Stiff Clay Subgrade Preparation

[roscoe]

We are constructing an AC Pavement sport court (2" AC with 4" base rock). The subgrade soil is stiff, thick adobe clay. The spec calls for scarifying 8 inches of the subgrade, and conditioning to a moisture content that will allow for compaction to the specified (90% MDD).

Based on my experience, it is better to not scarify the clay layer, but to make subgrade, then quickly add the aggregate base before the clay dries out, as to lock in the moisture. Does anyone have any comments on a procedure of dealing with this kind of subgrade. Also, what is the best test to do in the field to verify that subgrade has adequate moisture/compaction?

 

[pigdog]

Roscoe,
First thing is to find out if it is a swelling/shrinking clay. If so, you want to protect it from any change in the moisture content after it is built. If it is not a swelling clay, and it is residual soil, it is probably near the optimum density. A procter test would tell you. If it is an uncompacted clayey sediment, you would still need the procter test but the specified procedure would probably be appropriate.

roger

 

[MRM]

Roscoe,
I agree with pigdog, shrinking and swelling is an important issue especially since it is critical that the AC pavement remains servicable. If the clay has a lot of shrink/swell potential, the moisture content in the clay should be maintained after construction. Also, undercutting some of the clay and filling with a non-shrink/swell material such as a sand fill may help reduce the effect of some shrink/swell if the moisture content does change beneath the pavement.
Mark

 

[Jim6758]

If it is a stiff clay, it is likely expansive. Unless you can keep the water out, it will shrink and swell and that will reflect into the asphalt pavement. You would do well to lime stabilize the material before paving. Since an aggregate base is a prime path for water movement, you might consider a full depth HMAC pavement rather than asphalt over an aggregate base.

 

[Hambley]

The comments about shrink/swell potential are quite valid. However, there is one question that has NOT been asked, namely, where is this facility being constructed? The reason this becomes important is that in arid parts of the country, the shrink/swell potential of clays may be irrelevant. The same comment applies to sensitive shales.

It is always good practice to replace the top 2-3 feet of clay subbase with sand. Nuclear density in conjunction with a proctor curve is the best means of confirming moisture content and compaction adequacy.

 

[reddog99o]

Right on Hambley. Excellent points and insight, as are the others. R.A. Hassett, P.E.
rah1616@hotmail.com

 

[roscoe]

Thanks for your comments.

Yes, this clay is very expansive. It is adobe clay which is what is typically found in the San Francisco Bay Area. The project site is at an elevation of 8 feet above mean see level. Thanks again!