Question
We're trying to put in a Virginia native garden in the city of Charlottesville over a plot where an asphalt parking lot was and have discovered a drainage issue about eight inches down -- a layer that still has stone in it from the substructure. It won't drain at all over three days and we're not sure if that is the sub-base of the old asphalt (all asphalt removed) parking lot or whether the soil below can be that compact?

Driving a steel rod to the depth of three feet did not appear to help much. The soil itself has very little sand in it.

Is there a depth suggested for digging to get below old parking lot compaction? Can drainage at the layer where it's puddling be instituted by, for example, a series of post holes dug down to say four or five feet?

Thank you for whatever help you can locate.


Answer
Hi Randy, I'd rent a power auger and bore down 4 feet to get an accurate vies of the soil layers.  I suspect you will find a layer 6-8" deep of gravel base from the parking lot.  If that's true, then use the auger and bore holes through the gravel every 4' or so and fill with washed coarse gravel to allow water to get below the base.
I'd add another 6-8" of soil/organic matter over the entire area and till it as deep as possible.  Your rootzone needs to be deeper than 8 inches.
You'll also need to monitor your pH.  The limestone base may raise your pH to a point where your nutrients become unavailable to the plants.   Jim