Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Frostline

  • Please log in to reply
13 replies to this topic

#1 Joshuasnoesky

Joshuasnoesky

    Explorer 1

  • -----
  • topic starter
  • Posts: 78
  • Joined: 30 Oct 2018
  • Loc: Almere, The Netherlands, Europe

Posted 16 August 2019 - 02:44 AM

When talking about a frostline with regards to pouring a pier i suppose we are talking about open field.

In an observatorium the frostline won't be so deep i suppose. How much difference between the two?



#2 t-ara-fan

t-ara-fan

    Viking 1

  • -----
  • Posts: 790
  • Joined: 20 Sep 2017
  • Loc: 50° 13' N

Posted 16 August 2019 - 06:17 AM

A couple of factors come into play.

There is (hopefully) no snow in your observatory. Outside, snow insulates the ground. That makes the frost line less deep.

Grass and sod outside also insulate the ground. The observatory I have just started building will have a concrete slab floor. That conducts heat into the ground better than a wood floor would conduct. So if anything, the frost line will be deeper under my observatory than outside.

I am getting a guy to auger a 6' deep hole for my pier.

Edited by t-ara-fan, 16 August 2019 - 06:23 AM.


#3 lee14

lee14

    Viking 1

  • *****
  • Posts: 632
  • Joined: 19 Dec 2009
  • Loc: CNY

Posted 16 August 2019 - 07:06 AM

When talking about a frostline with regards to pouring a pier i suppose we are talking about open field.

In an observatorium the frostline won't be so deep i suppose. How much difference between the two?

 

A concrete slab conducts relatively little heat into the ground. Soil is a fairly good insulator, most of the heat is radiated away upwards and produces convection currents. The goal should be to prevent it from warming in the first place. It's true that a wooden floor will conduct little heat into the ground, but it shouldn't be in ground contact anyway. Wood will absorb relatively little warmth, and will radiate it away in short order. A deeply set concrete pour for a pier is a good idea for stability, but under a closed structure where the ground is not subject to rain or snow, the frost line for that location is not strictly relevant. Dry soil will be quite stable and unlikely to heave at all.

 

Lee



#4 t-ara-fan

t-ara-fan

    Viking 1

  • -----
  • Posts: 790
  • Joined: 20 Sep 2017
  • Loc: 50° 13' N

Posted 16 August 2019 - 12:08 PM

under a closed structure where the ground is not subject to rain or snow, the frost line for that location is not strictly relevant. Dry soil will be quite stable and unlikely to heave at all.

Lee


I doubt that. So there I am, in a field that stretches 1000' in every direction. The field is frozen to a depth of 5'. My 12' square unheated rolloff is in the middle of the field. I betcha the frost line is at 5' under where my pier sits in the rolloff.

The soil had been there for a thousand years and it is not 100% dry.

Anyway, OP you can ask a local contractor what is an appropriate depth for the pier. Add an extra foot to prepare for the coming ice age ;)

#5 spacemunkee

spacemunkee

    Gemini

  • -----
  • Posts: 3002
  • Joined: 22 Oct 2017
  • Loc: Cincinnati, Ohio

Posted 16 August 2019 - 12:17 PM

under a closed structure where the ground is not subject to rain or snow.

Lee


You mean the water table avoids moving into places under a structure?...
Tell that to someone with a flooded basement! :)

#6 lee14

lee14

    Viking 1

  • *****
  • Posts: 632
  • Joined: 19 Dec 2009
  • Loc: CNY

Posted 16 August 2019 - 12:51 PM

You mean the water table avoids moving into places under a structure?...
Tell that to someone with a flooded basement! smile.gif

A basement floor is typically 8 - 10 feet under grade. If your water table reaches to within a foot of the surface, you've built on a swamp. Ground moisture that causes heaving is produced from precipitation.

 

Lee



#7 lee14

lee14

    Viking 1

  • *****
  • Posts: 632
  • Joined: 19 Dec 2009
  • Loc: CNY

Posted 16 August 2019 - 01:01 PM

I doubt that. So there I am, in a field that stretches 1000' in every direction. The field is frozen to a depth of 5'. My 12' square unheated rolloff is in the middle of the field. I betcha the frost line is at 5' under where my pier sits in the rolloff.

