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The emergence of the “Three Caverns” 三洞 thought was a critical step in the formation of medieval Daoism. It proposed the first viable approach to integrating emerging Daoist scriptural traditions, enabling the creation of the first canonical Daoist catalog, and laying the foundation for the compilation of the Daozang and the establishment of the Ordination Ranks 法位 system. Scholars generally agree that the Shangqing taiji yinzhu yujing baojue 上清太極隱注玉經寶訣 played a significant role in the development of the Three Caverns thought. However, research on the formation of this scripture remains lacking. This study fills this gap by confirming the composition of the scripture through two independent lines of evidence. Then, based on new insights into its composition, this study discusses the historical context of the Three Caverns concept in this scripture and its direct impact on Lu Xiujing 陸修靜 (406–477)’s cataloging work. These discussions illustrate that, when confronted with the challenge posed by the newly composed Shangqing scriptures, the authors of the Shangqing taiji yinzhu yujing baojue employed the integrative approach commonly found in the Ancient Lingbao Scriptures to propose a more inclusive scriptural system. This approach played a crucial role in providing a theoretical foundation for the formation of medieval Daoism.

The mechanism of lagging water inrush in underground tunnel constructions due to the proximity of a karst cavern with confined water is investigated via large-scale physical three-dimensional (3D) model testing and 3D numerical simulations. A new method is proposed for the preparation of modeled karst caverns filled with confined water. The physical 3D model testing is divided into two stages: tunnel excavation and hydraulic pressure loading. Multivariate information is obtained at the two stages using multiple measurement techniques. The results indicate that the displacement, hydraulic pressure, and the developmental trend of the damage zone in the tunnel excavation process are related. It is evident from the physical 3D model testing results that the process of water inrush can be divided into three stages, which include the initiation of group cracks, the formation of a water inrush channel, and the complete collapse of the water-resistant slab. The 3D model testing in conjunction with the 3D numerical simulations reveal that the disturbance due to excavation has an obvious impact on water inrush channel formation. However, an increasing hydraulic pressure in the karst cavern has a greater impact on the collapse of the water-resistant slab. These test results can provide support and guidance for tunnel construction under conditions that are susceptible to water inrush events.

Salt caverns are internationally recognized as excellent facilities for underground energy storage. Creep shrinkage deformation will occur in deep salt caverns under the action of high-ground stress, and it is a key factor to evaluate the safety of salt caverns. However, there has been no salt cavern creep shrinkage mechanism research on ultra-deep salt caverns. In this paper, the creep shrinkage mechanism of an ultra-deep salt cavern is established from the theory, numerical simulation, and field application. First, the actual creep field experiments (pressure testing and sonar testing) of the salt cavern (depth ≥ 2000 m) are carried out. Second, theoretical models of salt cavern creep shrinkage are established from four influence aspects (creep shrinkage, heat conduction, salt dissolution, and brine permeability). Third, a 3D geological model is built to analyze the creep stability of deep salt caverns based on their field conditions. The novelty of this paper is analyzing the creep shrinkage of the ultra-salt cavern by the theoretical model, numerical model, and field experimental data systematically. The results show that the shrinkage of the salt cavern and brine thermal expansion is key factors leading to pressure lifting of the salt cavern, which accounts for 0.6121 and 0.2147 in the four influence aspects. Three creep phases are obtained: rapid rising stage, steady rising stage and decelerating rising stage. The study provides a reference for the creep shrinkage and field application of salt caverns.HighlightsCreep field experiments of the salt cavern are carried out.Theoretical models of ultra-deep salt cavern creep mechanisms are built.An ultra-deep 3D salt cavern creep numerical model is built to analyze the creep stability of salt caverns.A mathematical model is proposed to analyze the factors that influence salt cavern creep.

