Joong-Ho Synn

Regional Distribution Pattern and Geo-historical Transition of In-situ Stress Fields in the Korean Peninsula

We have analyzed the regional in-situ stress pattern using 460 stress measurement data at about 100 test sites in Korea, and suggested correlation equations of stress-depth and stress ratio-depth. We made Korea Stress Map(KSM) as in-situ stress fields of the Korean peninsula, combining with a paleo-stress analysis according to the geological period and a stress estimation from focal mechanism. We confirmed the reliability and applicability of correlation equations derived in this study, comparing with worldwide stress-depth patterns, and also estimated the pattern of in-situ stress fields of north-eastern Asia including Korea, China and Japan, comparing with World Stress Map.

Coupled Thermal-Hydrological-Mechanical Behavior of Rock Mass Surrounding Cavern Thermal Energy Storage

Abstract The thermal-hydrological-mechanical (T-H-M) behavior of rock mass surrounding a high-temperature cavern thermal energy storage (CTES) operated for a period of 30 years has been investigated by TOUGH2-FLAC3D simulator. As a fundamental study for the development of prediction and control technologies for the environmental change and rock mass behavior associated with CTES, the key concerns were focused on the hydrological-thermal multiphase flow and the consequential mechanical behavior of the surrounding rock mass, where the insulator performance was not taken into account. In the present study, we considered a large-scale cylindrical cavern at shallow depth storing thermal energy of 350℃. The numerical results showed that the dominant heat transfer mechanism was the conduction in rock mass, and the mechanical behavior of rock mass was influenced by thermal factor (heat) more than hydrological factor (pressure). The effective stress redistribution, displacement and surface uplift caused by heating of rock and boiling of ground-water were discussed, and the potential of shear failure was quantitatively examined. Thermal expansion of rock mass led to the ground-surface uplift on the order of a few centimeters and the development of tensile stress above the storage cavern, increasing the potential of shear failure. Key words TOUGH2-FLAC3D simulator, Cavern thermal energy storage (CTES), Thermal-hydrological-mechanical coupled analysis초 록 본 연구에서는 TOUGH2-FLAC3D 연계해석기법을 이용하여 암반공동에 고온의 열에너지를 30년간 저장하는 경우 주변 암반에 야기되는 열-수리-역학적 연계거동을 살펴보았다. 열에너지저장에 따른 암반의 거동 특성 및 환경 영향을 예측하고 이에 대한 제어기준을 수립하기 위한 기초 연구로서 , 저장소 주변 암반에서 발생하는 열-수리 흐름과 역학적 거동의 상호작용에 대하여 검토하였다 . 기본해석으로서 결정질 암반 내 원통형 공동에 350℃의 대용량 열에너지를 저장하는 경우를 모델링하였으며, 열에너지저장소의 단열성능은 고려하지 않았다. 암반 내 열전달의 주요 메카니즘은 암반의 전도에 의한 것으로 판단되며 , 암반의 역학적 거동은 수리적 요소보다는 열적 요소에 지배적인 영향을 받는 것으로 나타났다 . 암반과 지하수 가열에 따른 유효응력 재분포 양상과 열팽창으로 인한 암반 변위 및 지표 융기를 검토하였으며 , 주변 암반에서의 전단파괴 위험도를 정량적인 수치를 통해 제시하였다. 암반 가열에 따른 열팽창으로 인하여 지표면에서 수 cm의 융기가 발생하였으며, 저장공동 상부에 인장응력이 크게 발달하면서 전단파괴의 위험도가 증가하는 것으로 나타났다 .핵심어 TOUGH2-FLAC3D 연동해석, 암반공동 열에너지저장, 열-수리-역학적 연계해석

Thermal Energy Balance Analysis of a Packed Bed for Rock Cavern Thermal Energy Storage

A packed bed thermal energy storage (TES) consisting of solid storage medium of rock or concrete through which the heat transfer fluid is circulated is considered as an attractive alternative for high temperature sensible heat storage, because of the economical viability and chemical stability of storage medium and the simplicity of operation. This study introduces the technologies of packed bed thermal energy storage, and presents a numerical model to analyze the thermal energy balance and the performance efficiency of the storage system. In this model, one dimensional transient heat transfer problem in the storage tank is solved using finite difference method, and temperature distribution in a storage tank and thermal energy loss from the tank wall can be calculated during the repeated thermal charging and discharging modes. In this study, a high temperature thermal energy storage connected with AA-CAES (advanced adiabatic compressed air energy storage) was modeled and analyzed for the temperature and the energy balance in the storage tank. Rock cavern type TES and above-ground type TES were both simulated and their results were compared in terms of the discharging efficiency and heat loss ratio.

