Melting permafrost under concrete slab

Looking for recommended temp and flow rate to melt permafrost under concrete slab.

Conditions:
Temp above concrete slab 0°C
Permafrost roughly 10’ deep
2” diameter PEX 6” below bottom of concrete Multiple 2” PEX loops under slab
Each 2” PEX loop 1,000’ long
Running 40% glycol through loops

Welcome to PF.

Molon Lave said:

Looking for recommended temp and flow rate to melt permafrost under concrete slab.

Can you say why you want to do this? Won't melting the permafrost under a concrete slab cause the slab to become a sinkhole?


https://www.cbsnews.com/news/10-sinkhole-facts-that-could-save-your-life/

berkeman said:

Welcome to PF.Can you say why you want to do this? Won't melting the permafrost under a concrete slab cause the slab to become a sinkhole?

View attachment 338737
https://www.cbsnews.com/news/10-sinkhole-facts-that-could-save-your-life/

Permafrost, or frozen ground has developed under a cold storage facility. This frozen ground has heaved the building slab. Melting the ground will cause the slab heaving to slowly subside, not sink (definitely not lower than the original slab grade).

Where in the world; expansive soils/clays exist in a variety of geologies, and remediation strategies vary a bit.
https://nora.nerc.ac.uk/id/eprint/521301/1/EncyclopediaEngGeol_ExpansiveSoils_PostReview.pdf

Bystander said:

Where in the world; expansive soils/clays exist in a variety of geologies, and remediation strategies vary a bit.
https://nora.nerc.ac.uk/id/eprint/521301/1/EncyclopediaEngGeol_ExpansiveSoils_PostReview.pdf

So they can expand when they get wet, and then expand even more when that moisture freezes? Yikes.

Bystander said:

Where in the world; expansive soils/clays exist in a variety of geologies, and remediation strategies vary a bit.
https://nora.nerc.ac.uk/id/eprint/521301/1/EncyclopediaEngGeol_ExpansiveSoils_PostReview.pdf

Building is in central New Jersey.

How can frost happen under a slab in NJ if the temperature above is only 0C? And did you core drill to find it was 10' deep frost? I've worked on cold storage for pharma (including some in NJ...) and typically it is only the very cold storage (-40 or colder) where this issue is prevalent. The usual prevention solution is to insulate the slab and provide heating under it (with glycol or heat trace) in an amount equal to the heat transfer through the slab and insulation, thus cancelling the heat transfer and leaving the ground temperature unchanged.

Remediation though....adding the right amount of heat to stop cooling the ground will eventually allow it to warm up and melt the frost. But how long "eventually" is I don't know. You probably need to hire a geotechnical engineer for assistance with that, and would suggest an architect and mechanical engineer to re-design the slab.

russ_watters said:

How can frost happen under a slab in NJ if the temperature above is only 0C? And did you core drill to find it was 10' deep frost? I've worked on cold storage for pharma (including some in NJ...) and typically it is only the very cold storage (-40 or colder) where this issue is prevalent. The usual prevention solution is to insulate the slab and provide heating under it (with glycol or heat trace) in an amount equal to the heat transfer through the slab and insulation, thus cancelling the heat transfer and leaving the ground temperature unchanged.

Remediation though....adding the right amount of heat to stop cooling the ground will eventually allow it to warm up and melt the frost. But how long "eventually" is I don't know. You probably need to hire a geotechnical engineer for assistance with that, and would suggest an architect and mechanical engineer to re-design the slab.

Apologies: outside temp is 0°F (-15°C) . No core samples were taken. When water freezes however, it expands by about 9%. The floor is roughly 11” higher than its original elevation, so roughly 10’ deep.

https://en.wikipedia.org/wiki/Frost_heaving#In_refrigerated_buildings

https://en.wikipedia.org/wiki/Directional_boring

Drill across the site, every 2 metres, at a depth of 2 metres. Install heavy plastic pipes that will allow you to heat that underground isothermal surface to melt the ice below the slab.

Molon Lave said:

TL;DR Summary: Looking for recommended temp and flow rate to melt permafrost under concrete slab.

Temp above concrete slab 0°C
Permafrost roughly 10’ deep
2” diameter PEX 6” below bottom of concrete Multiple 2” PEX loops under slab
Each 2” PEX loop 1,000’ long
Running 40% glycol through loops

Molon Lave said:

outside temp is 0°F (-15°C) .

Do you mean the temperature in the building or the outside temperature is 0 deg F? It makes a difference.

