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

The purpose of a deep geological repository is to isolate radioactive isotopes of spent nuclear fuel from our biospheres. The isolation and shielding performance of a deep geological repository is ensured by engineered barriers, which are one of the components of a multi-barrier system. These engineered barriers are composed of disposal canister, buffer, and backfill that contain radioactive waste. Among these engineered barriers, plugs are designed to prevent the flow of water inflow during repository construction and gas flow during operation and after the closure of the facility, thereby preventing erosion of the bentonite buffer and sustaining a swelling pressure of the bentonite buffer. This paper reviews the results of a full-scale Domplu test conducted in Sweden to evaluate the sealing capacity and mechanical stability of plugs. The full-scale test results demonstrated that the concrete plug maintained its sealing and safety functions under groundwater pressure (4 MPa) and swelling pressure (2 MPa) conditions, which occur during the groundwater saturation process of the buffer and backfill materials at the disposal site depth. The study also introduces the potential improvements of field construction method of plug such as the prevention of void formation at the concrete plug's upper section due to the self-weight.

This report introduces the application of Agarwal’s method for analyzing recovery data following constant head injection tests in fractured rock. Agarwal’s method utilizes the concept of equivalent time to transform the pressure recovery curve, which represents the return of the test interval pressure to its initial state after the injection test, into a drawdown curve under constant pumping conditions. This transformed recovery curve can then be applied to various pumping test interpretation methods to estimate the hydraulic conductivity, which represents the in-situ permeability of the rock. Additionally, derivative diagnostic analysis can be used to evaluate the conceptual model of the fractured rock aquifer and groundwater flow patterns, providing a more detailed understanding of fracture network connectivity, flow mechanisms and boundary conditions. This report explains the fundamental principles and concepts of Agarwal’s method and presents case studies of its practical application. The use of Agarwal’s method for recovery data analysis offers an effective approach to overcoming the inherent limitations of field constant pressure injection test, where verification and validation of measured data are often challenging. This method enhances the reliability of hydraulic property estimation and is expected to be applicable across various fractured rock geological environments.

Tunnel excavation in sedimentary rock mass generally shows anisotropic behavior by the effects of bedding or other structures. This paper assesses abnormal tunnel deformation and damage phenomena during the excavation of construction adits for an oversea tunnel project in a sedimentary rock mass. Various inter-bedded sedimentary layers were encountered during the excavation and over 100 mm convergence was monitored. Such condition was totally different from the original design prediction of uniform sandstone formation. Severe tunnel damage including shotcrete cracks, roof slabbing and acoustic emission have also been observed. The reason for such abnormal behavior was identified as the results of combined mechanism of anisotropic and heterogeneous behavior of inter-bedded sedimentary structures and also unexpected over-stressed conditions. Detailed assessments on monitoring results and the tunnel face mapping support such complex mechanism. It also appears to be related to brittle fracturing, which seems to be unique phenomena in a sedimentary rock. A series of numerical analysis with real characterized ground conditions and heterogeneity could confirm that very similar behavior could happen on such conditions. More detailed assessments will be needed to understand the effect of anisotropy, heterogeneity and the brittle fracturing process in sedimentary rock.

Due to the limitation of national land area and the mountainous topography of Korea, the utilization of surface space is restricted, leading to active development of underground space. As underground structures become more widespread and their depth increases, it is necessary to consider in more detail the effects of blast-induced vibration on adjacent underground structures during blasting operations. Currently, various controlled blasting techniques are applied to reduce vibration; however, most of these techniques focus on reducing lateral vibration, and methods to attenuate vibration in the tunneling direction remain insufficient. In this study, to supplement this limitation, a medium with different physical properties from the surrounding rock mass was filled into the bottom of the blast hole to induce wave scattering and reflection due to impedance contrast at the interface, and to verify whether this can effectively attenuate forward-propagating vibration. For this purpose, Numerical simulations were conducted using ANSYS AUTODYN, applying sand with compaction characteristics as a filling material. The analysis was performed by varying parameters such as the length of the filling material and the type of explosive. As a result, it was found that the longer the length of the filling material, the greater the attenuation effect, and that the use of lower detonation velocity explosives showed a more distinct vibration reduction effect. This study suggests the potential of applying heterogeneous materials at the bottom of blast holes as a new design approach for forward-direction vibration control, and is expected to be utilized as fundamental data for minimizing blast-induced vibration in future blast design.

This study analyzes path errors caused by wheel slip in autonomous robots on rough terrain using three steering methods: Ackermann, differential, and tracked steering. Experiments in a mine revealed that Ackermann steering maintained stability at turning low speeds but exhibited significant path deviations at higher speeds, with a root mean square error (RMSE) of 0.2805 meters due to traction loss. Differential steering showed better path recovery, achieving an RMSE of 0.2311 meters. Tracked steering offered stability on rugged terrain but had the highest RMSE of 0.3119 meters, reflecting difficulties in fine direction adjustments. These findings underscore the need to select the right steering mechanism for optimized autonomous navigation. Insights from this research can improve robot navigation in mining, military, and other industries demanding robust performance in challenging environments.

In this study, a series of direct shear tests were conducted to investigate the shear characteristics of rock joints infilled with bentonite. Hwangdeung granite was used as the rock specimen, and splitting tensile tests were performed to create artificial tensile joints. The direct shear tests were carried out on two types of specimens: bentonite-infilled joints and unfilled joint. Prior to and after the tests, the joint surfaces were scanned using a 3D laser scanner to quantitatively analyze the joint roughness coefficient (JRC), the thickness of the bentonite layer after shearing, and the shear-induced degraded area. The results showed that both the shear strength and dilation angle decreased in the joints infilled with bentonite. However, for joints with high joint roughness coefficient, the shear characteristics of bentonite-infilled joints were found to be similar to those of unfilled joints.

High-level radioactive waste disposed of in rock mass continuously emits decay heat, leading to a rise in the temperature of the surrounding near field rock mass. Depending on the disposal concept, the temperature may reach up to 120℃. Therefore, such high temperature conditions must be considered in the design and operation of the engineered barrier system, and accurate measurements of the thermal properties of rock are essential. Among the various test methods proposed for measuring the thermal properties, some require the use of very small specimens. However, considering the inherent heterogeneity of rock, such small specimens may compromise practicality and/or representativeness. Thus, it is thought more appropriate to use single specimen of typical core size. In this study, thermal conductivity, specific heat, thermal diffusivity, and thermal expansion coefficient were measured using granite core specimens of standard NX size collected from the vincinity of the KURT. The DBM, TDBM, and strain gage method were employed. The results showed an average thermal conductivity of 2.79 W/m·K, specific heat of 0.78 J/g·K, thermal diffusivity of 1.35 mm2/s, and expansion coefficient of 6.59×10-6 1/K. Compared to previous studies conducted on similar location and rock type, the present results exhibited reduced dispersion, which was possibly attributed to the use of larger rock specimen so that the effect of heterogeneity was somewhat mitigated.

Electromagnetic radiation(EMR) generated during the failure of rock materials has been proposed as a novel technique for the early prediction and prevention of geotechnical hazards. However, the underlying mechanisms of EMR generation and its quantitative characterization have not yet been fully established. In this study, static and dynamic rock fracture tests were conducted on various rock materials using a universal testing machine and a Split Hopkinson pressure bar system to analyze the characteristics of EMR generated during the rock fracture. Based on the experimental results, the effects of quartz content, failure mode(compression and tension), and strain rate on EMR generation were investigated. A predictive model was proposed to estimate EMR generated during the failure of rocks under arbitrary loading rates, depending on the rock type.