Pavement structure

[MIGH]

I am working on a small parking lot which happens to be located on hard rock subgrade.
The owner wants it asphalt paved. Should I place the asphalt right on the bedrock, or should I place a layer of granular base first?

 

[Ron]

Add a layer of granular base. This will give you greater design flexibility.

 

[MIGH]

That's right. Big part of the site will have to be filled for grading purposes. Where fill will be applied there is no question, we apply the client's standard of 6" granular.
The part I am concerned with, is pretty high relative to the other parts of the site, any inches added in that area make the grading even more difficult then it already is.
We are in an area with freezing temperatures with lows down -20 celsius. How would it effect the asphalt placed on top of bedrock?

 

[Ron]

The asphalt will freeze and debond, most likely.

 

[oldestguy]

But the rock won't heave. If the gravel is on the dirty side, it will heave. Spec. a gravel that your highway dept uses for high quality roads.

 

[TGLG]

Bear in mind that water will eventually get under the asphalt and down into your fill, and that water will need someplace to go. Otherwise it will pool and freeze, destroying the pavement above.

 

[Dozerman56]

Feel free to tell your client to pave on top of the bedrock. They'll be reworking it in 3-5 years. Put some gravel on top of the rock, if it causes grading issues, remove some rock.

 

[dik]

Best to have a 6" min buffer for flexibility, else the asphalt may crack and delaminate... Use a rock eraser if necessary.

Dik

 

[dicksewerrat]

Explosives remove rock. And it is fun.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[fel3]

You can also rip it. Not as much fun, but easier to deal with.

Years ago, we had a contractor use a D-10 to excavate about 25,000 CY of unfractured basalt for a 2.5 million gallon reclaimed water tank in Southern California. The magic was a single ripper tooth on a hydraulic ram mounted to the back of the D-10. The ripper tooth was about 6" wide and the "point" had probably a 1/2" radius. The D-10 did the ripping and a D-9 moved the ripped material.

It worked like this....Stop the D-10 at an appropriate location; lower the ripper tooth until it contacts the rock; keep pushing with the ram until the ripper tooth lifts the back of the D-10 about 12"-18" in the air; wait maybe a minute or so until the weight of the D-10 forces the tooth into the rock and the D-10 settles back down on its tracks; then forward with 750 hp (?) to rip a furrow. Turn around and line up about 3' from the previous furrow, rinse and repeat. Over and over again. IIRC, a fully loaded D-10 weighs somewhere in the neighborhood of 90 tons.

One day I visited the site to check on the project. All work was stopped and our inspector was sitting there bored. I asked him what the situation was. He said "I'll show you" and walked me to the back of the D-10. The hydraulic ram was attached to the back of the D-10 with a 1-1/2" thick steel plate. Two weeks worth of cyclical stressing had torn the steel like it was a phone book.

BTW, the contract was set up for blasting, and I think the estimate for everything was about $350,000. This contractor was the only one to propose ripping. His bid was about 25% below our estimate and the 2nd low was only about 5% below. When he was done, he told our project manager that even with leaving so much on the table, he had made more money than he had planned.

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"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

 

[BigH]

Given that this is a parking lot - presumably cars - a 6 inch layer of crushed stone (DOT Granular base) would be appropriate. if this were a road, however, the variation of elevations of the rock - even if "nearly" level - should be built out with several layers of subbase and base course, in my view. (vertical variations or rock surface). Each layer will usually build out about 1/2 of the previous layer variations. (experience speaking!)