High water table

[Okiryu]

Do you have any references for engineering practices for earthwork and road construction in areas where water table is high? One of them is fill placement with good subdrain systems. Other ideas? Our site has clay and silt layers of 3 to 5 meters thick followed by gravels with occasionally limestone inclusions. Thanks.

 

[Ron]

Okiryu....attached is a paper I wrote about 20 years ago on this subject..hope it helps..

 

[Okiryu]

Hi Ron, finally I got the time to read your paper. It is a nice paper. Thanks for sharing it with us.

Some questions about the paper:

How do you recognize the quality of drainage shown in tables 1 and 2? There are five classes there, so assuming that normally base and sub base courses are composed of granular materials, how can I differentiate the quality of drainage? Does it depend on the percentage of fines?

 

[Ron]

Okiryu...the "quality of drainage" is defined in the AASHTO Pavement Design Guide in the following table:

 

[Okiryu]

Ron, thanks for the reply. I also noted that the paper tries to explain that base treatment is good as long as subgrade stabilization is provided. In the US, what are the minimum CBR requirements for subbase and base materials? I normally recommend to stabilize the subgrade (if it has less than CBR values of 3) and use granular subbase of minimum CBR of 20 and base of minimum CBR of 80. Luckily, in my location, all materials for subbase and base have CBR values greater than 80. Base and subbase materials are standard materials and therefore subbase and base stabilization (at least for what I have seen so far) is rare.

 

[Ron]

I specify subgrade stabilization that essentially creates a subbase. The minimum CBR for that would be 25 for light duty pavements and 30 for heavier pavements. Part of the reason for that is constructability. It is much easier to get compaction and stability in the base course if the subgrade is properly stabilized. Further, it helps to mitigate long-term rutting.

The base should have, as you specify, a minimum CBR of 80 at 98% compaction by the modified Proctor method.

 

[Okiryu]

Thanks Ron. Constructubility is a very good point. In your opinion, what is the minimum CBR that the subgrade may have in order to base or subabse courses can be properly compacted to say 98% modified proctor?

 

[Ron]

CBR of about 20

 

[Okiryu]

Ron, that means that if you have clays or silts for the subgrade you normally stabilized them? I have not seen clays or silts with CBR 20 so that is my assumption...

 

[Ron]

Yes. Typically with coarse, granular materials such as a percentage of crusher run rock or similar.

 

[Okiryu]

Thanks Ron.

 

[BigH]

Ron's the man! You might wish to look at Cedergren's book on drainage and flow nets for his chapter on roadways.

 

[Okiryu]

BigH, you reminded me that I do have the Cedergren's book available in my bookshelf (actually I bought it because you recommended it to me)...

 

[Ron]

Okiryu....I did the same on Cedergren's book. BigH is a guru on geotechnical and geotechnical construction!!