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

Deep geological disposal systems are considered for the permanent disposal of high-level radioactive waste by isolating the waste in stable deep geological formations. Among the components of the engineered barrier system, backfill fills the remaining space in deposition tunnels, limits groundwater flow and radionuclide migration, and restrains buffer deformation. Compacted mixtures of bentonite with non-swelling materials such as silica sand and crushed rock are commonly considered as backfill materials because bentonite provides high swelling capacity and low hydraulic conductivity. Their engineering properties can vary depending on material conditions and repository environmental conditions. This study summarizes the roles and constituent materials of bentonite-based backfill and the performance requirements proposed by SKB and Posiva. Focusing on compacted bentonite-based backfill mixtures reported in previous studies, the major influencing factors on swelling pressure, hydraulic conductivity, thermal conductivity, strength and stiffness, and elastic-wave characteristics were reviewed. In addition, swelling pressure and hydraulic conductivity data reported in the literature were reorganized and compared in terms of effective dry density (EDD). The review showed that increasing dry density and bentonite content generally increased swelling pressure, strength, and stiffness, while decreasing hydraulic conductivity. Thermal conductivity generally increased with increasing dry density and degree of saturation. Moisture condition, temperature, and groundwater chemistry also affected the engineering properties of the backfill, and elastic-wave characteristics showed potential for non-destructive condition assessment. In addition, swelling pressure generally increased and hydraulic conductivity decreased with increasing EDD. These results indicate that material selection and performance assessment of bentonite-based backfill should consider the combined effects of material conditions, repository conditions, and engineering properties.

The engineered barrier system (EBS) concept is applied for the safe disposal of high-level radioactive waste, and among its components the backfill performs an important function in securing the stability of the other engineered barriers, including the canister. While the backfill remains unsaturated immediately after emplacement, groundwater flowing in from rock fractures can cause piping and erosion, which lowers the installed density of the backfill and thereby affects the safety functions of the engineered barriers. Identifying the critical groundwater inflow rate below which piping is suppressed has accordingly been treated as a key requirement in backfill design. Previous studies on this subject, however, were mainly conducted on blocks and pellets, and studies including granular materials were performed under gap-fill conditions; they are therefore limited in their direct applicability to the present design context, in which the backfilling concept is shifting to granular bentonite. This technical report first introduces previous studies on piping and erosion carried out in countries leading the development of geological disposal, and then introduces the piping and erosion behavior observed in tests on GraFi and GBM (the granular backfill materials adopted by Posiva and Nagra respectively) performed at the lowest installation dry density required by Posiva for granular backfill and under two low inflow rates.

Fracture distribution within rock masses is known to exhibit spatial variability, and accounting for this variability is essential for reliably predicting the hydraulic behavior of fractured rock masses, particularly for large-scale, high-precision applications such as underground nuclear waste repositories. In this regard, this report reviews studies on structural domain classification based on the similarity of fracture properties, as well as DFN (discrete fracture network)-based studies on hydraulic characteristics conducted under the assumption that fracture properties follow a statistical distribution. It further examines a previous study that quantitatively linked spatial variation in fracture distribution to permeability variation using real outcrop trace maps compiled from the literature. The results showed that permeability differed distinctly between subdomains classified according to the similarity in fracture properties, indicating an association between spatial variability in fracture distribution and hydraulic heterogeneity within rock masses. Future research is needed to extend this understanding toward the coupled thermo-hydro-mechanical-chemical (THMC) behavior of rock masses.

Two-component tunnel backfill grout is commonly injected into the tail void formed during shield TBM tunnelling. Because the curing state and mechanical properties of the grout directly affect the quality of tail-void filling, nondestructive evaluation of these properties is required. In this study, two-component backfill grout specimens were prepared based on a mix proportion used in a power utility tunnel project, and the hardener-to-water ratio (H/W) was varied from 5.39% to 15.94%. P-wave and rod wave velocities were measured during the curing period. The curing completion time for each H/W condition was determined from the P-wave velocity, and Poisson’s ratio and Young’s modulus were calculated from the measured elastic-wave velocities. The relationship between P-wave velocity and Young’s modulus was also analyzed. The test results showed that the curing completion time ranged from approximately 10 to 23 days, and all specimens reached the curing completion criterion before 28 days. Both the curing completion time and P-wave velocity generally increased as H/W increased. Poisson’s ratio ranged from 0.40 to 0.48 and showed relatively small variations with H/W and curing period. Therefore, approximately 0.44 can be applied as a representative value for the tested backfill grout. In addition, P-wave velocity and Young’s modulus showed a positive correlation, and their relationship was represented by a quadratic polynomial. This study can provide fundamental data for evaluating the curing state of backfill grout nondestructively and estimating its Young’s modulus indirectly using P-wave velocity measured in the field.

Subsea road tunnels often adopt steeper longitudinal gradients than conventional tunnels due to topographic constraints, increasing mechanical load on vehicles and influencing breakdown rates. Using nine years of operational data(2016.12-2026.04) from the Eurasia Tunnel, a subsea road tunnel crossing the Bosphorus Strait in Istanbul with a maximum 5% U-shaped gradient, this study quantifies the relationship between longitudinal gradient and vehicle breakdown occurrence. Results show that 83.0% of vehicle breakdowns, which account for 58.9% of all recorded incidents, are concentrated on uphill sections, with breakdown density 4.87 times higher uphill than downhill overall. Applying CETU's official gradient correction still underestimates observed breakdowns by 1.65x; a fleet-age correction factor (β_fleet = 1.33, based on Türkiye's higher average vehicle age) suggests this residual may be partly explained, reducing it to 1.24x. A modified formula, N_B (revised) = Rate_B × AADT × 365 × L × (1+α·g) × β_fleet × 10^-8, is proposed for uphill sections. Under a best-case assumption that gradient relaxation(5% → 4%) eliminates all gradient-induced breakdowns, delay-cost savings recover only 23-34% of the additional construction cost($102 M), indicating that currently quantifiable operational costs alone cannot offset the additional cost of gradient relaxation.

