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Subbase vs. Base (AASHTO Design Guide)

Subbase vs. Base (AASHTO Design Guide)

Subbase vs. Base (AASHTO Design Guide)

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,




RE: Subbase vs. Base (AASHTO Design Guide)

AASHTO should clarify those figures a bit.  Even though the figure shows a straight line correlation between the different test parameters, that is not so, particularly with respect to the resilient modulus vs. CBR.  Secondly, if you have a base material with a CBR of 30, then you have a very poor base material (the basis of the correlations is for a base CBR=100).  The correlations do not hold when the base material is that poor.  In short, that figure should not even have base correlation values below a CBR of about 70 or 80.

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)

Hi Ron,

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 !


RE: Subbase vs. Base (AASHTO Design Guide)

On reason for the subbase or stabilized subgrade is constructability.  If you try to compact a granular base layer on top of a softer layer, it makes it more difficult to get good compaction, and thus stability, in the base layer.

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)

Something seems strange - not sure I've ever seen a base course with CBR 30% - unless it was natural sand and gravel of poor quality.  Our crushed stone base course material in Laos and in India all measured at more than 100.  Our subbase, being crushed rock was higher too.  I thought (from memory) that crushed stone subbase was 0.11 and crushed stone base was 0.14.  I'd say, if you have CBR 30 for base course, get a different base material or turn it into a cement stabilized base course (CTAB).

RE: Subbase vs. Base (AASHTO Design Guide)

BigH...I agree, but I think his dilemma is the use of the correlation figures in the AASHTO manual.  I agree with him that they don't particularly make sense, but as you noted, a CBR of 30 on the base material makes for a miserable base, so fix it!  The problem is that the figure actually shows values of 30 and below to be correlated with other tests or parameters.  In my opinion, they should not show these correlations for such low values of base materials, since they don't correlate anyway.

RE: Subbase vs. Base (AASHTO Design Guide)

Hi BigH,

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)

There is generally no problem if you are compacting a good base course (CBR >100) on a CBR=30 subbase.  The problem might lie with the subgrade.  In Ontario, we have a Granular A which is well graded - narrow band, crushed stone and it acts as the base course.  Granular B, the subbase, is a much wider graded (although only 8% fines (if I remember right) - there are two varieties - crushed and natural.  We used to use, successfully that [b] 25 mm of asphalt = 50 mm of Granular A = 75 mm of Granular B. There would be restrictions of course on the CBR of select subgrade overlying natural ground (if needed).  I do not have the AASHTO curves with me but they sound very fishy.

RE: Subbase vs. Base (AASHTO Design Guide)

Not sure on the numbers within the AASHTO spe, but as far as base vs. subbase:

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???


RE: Subbase vs. Base (AASHTO Design Guide)

Well, in the case of the FAA, the 209 material has much tighter  control. There are more issues creating the material, and it is actually a bit harder to place.  This all leads to extra cost.

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.

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