Stiff Clay Subgrade Preparation
Stiff Clay Subgrade Preparation
(OP)
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?
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?





RE: Stiff Clay Subgrade Preparation
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
RE: Stiff Clay Subgrade Preparation
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
RE: Stiff Clay Subgrade Preparation
RE: Stiff Clay Subgrade Preparation
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.
RE: Stiff Clay Subgrade Preparation
R.A. Hassett, P.E.
rah1616@hotmail.com
RE: Stiff Clay Subgrade Preparation
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!