Does 6" asphalt provide a "constrained" foundation? 2

[Evan5]

I'm looking at installing a large column foundation in a paved area and I'm referring to the CBC section 1805.7.2.2 where it states:
"The following formula shall be used to determine the depth of (foundation) embedment required to resist lateral loads where constriant is provided at the ground surface, such as a rigid floor or pavement."

Is asphalt rigid?
If so....how thin can it get before it's not considered rigid?

Thanks

 

[jike]

Asphalt is called "flexible" pavement for a reason. I would not use it to constrain any foundation.

 

[LobstaEata]

I must agree with Jike. Although bituminous pavement will offer initial restraint (below 70 F) under dynamic conditions, having good shock absorbing characteristics, it will most certainly yield under steady loads over a short time.

To estimate foundation depth you might try starting with a depth of rotation by considering about 2 feet of soil inactive from top of grade and using the Ruttledge Method to estimate foundation depth. Otherwise there are several (some pricey) software programs out there if you have an idea of lateral subgrade modulus. Some consider LPILE the gold standard.

Hope this helps.

 

[vmirat]

The lateral bearing formulas are designed to account for a certain limited amount of lateral deflection (I think it is 1/2"). Since asphalt can flow under stress (i.e. rutting and shoving), it would not be a good candidate for lateral resistance wheras concrete would. I have used asphalt as a restraint for such things as light pole foundations since their lateral force isn't that great and the lateral displacement isn't as critical as that for a building. Although I haven't look at this closely, one way of limiting the use of asphalt might be to use the stability and flow data of the asphalt as a limiting factor. For example, a mix design for asphalt may result in a stability of 1200 pounds with a flow of 0.08 inches. If you're dealing with existing pavement and you don't have the mix design, then you could assume minimum values for the type of use (i.e. 750 pounds, .18 inches, for a parking lot). These values are based on the Marshall mix design method which uses a 4" diameter by 2-1/2" high test specimen. If you use the cross sectional area of the test specimen, then the unit pressure would be 75 psi (750#/10s.i.). If you have a 24 inch pier against 3 inches of asphalt, that would equate to 72 square inches of bearing area and result in a load capacity of 5400 pounds.