Questions about frost heave ?

[James]

This is one subject that I find impossible to understand. Although the theory about having a firm foundation to a point just below the frost line is clear, everywhere I look I see construction of steps and sidewalks that I know is not built on a foundation or other support rested at a point below the local frost line.

I live in an area where the frost line is commonly known as two feet, but has been known to go down to three feet.

My first case in point is sidewalks and walkways. I see many sidewalks made either of concrete, flagstone, etc that I know for sure have not been built by first digging two or three down to get below the frost line. I see walkways made of flagstone placed in mortor, where I know for sure that the entire structure of the walkway is no more than five or six inches thick. How do they avoid frost heave ?

I would like to build some steps up a fairly steep slope. This would be about 14 steps over a span of about 25 feet (I am just estimating here). I would like to have flagstone steps, set in mortor. If I follow what I have read in prior posts here, I would have to build 14 separate footings, all 2 or 3 feet deep , upon which to place each steps. That would be a

**major** project !!

At one time, I thought that the frost heave problem was a consideration in structures that would carry a lot of weight, such as the foundation for a home (traditional footings) or a deck post to support a deck. But then I read that even steps, sidewalks, and walkways must also be built so as to avoid frost heave.

I must be missing something here, or somehow totally misunderstanding of how to deal with frost heave in constuction projects such as these discussed.

Can someone please clear this up for me ???

Thanks !!

James

[aemeijers]

The key with making flatwork like sidewalks and such survive frost heave, is good substrate and drainage. Well tamped dirt and good layer of gravel or whatever. Any water that runs under the concrete needs a place to go before it freezes. On something large, like a patio that is trapped up against a house foundation, you do sort of need footers. Heaved and cracked patios are dirt-common up north here, and often contribute to basement flooding. (If the concrete is thick enough and well reinforced, it tilts as a unit, raising up on the side away from the warm foundation, and you get a pond against the foundation.)

You can tell when a non-pro laid a sidewalk- squares lift and settle with the seasons, even if no tree roots are nearby. I've seen sidewalks less than 10 years old self-destruct, and I've also seen sidewalks 100 years old still in fine shape, other than the pitting from the salt splash from the street. Experience and not cutting corners makes the difference.

(Disclaimer- I ain't an expert, so this is one of the categories where I am willing to pay somebody who is....)

[Reno]

James;

Frost heave occurs only when there is a change in volume of the underlying soil when it freezes. As dry soil doesn't expand or contract when it freezes the problem occurs in the presence of water. If you can keep the supporting soil dry it won't heave in the winter. Thus you can prevent frost heave by constructing a good under-drainage system for your steps. Use a deep gravel base, 6 to 12 inches deep, to allow water on the slope to drain downwards out of the soil. Also use a good method to drain water out of the soil at the bottom of the steps. Since your frost penetration goes down 2 ft you need to keep the top 2 ft of soil dry so it won't heave.

Frost penetrates a lot deeper in dry soil than in wet soil. Thus it can become a vicious circle - the dryer you make it the deeper the frost goes, the deeper you have to keep it dry, etc. You can address this by good surface drainage. If water doesn't sit around near the steps the underlying soils will be dry and won't heave. The shallow gravel addresses the short term rainfalls and snowmelts and the surface drainage addresses the long- term wetness of the area and thus the deeper soils.

[James]

Good explanation Reno, many thanks !!

James

[hallerb]

well if a walk heaves its not a biggie repair or replace failrly cheap.

but imagine a homes structure heaving due to lack of footer.

could cost more than the price to build the home originally.

[lightingguru]

They key to any kind of concrete work is all in the prep work. You will want good drainage as well as a good base of gravel around 3 to 4 inches minumum. Also if you are woried about cracking then use rebar throught out your pad this will help minimize cracking. Because the only thing you can guarantee about concrete is that it is going to crack. Also put plent of joint in the concrete to make it crack where you want it to

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