Investigation of Historic Mining Infrastructure in the Upper Harz Mountains and Development of Repurposing Concepts
Significant historic mining infrastructure in the Upper Harz Mountains
A notable structure of the Upper Harz mining industry in Germany is the Ernst-August-Stollen, which played a key role in water management and ore transport. Completed in 1864 between the Gittelde portal (189.5 m ASL) and the Clausthal district mines, and later extended via Bockswiese to Lautenthal, the Ernst-August-Stollen is the most important and deepest water drainage adit in the Harz region. Its total length is about 40.2 km (Figure 1). The main tunnel sections have an almost uniform cross-section of 4.23Â m2, with a height of 2.52 m and a width of 1.68 m. The slope is a constant 1Â m over 1,580 m of adit length.
From the portal at Gittelde (189.5 m ASL), the drainage adit initially crosses the flat permian formations covered by quaternary strata gradually rising towards the Harz Mountains. Around the Fahlenberger Lichtschacht shaft, 800 m from the portal, the VarisÂcan folds of Culm greywackes and shales are reached. At 2,591 m from the portal, in the Hilfe Gottes shaft of the closed Grund ore mine (197 m ASL), the Silbernaaler vein system is encountered. Following this vein eastwards in the footwall, the Ernst-August gallery runs roughly beneath the valley of the Innerste river.
The northeast-facing section Wildemanner Flügelort connects with the Ernst-August blind shaft at Wildemann. From there the main gallery follows the Zellerfeld main vein to the south-east and joins the adit Tiefste Wasserstrecke. This continues along the Burgstaetter vein to the Kaiser Wilhelm II shaft and on to the former Caroliner shaft east of Clausthal. Here the Ernst-August-Stollen reaches its maximum depth of 392 m ASL At the St. Lorenzer shaft in Clausthal, a section branches off to the west into the Rosenhöfer district, connecting with the Thurm-Rosenhof, Silbersegen and Ottiliae shafts. From the Schreibfeder shaft near Zellerfeld, a section completed in 1871 leads north to Bockswiese and the Herzog August and Johann Friedrich mines. In 1892 a connection was made to the north-west to the mines on the Lautenthal vein system. The last mining activities in this area took place between 1945 and 1957. From the new Lautenthalsglück mine shaft, an exploratory section was driven westwards for over 3 km. The new discoveries of vein mineralization – mainly sphalerite – in the Bromberg ore zone are the last of their kind in the whole of the Upper Harz. After the end of mining at Clausthal in 1930, power stations were operated at the level of the Ernst-August-Stollen at the Kaiser Wilhelm II shaft and later at the Ottiliae shaft, using the water from the pond and ditch system and the gallery as a tailrace. The operation of the Clausthal mine power stations ended in 1980. Active maintenance of the adit was continued by Preussag AG Metall’s Grund, which also operated a power station at Achenbach shaft. With the closure of the Grund ore mine in 1992 and the closure of the mine’s power plant, the final section of the adit between the portal and Wiemannsbucht shaft was abandoned following the completion of the decommissioning work.
Today, the Ernst-August-Stollen is part of the Harz UNESCO World Heritage Site known as the “Rammelsberg Mine, Old Town of Goslar and Upper Harz Water Management”. The World Heritage element is still of significant water management relevance for the Upper Harz region today, as the adit continues to drain the entire Upper Harz mining area. The mine water that emerges at the portal in Gittelde is channelled into the nearby Markau receiving watercourse, which flows into the North Sea via the Söse, Rhume, Leine, Aller and Weser rivers.
Current condition of the Ernst-August-Stollen and the mine buildings in the Upper Harz Mountains
With the end of mining, dewatering ceased and the Upper Harz mine workings were flooded in a controlled manner. Today, the mine workings below the adit “Ernst-August” are completely flooded (Figure 2). The discharge rate at the Gittelde portal is highly dependent on the rainfall conditions in the Upper Harz and is approximately 160 l/s. Current continuous discharge measurements are still pending. The overflow point of the flooded mine workings of the Grund mine is located at the Achenbach shaft. For this reason, an oil separator has been installed here to retain potentially water-polluting elements. The central drainage system for the Zellerfeld and Clausthal mining districts is located at Neuer Johanneser shaft.
The accesses previously used for inspection purposes (Achenbach, Knesebeck, Wiemannsbuch, Neuer Johanneser, Kaiser Wilhelm II and Ottilia shafts) have now been sealed with concrete. Apart from the portal at Gittelde, the only other direct access to the drainage tunnels is through the shaft Neuer Förderschacht in the Lautenthalsglück Mining Museum. Indirect access is theoretically possible through some of the Clausthal shafts from the drainage adit Tiefer-Georg-Stollen.
