Bearing Capacity/Pressure of Existing Asphalt 1

[MMLJ]

What is a reasonable estimated bearing capacity/pressure of existing asphalt?

I must design the foundation for construction trailers that are being considered as a permanent commercial building that must be submitted for state approval.

The client wants to use 20-30" diameter x 8" thick concrete pole pads (cookies) for footings bearing on existing asphalt. I realize the bearing pressure cannot exceed the sub-grade soil capacity. I also realize i must use reasonable judgement regarding the condition of the existing asphalt.

Thank you!

 

[Ron]

This is not a "bearing capacity" issue of the asphalt. It is similar to a pavement design issue for wheel loads, except that you have VERY light loading.

A construction trailer has to be tied down to keep it from blowing away....its gravity load is small, both dead and live.

Since you have small pads to distribute the load, the contact stress on the asphalt is going to be very low. Compare that to a tire load for a typical truck that might be used to deliver these trailers and you'll see that the contact stress for the tire might be on the order of 100 psi. Assuming you have at least 4 of the "cookies", with a combined area of 1256 sq. in., your trailer could weigh up to 125,000 lbs. and not exceed the equivalent contact stress.

In short....that asphalt pavement will likely carry a lot more load than you're going to put on it with those trailers.

 

[MMLJ]

I DO have a bearing capacity issue on asphalt. The state plan reviewer is requiring proof or reasonable evidence for the 3,000 psf assumed allowable bearing capacity on the existing asphalt that i used in my footing calculations.

I have the tie down issue taken care of with acceptable, data-provided, minute-man anchors.

The client is constructing combined trailer units with point loads of up to 26 kips.

Ron, I appreciate your time and input, but, the state inspector will not accept the reasoning that the asphalt pavement will "likely" carry...

Therefore, my questions remains:

What is a reasonable estimated bearing capacity/pressure of existing asphalt?

Thank you!

 

[cvg]

there is not a reasonable answer to your question. get the boring logs and the as-builts and calculate it. the question cannot be answered without a lot more information. Or do as Ron suggested, check to be sure your pavement is designed for HS20 loading (or some other loading) and show that your trailer cookie loads are less.

 

[TpaRAF]

If this is for building design, the lateral wind loading forces tend to be much larger than the gravity loads. You'll get quite an uplift load on the windward size (and slack cables on the leeward side).

Regarding the bearing capacity, the prepared wearing + base courses should be much denser than plain ol' dirt. What are the typical soil bearing pressures for your region? The pavement should be at least that strong. Here in Florida, the soil allowable bearing capacity is usually 1 - 2 ksf (sandy soils). I'd raise questions about a 3ksf capacity -- here. Your region may quite be different.

As CVG suggested, review the geotechnical data, or consult with a geotech engineer on this. Often, they can provide testing for the _in-situ_ capacity.

Best regards, RAF

 

[BigH]

The bearing capacity - as Ron says, is not simply an "asphal" issue - the width of the pad will have a zone of stress influence that extends down below your thin asphalt layer. You will need to address, from a bearing capacity point of view the interaction of the footing on the aspalt, the base course, subbase and subgrade. How to do this - it is similar to that of a wheel on the asphalt surface. Further, it is very likely (almost assuredly so) that bearing capacity is not the factor - but the allowable bearing pressure - you will not have a capacity (shear) issue but a serviceability issue - and, if the pavement was designed, that would not really be an issue either but you can at least compute an expected settlement. you can transform approximate modulus values to a single layer so you don't have to do layered theory. Use 1" asphalt = 2" crushed base course = 3" subbase. (This latter discussion only if the state guy wants a number - likely he does).

 

[Ron]

Your state inspector doesn't understand what he's requiring. If you must "prove" a bearing capacity, then do a plate load test on the asphalt surface.

I understand that such inspectors don't necessarily apply reason to their requirements or interpretation. Your 3000 psf is achievable even in clean sands. The pavement layer just serves to buffer the loading on the soil even more.

 

[apsix]

Asphalt is subject to creep, especially in warmer climates.
I don't know how you can quantify that.

 

[Ron]

Creep is related to its "flow" under load when using the Marshall method of mix design testing. The higher the "flow", the high the tendency to creep under load when exposed to elevated temperatures. It also has to do with the stability of the mix.

Weathered asphalt has a much lower tendency to creep than newer asphalt.