Adfreeze on piles supporting concrete footing

[J1D]

It seems clear to me that the upward heaving force develops in two ways: 1 frost heave pressure on underside surface of foundation, 2 upward adfreeze force on the side surfaces of foundation AND on the pile perimeter.

But when some engineers suggested that the afreeze on pile can be ignored when the frost heave pressure develps on the underside of foundation, I found I don't have strong geotechnical explanation to him.

Can somebody shed some light on this? Thanks,

j1d

 

[ishvaaag]

Movement due to volumetric growth due to freeze won't be uniform in the ground halfspace under/near some building and its foundation. So for a start those saying that there is such case where all freeze effects "develop in the underside of the foundation" they are of course making one oversimplification. Consider, for example the case of a somewhat extense mat at which freeze effects are most likely to develop "from the underside" but clearly more pronunciated near the edges that under the center of the building. Considering heave uniform may lead to inadequately forfeit some effects. So this shows that horizontal variation has a say.

They may be thinking in cases where the heave is surmised to be quite uniform, like when the ground take the piles along for an unheated foundation and moves everything up, piles, foundations and house. In these cases if the overall movement is uniform it may not turn very relevant to distinguish what part is coming from bottom and what from the sides, you would be thinking more than anything on the strong points' movement.

Yet you are right in that an incrusted item such a pile not directly able to take the same deformation than the surrounding soil will sustain mechanical efforts that in some cases may need to be characterized; and there's no other way without experience to say how much than trhough evaluation. So if the surrounding soil expands piles will be compressed concentrically towards their axes and will be subject to tension induced through friction along their axes. Friction may be overcome or even lost upon repeated cycles etc so in all there is a subject of interest in your question.

 

[J1D]

The piles are designed to be deep enough to resist all the upward heaving force. So the piles with foundation won't move.
Regards,
j1d

 

[ishvaaag]

Say then that the bottom part of your piles is anchoring the upper part and above. The frozen part will experience volume growth, exerting pressure in the base of the foundation and also will be transmitting (as well) tensile forces axially through friction to the pile; the friction will be even more efficient since freezing the piles compresses the soil towards the piles. The composed effect of both kind of forces in the pile will be to bring the pile to a tensile (or diminished compression) status compatible with the stiffness presented by the pile: stiffer piles in concurrence with full top restraint won't allow as much heave displacement component as less stiff piles; in fact, respect heave, you are using your piles as anchored springs disposed to restraint heave. Depend on how you determine the effects on the pile, say, you may adjudicate the whole tensile stress of the heave to a mechanism in which the pile takes without friction the tension, or one that acknowledges the shaft friction. The first is the simplification of your colleagues and the second your more precise insight. In your view, the tensile forces exerted atop, grow to the total anchor force for heave at bottom as depht progresses, friction adding to tension. And in every case the integrated elongation must equal the vertical movement.

 

[J1D]

I see your good points.

There is a 12" thick voidfoam under the pilecap (i.e. foundation). The design soil heave under the pilecap is 4", this is also the compression amount of the voidfoam. My point (to support the adfreeze on pile still existing) is the soil is not restrained by the pilecap rigidly. The soil under pilecap will move upward gradually up to 4". This still allows the adfreeze/friction on the piles to happen.

 

[howardoark]

"the friction will be even more efficient since freezing the piles compresses the soil towards the piles."

Why is that? Isn't the pile also shrinking?

 

[ishvaaag]

There must be something real in what I say at least in the sense of that ice causes ice push in structures trying to lock the expansion. Since the piles are locked both up in the the foundation and bottom in restraining soil, push effects against the piles -hence friction- develop; and ice push can attain significant values.

There is also the issue of phase change in the water; the significant volume change of the water when passing to ice will be linearly something like 1.04^(1/3)=1.013000 m/m on a number of meters whereas say a 30ºC descent of temperature can bring on say a 30x0.000011=0.000330 m/m change on the size of the radius of the pile, this meaning that the free displacement towards a void of the frozen ground is 2 or 3 orders of magnitude greater than the ability of the concrete to shorten under the temperature descent, hence leading to a compression field on the perimeter of the piles.

 

[howardoark]

Sorry, I wasn't clear. I meant radial shrinkage of the pile in response to your statement that the frictional force on the pile increased as the temperature dropped.