The Lower Saxony State Office for Mining, Energy and Geology (LBEG) is responsible for hazard prevention and remediation in the Upper Harz mining area. UNDERyourfeet – Ingenieurgesellschaft für Geotechnik mbH was commissioned by the LBEG to carry out initial investigations and explorations of the current condition of the Ernst August adit. Information on the geometry of the gallery and the connected mine systems is available in the form of a comprehensive historical mine plan. The current condition of the first 5 km of the adit from the portal in Gittelde to the Knesebeck shaft is very well known and is in a very good state of preservation. From the portal to where it reaches the stable Zechstein limestone, the entire 1,050 m length of the adit is lined with a vault-shaped lining of cuboid dolomite stones. Wherever the rock is disturbed or where clay slate or greywacke layers tend to detach in a shell-like manner under slight rock pressure, lining was inserted. In addition to greywacke quarry stones, mainly slag stones were used to the east of the Achenbach shaft. In this way, various areas of the Ernst-August adit were lined with a vaulted slagstone wall for long-term durability. In the 1960s, a parallel drift was excavated 560 m from the portal to the Achenbach shaft, as long-term stability no longer appeared to be guaranteed. Later, parts of this section of the adit were secured with concrete (Figure 3). The current condition of the adit further to the east is not sufficiently known to make any statements about the hydraulic connection of the adit to the surrounding adits and underground systems and its complete stability.
Current research projects to explore (old) mining infrastructure
Due to difficult accessibility and complex sampling conditions, there is little data on the water quality of the water solution tunnels in the Upper Harz. Current investigations by Harzwasserwerke in 2016, Clausthal University of Technology (CUT), Clausthal-Zellerfeld/Germany, in 2020 and the Federal Institute for Geosciences and Natural Resources (GBR) in 2023 focus primarily on the front, approximately 5 km long tunnel section from the mouth to the former Grund ore mine. Information on the geometry of the gallery and pit systems is available in the form of partially digitized fissure structures, but the current condition of the cavities is not sufficiently known to allow conclusions to be drawn about the hydraulic connections between the galleries and pits and their stability. However, this is necessary to understand the behavior of the system in the event of potential future use.
Therefore, both spatial and qualitative investigations of the adit and pit systems need to be carried out using state-of-the-art measurement technologies. The spatial investigation of the adit systems requires the use of mobile measurement technology, while the investigation of the flooded pits requires the use of submersible measurement technology. Both techniques face the challenge of adapting to the demanding environmental conditions underground.
Applications of laser scanning for modelling historic mining in the Harz Mountains
Laser scanners have been used to a limited extent in the field of old mining, with documentation currently mainly carried out using the structure from motion method, a photogrammetry technique. Roos und Paffenholz (2) at the Institute of Geo-Engineering (IGE) of the CUT therefore investigated the acquisition and modelling of laser scanner-based 3D point clouds in historic mining in the Harz Mountains. In collaboration with the Roter Bär educational mine and the Rammelsberg World Heritage Site, various recording locations for this purpose were selected. The Zoller+Fröhlich (Z+F) IMAGER 5016 laser scanner for the underground data acquisition was used. Figure 4 shows the workflow from the acquisition of the 3D point cloud to the (pre-)processing of the acquired data and the derivation of a colored and meshed 3D model.
Table 1 addresses the particular challenges posed by underground use, especially in historic mines.
Once the data was acquired, the 3D point clouds were coloured and meshed. Then the meshed 3D models were visually evaluated and their suitability for large-scale applications was checked. Figure 5 shows a coloured 3D point cloud of the wheelhouse WennsglĂĽckt in St. Andreasberg.
The laser scanner-based 3D models were compared with the photogrammetry models to demonstrate the advantages and disadvantages of both methods in historic mining. This proved that terrestrial laser scanners are ideal for use in underground mining infrastructures. The method was tested for a range of applications, including mining, archaeological documentation, geological identification, monitoring, surveying and virtual tours. Initially, a large number of viewpoints were used to determine the optimal number for achieving a high level of detail. A lower number of viewpoints can significantly reduce the survey time. Furthermore, the use of mobile multi-sensor systems, which allow data to be captured during movement, significantly increases efficiency. However, the cramped conditions in historic mines must be considered. Figure 6 (left) shows a first impression of a 3D point cloud captured with a mobile, person-carrying system for the power station in the Ernst August gallery. The Z+F FlexScan22 was used for the acquisition, with a Z+F IMAGER 5016 recording the 3D point cloud (Figure 6 right).