Reliable prediction of coupled thermo-hydro-mechanical (THM) behavior induced by decay heat is essential for the safety assessment of deep geological repositories for high-level radioactive waste. OGS-FLAC3D, a sequential coupling simulator that combines OpenGeoSys for thermo-hydraulic analysis with FLAC3D equipped with various geomechanical constitutive models, offers the practical advantages of a modular framework; however, the convergence and stability of sequential iterations may deteriorate under strongly coupled conditions. In this study, a fixed-stress split stabilization term was introduced into the fluid mass balance equation, and the convergence and accuracy of the coupled analysis were verified against the analytical solution of the Booker-Savvidou point heat source problem. The numerical results for temperature, pore pressure, displacement, and stress showed good agreement with the analytical solution, with peak relative errors in pore pressure of −2.73% and −1.60% at two monitoring points located at different distances from the heat source. In the unstabilized fixed-strain split analysis, in which a small specific storage coefficient was introduced, the prescribed convergence criterion was not satisfied within the given number of coupling iterations, and the pore pressure was overestimated by approximately 67% relative to the analytical solution. This result demonstrates that fixed-stress stabilization plays an important role in achieving convergence within a practical number of coupling iterations for the strongly coupled benchmark considered in this study. For the stabilized cases, the final converged solutions showed only negligible differences among the tested stabilization coefficients and the T-H-M and H-T-M coupling sequences. These results confirm that the fixed-stress- stabilized OGS-FLAC3D framework can reliably reproduce the coupled THM response of the present benchmark problem.

The OO Tunnel on the OO Line was opened in April 2015, and continuous invert heaving has been observed since 2017. Through seven types of geotechnical investigations and geophysical surveys, the depth of the deteriorated stratum was identified to be approximately 4.5 m, indicating the occurrence of ground squeezing. To establish an effective reinforcement plan for invert heaving, a Parameter Study was conducted focusing on the spacing, length, installation angle, and quantity of rockbolts. The proposed reinforcement method derived from this Parameter Study successfully secured the geotechnical stability of the heaved tunnel. This study proposes economically optimized selection criteria for determining the optimal reinforcement alternative in rockbolt reinforcement works for railway tunnel heaving remediation.

The initial stress of a site is needed for the design and stability analysis of underground facilities but is rarely measured at the early stage of site investigation. This study proposes a procedure for establishing the Best Estimate Stress Model (BESM) of the ISRM suggested method from existing data only, such as the Korea Stress Map and published focal mechanism solutions, and applies it to the Taebaek Underground Research Laboratory (URL) site. Inversion of 24 focal mechanisms within 80 km of the site indicates a strike-slip regime with the maximum horizontal stress (SHmax) oriented N74°E and a stress ratio R of 0.68. The frictional limit of faults gives Shmin/Sv of 0.63-0.83 and SHmax/Sv of 1.5-2.2. Comparison with hydraulic fracturing data at 508-849 m depth in the nearby Sinyemi mine shows that the median Shmin/Sv lies within the model range, whereas most of the measured SHmax/Sv lies near or below the lower bound of the model. The frictional-limit SHmax/Sv is therefore used as an upper-bound case rather than as the estimate, and SHmax/Sv of 1.4-1.8, based on the nearby measurements rather than on measurements at the site, is proposed as a provisional reference range for numerical analyses. The procedure can be applied to other sites in Korea where the Korea Stress Map data and enough focal mechanisms for the inversion are available.

In this study, unused underground spaces of mines are being considered for underground smart farms because of their stable temperature and humidity, but their environment in summer, when the outdoor air is hottest and most humid, has rarely been confirmed with measured data. In this study, the data measured from June to September 2026 by the monitoring system installed at the idle space of a room-and-pillar mine in a previous study were analyzed. Temperature, humidity, atmospheric pressure, CO₂, CO and particulate matter were compared between the mine entrance and the underground test section in terms of time series, monthly statistics and diurnal variation, and the compliance with the cultivation conditions of the previous study was evaluated. The underground temperature was 15.2 ± 0.7°C against 23.9 ± 4.7°C outside, with its variability attenuated to 16~28% of that outside. The underground relative humidity remained at 94.4 ± 0.9%, but the absolute humidity(12.3 g/m³) was lower than outside(16.4 g/m³), so the high relative humidity results from the low temperature rather than from excess moisture; the outdoor dew point exceeded the underground temperature for 87% of the period, indicating condensation when outdoor air is introduced. Atmospheric pressure followed the outside without lag, CO₂ and CO remained low, and particulate matter readings were high under the near-saturated humidity. Temperature, pressure and CO complied with the cultivation conditions, whereas relative humidity, CO₂ and particulate matter did not; the relative humidity, however, fell within the ranges when the underground air was warmed to the cultivation temperature. The results show that temperature-dependent humidity control and the handling of outdoor air are the key considerations for underground smart farm units in summer.