Frost line questions


Old 07-03-15, 02:50 PM
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Frost line questions

not sure where to post this question.

Does the "frost line" mean solid ground/dirt to the point of no freeze or does it include ground cover like mulch, landscaping, concrete etc?

also, does sandy soil freeze easier than say black dirt soil? Does the frost line change for sandy soil, clay etc?
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Old 07-03-15, 03:17 PM
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It's a very general guideline used mainly in construction to know how deep footers should be to avoid frost heave. But as you suspect different soils freeze at different rates and ground cover can affect freezing. Mulch, shrubbery and snow can act as an insulator and slow freezing which affects how deep the freeze goes before temps warm in spring.
Old 07-03-15, 05:44 PM
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I never included mulch in my frost line calculations. I don't even know why mulch exists. I think it was a worthless invention, except for the people who sell it.
Old 07-03-15, 06:11 PM
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The specific answer can depend upon why you ask. Freezing vs frost is the question. If you are running a pipe underground that will carry water you are more interested in the temperatures that can occur at some depth. If you are concerned about frost lift, then the soil types as well as moisture and temperature get involved.

My area will list a frost line anywhere from 4' to 6' depending upon who and where you ask. But even our 5' frost line has failed more frequently than the supposed 100 year event often used as a timeline. Those major failures often occur under paved roads where there is no snow or other cover above the ground line.

Old 07-03-15, 09:23 PM
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Building code frost depths are for the purposes of preventing damage to a structure in the 50 to 100 year range. Most structures (heated or not) protect the soil and emanate heat.

Unfortunately people equate this to many other situations other than structures.

The ground cover makes a dramatic difference. I was surveying on a construction site in northern MN. In late February that has already been through 4 months of frigid freezing weather and the previous month had lows of -5F to -30F, Is was setting a pin in the soil. There was about 18" of undisturbed snow and the ground was slightly frozen to a depth of 6" (yes - inches). I had hunted in that area in early November when it was -10F one morning.

The soil type also makes a difference Silty sands can be horrible. I have seen country gravel roads that heaved 5' to 10' in wet areas. In the spring they had "mud vacations" because the roads were impossible. - a period was even built into the school schedules as a lost week.

Old 07-09-15, 06:52 AM
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I ask because I dug down to my sewer pipe as it exits the house. Its only about 2.5-3' deep... I assume the freeze line is much deeper in Chicagoland.

Its been that way for 60 years though. however, I disrupted the soil to make a repair and I am nervous now ...

It exits the house directly next to a 3' concrete stoop/porch and I was wondering about the added ground cover question to find out if that stoop provides any additional insulation.
Old 07-09-15, 07:17 AM
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One thing to remember is the sewer pipe shouldn't ever have standing water in it, just water/sewage passing thru to the main sewer down the line. I wouldn't be overly concerned .... but then I don't live that far north so my thinking could be flawed
Old 07-09-15, 07:21 AM
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A sewer pipe isn't totally full with water the way a feed pipe is. AFAIK, that lessens the chances that it will freeze & break. As you said, it's been that way for 60 years. I wouldn't worry about it.

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