Table 2 summarizes the recommended minimum settings for the Z+F IMAGER 5016 for underground mine surveys using the classic static approach. These settings have proven their worth in the historic mines of the Harz Mountains.
Select the “high” quality option for the Z+F IMAGER 5016 for most extensions and objects in the cavities to ensure minimal recording of mixed pixels. A procedure for processing and merging the 3D point clouds was also developed. The Cloud-to-cloud (C2C) function in CloudCompare allows to create a 3D model with a high point density without point doubling. This enables to create a very detailed model with little effort and in a short time. The Institute of Geo-Engineering (IGE) is currently researching how to post-colour laser scanner-based 3D point clouds with high-resolution digital images from photogrammetry for selected areas with high montane-historical value.
Development of floating and diving multi-sensor-systems
The use of autonomous systems, robots and intelligent solutions for forecasting and monitoring water quality parameters in mining is currently the subject of research. From 2017 to 2020, the cross-institute ESF junior research group ARIDuA (Autonomous Robots and Internet of Things in Underground Facilities) at TU Bergakademie Freiberg conducted research into the use of mobile robots and temporary sensor networks as well as data communication infrastructures in underground mining. The Institute for Advanced Mining Technologies (AMT) at RWTH Aachen University develops robust, autonomous systems and smart sensor-based applications for navigation, object recognition and process optimisation in modern mining. Underwater cameras specially developed for underground mines are already being used to explore shafts in flooded mines. The Robotic Explorer (UX-1) robot was specially developed for autonomous 3D mapping of flooded mines (4, 5). The biggest challenges include communication and navigation as well as the safe recovery of the technology.
Most research projects in the field of exploration of flooded mines focus on the development of a highly complex diving robot equipped with various sensors to collect data on water quality, geological structures and environmental conditions. The loss of such a diving robot due to possible complications during the dive can not only endanger the entire exploration, but also lead to substantial financial losses. The UNEXMIN research project has given rise to the start-up UNEX-UP, which offers exploration services with various robots. Highly specialised robots are used here with the existing risk of loss and without the technology being able to be used on site in the long term for targeted or flexible monitoring as required. As each mine is unique, the diving robots need to be adapted to the specific environmental conditions or issues. In order to minimise reinvestment costs, the development of a modular or swarm-based system could help. As part of a pilot project, the Institute of Mining (IBB) at CUT, in collaboration with the Department of Underwater Robotics at Fraunhofer IOSB-AST and RAG Aktiengesellschaft, is introducing a modular submersible robot system at a test site for the first time. The project focusses on the safe positioning and removal of the technology.
A future joint project between CUT, TU Braunschweig and Ostfalia University of Applied Sciences is focussing on the exploration of the Ernst August adit. A floating multi-sensor system is being developed which will be able to carry out both spatial and water quality measurements along the first 5 km of the main section of the adit. When recording cavities that are partially filled with water, there is the problem of reflection and the inability to scan below the water surface with the large number of laser scanners with a near-infrared wavelength. For this reason, a laser scanner specially tailored to the water area, developed by the Fraunhofer IPM´s Object and Shape Detection department, is to be used in the project to enable the combination of above and underwater images of the floating multi-sensor system.
At the same time, a stationary monitoring network is to be established within the adit, which will be supported by virtual artificial intelligence (AI) sensors. Stationary, physical water sensors are susceptible to deposits such as iron precipitation on the sensor surfaces and need to be cleaned at regular intervals. Optimising the sensor technology for this area of application and supporting it with virtual sensor technology can reduce the maintenance effort here. The aim of the qualitative measurement technology is a permanent monitoring concept for the water management-relevant world heritage element. The knowledge and methods gained from the project can be extended to other adit systems in follow-up projects and used to analyse the potential for the utilisation of the excavation structures.
Previous re-use strategies in the Upper Harz Mountains and required research
Cultural reuse is the most widespread use of old mining sites in the Upper Harz. Numerous mining museums and visitor mines illustrate the rich mining history of the region and allow locals and tourists to experience it. Further reuse options for the old mining infrastructure, in particular integration into the local water and energy supply network, could create additional added value for the region. There are many potential end-users along the Ernst August mine adit, including local communities, industrial companies and water and energy utilities. However, comprehensive studies of the current state of the infrastructure and water quality are required. The integration of micro-hydropower could be considered, as up to 160 l/s of water are available at the mouth of the Gittelde. However, the slope of the tunnel is only 1:1,500. Damming the water along the tunnel to increase the hydraulic potential is risky, as the primary function of the water solution must not be compromised. Therefore, a lateral relief channel would have to be installed if a hydroelectric power station was to be built. Structural measures on the tunnel are also in conflict with the preservation of the World Heritage character. A detailed study should be carried out to analyse how a subsequent use for power generation could be successfully integrated into the World Heritage Site.
