Grid project Beznau – Birr

Soil is a precious resource

How underground cables affect the soil and the environment

Guest author: Dr. Beat Frey*

Our soil carries out many important services for humans and the environment. In fact, healthy soil offers us a wide range of benefits. The nutrient cycle, for example, is vital for the production of food and fibre. There are also clear links to the water cycle. If the soil structure is altered or destroyed, the soil’s ability to purify, absorb and hold water is compromised. Intact, uncompacted soil can store water during heavy precipitation, which helps to mitigate damage from flooding and to provide water over longer periods of time and during dry spells.

Consequently, soil makes a significant contribution to the provision of numerous ecosystem services and therefore also to human well-being. Ecosystem services are ecological services that we humans receive as free services from nature. These services from the soil provide the basis for the production of food or wood, offer space for settlements and infrastructure, and allow uses in the field of leisure, sport, recreation or tourism. However, the long-term usability of soil presupposes that its use is sustainable and therefore that the ecological efficiency of the soil is ensured. This makes functional soil crucial to the well-being of humans and the environment.

Soil organisms are considered excellent early warning systems for disturbances.

A significant part of the valuable work is done by the billions of creatures that live in the soil. Soil contains a multitude of inconspicuous microorganisms and soil animals that are despised rather than respected. They include bacteria, algae, fungi, various worms, springtails, woodlice and many others. Probably the most prominent representative is the earthworm. Soil organisms are responsible for the regeneration of the soil. Bacteria, fungi and earthworms create fertile soil and make nutrients available for plant growth.

Soil organisms respond to human activities through changes in their diversity and are therefore considered excellent early warning systems for disturbances. Soil is threatened in several ways. The biggest risk factors for soil are physical changes such as erosion, humus loss and compaction, and chemical changes due to external inputs of pollutants and pesticides. Climate change, for instance bringing about significant variations in precipitation (drought or floods), can also affect soil biodiversity. In all human soil activities, it is important to bear in mind that it is virtually impossible for soil to be reproduced, as its regeneration is extremely slow. When soil disturbances reduce biodiversity, the quality of the goods and services that ecosystems provide also declines. We should therefore be very careful not to adversely change our soil biodiversity.

Underground extra-high-voltage line goes live

Swissgrid laid its very first extended section of a 380 kV extra-high-voltage line underground at the «Gäbihübel» in the Bözberg/Riniken area, at Bözberg in the Canton of Aargau. Transition structures were built at both ends of the 1.3-kilometre section in order to connect the underground cable and overhead line. Even though the cable trench has already been filled in again and the conduit blocks are no longer visible, the dimensions of the cable conduit blocks remain impressive. Around 55,000 m3 of earth had to be excavated and taken away in lorries to construct the blocks. This means that when laying underground cables in conduit blocks, extensive transformation of terrain takes place. These interventions in naturally laid soil can lead to soil compaction or humus loss, which negatively affect long-term soil fertility.

After a construction period of around two years, the Swissgrid grid control room connected the new 380-kV line to the transmission grid on 19 May 2020. Since then, large amounts of electricity have been flowing through the soil via the first underground extra-high-voltage line. This raises the question of the extent to which these changes can lead to long-term impairments in soil quality. Swissgrid is now using this partial cable section to gain important knowledge about the operation of underground cables. The physical properties of underground cables differ from those of overhead lines. In periods of drought, the location of the cable conduit block may become visible, as the soil above the conduit block dries out more quickly than the surrounding soil. When the snow melts in the spring, the soil above the conduit block melts earlier because the soil above the conduit block is warmer than the surrounding soil. Not much is known about the effects of operational heat emissions from underground cables on the living soil layer, as the subject has hardly ever been studied until now.

Measuring station
Measuring stations monitor the underground cable

After completion of the recultivation work in December 2019, three measuring stations were set up along the cable route to measure soil temperatures down to a soil depth of one metre. Investigations were carried out into aspects including the temperature behaviour of the underground cable conductors depending on the electricity load, temperature variation in the soil above the conduit block and in the wider area, as well as biodiversity in the soil. Earthworms are particularly useful in these studies, as they are important soil organisms that carry out essential functions in the soil, and are able to escape unfavourable soil conditions. Two years after the commissioning of the underground cables, the earthworm populations at monitoring stations 1, 2 and 3 at the «Gäbihübel» in the Bözberg/Riniken area were recorded on 10 May 2022. In addition, the biological activity of the soil (soil respiration, compaction indicators) was investigated by laboratory analysis. The temperature variation in the soil above the conduit block and in the wider area was documented in the two-year period following commissioning (electricity flow through the underground cables).

