Heave Uplift force 1

[DaveHolder]

Hi Guys,


I am just not sure how would I work out the uplift force induced by the heave? I have ran a calculation check to find out the minimum foundation depth required when it is close to the trees.


Based on my calcs the minimum foundation depth required is 2m. Water level was 1m below the ground.


Thanks

 

[r13]

How deep is the frost depth? You shall place your foundation below the frost line, or improve the soil to eliminate the root problem.

 

[DaveHolder]

frost depth 0.9m and 2m minimum foundation depth. my question is how to work out the uplift force?

 

[KootK]

In the absence of an interstitial element with serious give, like frost cushion, I've always treated frost heave as an applied movement to the underside of foundation elements and not attempted to restrain it structurally. I would expect the restraining forces to be astronomical for conventionally stiff structural elements.

Or are you talking about the drag imposed on the sides of your foundation elements by the adjacent soil heaving beside them and dragging them upwards? If it's this, I've typically gotten these values from project geotechnical reports and would be quite unsure of how to calculate them myself. If you don't get the response that you see here, you might try moving your thread to the geotechnical forum.

 

[r13]

If the base of your foundation is below frost, where is heaving in your mind? The freeze and thaw will happen above your footing only. You need to carefully select a suitable backfill to avoid upward mitigation of the ground water. Thermal protection may be required on inside face of the foundation (for turndowns or grade beams), and the bottom face of SOG around perimeter of the building.

 

[dik]

Freezing water can expand and produce pressures in excess of 1000 psi...

Dik

 

[r13]

dik,

That's very true. But wasn't that's part of the reason the bottom of foundation must placed below frost, so no freezing will occur? Insulation form should be installed beneath the upper structures to prevent freezing of the underlaying soil/ground water.

 

[1503-44]

Stopping frost heave is a fools errand worse than shoveling back ocean waves. Bulk modulus of ice is 300,000 psi, so in restraining its expansion, pressures could reach 43,000,000 lbs/ft2. Obviously whatever you put in the way will move or be absolutely crushed. So to answer your question, the pressure generated by expanding ice under your foundation will be the weight of the foundation and all loads it supports, up to a possile maximum of 43,000,000 psf.

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"

 

[phamENG]

ax1e - I take your point and agree that attempting to restrain it is essentially futile, but your figures seem a little extreme. To mobilize the entire bulk modulus doesn't the ice need to be under uniform compression from all sides? So really, the ice can only exert as much pressure on the building above as the soil is able to support from below. And, if the ice is not confined on the sides, it's even less as the water will expand out to areas of lower pressure as it freezes (to a point). Right? I'm not an ice expert - we get about 1/2" of snow here every 2 years, and I grew up on the beach in Florida.

 

[1503-44]

As I said, that is a maximum pressure. Naturally, and as you say, to develope that pressure, it must have the same confining pressure on all sides of the "ice cube". Yes it is extreme in that sense, because that pressure is effectively never reached, because what can resist that much pressure against the ice without giving way, or being crushed itself? That said, if you consider an infinite ice field in width and depth, each adjacent cubic differential element of ice within the field is capable of generating that immense confining pressure on the next adjacent element as each freeze and attempt to expand to 1.09% of their original liquid volume. In that case the only direction to which the elements can expand is upwards, which is why foundations experience an upward heave. In the case of ice forming above the soil, the ice will still expand upwards, because there is ice forming simultaneouosly on all lateral sides supplying equal pressure and the soil pressure below the ice layer is usually capable of generating more resisting pressure than the atmospheric pressure above, so up she goes, at least until the weight of accumulated ice layer above the soil starts to depress the other soil layers below, as in the case of many areas in Greenland and Antarctica, where the soil layer's surface has been depressed for perhaps several thousand feet in some cases. In the case of a retaining wall with water being frozen behind it, the wall will most likely buldge outwards as the ice expands in the horizontal plane as well as the vertical, simply because the lateral confining soil pressure on the interior is greater than the atmospheric pressure on the exterior of the retaining wall.

So, yes you are correct, an equal confining pressure is required on all sides to get to 43,000,000 psf, but remember just about nothing short of a black hole (stellar version) can provide the kind of pressure needed to resist the expansion of ice, so such pressure will probably never be reached in practical situations. That's why a freezing engine block always splits. None the less ice is capable of generating such pressures if it needs to move any adjacent element, or a black hole. If the object moves, maybe only the friction force needed to move it was generated in the process.

This is becoming a huge problem for Russian, North Slope and other arctic pipelines with the onset of climate change. The permafrost in which they were installed never melted, so never heaved. Now with the warmer temperatures, the permafrost is no longer permanently frozen. The pipelines have become exposed to the freeeze, thaw cycle and many pipelines in Russia have popped up above the original surface level. Some have burst, some have not... but all have moved with the expanding ice.

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"

 

[phamENG]

Thanks, for the detailed explanation. I've read a few articles on the not-so-permafrost problem as it relates to building foundations, but hadn't considered the impact on pipelines (nobody thinks of the buried infrastructure, after all).

 

[1503-44]

Yeah. "Out of sight, out of mind" ... as they say. We like it that way. Those "pop-ups" are very annoying.



“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"

 

[r13]

Ice expansion is very important consideration for structures in the water ways. It is quite difficult to overcome in cold regions, and may require thermo-mechanical device to prevent ice from forming near the rigid barriers to eliminate the risk. On land, place foundation below frost is the typical practice and required by codes. If it is not economically practical, use deep foundations with adequate consideration given to against heaving.

 

[1503-44]

One might also try to keep water from entering the area below foundations that might be subject to freezing by placing water dam around the outside and using an insulating material layer in the foundation construction thereby keeping the heat from the building above from thawing permafrost below.

North Slope and the TransAlaska pipelines were not placed in permafrost. Where permafrost was encountered, the pipelines were elevated on insulated pipe supports so as to avoid any melting of the permafrost by the warmer oil within. In Russia, many pipelines were simply placed directly in permafrost ground and immediately started melting it, a problem that has only gotten much worse with time.

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"

 

[1503-44]

Tank failure in Russia linked to thawing permafrost.

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"

 

[haynewp]

I'm not sure what the situation is but I think figuring out the deflection that would result from the heave and designing for it would be better than trying to come up with a force. It doesn't seem that much different than designing a reinforced slab from movement from a heaving clay. But I have never considered this with ice and have no idea how to calculate the movement.

https://www.wbdg.org/FFC/ARMYCOE/COETM/ARCHIVES/tm_5_818_7.pdf

 

[1503-44]

Ice expands to 1.09 of its equivalent mass of liquid water, so a frozen, laterally restrained column of ice would be 1.09 x as tall. If we were talking about freezing a semisaturated column of soil, I would assume that most all of any void space in column would be compressed to nothing first, then the froeze zone would continue to expand until the volume of frozen water in the zone reached 1.09 x its original volume.

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"