Effect of compaction on variations in soil thermal conductivity

[Silty]

I understand that the thermal conductivity of a soil sample that is mostly clay will increase about three fold when transitioning from a dry state to a saturated state while one that is 100% sand may increase as much as ten fold.  In the latter case, e.g., in a sand dune or gravel bed, these properties may not depend on depth. For most soil mixtures, however, I am hypothesizing that the sensitivity of the thermal conductivity of the soil to moisture changes will decrease with depth due to the reduction and then elimination of root aeration, and the general compaction of the soil which will reduce the voids where water can displace air. If my logic is correct, has anyone measured this effect versus depth for different soil types?

[studiot]

5 minutes ago, Silty said:

I understand that the thermal conductivity of a soil sample that is mostly clay will increase about three fold when transitioning from a dry state to a saturated state while one that is 100% sand may increase as much as ten fold.  In the latter case, e.g., in a sand dune or gravel bed, these properties may not depend on depth. For most soil mixtures, however, I am hypothesizing that the sensitivity of the thermal conductivity of the soil to moisture changes will decrease with depth due to the reduction and then elimination of root aeration, and the general compaction of the soil which will reduce the voids where water can displace air. If my logic is correct, has anyone measured this effect versus depth for different soil types?

I haven't seen any work on this but I will look around tomorrow as I may have some specialised material on this.

Meanwhile perhaps you could elaborate on

Quote

sensitivity of the thermal conductivity of the soil to moisture changes will decrease with depth due to the reduction and then elimination of root aeration, and the general compaction of the soil which will reduce the voids where water can displace air.

If your first statement is correct would you not expect a step change in sensitivity at the phreatic surface ?

 

Edit

 

Oh and welcome to Science Forums !

Edited Tuesday at 09:54 PM by studiot

[Mordred]

34 minutes ago, Silty said:

I understand that the thermal conductivity of a soil sample that is mostly clay will increase about three fold when transitioning from a dry state to a saturated state while one that is 100% sand may increase as much as ten fold.  In the latter case, e.g., in a sand dune or gravel bed, these properties may not depend on depth. For most soil mixtures, however, I am hypothesizing that the sensitivity of the thermal conductivity of the soil to moisture changes will decrease with depth due to the reduction and then elimination of root aeration, and the general compaction of the soil which will reduce the voids where water can displace air. If my logic is correct, has anyone measured this effect versus depth for different soil types?

I have done soil conductivity sampling for radio towers. You really want to remove any salt and water from the soil to get an accurate reading.

 Typically the water content will be conductive due to the additional salt content.

A lot depends on salt content which will vary layer per layer.

Edited Tuesday at 10:23 PM by Mordred

[Silty]

58 minutes ago, studiot said:

I haven't seen any work on this but I will look around tomorrow as I may have some specialised material on this.

Meanwhile perhaps you could elaborate on

If your first statement is correct would you not expect a step change in sensitivity at the phreatic surface ?

 

Edit

 

Oh and welcome to Science Forums !

Thanks! Yes, I am referring to the region above the capillary zone where there are varying degrees of saturation. I am assuming that the thermal conductivity will be maximized below the capillary zone. However, above the capillary zone, my thought is that, even if the material composition of the soil is the same through a vertical column, because of reduces aeration the effect of moisture content on thermal conductivity will decrease with depth. In other words, with greater depth, the thermal conductivity becomes closer to that of the soil material and less on the mix of that material and water or air. Near the surface, where the soil will likely contain more air, substituting water for air greatly increases the thermal conductivity, but if the capillary zone is below the root line, then the soil is not going to be broken up by the roots, and the soil will be compacted by the weight of the column of soil above it so there will be less air volume to displace with water volume as the moisture content changes so the soil will never be as insulating as the same soil was at the top. In summary, I am thinking the thermal conductivity will be its maximum at the capillary zone. Moving up to the ground surface from there it will become increasing sensitive to moisture content with the largest sensitivity at the surface. 

[Mordred]

What you will likely notice is that the higher porosity the higher the moisture content the higher the electrical conductivity due to higher concentration of dissolved cations and anions.

Soil layers with higher drainage will contribute to higher removal of conductive salts as opposed to soil content with poor drainage. So poor drainage soils will lead to higher salt accumulation.

