Proper Subgrade Prep for Asphalt Parking Pad

[Signious]

Hello!

I have a quick question regarding standard subgrade prep work for a run of the mill asphalt parking pad. Rough backstory, a family member is running a commercial reno project & is installing a new parking surface over what was previously greenspace. I do not have a stamp in his jurisdiction.

The soils report indicates:
-No Organics
-Slightly plastic soils (~12 P.I.) @ 2' depth extending roughly 8"
-7'-0" Frost depth at an unheated space

The structural firm is recommending removing soils to a depth of 4'-0" and backfill with clean rock for the full depth.

To me, this seems absolutely ridiculous for a parking pad, I just want to make sure I am not missing something silly before I call up the designer and have a discussion. This is somewhat skewed as it is a design-build shop and I think they are just trying to run their bill up.

Edit. To me this seems like something that can be addressed with proper compaction of the site soils, a ~6" layer of washed rock, and properly designed drainage & grading on the pad.

 

[oldestguy]

Signius: ya got it right!!

 

[Ron]

Agree....stupid recommendation by structural engineer. Most often the pavement recommendations are from a geotechnical or civil engineer.

For light duty parking lot with no trucks or heavier traffic, then removed topsoil, excavate enough for competent base material, generally 6 to 8 inches of graded aggregate base, compact the subgrade to 98 percent of the modified Proctor maximum dry density, install base material compacted similarly, then add pavement surface. Essentially what you noted....

 

[ACtrafficengr]

That's a heck of a frost depth. If you have silty soils, it could be an issue. My guess is the structural firm's intent is to break the capillary action to prevent frost heave. Why don't you ask them?

I see a lot of rutting in parking stalls around here, and we only get 4' of frost. Spring thaw mushimess is a likely culprit. Can you daylight your subbase, use underdrain or otherwise give meltwater a path out?

 

[Signious]

When I talked to the stamp engineer he forwarded me to the geotech on the project, the reasoning given was as follows:

-Remove any potentially unwanted bearing material (there is a history of finding random old farm dumps in excavations in the area)
-Provide adequate frost coverage to prevent heave
-Ensure that no surcharge from the parking is seen by the building

This made me even more curious, because (as I understand it) frost will penetrate clean rock fairly well - it isn't insulation, This is exposing the existing grade beam system for the building to MORE freezing potential. Building is an old single story CMU school ontop of grade beam which has been micro piled 17 years ago.

The idea that adding a gravel will reduce any surcharge lateral loads also blows me away - higher friction angle =/= lower lateral loads seen by the grade beam.

As the city is not requiring an engineering stamp on the pavement design I think I will advise the family member to go to a different company that I know does good work. Hell - the cost of the excavation & rock fill would cost the same as doing the pad the right way twice anyways!

ACtrafficengr: Southern Saskatchewan, I have personally seen frost as deep as 6'-0" during excavation, haven't seen 7'-0" but I'd believe it in one of the bitter winters. There aren't many freeze-thaw cycles but it typically hangs around -25 deg.C for 5 months straight, with at least 3 weeks of -45 deg.C for a month totalled.

 

[ACtrafficengr]

It's a bit out of my field, but I used to work with a guy who researched spring thaw effects on pavement with a falling weight deflectometer. The problem isn't just freeze/thaw cycles. If you have a water table within capillary action range (up to 9m for silty soils), it will draw water up into your subgrade, where it freezes.

In spring, it melts from the top down and bottom up, so you have saturated soil that can't drain until the frozen ground below it completely thaws. That's why I mentioned daylighting the subbase for lateral drainage.