The frostline here is four feet. I have a 10 x 12 foot observatory whose frame sits on solid concrete blocks 18 inches below grade. The pier is a solid pour two feet below grade, and another foot above so it's level with the floor. The observatory has been here for 24 years, and neither the structure nor the pier has ever moved at all. Overhangs and proper grading keep water away from the foundation, and the soil underneath is dry as a bone. No matter how much it rains, or how fast several feet of snow melt, moisture does not migrate horizontally, it goes directly down. The pier has not moved at all. The photo-quality polar alignment I did two decades ago is as accurate as ever. Prevent moisture from entering your soil and frost upheaval will not be an issue.

 

Lee



#8 Joshuasnoesky

Joshuasnoesky

    Explorer 1

  • -----
  • topic starter
  • Posts: 78
  • Joined: 30 Oct 2018
  • Loc: Almere, The Netherlands, Europe

Posted 16 August 2019 - 01:20 PM

Moisture isn't an issue here. The observatorium is 70 cm above the rest of the garden and i prevented water to enter the ground beneath the obsy.

The ground beneath the shed that stood here before was as dry as can be.

Coming winter i may conduct some tests with ground temperatures outside the obsy and beneath it.

 

Interesting experiment.



#9 lee14

lee14

    Viking 1

  • *****
  • Posts: 632
  • Joined: 19 Dec 2009
  • Loc: CNY

Posted 16 August 2019 - 01:28 PM

Moisture isn't an issue here. The observatorium is 70 cm above the rest of the garden and i prevented water to enter the ground beneath the obsy.

The ground beneath the shed that stood here before was as dry as can be.

Coming winter i may conduct some tests with ground temperatures outside the obsy and beneath it.

 

Interesting experiment.

Empirical data. Indeed. That's the proper way to arrive at conclusions!

 

Lee



#10 mich_al

mich_al

    Fly Me to the Moon

  • *****
  • Posts: 5709
  • Joined: 10 May 2009
  • Loc: Rural central lower Michigan Yellow Skies

Posted 16 August 2019 - 07:09 PM

Empirical data. Indeed. That's the proper way to arrive at conclusions!

 

Lee

Yes but keep in mind that will be short term data ie weather not climate.



#11 speedster

speedster

    Vostok 1

  • *****
  • Posts: 157
  • Joined: 13 Aug 2018
  • Loc: Abilene, Texas

Posted 16 August 2019 - 11:27 PM

+1 to t-ara-fan.  Even dry soil has moisture and it freezes and heaves.  An unconditioned building like an observatory will not affect the frost depth.  There is little difference between wet, dry, and normal soil temps and the difference decreases as you get below active surface soils.  In the graph below, Tm is approximately the annual average air temp.  The depth at which soil temps are constant is the far field temp. 

 

Will your telescope break if not founded below the frost line?  Certainly not.  But, if you want the least movement, get below the frost line.

 

Both freezing and moisture content cause soil movement and it moves much more than most people realize.  When everything in the neighborhood moves together, no one notices.  It's the differential movements that cause fits and that can be something as simple as a driveway on one side of a building and turf on the other.  There are things that can be done to minimize it and they all involve draining surface water a good distance away from the building quickly.

 

 

amplitude-vs-depth.gif



#12 Steve Haverl

Steve Haverl

    Vostok 1

  • *****
  • Posts: 103
  • Joined: 16 Mar 2018

Posted 17 August 2019 - 10:54 AM

I’m amazed at the level of overthinking exhibited in this string. At most you would shorten the pier depth a foot or two, so is it really worth all this thought and conducting “experiments “ ?

Dig the @#$& pier in accordance with code or local practice and be at peace !
  • guyroch, lee14 and t-ara-fan like this

#13 mich_al

mich_al

    Fly Me to the Moon

  • *****
  • Posts: 5709
  • Joined: 10 May 2009
  • Loc: Rural central lower Michigan Yellow Skies

Posted 17 August 2019 - 12:24 PM

I’m amazed at the level of overthinking exhibited in this string. At most you would shorten the pier depth a foot or two, so is it really worth all this thought and conducting “experiments “ ?

Dig the @#$& pier in accordance with code or local practice and be at peace !

  Agreed, seems to be some efforts to justify saving a foot or so of digging.  Don't.



#14 t-ara-fan

t-ara-fan

    Viking 1

  • -----
  • Posts: 790
  • Joined: 20 Sep 2017
  • Loc: 50° 13' N

Posted 17 August 2019 - 02:35 PM

Agreed, seems to be some efforts to justify saving a foot or so of digging. Don't.


And scrape the loose dirt from the bottom of the hole so the pier doesn't settle over time.

Edited by t-ara-fan, 17 August 2019 - 03:19 PM.



CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics