Modeling soil temperature at a depth of 5ft

[Jyan]

Hello everyone,

This isn't really my area of expertise, so I am looking for a bit of guidance. I wish to estimate the temperature underground at a depth of 5ft. The climate is Saskatchewan (Canada) so it is covered with snow in the winter, and fairly hot in the summer. There is no tree coverage, only grass. The widest average temperature swings are from about -28C to +32C.

Currently, I'm attempting to apply the model described here: http://archive.nrc-cnrc.gc.ca/eng/ibp/irc/cbd/building-digest-180.html#archived

My results however are pretty questionable. I am using an average yearly temperature of 2.6C, and a thermal diffusivity of between .8 and .15. I've calculated that the temperature swings from -35C in the winter, to +42C in the summer, obviously this doesn't make a lot of sense. Can anyone point me in the right direction for estimating underground temperatures? The model doesn't need to be accurate either, +/- 10 degrees is pretty acceptable.

Thank you,

 

[mfb]

I've calculated that the temperature swings from -35C in the winter, to +42C in the summer, obviously this doesn't make a lot of sense.

Looks like a wrong sign in the exponential function.

 

[Jyan]

I think the problem is with the value "A". " A is the difference between the maximum and minimum temperatures for the period", shouldn't it be the difference between the max/min temp and the average (The amplitude)? I used 60 for A, but I think it should be 30 instead. 2.6C average, with a 32.6C max and 27.4C min. This gives me a temperature swing 5ft below ground of about +23C and -17C. Do these values seem reasonable? What about the snow coverage? Snow is an excellent insulator.

 

[AlephZero]

I think the problem is with the value "A". " A is the difference between the maximum and minimum temperatures for the period", shouldn't it be the difference between the max/min temp and the average (The amplitude)?

I agree with you. The web page description is wrong.

Do these values seem reasonable?

Check that you can reproduce the numbers in Table 1.

What about the snow coverage? Snow is an excellent insulator.

So is earth. Since the web page doesn't mention snow, maybe it doesn't make much difference. But I'm not an expert on soil mechanics or civil engineering.

 

[256bits]

Insulating snow is mentioned directly under Table I, and the effect upon the average soil temperature being elevated from that of a surface with no surface snow.