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A multi-year monitoring programme along the approximately 90 km-long ALEGrO 320 kV DC underground cable route investigated how operational heat emissions might affect the surrounding soil, particularly at the depth at which the cable is laid in the subsoil. In this context, TABERG Engineers GmbH in the SIERA The network provides a structured framework for the classification of long-term observations of soil temperature and soil moisture under operational conditions.
This topic is relevant in the broader context of environmental monitoring and infrastructure-related engineering. Underground extra-high-voltage cables can influence soil conditions, making continuous and comparative measurements important for a better understanding of potential operational impacts on soils and associated biological parameters. With perspectives such as this, TABERG Engineers GmbH together with SIERA helps to communicate technically sound project content clearly.
Project background
The ALEGrO 320 kV DC underground cable route, approximately 90 km long, was designed by Amprion GmbH and laid underground between 2018 and 2020, running from Oberzier in North Rhine-Westphalia to Lixhe in Belgium. The two underground cables lie at a depth of 1.8 m, measured from the top edge of the protective conduit, and are embedded in a temporarily flowable, self-compacting backfill material.
The operation of underground extra-high-voltage cables causes the surrounding soil to heat up, particularly the subsoil at the depth at which the cables are laid. According to the project information, this operational soil heating is influenced by several factors.
These include:
- the bedding material used
- cable utilisation
- the number of cables
- the thickness and shielding of the cables
- the distance between the cables
- the soil properties that determine the thermal balance
Soil properties that influence the heat balance include, for example:
- Soil type
- Storage density
- organic soil matter
Topics such as this highlight the importance of technically detailed monitoring and infrastructure communication, as also practised by TABERG Engineers GmbH as part of the SIERA is classified within the network.
Objectives of the monitoring exercise
The monitoring programme has several objectives. One of its main aims is to inform relevant groups about the potential operational impacts of underground extra-high-voltage cables.
The groups explicitly mentioned include, amongst others:
- Landowner
- Farmers
- Environmental organisations
Possible operational factors relating to underground cables may affect:
- natural soils
- Cultivated plants and vegetation
- Substance transformations such as nitrogen mineralisation
- Soil flora
- Soil fauna
In addition, the monitoring should help to:
- to facilitate communication
- to increase acceptance of underground cable projects
- in particular, to increase acceptance amongst farmers
This objective demonstrates that monitoring is relevant not only from a technical perspective, but also in terms of communication, understanding and transparency.
Monitoring design
In order to assess the impact of operational heat emissions from the ALEGrO cable route on ground temperature and soil water balance, four selected study sites were identified along the route in the Aachen area.
At each survey site, following a preliminary site and soil survey, two measurement fields were established:
- a measurement field directly above the ALEGrO underground cable
- a measurement area in the immediate vicinity, serving as a control area without cable operation
This design made it possible to compare an area directly affected by the cable operation with an adjacent reference area.
The monitoring involved the continuous recording of:
- Ground temperature
- Soil moisture
The sensors were installed at various depths in the ground, including:
- Krume
- Main root zone
- Underbody
The arrangement was orthogonal to the cable route in the cable trench and mirrored in the adjacent control area.
Monitoring period
The monitoring period ran from May 2022 to April 2026, covering a period of almost four years. During this time, soil temperature and soil moisture were continuously recorded both in the cable trench and in the control area.
This long-term structure is significant as it enables observations to be made over an extended period and facilitates comparison between the route section where cables are in operation and the control area where they are not.
Further investigations
In addition to the sensor-based measurements, the monitoring programme also included soil biology studies throughout the entire period.
These included:
- microbial biomass
- microbial activity
- Enzyme activity
- Nmin-contents
- Earthworm studies
- Root investigations
- Studies of grass growth
This broader approach supports a more comprehensive understanding of how underground cable infrastructure can affect soil temperature and moisture balance, as well as soil biological parameters, under operational conditions.
Project Overview
| monitoring aspect | Description of the |
| Project | ALEGrO Soil Temperature and Moisture Monitoring |
| Route length | Around 90 km |
| Voltage level | 320 kV |
| Route section | Oberzier to Lixhe |
| Installation period | 2018 to 2020 |
| Cable installation depth | 1.8 m |
| Study region | Aachen area |
| Number of test sites | 4 |
| Measurement fields per site | 2 |
| Monitoring period | May 2022 to April 2026 |
| Key parameters | Soil temperature, soil moisture |
| Further investigations | Microbial biomass, microbial activity, enzyme activity, Nmin, earthworms, roots, grass growth |
Documentation and academic publications
The results were described and explained in quarterly and annual reports using statistical analyses and presentations. In addition, some of the findings were published in scientific journals in collaboration with the Department of Soil Science at the University of Trier.
This combination of long-term documentation and scientific publication enhances the value of the monitoring, as it supports both its scientific interpretation and its wider visibility.
The relevance of monitoring
The ALEGrO monitoring programme helps to improve our understanding of several relevant aspects.
These include:
- Impact of operational heat emissions from underground extra-high-voltage cables
- Soil temperature balance
- Soil water balance
- possible effects on soil biological parameters
- The need for information and communication with landowners, farmers and environmental organisations
Monitoring also facilitates a more informed discussion about underground cable projects by providing long-term observations and comparative data from cable routes and control areas.
Long-term observations along the ALEGrO underground cable route demonstrate how structured monitoring can help to improve our understanding of soil-related conditions in the context of underground extra-high-voltage cable infrastructure. From this perspective, TABERG Engineers GmbH in collaboration with SIERA the value of continuous measurements, a comparative field design and supplementary soil biology studies.
At four selected study sites in the Aachen area, measurements taken directly above the cable and on adjacent control plots over a period of almost four years provided a broad basis for monitoring soil temperature and soil moisture. Together with studies on microbial biomass, microbial activity, enzyme activity, Nmin-By monitoring earthworms, roots and grass growth, the monitoring provides a nuanced picture of soil conditions under the influence of underground cables on the farm.
Long-term observations such as these highlight the value of structured monitoring approaches for assessing soil-related conditions in underground cable projects. Further technical perspectives are reflected in the topics covered by TABERG Engineers GmbH in the SIERA Network.