That's been my experience 

 

Edited Wednesday at 12:06 AM by Mordred

[Silty]

1 hour ago, Mordred said:

What you will likely notice is that the higher porosity the higher the moisture content the higher the electrical conductivity due to higher concentration of dissolved cations and anions.

Soil layers with higher drainage will contribute to higher removal of conductive salts as opposed to soil content with poor drainage. So poor drainage soils will lead to higher salt accumulation.

That's been my experience 

 

Thanks for your input. I want to clarify that I am interested in the soil's thermal conductivity rather than its electrical conductivity. 

[Mordred]

Fair enough your on the right track with your last post. I don't have personal experience involving thermal conductivity. Any tests I've done has been electrical conductivity.

[studiot]

13 hours ago, Silty said:

Thanks! Yes, I am referring to the region above the capillary zone where there are varying degrees of saturation. I am assuming that the thermal conductivity will be maximized below the capillary zone. However, above the capillary zone, my thought is that, even if the material composition of the soil is the same through a vertical column, because of reduces aeration the effect of moisture content on thermal conductivity will decrease with depth. In other words, with greater depth, the thermal conductivity becomes closer to that of the soil material and less on the mix of that material and water or air. Near the surface, where the soil will likely contain more air, substituting water for air greatly increases the thermal conductivity, but if the capillary zone is below the root line, then the soil is not going to be broken up by the roots, and the soil will be compacted by the weight of the column of soil above it so there will be less air volume to displace with water volume as the moisture content changes so the soil will never be as insulating as the same soil was at the top. In summary, I am thinking the thermal conductivity will be its maximum at the capillary zone. Moving up to the ground surface from there it will become increasing sensitive to moisture content with the largest sensitivity at the surface. 

 Ok so the next thing is elaborate on what you want to do with the thermal conductivity, where you are coming from and where you want to go to.

So is this about agricultural, environmental or some other science and what is you background in Mathematics?

The subject can be as complicated or simple as you wish or need to make it.

 

Generally thermal conductivity arises not directly, but in connection with heat flux.

The equation for this is called the heat equation or sometimes the diffusion equation.

This is normally used as a particularly simple first order differential equation with the thermal conductivity being one of its constants.

However constancy implies homogeneity a property which soil is anything but.

Further complications arise because input solar heat flux cause loss of (latent) heat by evaporation of pore and adsorbed water.

You mention roots and these are also known to modify the environment local to them, a phenomenon known as the rhizosphere.

 

 

These, and perhaps other factors (such as compositional variation, compaction etc) mean that the thermal conductivity can no longer be considered as a constant by becomes a coordinate system dependant variable.

 

Depending upon the application, discipline and complexity of the model adopted I can find various numerical solutions in the literature.

 

Some starter books to ask your librarian for

 

Soils and the Environment  Alan Wild  Cambridge University Press

Heat Transfer  J P Holman  McGraw Hill

Dynamics of Fluids in Porous Media  Jacob Bear  Elsevier /  Dover

 

 

The electromagnetic equations Mordred would need for electrical conductivity analysis are of higher order and not similar as are not the stress equations you would find in Soil Mechanics  texts (though Lambe does discuss the effect of thermal conductivity on soil structure)

[Mordred]

3 hours ago, studiot said:

 

The electromagnetic equations Mordred would need for electrical conductivity analysis are of higher order and not similar as are not the stress equations you would find in Soil Mechanics  texts (though Lambe does discuss the effect of thermal conductivity on soil structure)

Lol some of those equations get downright brutal even for first order approximations. Glad it's a rare occasion for me nowadays. Haven't had to design a ground grid for a tower in near a decade.

Edited Wednesday at 04:14 PM by Mordred

[Silty]

On 9/18/2024 at 8:49 AM, studiot said:

 Ok so the next thing is elaborate on what you want to do with the thermal conductivity, where you are coming from and where you want to go to.

So is this about agricultural, environmental or some other science and what is you background in Mathematics?

The subject can be as complicated or simple as you wish or need to make it.

 

Generally thermal conductivity arises not directly, but in connection with heat flux.

The equation for this is called the heat equation or sometimes the diffusion equation.

This is normally used as a particularly simple first order differential equation with the thermal conductivity being one of its constants.

However constancy implies homogeneity a property which soil is anything but.

