Subbase vs. Base (AASHTO Design Guide)
Subbase vs. Base (AASHTO Design Guide)
(OP)
Dear Colleagues,
I have a problem by understanding the following:
The granular Subbase is supposed to be a lower quality material than the granular base.
If I refer to the "1993 AASHTO Guide for Design of Pavement Structures" to obtain the layer coefficients, I get the following:
a) Layer coefficient a2 for a granular base with CBR 30% is 0.095 (Part II, Table 2.6)
b) Layer coefficient a3 for a granular Subbase with CBR 30% is 0.11 (Part II, Table 2.7).
Hence a granular Subbase with same thickness and same CBR would create a higher SN than a granular Base though it is in general "lower quality".
What have I missed ???
Thank you,
MARTIN
I have a problem by understanding the following:
The granular Subbase is supposed to be a lower quality material than the granular base.
If I refer to the "1993 AASHTO Guide for Design of Pavement Structures" to obtain the layer coefficients, I get the following:
a) Layer coefficient a2 for a granular base with CBR 30% is 0.095 (Part II, Table 2.6)
b) Layer coefficient a3 for a granular Subbase with CBR 30% is 0.11 (Part II, Table 2.7).
Hence a granular Subbase with same thickness and same CBR would create a higher SN than a granular Base though it is in general "lower quality".
What have I missed ???
Thank you,
MARTIN





RE: Subbase vs. Base (AASHTO Design Guide)
Use the correlation equation and compute the a2 value based on a resilient modulus of 15ksi being equal to CBR 30....you'll find the a2 value will be even lower than the figure shows!
RE: Subbase vs. Base (AASHTO Design Guide)
thanks for your quick reply. I have plot a graph "Modulus vs. Layer coefficient" (see enclosed pdf) and obviously for each value the Sub-base has a higher coefficient !?!.
Our Specification says that a granular base bayer shall have a minimum CBR of 60% and a granular subbase layer of 30%. Are the graphs/equations reliable for these values?
Actually I do not really understand the purpose of the subbase as the granular base meets all the requirements (strenght, drainage, frost...). So why not just provide a base and use the a2.?
Thank you !
Martin
RE: Subbase vs. Base (AASHTO Design Guide)
Re-post your attachment...it didn't come through.
RE: Subbase vs. Base (AASHTO Design Guide)
Sorry, it must have been my browser. I was able to open your chart.
I'll check it a bit later, but the straight portion of the curve is the only valid part.
RE: Subbase vs. Base (AASHTO Design Guide)
RE: Subbase vs. Base (AASHTO Design Guide)
RE: Subbase vs. Base (AASHTO Design Guide)
to make it clear:
I had (still have) two issues that I do not entirely understand and I am surely not a Pavement Specialist:
1. The coefficient graphs in the AASHTO Guide:
For all CBR values a granular subbase has a higher coefficient than a granular base - means a subbase contributes more to the bearing capacity though it is supposed to be the cheaper material.
2. The purpose of the subbase:
I have refered to various specifications and books and my understanding is, that a granular base layer meets all requirements that are expected from a granular subbase. So why to use it???
The hint from Ron was very helpfull: It can be hard/impossible to compact a granular base on a (weak) subgrade, so a medium stiff subbase acts as a "transition" for constructability.
In general I found sometimes a different use/understanding of the terms. We (in Germany) use normaly the following composition: subgrade - granular antifrost layer - granular base - bituminous base course - bituminous wearing course. In the international (american) sources, sometimes either a granular or bituminous base it used not both combined.
Anyhow, thanks to you both. I think I have learned something and understood this topic now a little more.
Regards from Germany, 2010 World Cup Champion :)
RE: Subbase vs. Base (AASHTO Design Guide)
RE: Subbase vs. Base (AASHTO Design Guide)
Subbase generally has less strict criteria (such as gradation) than base material. This usually leads to lower production costs, and a savings to the project.
For a comparison, look at the template FAA specs for P-154 subbase coarse and P-209 base coarse (Google it). You will see a major difference in the material specs.
RE: Subbase vs. Base (AASHTO Design Guide)
hm, in general I fell that gravel or crushed stone is not that expensive. We have used ASTM D2940 for our final grading requirements for base and subbase course. O.K. I see a difference in the grading curves but does that lead to dramatic cost changes???
Martin
RE: Subbase vs. Base (AASHTO Design Guide)
Where I am at, we can use "pit run" material for subbase. This is cheap, since there is little done with it prior to use. Once you move up to a base, there is more crushing and gradation control. The maximum size is smaller as well.
The base will help you get a smoother surface to pave on than the subbase.