A considerable number of deep engineering cases show that strain rockbursts occur on both sidewalls of circular caverns and ultimately form a symmetrical V-shaped notch. To investigate the occurrence process and mechanism of strain rockbursts in deep circular cavern under high stresses, simulation experiments under four different three-dimensional (3D) stress conditions were conducted on cubic granite specimens with a prefabricated circular hole using the true-triaxial electro-hydraulic servo mutagenesis testing system. The rock bursting process was monitored on the sidewalls of the hole and recorded in real-time with a wireless microcamera. The results show that the entire rock bursting process on the sidewalls can be divided into four distinct periods for every 3D stress condition: calm period, pellet ejection period, rock fragment exfoliation period, and rock bursting period. Under the conditions that the vertical stress is constant and the horizontal radial stress is equal to the horizontal axial stress, the rockburst severity of the sidewalls clearly decreases with increasing horizontal stress. When the vertical stress is constant and the horizontal axial stress is low, the rockburst severity clearly decreases with the increase of horizontal radial stress. In contrast, when the horizontal axial stress is high, the rockburst severity becomes more serious with increasing horizontal radial stress. During the process of rock fracture and bursting, the damage zones on both sidewalls form two symmetrical V-shaped notches, and the line connecting the centres of the two V-shaped notches is perpendicular to the maximum principal stress direction. The symmetrical V-shaped failure modes on both sidewalls under the four stress conditions are in line with the statistical relationship of the far-field stress state and failure mode of a deep circular cavern without support (Martin et al. in Can Geotech J 36(1):136–151, 1999).

Quantitative stability evaluation of multi-tunnel structure is an important issue related to the safety assessment and stable construction of geotechnical underground tunnels. In this work, the overall failure process of twin tunnels were exhibited by a physical simulation based on the 3D printing (3DP) sandstone analogues model, and a safety factor method was also presented for evaluating the general safety of multi-tunnel structure. For checking the 3DP material performance of physical model, uniaxial and triaxial compressions for the 3DP cylinder specimens were first tested and showed that their mechanical properties and failure characteristics were similar to natural rock in general. Then, the overloading tests for twin-tunnel model were carried out and have exposed the critical position of overall failure of twin-tunnel structure through visual observation and automatic measurement. Testing results and corresponding numerical back analysis indicated that the connectivity of plastic strain between tunnels can be deemed as the conservative instability criterion (i.e. yielding of material) and the inflection point of tunnels’ displacement can be deemed as overall failure criterion (i.e. structure failure) for twin-tunnel structure. The safety analysis for underground hydraulic caverns indicated that this method can provide a reference for quantitative and reasonable evaluation of the general safety of multi-tunnels or caverns and the local instability zone of surrounding rock.

Recent in situ pressure test indicates that there is a mudstone interlayer with high permeability in the open hole of the underground gas storage (UGS) salt caverns in Jintan, China. The interlayer is called the “micro-leakage interlayer (MLI).” MLI brings a great new challenge for UGSs construction and operation. The stability evaluation is the main research target of this paper. Laboratory tests have been carried out on samples collected from the target formation to determine the mechanical properties. A 3D geomechanical model of the two adjacent caverns with MLI is established based on the geological data and the laboratory test results. The minimum and maximum limit operating pressures are determined as 6 MPa and 18 MPa based on the numerical simulation results of six operating conditions. Two operating conditions (synchronous and asynchronous injection–production) are designed and discussed. The result shows that the MLI has little effect on the stability of the caverns and can be ignored. The stability under the two operating conditions is quite good, suggesting that asynchronous injection–production can be used in the actual operation. This makes the operation more flexible to meet unpredictable situations. The tightness analysis under the two operating conditions will be the subject of future investigations.