3-D Visualization of dynamic loading induced damage of rock-like materials using Micro-focus X-ray CT scanner

ABSTRACT 1.5″ Pulse shape-controlled Split Hopkinson Pressure Bar system (PS SHPB) was used to impact cement paste samples in order to make dynamic loading-induced damaged samples. The microfocus X-ray Computerized Tomography (CT) was used to scan the damaged samples. The scanned slices were rendered to build 3D dimensional images of the samples. The number of cracks increases as impact velocity increase. The most of the cracks generate alongside the boundaries of the material constitutes and connect the pores. The density of the vertical cracks increased with increasing impact velocity. A number of cracks which are not connected each other are visible. From the 3 dimensional images of pores and cracks through the removal of minerals and cement paste phase, it was revealed that pores are distributed from several 10 μm to several mm and planar cracks (parallel to loading axis) are increased with impact velocity increment but there is a tendency toward decreasing the cracks length.

Effects of Hydrological Condition on the Coupled Thermal-Hydrological-Mechanical Behavior of Rock Mass Surrounding Cavern Thermal Energy Storage

The thermal-hydrological-mechanical (T-H-M) behavior of rock mass surrounding a large-scale high-temperature cavern thermal energy storage (CTES) at a shallow depth has been investigated, and the effects of hydrological conditions such as water table and rock permeability on the behavior have been examined. The liquid saturation of ground water around a storage cavern may have a small impact on the overall heat transfer and mechanical behavior of surrounding rock mass for a relatively low rock permeability of . In terms of the distributions of temperature, stress and displacement of the surrounding rock mass, the results expected from the simulation with the cavern below the water table were almost identical to that obtained from the simulation with the cavern in the unsaturated zone. The heat transfer in the rock mass with reasonable permeability was dominated by the conduction. In the simulation with rock permeability of , however, the convective heat transfer by ground-water was dominant, accompanying the upward heat flow to near-ground surface. The temperature and pressure around a storage cavern showed different distributions according to the rock permeability, as a result of the complex coupled processes such as the heat transfer by multi-phase flow and the evaporation of ground-water.

A Comparative Study on Heat Loss in Rock Cavern Type and Above-Ground Type Thermal Energy Storages

A large-scale high-temperature thermal energy storage(TES) was numerically modeled and the heat loss through storage tank walls was analyzed using a commercial code, FLAC3D. The operations of rock cavern type and above-ground type thermal energy storages with identical operating condition were simulated for a period of five consecutive years, in which it was assumed that the dominant heat transfer mechanism would be conduction in massive rock for the former and convection in the atmosphere for the latter. The variation of storage temperature resulting from periodic charging and discharging of thermal energy was considered in each simulation, and the effect of insulation thickness on the characteristics of heat loss was also examined. A comparison of the simulation results of different storage models presented that the heat loss rate of above-ground type TES was maintained constant over the operation period, while that of rock cavern type TES decreased rapidly in the early operation stage and tended to converge towards a certain value. The decrease in heat loss rate of rock cavern type TES can be attributed to the reduction in heat flux through storage tank walls followed by increase in surrounding rock mass temperature. The amount of cumulative heat loss from rock cavern type TES over a period of five-year operation was 72.7% of that from above-ground type TES. The heat loss rate of rock cavern type obtained in long-period operation showed less sensitive variations to insulation thickness than that of above-ground type TES.

Determination of failure initiation stress and failure grade using Acoustic emission

ABSTRACT The excavation of rock structures in hard rock involves three safety issues: structural failure such as wedge-type failures, brittle failures such as slabbing and spalling, and a combination of these two. Especially at highly stressed levels, the failure process of rock structures are mainly affected and eventually dominated by stress-induced fractures growing preferentially parallel to the excavation boundary. This brittle failure is usually occurred within very small displacement or strain, and thus it is not easy to determine the precursor and initiation stress level of failure in displacement detection method. To overcome this problem, some researchers have tried to adopt the acoustic emission (AE) detection. This study carried out physical model experiments and determined brittle failure initiation stress and failure grade, which are the two important characteristics of brittle failure. Failure initiation stress and failure grade are determined using AE detection as well as visual observation. And the locations of failure were detected in real time by the AE source location. This study showed that AE detection method could be usefully applied to determine the brittle failure initiation.

Design Criteria for the Reinforcement of a Water Pressure Tunnel Driven by TBM

ABSTRACT In construction of domestic water pressure tunnels, concerns on the lining reinforcement is being increased with relation to the mechanical stability of the tunnel. In this study, the design of lining and unlining of a water pressure tunnel is carried out through the detailed estimation of rock mechanical and hydrological factors and the establishment of the design guideline. The field investigation for rock mass rating and joint pattern, laboratory rock test, numerical analysis on mechanical stability using UDEC, wedge analysis for the estimation of rock block falling according to joint pattern, and hydrological analysis for the lining and unlining conditions are carried out. The design criteria are evaluated for the factors such as support pattern related with RMR range, hydraulic jacking and leakage according to the condition of rock stress and inner/outer water pressure. With these guidelines, the lining and unlining sections are designed for the water pressure tunnel with the diameter of 2.6 m and the length of 5.35 km. This assessment technique and the design criterion can be used as a guideline in the design of lining reinforcement of the water pressure tunnel.