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AASHTO Flexible Pavement design question

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Joe-Bouza
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AASHTO Flexible Pavement design question

forgive me this isn't really a civil3d question, but I know this is a well os vast knowledge.

 

Is any one familiar with the AASHTO Flexibl Pavement design process?

 

Have used county and state asphalt sandwiches as the design norm for the facilities we typically work on. I have recently been tasked to do an asphalt study for a few sites that see a very small amount of heavy truck traffic but the client wants the pavement designed to support an H20 load.

 

We had soil test done and got CBR data for each site, and estimated from trip generation a number of trucks.

 

I have taken the CBR values and used the nomographs to get a subgrade number "S" and the Structural number "SN"

 

1. I use appendix A to get a k value from the CBR correlation

2.I use appendix C to get  a California R-value

3 I use appendix B to get  a Soil support value "S"

4. I use a published table for the Regional factor "R"

 

once the parameters are obtained you use a simple polynomial to verify the pavement thickness with the thickness formula

 

t(1)a(1) + t(2)a(2)+t(3)a(3) = SN

 

Here is my problem;

 

the CBR is a measure of subgrade strenght relative to California limestone. So the higher a CBR the stronger the subgrade and the less pavement would be required to carry the load.  But when go through this exercise I have to use considerably more asphalt then the standard details we use all over our area. The charts seem backwards; inorder to get a SN to match the thickness we typically use arounf these parts I would need a lower CBR to get a lower k to get a lower S. something doesn't make sense

 

Thank you

Joseph D. Bouza, P.E. (one of 'THOSE' People)

HP Z210 Workstation
Intel Xeon CPU E31240 @ 3.30 Hz
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Note: Its all Resistentialism, so keep calm and carry on

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