The key to preventing the seepage of hydrocarbons from underground water-sealed oil storage facilities is to maintain a certain thickness of groundwater on the top of the caverns so that the groundwater pressure can maintain their tightness. However, the presence of fractures in the rockmass that surrounds the caverns complicates the calculation of the seepage field in the oil storage area. Conventional two-dimensional or equivalent three-dimensional seepage models cannot accurately reflect the impact of the fractures on the seepage field. Therefore, in this study, a dual-porosity medium modeling approach was used to develop an unsteady-state seepage model using the three-dimensional discrete element software 3DEC. The model was developed to assess the effect of grouting on the rock mass that surrounds oil storage caverns that took into account the possible presence of non-persistent fractures. Thus, the simulation of the three-dimensional seepage field of large-scale water-sealed oil storage caverns was realized. The three-dimensional seepage field evolution law and seepage control effect under different working conditions during the construction and storage phases were studied. The results can provide a particular reference for the construction and operation of large-scale underground water-sealed oil storage caverns.

Caves have a great archaeological importance: they were used as a dwelling, as a shelter of animals, as an occasional refuge both for funerary and religious purposes. A cave survey is the first step towards their exploration. This study describes the San Giorgio cave’s survey that is the object of an archaeological research that concerns both the area above it and the underground environments. The cave, located in the north-west of Sardinia, has an extension of about 140 meters and has a maximum depth (surveyed) of -15 meters. Sixty-two TLS scans were carried out producing 1.5 milliard points. The Poisson surface reconstruction algorithm [3] is used to produce the 3D Model. A 3D model in low resolution can be adopted for aims of public archaeology; however archaeologists should take advantage of all the information available in the original point cloud.

New techniques and methods for energy storage are required for the transition to a renewable power supply, termed “Energiewende” in Germany. Energy storage in the geological subsurface provides large potential capacities to bridge temporal gaps between periods of production of solar or wind power and consumer demand and may also help to relieve the power grids. Storage options include storage of synthetic methane, hydrogen or compressed air in salt caverns or porous formations as well as heat storage in porous formations. In the ANGUS+ project, heat and gas storage in porous media and salt caverns and aspects of their use on subsurface spatial planning concepts are investigated. The optimal dimensioning of storage sites, the achievable charging and discharging rates and the effective storage capacity as well as the induced thermal, hydraulic, mechanical, geochemical and microbial effects are studied. The geological structures, the surface energy infrastructure and the governing processes are parameterized, using either literature data or own experimental studies. Numerical modeling tools are developed for the simulation of realistically defined synthetic storage scenarios. The feasible dimensioning of storage applications is assessed in site-specific numerical scenario analyses, and the related spatial extents and time scales of induced effects connected with the respective storage application are quantified. Additionally, geophysical monitoring methods, which allow for a better spatial resolution of the storage operation, induced effects or leakages, are evaluated based on these scenario simulations. Methods for the assessment of such subsurface geological storage sites are thus developed, which account for the spatial extension of the subsurface operation itself as well as its induced effects and the spatial requirements of adequate monitoring methods.

A water curtain system is widely adopted in the underground storage of gas/oil in unlined rock caverns to maintain a stable groundwater level and ensure a good water sealing condition for the storage facility. Hydrogeological tests were conventionally carried out in practice to assess the hydraulic conductivity of the surrounding rocks and evaluate the performance of the water curtain system. Among them, the hydraulic connectivity between adjacent boreholes is assessed using interconnectivity test, and it is commonly thought that, if the measured connectivity between two neighboring boreholes is low, a new borehole has to be added in the middle of them to improve the local efficiency of water curtain system. This paper reexamines the viewpoint of using interconnectivity test to determine the necessity of adding borehole. To this end, the conditions of water sealing were firstly discussed and the deficiency of existing water-sealing criterion was presented, with a new rigorous criterion proposed. The equivalent porous medium (EPM) flow model and fractured porous medium (FPM) flow model, two representative models for groundwater flow analysis, were then used to investigate the influences of borehole spacing and stored gas pressure on seepage field. These numerical results were employed to validate the proposed water-sealing criterion and suggested that, in terms of forming a stable water covering layer, the addition of borehole based on interconnectivity test is unnecessary and should be canceled for saving time and reducing cost.