What is the impact of underground cables on the soil environment? Results after two years of electricity flow

This pioneering study for Switzerland investigated how earthworm populations and soil quality in the soil above the conduit block had been affected two years after commissioning. In addition, initial data on temperature variation above the underground cable was evaluated.

Soil temperatures

From June/July 2020, slight but nevertheless systematic deviations in soil temperatures were measured between the underground cables and undisturbed control soil at a depth of one metre. In general, however, the deviations in soil temperatures between the control soil and above the conduit block remained small. The increase in soil temperatures during normal operating conditions on the line represented a maximum of less than 1 °C in the two years from May 2020 to 2022. This occurred in the measured soil depth closest to the cable conduit block. Especially in winter, it can become slightly warmer in the soil above the cables. The temperature increase above the underground cables is lower in summer, and barely reaches +0.5 °C at a soil depth of one metre. Whether or not the slightly higher soil temperature above the cable conduit block can be explained by the heat emissions from the cables can only be proven by longer-term measurements.

Soil temperature chart

Relationship between intensity of current and cable temperature

There was a weak positive relationship between intensity of current and cable temperature over the entire measurement period (two years). The greater the intensity of current, the higher the cable temperature. This relationship was more pronounced in winter than in summer.

Relationship between intensity of current and soil temperature

In contrast, there was hardly any relationship between intensity of current and soil temperature at a soil depth of one metre over the entire two-year measurement period. This suggests that possible heat emissions from the cables are cushioned by the insulating cable conduit block. For this very reason, it is reasonable to assume that the slightly increased soil temperature around the cable conduit block has no or only very little effect on soil biology and soil quality.

Earthworm populations (individual density, biomass and species diversity)

In general, earthworm populations have not been negatively affected by the transformation of terrain and underground cables. Individual densities of between 224 and 576, on average 357 ind. m-2, and biomasses of between 18.3 and 42.4 g m-2, on average 27.3 g m-2, were measured. These values are comparable to those of other permanent grassland sites in spring with similar annual temperatures and precipitation. On average, six different earthworm species were found at the three sites, with Aporrectodea caliginosa, Aporrectodea longa and Lumbricus terrestris being the most commonly identified. These three earthworm species are typical of these types of soil. There were no significant differences between the undisturbed control soil and the soil above the underground cables. Individual density and biomass were generally elevated in the soil above the underground cables. Likewise, the number of earthworm species in the control soil did not differ significantly from that in the soil above the cable conduit block.

Effects on soil quality

Our soil quality investigations revealed no negative impacts due to the recultivation of soil above the cable conduit block in the first two years after electricity flow. Measurements of soil respiration (a proxy for the biological activity of soil) showed little difference between undisturbed soil and the soil above the conduit block. The soil above the cable conduit block even indicated slightly increased soil respiration activity above the underground cables compared to the control soil, indicating increased biological activity in this soil.

Likewise, the soil is well aerated and not compacted by the large transformation of terrain by heavy machinery. Methane-producing microorganisms were investigated as compaction indicators because they can only multiply in oxygen-limiting conditions. All other soil organisms depend on oxygen for energy production and reproduction. In all soil samples, these microbial compaction indicators were below the detection limit of the method, indicating that the soil is well aerated (i.e. not compacted) after the transformation of terrain and can be used for agricultural purposes without restriction.

In conclusion, soil quality and earthworm populations were not affected by the transformation of terrain and the slightly increased soil temperature in the first two years after electricity flow. The soil above the cable conduit block has been perfectly recultivated, is not compacted and is biologically active. The soil is well populated with earthworms, which is attributable to favourable soil conditions. However, as roots can endanger underground cables, the soil above the cable conduit block must be kept free of tall or deep-rooted trees. Only longer-term studies (> five years) will show whether the increased biological activity in the soil above the conduit block leads to greater loss of humus, which could affect soil functions and biodiversity.

Guest author

Michael Frank
Dr. Beat Frey*

Senior Scientist at WSL Swiss Federal Research Institute

* Dialogue is very important for Swissgrid. The mutual exchange of ideas helps to develop understanding and increase knowledge. That is why we also publish articles by guest authors on relevant topics on the Swissgrid blog.

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