Further complications arise because input solar heat flux cause loss of (latent) heat by evaporation of pore and adsorbed water.

You mention roots and these are also known to modify the environment local to them, a phenomenon known as the rhizosphere.

 

 

These, and perhaps other factors (such as compositional variation, compaction etc) mean that the thermal conductivity can no longer be considered as a constant by becomes a coordinate system dependant variable.

 

Depending upon the application, discipline and complexity of the model adopted I can find various numerical solutions in the literature.

 

Some starter books to ask your librarian for

 

Soils and the Environment  Alan Wild  Cambridge University Press

Heat Transfer  J P Holman  McGraw Hill

Dynamics of Fluids in Porous Media  Jacob Bear  Elsevier /  Dover

 

 

The electromagnetic equations Mordred would need for electrical conductivity analysis are of higher order and not similar as are not the stress equations you would find in Soil Mechanics  texts (though Lambe does discuss the effect of thermal conductivity on soil structure)

Sorry for my delayed response. I didn't get alerted to your post. My application is ground-loop design for a ground-source heat pump. I just wanted to get a top-level understanding of how moisture content increased soil thermal conductivity at different depths. I will check out your references. Thanks.

[studiot]

18 minutes ago, Silty said:

Sorry for my delayed response. I didn't get alerted to your post. My application is ground-loop design for a ground-source heat pump. I just wanted to get a top-level understanding of how moisture content increased soil thermal conductivity at different depths. I will check out your references. Thanks.

I think there is a switch somewhere in your settings to send you an email when you have a response, but few use it as it gets tedious after a while.

On the other hand I usually get a notification as soon as I log on so if you are looking for responses you need to log on.

 

I did have an afterthought that your question might be about ground source heat pump design.

We at SF have discussed this topic a few time over the past few years and I seem to remember posting some design calculations.

 

Again you are short on detail but I don't think compaction will have a significant effect on the heat transfer to your working fluid, which I take it is is water.

Conditions will be very different if the source is under the foundations of a (large) building or via d deep borehole. I have a friend in Germany with a borehole version.

When I worked it out for my house I would have required about 100 metres of buried pipe, space about 1m apart to gain enough heat to operate satisfactorily.

This is why ground source has a high capital cost unless it can be incorporated within the foundations.

[Silty]

On 9/19/2024 at 3:09 PM, studiot said:

I think there is a switch somewhere in your settings to send you an email when you have a response, but few use it as it gets tedious after a while.

On the other hand I usually get a notification as soon as I log on so if you are looking for responses you need to log on.

 

I did have an afterthought that your question might be about ground source heat pump design.

We at SF have discussed this topic a few time over the past few years and I seem to remember posting some design calculations.

 

Again you are short on detail but I don't think compaction will have a significant effect on the heat transfer to your working fluid, which I take it is is water.

Conditions will be very different if the source is under the foundations of a (large) building or via d deep borehole. I have a friend in Germany with a borehole version.

When I worked it out for my house I would have required about 100 metres of buried pipe, space about 1m apart to gain enough heat to operate satisfactorily.

This is why ground source has a high capital cost unless it can be incorporated within the foundations.

You are correct that the high installation cost is what is inhibiting widespread GSHP use. I am looking for a way to reduce it. For the borehole architecture, I want to better match the piping to the rock thermal conductivity, but bedrock also has a wide TC, apparently ranging from about 0.5 W/mK for sedimentary to more than 5 W/mK for metamorphic and igneous.

[studiot]

51 minutes ago, Silty said:

You are correct that the high installation cost is what is inhibiting widespread GSHP use. I am looking for a way to reduce it. For the borehole architecture, I want to better match the piping to the rock thermal conductivity, but bedrock also has a wide TC, apparently ranging from about 0.5 W/mK for sedimentary to more than 5 W/mK for metamorphic and igneous.

Rock ?

You started by asking about partly saturated soil.

I very much doubt that the cost of 100m of 2 - 3 inch pipe in igneous rock would be economic.

However limiting factors are twofold.

Unless you live in Iceland or NZ your output water would be in the 16^oC  range  so not a great deal of temperature different to drive the heat exchange.

The actual rate of takeup would be limited by the pipe material  - probably plastic. But stainless steel has a relatively low conductivity as well.

In Mordredland you may also be fighting permafrost.

 

You still haven't said much about where you are coming from.