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

This report presents the Transmissivity Normalized Plot (TNP) method, developed for the visual comparison and analysis of field hydraulic test data that reflect flow characteristics in fractured rock. This method is based on a derivative normalization approach that complements traditional type curve analysis. Derivative data are normalized with respect to the dominant inner boundary condition and then converted into units of radial flow equivalent transmissivity. The resulting TNP expresses transmissivity as a function of time on a log-log plot, allowing for the visual comparison of hydraulic test data obtained under various conditions. For the case when the hydraulic test displays infinite acting radial flow (flat derivative), the transmissivity can be derived from the y-axis coordinate of the flat portion of the derivative displayed on the TNP. The basis for the visual evaluation of transmissivity is that during radial flow the derivative has a slope of zero and its vertical position in log-log coordinates is an inverse reflection of the transmissivity. When analyzing transient hydraulic test data from different field settings, the TNP technique enables visual comparison of similar formation responses in terms of both flow model characteristics and transmissivity magnitude. If normalized derivative curves from different tests converge in the late-time region, it suggests that the tests are capturing the same large-scale transmissivity and flow structure. This report outlines the fundamental concept and principles of the TNP method and examines its applicability through case studies covering a range of fractured rock environments and hydraulic properties. This TNP approach enables the visual integration and direct evaluation of various hydraulic test data within a single plot, and can be effectively utilized for constructing consistent conceptual flow models and accurately estimating hydraulic parameters.

In continuous excavation type TBM, thrust jacks contact with the inclined surface of helical segments, where they are subjected to lateral loads. When this condition persists, buckling and fatigue stresses accumulate in the major components, compromising the structural stability and reliability of the thrust jack. In this study, the primary failure modes of hydraulic cylinders were investigated to identify the main failure factors of the thrust jacks. Based on the target durability assurance period, the required failure-free test time was calculated, and an accelerated life testing protocol was designed. A dedicated test rig for the accelerated life test was constructed, and cyclic endurance tests were performed on refurbished thrust jacks. The results showed no signs of fracture or oil leakage, confirming that the jacks satisfied the reliability requirement corresponding to 4,500 hours of operation.

This study aims to evaluate the rock mechanical feasibility of deep geological disposal facilities by constructing a numerical model that reflects the rock properties of representative domestic crystalline rock formations. Representative cases were established for both vertical and horizontal disposal concepts. Rock properties, lateral stress ratios, and initial in-situ stresses reflecting the domestic deep geological conditions were applied as input parameters for the simulation. Stress redistribution and strain distribution around disposal tunnels and deposition holes during excavation were analyzed, and key stability indicators such as localized zones of maximum principal stress, strain concentration, and plastic deformation regions were identified. In the reference case, under the initial condition without engineered barriers, the maximum principal stress acting on the surrounding rock remained lower than the rock strength, indicating an overall stable stress condition. In contrast, under conditions with lower rock strength and a higher lateral stress coefficient, localized plastic deformation was observed around the disposal tunnels and boreholes. These results demonstrate that the rock mechanical stability of the disposal system is sensitive to rock properties and initial stress states. The findings of this study can serve as a quantitative basis for assessing rock mechanical stability in the conceptual design stage of disposal facilities and contribute to evaluating the safety margin of the natural barrier.

Water infiltration into slopes during rainfall can cause an increase in the moisture content and saturation of the ground and a rise in the groundwater level, which may lead to a decrease in slope stability. In the present study, changes in slope stability and ground behavior over time due to rainfall infiltration were investigated using a coupled hydro-mechanical analysis based on the TOUGH-FLAC approach. A slope under unsaturated conditions where the ground was not fully saturated to the ground surface was considered, and rainfall conditions were set based on a probable rainfall intensity formula in Korea. The slope stability was evaluated by the factor of safety determined by the shear strength reduction method, and ground responses such as saturation, stress, displacement, and strength-stress ratio were investigated. Numerical simulations were performed by varying the initial saturation, relative permeability model, and saturated permeability of the ground, and the effect of coupling terms for the hydro-mechanical analysis on the slope safety factor calculation was examined.

This study investigates the effects of three backfill material designs with varying bentonite- silica sand mix ratios on the thermal, hydraulic, and mechanical interactions in the multi- barrier system of the Improved Korean reference disposal system (KRS⁺). Numerical modeling results reveal that while the backfill material designs have minor effect on the temperature evolution within the repository, the saturation evolution shows notable differences based on the backfill properties. Specifically, increasing the bentonite content in the backfill accelerates backfill saturation, yet delays buffer saturation. The capability of the backfill to restrain upward movement of the buffer is enhanced by higher swelling pressure and stiffness of the backfill. However, stiffness increases tend to be more effective than additional swelling pressure for restaining upward movement of the buffer. Consequently, Designing an optimal mix ratio is crucial to ensure sufficient swelling pressure, fullfill performance criteria, and maintain relatively high stiffness. The backfill swelling pressure can act as a confining stress on the excavation surfaces, thereby enforcing the rock mass strength and reducing the extent of failure in the near-field rock mass, as the bentonite content in the backfill increases.

There has been an increasing demand for 2-Arch tunnels in urban areas. However, as the collapse of 2-Arch tunnels due to uneven pressure occurred recently, interest in the stability of 2-Arch tunnels is being focused. At the construction site, considering the equipment operation and construction period, it is expected that the 2-Arch tunnel will be excavated on one side, causing uneven pressure to be generated on the central column. However, when evaluating the stability of the central column, Matsuda's relaxed rock load, which only considers the vertical load, is applied, resulting in under-applied load conditions. In this study, the stability assessment method of the central column was improved by considering the effect of asymmetric excavation. A numerical analysis was conducted simulating the construction stages according to asymmetric excavation, and the effects of uneven pressure on the central column according to parameters were analyzed. The element force generated in the central column was analyzed by linking the numerical analysis results to the structural analysis. The structural analysis results demonstrated that the section force of the central column increased due to the uneven pressure and exceeded the design strength under the unfavorable conditions. In addition, an Uneven Pressure Factor(U.P.F) was proposed as a method to consider the effect of uneven pressure on the existing under-applied relaxed rock load.

