A research on excavation compensation theory for large deformation disaster control and a review on the multiphysical–multiscale responses of salt rock for underground gas storage

We highlight two articles in this issue: A research article titled “Excavation compensation theory and supplementary technology system for large deformation disasters” by Manchao He et al. and a review article titled “Mineralogy, microstructures and geomechanics of rock salt for underground gas storage” by Veerle Vandeginste et al.

The research article “Excavation compensation theory and supplementary technology system for large deformation disasters” by the team of Academician Manchao He comprehensively and systemically summarized their long-term research outcomes on excavation compensation theory and its supporting technology system. The capability of excavation compensation theory in finding effective solutions to large deformation disaster control in underground engineering was demonstrated through its successful applications in various engineering projects. We are sure that this theory and its supporting technologies as well as equipment represent valuable contributions to geotechnical and deep underground engineering.

This excavation compensation theory for the large deformation disaster control is based on the concept that “all damage in tunnel engineering is caused by excavation.” The authors systematically summarized its five components: concept, equipment, technique, design methods with large deformation mechanics, and engineering applications. According to the excavation compensation theory, any supporting system can provide a compensation force for the restoration of the stress state in the surrounding rock to its original stress state as much as possible. Through compensation force calculations, the authors found that high-stress compensation was the most effective means for excavation disturbance control, which could prevent damage in deeply burial tunnels. They proposed a design method and a small deformation criterion for large deformation disaster control based on large deformation mechanics. This design method largely extends the traditional design methods for the excavation of shallow tunnel to deep tunnels.

The authors described their efforts toward the development of supporting equipment such as NPR anchor rods/cables with high resistance, large deformation, and shock resistance. These mechanical properties can effectively achieve the goal of high-stress compensation. They developed an integrated system for comprehensive monitoring, early warning, and control of rock mass large deformation disasters, a tunnel intelligent monitoring and early warning cloud platform system, and a Newton force remote monitoring and early warning system.

The authors developed a series of supporting technologies for different geological conditions. The dual-gradient advanced grouting technology can effectively improve the strength of surrounding rocks in fault fracture zones. The NPR materials can achieve a high-stress compensation for large deformations in surrounding rocks of fault fracture zone tunnels. Two-dimensional blasting technology can convert the destructive nature of blasting technology into a constructive one. The use of NPR materials can effectively solve the difficult problem of controlling large deformations in roadways caused by impact ground pressure in traditional coal mining.

The authors described successful applications of their excavation compensation theory and its supporting technology system in practical engineering projects. These applications include the Mudongzhai Highway Tunnel and the Changning Highway Tunnel in tunnel engineering, the Anju Coal Mine kilometer-deep well in energy engineering, and the Nanfen Open-pit Mine landslide monitoring and warning engineering in slope engineering. These applications prove that the excavation compensation theory can effectively solve the problem of large deformation disaster control, thus resulting in significant social and economic benefits.

The review article “Mineralogy, microstructures and geomechanics of rock salt for underground gas storage” is written by Professor Veerle Vandeginste's team from the Katholieke Universiteit Leuven (Belgium) and associate professor Yukun Ji's team from the China University of Mining and Technology (China). They comprehensively reviewed the multiphysical–multiscale responses to large-scale underground energy storage with salt caverns (scaling up of hydrogen utilization). This review article is based on their long-term research outcomes on underground energy storage, water–rock interactions, and multiphysical coupling. Hence, the methods outlined here can provide a deep insight into cavern formation and operation maintenance.

This article discussed the significance of mineralogy, geochemistry, microstructure, and geological mechanical properties of salt rock for the construction of gas storage facilities. Underground space resources are key options for large-scale energy storage and hydrogen energy utilization and can overcome the issues related to nonsustainability of clean energy supply such as wind, solar, and water. Salt rock has low permeability and good self-healing property, thus being an ideal porous medium for underground gas storage.

This article summarized the research advances in the identification methods and geochemical indicators of typical salt rock minerals. These methods and indicators can be used to quantitatively characterize salt rock mineral composition and identify the source of diagenetic brine (distinguishing marine and nonmarine). Mineralogy and geochemistry analyses can identify the mineral types in salt rock and reveal the sedimentary environment and diagenetic evolution.

This article focused on the micromechanisms of salt rock deformation and elucidated the fragmentation deformation accompanied by microcrack development and grain rotation, the intergranular sliding and diffusion induced by dissolution-diffusion mass transfer under high-temperature and high-stress environments, and the plastic behavior of intracrystalline dislocation movement. The visualization methods for microstructure analysis were highlighted in the investigation of the macromechanical behavior and permeability evolution of salt rock.

This article further reviewed the investigations on the geomechanical behaviors of salt rocks and the mechanisms linked to how low confining pressure and high differential stress-induced salt expansion can affect salt rock sealing and the safety of underground gas storage, and how impurity differentiation distribution can induce different creep behaviors. It was revealed that the steady-state creep rate is affected by both stress-controlled dislocation creep and pressure solution creep controlled by grain size. The geomechanical behaviors of salt rocks under high temperature and pressure loads are unique.

This article finally discussed the challenges of salt cavity construction and gas storage. The authors explored the impact of water jets on salt cavity shapes under the differential dissolution kinetics of fluid mechanics and salt minerals, and discussed the important influence of interlayer permeability on the gas migration range in salt rock. On this basis, it was further demonstrated that gas injection and extraction can induce microcracks in surrounding rocks due to thermal cycling.