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Re: Forces due to Frost Effects

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Bhavin Shaw,

The physics of ice is a complicated science and many people have spent their lifetimes trying to sort it out. What I am going to tell you below should be considered as a very shallow introduction of that part dealing with the action of ice on civil engineering structures.

The following explanation may be a bit lengthy for those not interested in the subject.

1. As water cools it reaches a maximum density at 4 degrees C. )or about 40 degrees F.). Although this fact is vitally important to virtually all life forms on the planet it doesn't seem to be particularly important to the reaction with engineered structures.

2. The formation of ice, which is a phase change, occurs at 4 degrees C. (32 degrees F.). This phase change results in a virtually instantaneous volume increase (density decrease) of about 10%. This volume increase causes most of the problems civil engineers experience.

3. The formation of ice crystals results in the exclusion of other materials to the extent that freezing can even be used for the removal of dissolved salt in sea water. This is important in understanding the formation of ice lenses (hence pot holes and other problems for earth structures).

4. Ice near the freezing point, say between 0 and -1 or -2 degrees C. (32 and 30 or 28 degrees F.) has very little shear strength, hence, it can virtually flow like a liquid. This is important to the protection of concrete against freeze-thaw damage by using air entrainment; the "ice" can flow, or push water ahead of itself, into the air voids thus avoiding damage to the concrete structure. By the same mechanism it also helps to reduce the volume change of freezing soil in some cases.

5. The formation of ice lenzes which you refer to results from the freezing action excluding earth and other materials from the ice formation; and a source of water which can migrate, usually upwards, through the soil. Water can not migrate upward through granular material (gravel) because the voids are too large to support this migration; it also can not migrate through clay because the clay is generally impermeable. In fact, it is only silt type soil which is susceptible to this problem. The problem can be eliminated by either of two solutions: get rid of the water by draining the site; or replace the silt with a more suitable material.

Most structural damage due to frost is caused by non uniform lifting of the structure foundation due to the formation of ice in critical locations. In Canada we prevent this problem by setting our foundations below the deepest seasonal frost penetration depth. Mind you, this is not necessary if there is no water to freeze such as well draining granular soil or rock which contains no water. It's also not necessary to do this if you have a mat type foundation which is strong enough and rigid enough to resist differential soil movement (although the building officials may give you a hard time with this); but the structure will move up and down so anything connected to it must be able to accommodate these movements. Forces from ice formation can act in any direction. Retaining walls are subject to frost action just as building footings are.

"Addfreeze" is basically a problem for piles. As the ground freezes starting at the surface it bonds to the pile. As the freezing progresses the surface is lifted; this produces a shear force (skin friction) on the side of the pile generally in the upward direction. When the ground thaws out the process does not reverse, hence, piles or posts can be "jacked up" to the extent that they even come out of the ground. I have not witnessed this extreme myself but I have seen as much as two inches (50 mm) for sundeck posts with six feet embedment. Ice undergoes creep or stress relaxation at a very high rate, therefore, the effective uplift stresses are not as high as they might be; it's somewhat less than double the skin friction values for unfrozen soil. There are two methods of avoidance. In Canada we set our piles a minimum of three times the depth of frost penetration; for six feet of frost penetration we would use piles with a minimum of eighteen feet embedment. Alternatively we can make sure that the static minimum dead load is greater than the "addfreeze" uplift. Note: for dynamic loads like wind or earthquake it is not necessary to add the frost loading to the dynamic uplift; the soil will resist dynamic uplift even better when it is frozen. "Addfreeze" would be additive with continuous uplift say from anchors.

I can not visualize a situation where addfreeze and soil expansion would act together except, possibly, a retaining wall where the vertical action of addfreeze might combine with the horizontal action of soil expansion. In any case the two would never counteract one another.

       I hope this is helpful

Regards,

H. Daryl Richardson

----- Original Message ----- From: "Bhavin Shah" <bhavin.design(--nospam--at)gmail.com>
To: <seaint(--nospam--at)seaint.org>
Sent: Monday, October 17, 2005 7:51 AM
Subject: Forces due to Frost Effects


Dear All,

Sub : Forces due to Frost Effects

There will be two type of forces due to frost effects as per my understanding :

1) Heave Force :

In soil (finer particles) containing water, due to frost effects water
from the unfrozen zone will be attracted towards frozen zone. This
will cause uplift force on the structure. This force will be acting on
the bottom (horizontal) surface of the foundation. Please discuss.

2) Addfreeze force :

Due to frost effects water containing in the soil particles will be
frozen. Force generated due to this effect is known as Adfreeze force.
The adfreeze force will act on the vertical surfaces of the foundation
and in vertical direction.

Please discuss whether Heave force and Addfreeze force will be acting
in the same (vertical) direction or opposite to each other.

Regards,

Bhavin Shah

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