Adfreeze / frost heave on caissons supporting solar canopy structure

[rharashi]

I have a project in Connecticut for the foundation design on a solar parking lot canopy. I have soils values from a geotech report. I also have reactions from the steel manufacturer. The soils report calls for adfreeze behavior to a depth of 42" and gives values from the Canadian Foundation Engineering Manual. The value on concrete is 65 kPa or 1300 psf.

Being a canopy, there is also a significant wind uplift load on the footings. Where do I find an appropriate ASD load combination that takes into account resisting dead load, wind load and adfreeze?

 

[BridgeSmith]

Not sure about the loading combination, but accounting for frost heave is fairly straightforward - neglect the lateral resistance of the soil above the frost depth, i.e. set the phi angle (or cohesion) and soil modulus to very small values (or zero, if the analysis method/software allows). Some designers just ignore the soil completely (set the ground elevation equal to the frost depth), which is conservative since it ignores the contribution of the weight of that layer to the confining effect it provides to the soil layers below.

 

[rharashi]

I'm using AllPile to analyze it. You're saying that for the first 42" to set the phi angle to zero, that way the caisson isn't relying on that layer for uplift resistance too? Thanks.

 

[BridgeSmith]

I wouldn't rely on soil subject to frost heave for uplift resistance. Frost-susceptible soils will become loose and lose interlock when they freeze and then thaw. I don't know enough to put any kind of a percentage on what the loss in friction would be, so I just assume it's a 100% loss.

Honestly, the cost difference of drilling an extra 3 or 4 feet of shaft is minor. The mobilization and setups costs are the major items; the little bit of extra time, concrete, and rebar to go deeper once the hole is started, is a small fraction of the total cost, especially for a small number of shafts.

 

[oldestguy]

The value you get from he Canadian Manual probably is rough number, but at least something. For example the "piles" used for anchorage of airplanes sitting on pavements at air bases in Greenland during WWII were later found to have been completely jacked out of the ground with annual freezing and heaving. You can't put zero on that number fir uplift. Did the soil report give any indication of the frost heave potential might be? For instance so it may freeze to 42", but if the ground has little heave potential (low silt content), there may be no heaving. This is a separate issue than uplift resistance.

 

[rharashi]

The geotech report specifically gave the numbers from the values from the Canadian Foundation Engineering Manual and called out the depth as 42". So I'm ok with how to determine that number. Basically, 30" diameter caissons, multiply the 1300 psf by the circumference by the 42" depth. That yields a really big number: 35.7 kips.

The dead load per column is only 9.6 kips and I have to deal with a wind uplift of -74.7 kips (unfactored).

I'm mainly wondering do I need to combine that full heave value with the wind uplift? Without heave, the controlling uplift combination in ASD would be 0.6D + 0.6W. Would it be appropriate to add the full heave value into this equation? That seems like a really gigantic caisson.

Also, I have a previous design by another engineer for solar canopies on the same property. On this older design, the tributary canopy area to each column is around 1200 sf (on mine, that number is closer to 1500 sf). The design for the caissons on that one are 36" diameter x 10'9" deep. If I understand correctly, the heave force on this design would b 42.9 kips. Assuming similar wind uplift conditions, the wind uplift force would be around 60 kips. How does this caisson resist that combination of forces?

 

[aeoliantexan]

One approach would be to extend the drilled shafts deep enough to resist the uplift in tension using skin friction below the frost zone. This is commonly done to resist heave from swelling soils. How to combine the loads - I'm not sure, but conservatively.

Spread footings should be feasible, but to resist the wind uplift, they need to be about 13 feet square unless you place them deeper than frost depth. frost uplift on the column stub can be ignored, as the soil is pushing down on the footing while it pushes up on the stub.

Belled piers may be an option.

How about helical piles? A small shaft means small adhesion force.

I would be conservative about frost depth. Unheated structure plus shade plus snow removal equals extreme conditions.

 

[BridgeSmith]

You could use a sonotube to form it as a column to below frost depth. That would eliminate most of the uplift for frost heave, as the soil wouldn't be able to get much of a 'grip' on the the smooth concrete surface.

 

[oldestguy]

First off, what is the gradation of the soil? If the P-200 fraction is low, say 10%, there will be minimal, if any frost heave. If severe (high silt content), you might want to insulate the ground with sheets of buried Styrofoam extending 3 feet out from each post (at one foot depth). Then no heave. Or replace the soil to 4 ft. depth with clean gravel or sand.