Soil Selection For Horizontally Trenched

Thursday, February 18, 2016

Soil selection has long been a sticking point for a lot of LoopLink® RLC users when designing a horizontally trenched system. For our first try at simplifying the process, we posted the soil selection table from the Residential & Light Commercial Design & Installation Manual. The table is admittedly a bit dense and we were asked to make things a little easier.

So we created, what we thought at the time, was a much simpler method of selecting a soil. We supplied drop downs for soil density and moisture content which filtered the table down to a single thermal conductivity and thermal diffusivity for each soil type. The user then adjusted a slider to define the soil composition. LoopLink® RLC found the closest matches from the table and returned at most three options to choose from.

It was that last part of the interface that seemed to over complicate things. We (I) may have been too interested in creating something neat to look at versus something that did its job well. So, this week we removed the last step. Now you select a soil density and a moisture content and we return the entire table of soil types with just one thermal conductivity and one thermal diffusivity for each.

With this change I thought it might be worth a review of why soil selection matters and generally how to go about it.

Why Soil Selection Matters

Selecting the correct soil for your horizontally trenched installation is important because it defines two key variables in the calculation of loop lengths—soil thermal conductivity and soil thermal diffusivity. Both variables are a function of soil composition, density and moisture content.

Soil Thermal Conductivity
How fast heat moves through the soil given a specific temperature difference.
Soil Thermal Diffusivity
The measure of how fast heat moves relative to the capacity of the soil to store heat.

Determining Soil Composition

Soil composition is the single most important component of soil identification and it is also the one that tends to be the most intimidating. There are hundred page books on soil classification systems or we can point you to a very simple and direct explanation from Colorado State University Extension.

The page is written with gardeners in mind but the concepts are universal. If you are after more depth, refer to the IGSHPA Soil & Rock Classification Field Manual. We do also cover this topic thoroughly in our IGSHPA Geothermal Installer Certification Training which we offer through HeatSpring.

Once you determine your soil composition, you need to determine its density and moisture content.

Determining Soil Density

The density of your soil as it relates to the tabled values in LoopLink® RLC is determined as a dry bulk density. To find the dry bulk density, you extract a known volume of soil using a bulk density soil sampling kit. Dry the sample completely and then divide the weight of the dry sample by the volume extracted.

Practically speaking, if the soil density test is skipped you should know that in general lower soil density tends to correlate with lower thermal conductivity and diffusivity. So assuming low density is more conservative.

Determining Moisture Content

For LoopLink RLC we have reduced the moisture content selection to values of dry, typical and wet. Where dry soil is at the wilting point, wet soil is at field capacity and typical soil is the average of the two.

To understand these terms a little better you should know that wilting point is the level of soil moisture at which plants can no longer draw water through their roots and thus begin wilting. Field capacity simply means that the soil can absorb absolutely no more water because it is saturated.

The dryer the soil the worse it will perform with respect to our thermal properties. It should be noted that unless it is known that droughts are prevalent or the formation has a tendency to dry out, it is rare that 6 feet down the soil will dry out to the wilting point. It can however be overly optimistic to assume that a soil will always be at field capacity.

You Can Look It Up

The USDA maintains very detailed soil maps for the entire United States. If you are unsure about your soil composition reach out to your county USDA office to get this information or look it up through the Web Soil Survey. A quick search of the internet should also result in a few other tools that will help you in your pursuit of determining the correct soil properties.