Is there any insulation below the slab? Is the building supported by the slab, or does it have a deeper foundation? How long has the building been refrigerated - months, years, decades? What is the soil under the building - sand, loam, clay, gravel? Is it a frost susceptible soil? Read carefully the link in Post #9. Need the soil type to at least the frozen depth. How far down is the water table?

Do you want melt the permafrost, or just keep it from freezing deeper? Is that the tubing in there now, or is that what you are planning to install? If the tubing is in there now, a sketch showing the placement and spacing would be very helpful because if that permafrost is melted unevenly, the building will settle unevenly and possibly be severely damaged.

Frost heaving is from ice lenses, that can turn the soil into quicksand when melting. You could create sinkholes under the building even if you melt it evenly and slowly. Try search terms sinkholes in permafrost to find what could happen.

Molon Lave said:

Permafrost, or frozen ground has developed under a cold storage facility. This frozen ground has heaved the building slab. Melting the ground will cause the slab heaving to slowly subside, not sink (definitely not lower than the original slab grade).

Maybe. But don't count on it. I strongly suggest finding a civil engineer with experience in permafrost before you attempt any melting. Don't be surprised if the recommendation is to demolish the building, then wait until the permafrost is fully melted before doing anything with the site.

jrmichler said:

Do you mean the temperature in the building or the outside temperature is 0 deg F? It makes a difference.

Is there any insulation below the slab? Is the building supported by the slab, or does it have a deeper foundation? How long has the building been refrigerated - months, years, decades? What is the soil under the building - sand, loam, clay, gravel? Is it a frost susceptible soil? Read carefully the link in Post #9. Need the soil type to at least the frozen depth. How far down is the water table?

Do you want melt the permafrost, or just keep it from freezing deeper? Is that the tubing in there now, or is that what you are planning to install? If the tubing is in there now, a sketch showing the placement and spacing would be very helpful because if that permafrost is melted unevenly, the building will settle unevenly and possibly be severely damaged.

Frost heaving is from ice lenses, that can turn the soil into quicksand when melting. You could create sinkholes under the building even if you melt it evenly and slowly. Try search terms sinkholes in permafrost to find what could happen. Maybe. But don't count on it. I strongly suggest finding a civil engineer with experience in permafrost before you attempt any melting. Don't be surprised if the recommendation is to demolish the building, then wait until the permafrost is fully melted before doing anything with the site.

The temperature is in the building, outside temp varies from 10-90°F (New Jersey). Lower end typically during January-February. Designated Frost line in this region is 36”.

The slab is an independent component and does not support the building. Heaving appears to be concentrated in the center of the building, perimeter of foundation mostly unaffected (building is elevated with loading docks). Slab heaving first presented 10 years, where cracks in slab were noted.

2” PEX is in place. Soils mostly DGA 6’ deep with typical downer soil below. Underground temperature (3’-10’) typically above 50°F.

The heaving condition has presented in roughly 30% of this facility. Objective is to prevent freezing under the unaffected section, also stop and reverse the freezing where it has developed.

Molon Lave said:

Looking for recommended temp and flow rate to melt permafrost under concrete slab.

In order to even attempt to answer this, we need good, complete answers to the remaining questions in Post #10. What is "DGA" soil?

jrmichler said:

In order to even attempt to answer this, we need good, complete answers to the remaining questions in Post #10. What is "DGA" soil?

DGA Dense Grade Aggregate, uncrushed, crushed, or partially crushed gravel or stone, most of the aggregate passes a 3/4” sieve.

Let me summarize my understanding so far:

The building was apparently designed as a cold storage building.
It has a concrete slab floor.
There may or may not be insulation under the slab.
The floor is on gravel.
The gravel is over some sort of dirt. That dirt may have enough fines to wick water and form ice lenses.
There exists plastic tubing under the slab for the purpose of adding heat to prevent soil freezing and ice heaving.
The engineer that designed the building knew exactly how much heat needed to be added to accomplish this.
That information should be in the original design drawings and specifications.
If you do not have a copy of those, a copy is in the building permit file with your local building inspector and/or the state bureau that checks the designs of commercial buildings.
The heaving apparently started 10 years ago.
The building was built some time before that.
The building owners / leasers / renters never ran the heating system.
If that heating system was never installed because somebody thought they could save money, the specifications for it are on file with the building permit.
Find it, buy and install the heating system if necessary, then operate according to the original instructions.

And that is as much as we can do at PF because of professional liability. If the owners cry and moan about the cost, show them this thread. This experienced engineer has no sympathy for them at all.

This thread is therefore closed.