Tunnel and Underground SpaceISSN:1225-1275(Print) 2287-1748(Online)한국암반공학회

As the transition to carbon-free energy progresses, the need for long-duration Energy Storage System (ESS) is increasing. Underground Pumped Hydro Energy Storage (UPHES) is one type of long-duration energy storage system that utilizes an underground reservoir. It offers several advantages, including minimizing environmental impact, increasing installed capacity by utilizing old Pumped Hydro Energy Storage (PHES) reservoirs, and utilizing unused underground spaces such as mines. Considering the current situation in Korea, where there are mines with vertical shafts and PHES with a service life of 30 years or more, UPHES is expected to meet the demand for long-term ESS in Korea. Therefore, we investigated the research trends of UPHES through literature review and performed quantitative bibliometric analysis using VOSviewer. In particular, we collected numerical modeling studies necessary for the realization of underground pumped storage power generation and analyzed the modeling targets and software used. The analysis revealed that UPHES research utilizing coal mines was predominant, with the primary modeling targets being rock stability, fluid flow of water and air, and interactions between reservoirs and aquifers. It is anticipated that this study on UPHES research trends will serve as a catalyst for domestic research in the field.

The mechanical behavior of compacted bentonite with high swelling potential is a key factor in the design of repositories for high-level radioactive waste disposal, particularly in the design of engineered barriers system (EBS). In this context, predicting and evaluating the long-term performance of EBS by numerical analysis requires an appropriate constitutive model. The Barcelona Basic Model (BBM), which is suitable for simulating the mechanical behavior of unsaturated soils with slightly or moderate swelling characteristics, is widely used for this purpose. However, the BBM has limitations in representing some important phenomena observed in highly expansive unsaturated soils. To overcome these limitations, the Barcelona Expansive Model (BExM) was proposed to model highly expansive soils such as clays and bentonites as two distinct levels: micro- and macro-structure. In this technical note, the basic concepts of the BExM and the mathematical formulas used to simulate mechanical behavior are first summarized, followed by two examples: strain accumulation due to drying-wetting cycles and controlled-suction oedometer tests. The BExM introduced in this report can be adopted as a constitutive numerical model to simulate the coupled thermo-hydro-mechanical (THM) behavior of compacted bentonite buffer blocks and backfill materials. It is anticipated that the examples described in this report can be used for verification of BExM module in TOUGH2-MP/FLAC3D.

Hydrogen energy is recognized as a critical solution for achieving carbon neutrality due to its high specific energy and zero carbon emissions upon combustion. This study reviews current technologies for underground hydrogen storage to support large-scale, long-duration storage systems essential for a stable hydrogen supply. Various geological storage methods, including salt caverns, depleted oil and gas reservoirs, deep saline aquifers, and lined rock caverns (LRC), were analyzed for their technical feasibility and applicability under Korean geological conditions. Among these options, LRC systems emerged as the most promising approach, offering design flexibility and minimal dependence on specific geological formations. Additionally, this study evaluated different hydrogen storage carriers, such as compressed gaseous hydrogen, liquid hydrogen, liquid ammonia, and liquid organic hydrogen carriers. Liquid hydrogen was identified as an optimal storage carrier for underground applications due to its high volumetric energy density, despite challenges related to cryogenic storage and material embrittlement. These findings suggest that liquid hydrogen storage in LRC systems can be a practical strategy to enhance Korea’s energy security and advance its hydrogen economy. This review provides essential insights for the development of safe, efficient underground hydrogen storage technologies.

This study was conducted to quantitatively analyze the shear creep behavior of granite joint surfaces in Korea. Hwangdeung granite was used in the tests, and direct shear creep tests were performed on discontinuity specimens created through wedge-type tensile splitting. The shear creep tests were carried out under stepwise shear stress levels ranging from 85% to 65% of the peak shear strength. Prior to and following the tests, a portable 3D scanner was used to measure the statistical roughness parameter (Z₂), from which the JRC and peak shear strength were calculated using Barton’s empirical equation. The results revealed localized degradation of surface roughness, with the degree of degradation varying depending on the applied shear stress level. At stress levels above 75%, tertiary creep was observed, indicating a potential reduction in structural stability due to exceedance of the critical stress threshold. The shear creep rate increased in the order of tertiary > primary > secondary, suggesting differences in deformation mechanisms across creep stages. Among the creep models applied, the Burger model showed the best fit with the experimental data, while the Kelvin model tended to underestimate the creep deformation. However, the derived creep parameters did not exhibit a clear trend under the limited test conditions, indicating the need for further studies under various rock and stress conditions. This study is expected to provide fundamental data applicable to long-term shear behavior assessment of discontinuities and verification of creep model applicability.