Another option for supporting the local energy supply could be to exploit the geothermal potential of the flooded mine workings. The amount of heat provided by the plant can cover the needs of individual buildings, such as museums or university facilities (6). The water that emerges from the tunnel mouth at Gittelde has a temperature of 11 to 12 °C for most of the year (Figure 7). Looking at the deeper flooded mine workings and assuming a temperature increase of 3 °C per 100 m depth, a temperature of approximately 27 °C can be estimated for the Achenbach shaft at the 19th level. The machine room of the Achenbach shaft also offers sufficient space for the necessary technical equipment. Other locations along the gallery route could also have geothermal potential with appropriate end users. However, difficult access is a major hurdle, as all the shafts have been sealed off. In order to explore and exploit the geothermal potential, boreholes would need to be drilled and the local power grid adapted. In addition, an extensive campaign of water quality measurements would be required to assess the impact of flooding. Nevertheless, the geothermal use of mine water in the Upper Harz region could provide a sustainable energy supply for a large number of end users and also make a significant contribution to reducing CO2 emissions in the region, which in turn would outweigh the costs of implementation in the long term.
The use of old mining infrastructure for energy storage in the form of underground pumped storage power plants was already investigated in 2011 in a joint project of the Lower Saxony Energy Research Centre (EFZN) in Goslar (8). The Bad Grund ore mine was used as an example in the study. However, it has been shown that old mines can no longer be used without restrictions for the construction of a modern pumped storage power plant, as the old mining infrastructure does not meet the safety and stability requirements and would therefore have to undergo extensive rehabilitation (9).
In the past, the mine water discharged from the Ernst August mine into the Markau was used as process and industrial water by the company Fuba, Hans Kolbe & Co (now MPM Environment Intelligence GMBH). At the time, the company manufactured components for radio technology and used the water for cooling and rinsing purposes. Even today, the stable hydrogeochemical properties of the water at the mouth of the adit mean that it could be used to support local industry. However, each application must be assessed to determine whether the water meets the quality requirements or whether appropriate treatment measures need to be implemented in the interim. Of course, permission must be obtained in advance, and water extraction must not exceed the low water level of the Markau. To support the drinking water supply of Harzwasserwerke GmbH, the water would be fed into the Söse Nord pipeline. To do this, the water from the mine must have the same degree of hardness as the water in the Söse I waterworks and must not lead to a deterioration in water quality. Harzwasserwerke carried out extensive water quality tests between 2016 and 2017 and concluded that although the water does not contain any toxic elements, the overall hardness and treatment of the water from the Ernst August mine to produce drinking water is too complex.
Summary and outlook
The discussion about the future use of the mining infrastructure in the Upper Harz region, which goes beyond cultural use, is an example of the challenges faced when dealing with industrial monuments. On the one hand, there is the effort to preserve these witnesses of technical and social history for future generations. On the other hand, innovative reuse concepts open up opportunities to integrate the cultural heritage into the present and to create new economic and social prospects for the region. The Ernst August Gallery, a central element of the UNESCO World Heritage Site, is the subject of several studies by the IGE and IBB at CUT. The gallery, which has played an important role in water management since 1864, is still essential for the drainage of the region. Despite its good state of preservation, there is a lack of comprehensive current data, especially for the eastern sections of the tunnel. Modern survey methods, such as the combination of laser scanning and photogrammetry, allow accurate 3D models of the underground infrastructure to be created in a shorter time. The development of mobile multi-sensor systems (MSS) offers innovative approaches to the exploration and monitoring of flooded mines. In addition to the methodological and technical development of MSS, future research will focus on detailed mapping and condition assessment of mine infrastructure. Detailed feasibility studies are needed to fully assess the potential for energy and industrial reuse. The potential for conflict arises in particular from the different interests involved in heritage protection, economic use and environmental sustainability. Successful reuse of old mining infrastructure therefore requires interdisciplinary cooperation between local authorities, investors, environmental organisations, the local community and scientists.
References / Quellenverzeichnis
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(7) Bothe-Fiekert, M. (2021): Hydrogeochemische Studie des Ernst-August-Stollens. Masterarbeit. Technische Universität Clausthal, Clausthal-Zellerfeld. Institut für Endlagerforschung.
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