a load of about 200 tons. The compacted moisture content is somewhat close to optimum (maybe a wee bit drier). We are now wetting for another check. Further information will be reported.

 

[VAD]

For subgrade materials, we are genearlly accustomed to see CBR results that are normally less than 100%. These tests are generally based on specimens compacted to standard Proctor density. Hence, when a value of 150% is reported we are obviously surprised. However, CBR increases dramatically with increased density and for the material in question, the surcharge pressure used on top of the mould.

It is assumed that the test procedure is correct and results have been checked to ensure that corrections are made etc. This being the case, one may wish to believe that the reported result is reasonable noting as well that most of us have been calibrated with the standard Proctor CBR.

It would be of interest if the soil proposed can be compacted to modified Proctor values in the field. This may not be readily achieved. I would suggest that the CBR test be undertaken at 95 and 97% of modified density at the corresponding moistures. Life in the field is hardly ever as controlled as in the lab - not only moisture but variability of material,Contractor and Client pressures.

I also presume that this temporary road would be capped, if not then you may well be advised to undertake a soaked CBR test. On sands compacted to standard Proctor, I have noted that the CBR of the soaked sample drops to about half of the unsoaked value.

Question: The load of 200 tons how will this be distributed and what is the proposed pavement structure.

 

[fattdad]

Yet another followup: If this is a temporary road and you are using the AASHTO design method for a particular traffic loading, just how sensitive is the design to the actual CBR value? For example, if you proceeded with a 1 year design using a CBR of 100 (i.e., 2/3rds of the lab value of 150) and also proceeded with the design using 20 (for example as if the lab CBR was 30), just how different would the design really be?

Irrespective of the appropriate design CBR value what method will you be using to convert the 200 ton vehicle weight into equivalent 18-kip axle loads? Most design methods show 18-kips equivalancies (sp) for trucks up to 40 tons (i.e., tractor-trailer combinations). What you need to identify is how the off-road tires, axle loads, etc. convert to equivalent 18-kip axle loads.

As an important asside do you plan on completing a design that is just gravel or will there be some asphalt on the surface? For my practice, I would never consider a heavy-duty asphalt pavement section thinner than 2 in surface, 3 in base and 6 in aggregate. Depending on your design life, 18-kip axle loads and anticipated life, no matter what CBR value you use this "minimum" pavement section may be adequate. For a gravel-only pavement (and considering just how big this vehicle seems to be), I'd think you'd need at least 12 to 14 inches of dense-graded aggregate.

Good luck.

f-d, p.e.

¡papá gordo ain’t no madre flaca!

 

[BigH]

You might want to do some research into mine haul roads - they deal with heavy loads with the mining haul trucks. Might be more appropriate than trying to "dumb" down to highway type loadings.

 

[avirut]

Billion thanks for fruitful discussions from ever person.
I am just back from another construction site and now awaiting to see a repeated result from laboratory. Hopefully it will be available within today. The repeated result will be informed here.

 

[avirut]

Regarding to VAD's post, the about 200 tons load is Boeing 747.