In-situ stress evaluation of deep rock masses is a critical factor for the stability and constructability of high-level radioactive waste disposal facilities. In this study, in-situ stress in deep rock masses was evaluated through Cavity Contraction Creep (CCC) tests using a High-Pressure Dilatometer (HPD). The in-situ stress evaluation was conducted in a borehole at 757.3 m depth in the Wonju, using a 73 mm High-Pressure Dilatometer manufactured by Cambridge Insitu. Tests were performed at five locations up to 450 m depth. The CCC test is a method for determining in-situ stress by measuring pressure and displacement at the cavity wall and analyzing creep movement and creep rate. This method has the advantage of simultaneously evaluating in-situ stress while measuring the deformation modulus of rock masses. Test results showed maximum horizontal stress of 3.29 MPa and minimum horizontal stress of 2.06 MPa at 101.5 m depth, and maximum horizontal stress of 12.63 MPa and minimum horizontal stress of 8.83 MPa at 449 m depth, confirming the trend of increasing stress with depth. When compared with hydraulic fracturing test results conducted in the same area, the CCC tests yielded relatively lower horizontal stress values, which was attributed to the characteristic of CCC tests that do not induce rock failure. Although CCC tests have limitations such as subjectivity in creep analysis and the importance of equipment calibration, they can be utilized as a useful alternative method for determining the range of in-situ stress in deep rock masses in conjunction with conventional hydraulic fracturing methods.

Traditional manual methods for marking blasting patterns are time-consuming and error- prone, with physical access limitations. This research presents a pattern projection system for the tunnel face that improves precision and efficiency in underground excavation. The software solution developed in this research enables the projection of blasting patterns directly onto tunnel faces using standard projectors, significantly reducing layout time and improving drilling accuracy. Three projected pattern alignment methodologies were evaluated: manual, automatic, and semi-automatic, with the semi-automatic approach proving most effective in balancing precision with operational practicality in challenging underground environments. Field validation demonstrated the system’s ability to visualize drilling patterns, geological information, and blast design parameters, resulting in reduced positioning errors, improved adaptability to field conditions, which could result in enhanced blasting outcomes. The system offers considerable advantages over previous projection methods, requiring no specialized hardware integration or expert personnel, while maintaining safety and operational flexibility. This technology effectively bridges the gap between digital design and physical implementation in tunnel excavation, supporting the digital transformation of underground construction practices.

The orientation of fractures distributed within rock masses is an important factor that affects the behavior of rock masses. Therefore, the orientation data of the borehole images measured in the field are data that must be closely analyzed in engineering activities targeting rock masses. These fractures can be classified into clusters with similar orientations, and the representative characteristics of fractures in each cluster are used to reflect the influence of the fractures on the behavior of the rock mass in engineering analysis. In this study, the orientation data obtained from the borehole image were used to automatically cluster the fractures distributed in the target rock mass, and the results were visualized to present the distribution and statistical characteristics of the fractures in the rock mass. In this process, the Fisher mixture model method was developed as an automatic clustering algorithm based on Fisher distribution, and several alternatives for rational and efficient setting of initial parameters were presented.

This study evaluated the hydraulic properties during the recovery phase following constant pressure injection test in deep fractured rock formations in Korea using Agarwal’s method. High-performance hydraulic testing equipment was used to conduct constant pressure injection-recovery test in deep boreholes drilled in volcanic rock and granite regions. Agarwal’s method was applied to drawdown data obtained during the recovery phase to estimate hydraulic conductivity (representative hydraulic parameter of fractured rocks) and to analyze groundwater flow patterns. The analysis indicated significant variations in hydraulic conductivity values depending on the calculation type of equivalent time used in Agarwal’s method. The influence of equivalent time on hydraulic properties was found to vary with the permeability of the test interval. In high-permeability zones, the shapes of recovery pressure curves and the estimated hydraulic conductivity values were generally consistent across different types of equivalent time. However, in medium- and particularly low-permeability conditions, the resulting hydraulic properties varied considerably depending on the type of equivalent time applied in the analysis. These findings highlight the importance of understanding the concept of equivalent time within Agarwal’s method and selecting the appropriate calculation method to ensure reliable evaluation of hydraulic property during the recovery phase of constant pressure injection tests, particularly in varying permeability environments.