Search

The exploitation of the underground is currently undergoing a major transformation process. Besides mining, alternative ways of utilization are playing an increasingly important role. These alternative utilizations include the storage of gases, the disposal of radioactive wastes, and the implementation of geothermal storage facilities. In addition, more and more mining operations are being shut down, like the German hard coal mining industry in 2018. But the integrity of closed mines must be ensured in the post-mining phase. In order to capture such large and complex structures efficiently and reliably, new surveying techniques are required. A promising approach is the use of photogrammetric methods to generate three-dimensional models of the cavity. Photogrammetry is currently known mainly from aerial photography. It is conceivable to use such methods for underground applications. In recent years, significant progress has been made in the algorithm-based orientation of unstructured image sets. More powerful computer systems and the growing range of unmanned aerial vehicles could enable partially automated capture. However, a number of problems have to be solved for underground imaging techniques. Unfavorable exposure conditions complicate image alignment. Furthermore, the models must be integrated into existing surveying systems with a sufficiently high degree of accuracy. In order to consider the numerous technical aspects, the development of an integrated capture and evaluation approach is necessary. This could make an important contribution to the monitoring of critical cavities of different commodities. Such an approach would also fit into the current digitization trend in the mining industry and provide new opportunities for ground control in particular.

The topic of ground movements in Germany has been studied extensively in the past, especially in the field of active mines. The active hard coal mines in Germany were finally shut down in 2018 and lignite mining is expected to take place only until 2038. The so-called long-term liabilities of the mine operators in Germany include, among other things, the long-term guarantee of stability and thus the monitoring of ground motion. So far, the economic use of underground mining in Germany was mainly the supply of raw materials. In the future, the underground storage of compressed air, methane or hydrogen will play an important role in renewable energy supply and climate change. Therefore, the underground storage space will become more important and the spatial planning is essential to ensure availability of safe underground openings for the various options of environmentally friendly energy storage. However, this renewed usage of underground openings may also bring new and sometimes unknown challenges of geomechanical influence. The aftermath of hard coal and lignite mining will be an increasing challenge in mining subsidence engineering. On the other hand, new possibilities due to underground spatial planning may lead to subsidence and/or heaving of the upper surface.

In 2010, the fracking discussion in Germany caused a number of changes in German law, which came into force in 2016. Especially the production of gas had to be regulated. With the legislation amendment, the Subsidence-Area Mining Regulationhas been revised, too. The changes expand the compensation of mining damages, especially to the extraction with drilling from the surface and underground storage. Although the Subsidence-Area Mining Regulationhas been revised, the area of main influence (subsidence of 10 cm) remains to determine a relevant boundary for mining damages. The determination and prediction of this boundary above caverns are presented in this paper. In addition, further elements of ground movements and their relevance to mine damages are analyzed. The usage of the area of main influence to fix a relevant boundary for mining damages does not correspond to the relevant elements of ground movements. A limit for differences in subsidence (tilt) or horizontal changes in length should be preferred to describe the relevance of mining damages on buildings. Furthermore, this paper outlines the meaning of using the area of main influence to fix a relevant boundary for mining damages.

The utilization of Underground Pumped Storage Power Systems (UPSP) addresses the growing need for energy storage in the face of increasing intermittent energy sources. Simultaneously, the closure of mining activities has resulted in vast underground spaces potentially becoming available for alternative purposes. This paper explores the potential of repurposing abandoned mines, particularly coal mines, as lower reservoirs for UPSPs.

Abstract: In order to understand the permeability evolution of coal under different temperature and pore pressure conditions, a series of experiments are carried out by simulating stress state and temperature conditions of coal in depth, meanwhile the permeability is obtained by calculation using fractional derivative calculation method. A damage-induced permeability model for the complete stress–strain process of deep coal considering temperature and pore pressure is proposed by introducing the non-uniform coefficient. The model is validated by experimental data from different perspectives. Results show that the initial permeability decreases with increasing temperature while it increases with pore pressure rising. Furthermore, the developed permeability model is proved to agree better with the experimental data of permeability evolution with pore pressure in comparison with the traditional models. The experimental results of permeability evolution caused by temperature and axial stress can be characterized by the proposed permeability model well. Finally, the model can accurately characterize the piecewise permeability evolution that it decreases in compaction stage and increases in dilation stage with axial strain. Discussions on the effects of non-uniform deformation coefficient and permeability-damage coefficient on the permeability model indicate that the increasing non-uniform deformation coefficient will accelerate permeability evolution. The permeability trend in the volumetric dilation stage is found to be more sensitive to the permeability-damage coefficient in comparison with the compaction stage. Article highlights:

Considering temperature and pore pressure, a damage-based permeability model for the whole stress–strain process is established.

The experiments under different conditions are carried out to verify the permeability model.

The sensitivity of non-uniform deformation and the permeability-damage coefficient is analyzed.