A buffer material, a key component of the engineered barrier in a geological disposal system, exhibits swelling behavior when saturated. The chemical composition of groundwater can significantly influence its swelling capacity. Therefore, numerical analysis of the buffer’s swelling behavior, incorporating chemical effects, is essential to ensure the long-term stability of the disposal system over hundreds of thousands of years. In this study, we developed the OGSPHREEQC-FLAC3D simulator, which enables coupled Hydro-Mechanical-Chemical (HMC) analysis. OGSPHREEQC-FLAC3D integrates OGS-FLAC3D for thermal-hydro-mechanical (THM) simulation with PHREEQC for chemical analysis using a sequential coupling approach. The Barcelona Basic Model was employed to simulate swelling behavior induced by suction pressure. Chemical effects were incorporated by considering changes in ion concentration in the solute, and a coefficient reflecting montmorillonite concentration variation was applied. A series of numerical simulations were conducted to evaluate the swelling behavior of Na-type bentonite under one-directional flow conditions. To investigate the effect of potassium ion (K⁺) concentration in the injection water, K⁺ concentration was increased in the sample through advection and diffusion. As K⁺ concentration rose, the amount of illite produced by chemical alteration also increased, leading to a reduction in swelling pressure. These results demonstrate the capability of the developed simulator and provide a foundation for future research aimed at advancing fully coupled thermal-hydro-mechanical-chemical (THMC) numerical simulation for geological disposal systems.

The present study developed a numerical model to accurately simulate the coupled hydro–mechanical behavior of bentonite, a key material in the engineered barrier system (EBS) of high-level radioactive waste repositories. For this purpose, the constitutive equations of the Barcelona Basic Model (BBM), a representative elasto-plastic model for unsaturated soils, were implemented in FLAC3D as a user-defined model, and a coupled OGS–FLAC analysis framework was established by linking with OpenGeoSys. The developed module was first validated against the Modified Cam-Clay (MCC) model under saturated conditions, and subsequently verified through representative benchmark scenarios, including swelling–collapse behavior induced by suction decrease, path dependency under combined loading–suction variations, and the interaction of SI–LC yield surfaces. These comparisons confirmed the theoretical soundness and numerical reliability of the implementation. Furthermore, by incorporating suction and degree of saturation computed in OGS into the BBM stress analysis, a stepwise OGS–FLAC coupling approach was proposed to simulate hydro–mechanical interactions under unsaturated conditions. The framework was applied to the international DECOVALEX-2027 Sandwich Task Step 0a, in which swelling pressure tests on Calcigel bentonite at three initial dry densities were modeled. The simulations reproduced the observed evolution of swelling pressure, degree of saturation, axial strain, and water uptake with reasonable accuracy. The developed BBM-based OGS–FLAC module was demonstrated to reliably capture the path-dependent volumetric deformations and coupled hydro–mechanical mechanisms of bentonite. The proposed modeling approach and calibrated parameters are expected to provide a robust basis for future applications to larger-scale experiments, including MiniSandwich, HTV-7, and Mont Terri in situ tests, and for international comparative studies.

This study investigates the integrity of cements used in deep injection wells for geological CO₂ storage by evaluating the mechanical and hydraulic properties of Portland cement and G-Class cement, as well as analyzing their fracturing behavior and injectivity under CO₂ exposure. To simulate marine geological storage conditions, cement specimens were prepared using seawater with varying water-to-cement ratios, supplemented with functional additives, and subsequently cured in seawater. Both Portland and G-Class cements exhibited increased density, uniaxial compressive strength, and Young’s modulus with lower water-to-cement ratios. While porosity values tended to be slightly higher in specimens with lower water-to-cement ratios regardless of cement type, G-Class cement consistently showed lower permeability, a key parameter influencing potential leakage pathways. To assess the potential fracturing and cracking of cement surrounding injection wells under deep reservoir CO₂ injection, CO₂ fracturing tests were conducted, revealing that G-Class cement exhibited higher CO₂ fracturing resistance compared to Portland cement. In addition, casing–reservoir simulation experiments were performed to analyze injection pressure variations and assess leakage risks along the wellbore. The findings indicate that G-Class cement, characterized by its lower permeability, higher CO₂ fracturing resistance, and stable injection pressure profile, is more suitable for ensuring wellbore cement integrity in CO₂ geological storage applications.

Groundwater flow in crystalline rock masses occurs predominantly along joints. Therefore, detailed characterization of flow through individual joint is critical to the long-term safety of deep geological repositories for high-level radioactive waste. The geometry of joint surfaces can be described by various properties and exert a significant influence on hydromechanical behavior. This study investigates, experimentally and numerically, the effects of such geometric properties on linear and nonlinear flow through a rock joint. Joint specimens were created by inducing tensile failure in intact rock, and their surface geometries were measured using a laser scanner. Then hydraulic tests were conducted under conditions representative of a repository environment. The results indicate that the spatial correlation of aperture can affect the overall flow behavior. Numerical simulations of the aperture field further show that small spatial correlation strengthens flow channeling, giving rise to flow nonlinearity. In addition, localized eddy flows that develop under high pressure can contribute to nonlinearity as well. The findings provide fundamental insights into flow through single rock joint and can serve as reference data for subsequent safety assessments.