Soil-Testing

Thermal Conductivity Test of Soil

Thermal conductivity of soil is basically done in the agricultural as well as the constructional field. It is a known rule that with the increasing moisture content, the thermal conductivity of soil would also increase. Someone a person working in the construction industry would be able to tell you the role of result buried cables and pipelines pose as a potential threats underground there. So, when the underground systems of these construction companies are being built, it is really important that they should pay close attention to the levels of moisture as well as the thermal conductive properties that basically surrounds the soil. There might be situations where the cables and pipelines might experience intense form of overheating and also they may probably combust. However, surrounding soil of low thermal conductivity is needed when dealing with buried hot water pipes. This is performed to minimize the heat loss from pipes to the ground. For this reason, authentic and detailed thermal testing and moreover a perfect selection of backfilling materials is the ultimate key for  inexhaustible utility of underground systems.

How do you measure the thermal conductivity of soil?

For measuring the thermal conductivity of soil there are many options. One of them is the Thermtest TLS- 100 portable thermal resistivity meter. Their main aim is to equip the most convenient, comfortable, efficient and easy to use thermal conductivity system. It is used to measure the thermal properties of soil. They are designed in such a way that they could test soils and other soft materials through an advanced and extensive research and modelling. It has a superb accuracy of 5 percent and reproducibility of 2 percent. This device is also schemed in a way that they account for isothermal drift in a sample which would be prior to its measurement. The temperatures of such samples are closely monitored and recorded prior to each test. The recording of this temperature drift is done as a part of measurement to let on the drift compensation if required.

However, if the physical properties of the device experience some variations then the thermal conductivity of soils could have significant effects. As a general rule, the thermal conductivity of soil is proportionate to the moisture content. In other words, we could also say that higher the thermal conductivity, higher would be the moisture content of the soil or it can also go vice versa.

Thermal conductivity of soil at different moisture levels

To perform the thermal conductivity test of soil at different moisture levels, various tests are performed. One of them is to check the thermal properties of soil, two soil samples are taken. They would have two different moisture levels, 0 percent and 100 percent respectively. To start this procedure, a small sample of soil is taken from the thermtest lab. It is then left to dry naturally for a period of three weeks under good atmospheric conditions. To strongly impersonate a noted New Brunswick soil density, there is a method used known as impact compaction which is used to compact the soil in the sample holder which would have two separate layers. If the soil sample of 0 percent saturated soil is taken, then the TLS 100 probe would be carefully inserted inside the center of that dry sample. Before taking any readings , the sample set up or the probe is required to be isothermally stable for around 10 minutes.

On the other hand, if the tests are performed on the 100 percent saturated soil sample, the exact procedure like the one mentioned above is followed. The only difference is that the soil is compacted into a bottomless sample holder which is pasted on with cheesecloth. Then this new sample holder is transferred into a larger dish which contains water. This sample is left aside untouched for approximately 12 hours so that the soil sample manages to achieve 100 percent saturation. It could also be termed as a form of a capillary action. Before this probe is inserted, the sample holder is replaced from the larger dish to be placed on a dry saucer. The TLS 100 probe is then inserted into the core of this sample and left as such for a period of approximately 10 minutes.

Time taken to test soil thermal conductivity

Because this method is transient in nature, the testing time is comparatively low, that is just two minutes. It is often considered to be an essential method for  testing materials with high moisture content. While this test happens, the TLS 100 probe that is inserted gives out heat over a period of time that is set. All this while, multiple temperature readings would be noticed and noted down at regular intervals. When cooling starts to happen the temperature readings would be noticed to be repeated at a regular interval of time once again. Thus, the soil thermal conductivity test is calculated according to their temperature readings in the following equation.

k = q / 4πa

k here stands for thermal conductivity

q denotes the heating power of the needle

And a stands for the slope of temperature rise over the log of time.

After many such tests are done on the 0 percent and 100 percent saturation, it was proven that the thermal conductivity  of soil is largely dependent on various physical properties like water content.


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