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Hard Rock Thermal Properties? Stumped!

old-iron-habit

Senior Member
Joined
Nov 22, 2012
Messages
4,233
Location
Moose Lake, MN
Occupation
Retired Cons't. Supt./Hospitals
On a hospital project I am consulting on an interesting conversation took place today. This is not in my juristiction but it is something I have never came accross before. There is a section of the new hospital that can not be built until next spring because of access issues with the existing hospital. The contractor is allowed by the licensing authorities to put the foundations in but can not build on them until spring. Code does not allow hospital patient windows to be blocked (physically or view) while they are occupied. The contractor wants to put the foundations in this fall to save time but the footing will set on solid slate rock that was blasted to a depth of 2 to 5 feet below slab on grade. Two inches of sand below the footing is required for separation from the rock. The frost depth in this locale can be up to 8 ft dependent on the winter, 5 to 6 feet is normal. While discussing this today the structural engineers started getting concerned about frost mitigating into the footings after the facility is completed. So finally, heres the question. What is the thermal properties of rock as compared to gravel or other soil. Normally we would put 4 inches of styrofoam horizontal out from the footing to a width of 8 ft. and with the footing exterior insulated it would never freeze beneath the footing when the building is heated because of the thermal properties of the ground. Does rock transfer heat or cold different than soil? Would frost travel deeper in rock, would thermal ground heat come up more or less? Thanks in advance for any thoughts or theories.
 

Delmer

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Jan 3, 2013
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8,863
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WI
The thermal conductivity is in the same range as soil. A little more conductive than clay soils and less conductive than damp sand. Basalt or Granite are much more conductive so that would be different. I've never heard of a building code distinction for soils with regard to insulation. In the geothermal ground loop heat pump business they pay closer attention to this than anybody I can think of, you could try researching that angle.

Why are they concerned about after the building is completed? I'd be more concerned about this winter, or do they have a plan for that?
 

td25c

Senior Member
Joined
Feb 14, 2009
Messages
5,250
Location
indiana
Interesting issue , I would think the solid rock would conduct better than soil .

Then again so many variables as well . If the rock is deep in the ground then sub surface temps will rise through it and the above cold freeze will push down . The frost line may very well be similar to bare dirt .

Thick sod & tall grass have a good insulating effect as compared to a rock road .

Good question old -iron -habit ? :beatsme


We have a lot of limestone in my area and I will say frost line wont be as deep around it but that is probably due to ground water weeping through it keeping it a little warmer .
 

old-iron-habit

Senior Member
Joined
Nov 22, 2012
Messages
4,233
Location
Moose Lake, MN
Occupation
Retired Cons't. Supt./Hospitals
The thermal conductivity is in the same range as soil. A little more conductive than clay soils and less conductive than damp sand. Basalt or Granite are much more conductive so that would be different. I've never heard of a building code distinction for soils with regard to insulation. In the geothermal ground loop heat pump business they pay closer attention to this than anybody I can think of, you could try researching that angle.

Why are they concerned about after the building is completed? I'd be more concerned about this winter, or do they have a plan for that?

Thanks for your prompt reply. I already learned more than I did all day. There concern is that the frost may penetrate under the footing from the outside when its cold. This area normally experiances some -30 to -40 F tempuratures every winter leading to frost heave that will cause cracks. Some of the exterior walls are in mechanical and electrical room space that have minimal heat near the building exterior. This project is near the Canadian border on the north shore of Lake Superior. The engineer in the meeting was cafeful in not committing on the effective value of the insulation on rock with out doing more research.
 

rsherril

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Joined
May 2, 2009
Messages
264
Location
Far West Colorado
Occupation
Geologist, Retired from teaching sciences
Quick review of some basics for thermodynamics and heat flow. Heat moves from hot to cold, the difference in temperatures determines the rate of movement in similar substance. Insulation moderates the flow of heat.

If your frost penetration is 8 ft. then we can assume that the temperature at eight feet is about freezing point. Move eight feet towards the surface and we have a seventy degree temperature differential in eight feet. Pretty steep slope on the graph. Add to that the fact that sand will provide even more of a heat barrier because of the dead air space between grains. To achieve a seventy degree temperature differential in the Earth use the oil field standard of one degree per one hundred feet and you would have to go about seven hundred feet into solid rock to achieve the same difference.

Other variables include the temperature loss rate of the concrete as it cures which is a function of surface area and volume. Rebar in the concrete would accelerate the heat loss from the center towards the cold.

Conclusion. No way would a professional engineer risk their licence and approve this without a lot more thought and expense to insulate the concrete from those kinds of surface temperatures. My two cents worth.
 
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