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

This report reviews key recovery analysis methods for interpreting drawdown recovery data following pumping tests conducted in fractured bedrock. Pumping tests are widely used for characterizing fractured aquifers, employing diagnostic plots to enhance the conceptual understanding of groundwater flow systems and to estimate hydraulic parameters. Recovery testing involves measuring the water level rebound in pumping and observation wells after the termination of pumping and is particularly useful under field conditions where precise control of discharge rates is difficult. Traditionally, recovery data have been analyzed based on the Theis recovery method, which has provided the foundation for various extended analytical approaches. While these methods offer reliable estimates of effective transmissivity, they are limited in providing specific insights into the conceptual flow models of fractured aquifers. Agarwal introduced an analysis technique capable of incorporating both early and late time hydraulic responses using recovery data. This method has since become a standard approach in both hydrogeological and petroleum reservoir engineering fields. The Agarwal method transforms recovery curves into equivalent drawdown curves under constant-rate pumping, thereby enhancing the accuracy of parameter estimation and contributing to the identification of appropriate conceptual flow models. This report examines case studies where the Agarwal method was applied to pumping-recovery test data, evaluating its applicability for interpreting limited and uncertain test results under complex, non-ideal flow conditions. The findings suggest that the Agarwal approach holds significant potential as an effective analytical tool for improving the accuracy of hydraulic characterization in fractured rock environments.

This study evaluates the influence of buffer material infiltration on the long-term stability of rock discontinuities in deep geological disposal environments. Shear creep tests were performed on Hwangdeung granite joints infilled with buffer material under a normal stress of 3.0 MPa. Shear stress levels were applied stepwise from 75% to 94% of the peak shear strength, and joint roughness was analyzed using 3D scanning before and after testing. Results indicated that buffer infilling led to localized reductions in joint roughness, and no transition to tertiary creep was observed during the 4-day observation period. In the suitability analysis of various creep models, the Nishihara model demonstrated higher fitting accuracy than the Burger model, which yielded physically inconsistent creep constants. Due to the inconsistent trends of constants regarding buffer thickness and stress levels, further experimental research under diverse conditions is necessary for a precise analysis of creep behavior.

Brittle failure and spalling are typically associated with high-strength crystalline rocks at great depths. However, this study investigates an exceptional case of spalling and support failure in a weak sedimentary rock mass at a shallow depth of 90 m. Despite a low compressive strength (~30 MPa), excessive displacements, shotcrete cracking, and rock bolt ruptures were observed. Field evaluations and borehole endoscopy confirmed that the damage resulted from spalling, triggered by brittle rock characteristics and anomalous stress conditions from specific topographical features. The study also found that bedding planes can exacerbate this by combining spalling with slabbing mechanisms. Numerical analyses validated that the stress-spalling depth relationship established for deep crystalline rocks remains applicable here. These findings highlight the necessity of considering brittle behavior in weak rocks to optimize tunnel support design in unconventional stress environments.

In South Korea, 79.6% of the storage capacity for High-radioactive waste (HLW) has been reached, with some nuclear power plants expected to face full saturation by the 2030s. Since HLW continuously emits heat and radiation over extended periods, deep disposal at depths of approximately 500 m is being considered as a viable solution. Consequently, it is essential to evaluate the thermal properties of the rock mass during the site selection process. However, there is a lack of measured data regarding the thermal properties of deep rock masses in South Korea. In this study, rock samples were obtained from boreholes drilled to depths of up to 750 m Namwon, Wonju, and Chuncheon, and their thermal properties were measured using Transient plane source method (TPS) and Laser flash method (LFA). The results indicated no significant trend in thermal conductivity with respect to depth; however, a proportional relationship between thermal conductivity and thermal diffusivity was confirmed. Furthermore, X-ray diffraction (XRD) analysis revealed that the mineral composition is a primary factor influencing the thermal conductivity of the rocks. These findings are expected to serve as foundational data for the future selection of deep disposal sites.

Deep rock masses are complex structures characterized by numerous joints. These discontinuities serve as mechanical weaknesses and primary pathways for groundwater flow, making it essential to characterize their coupled thermo-hydro-mechanical behavior. In this study, shear-flow coupled tests were conducted on granite joints at both ambient and elevated temperatures to investigate the influence of thermal effects on shear-flow behavior. The experimental procedure was categorized into three distinct stages: pre-shear, during-shear, and residual. During each stage, permeability characteristics were precisely measured in response to changes in joint normal stress and injection pressure. In the pre-shear stage, permeability decreased due to the thermal closure effect caused by thermal expansion and increased normal stress. However, during shear failure, the flow rate surged by more than 100 times compared to the initial state, as the effect of shear dilation significantly overwhelmed thermal expansion. In the residual stage, shear dilation remained the dominant factor, leading to convergent flow behaviors regardless of the temperature conditions.