Yong-Bok Jung

Thermal–mechanical analysis of the fracture initiation and propagation around the underground pilot LNG storage cavern

Thermal–mechanical integrity of rock mass and lining system is a crucial factor in the construction and operation of the underground LNG storage facilities. The pilot LNG storage cavern was successfully constructed and operated for the validation of the suggested technologies. In order to simulate the fracture initiation and propagation of thermal fractures around the pilot LNG storage cavern, thermal–mechanical coupled analyses have been carried out using FRACOD and the capability of the code has been verified by a comparison of numerical results with in situ measurement data. Temperature and fracture initiations were well matched with the field data. In case of displacement, the trend was similar but the magnitude showed noticeable difference. This was partially attributed to the volume expansion during water–ice phase change because this model did not consider this phenomenon. According to the numerical analyses, the different behaviors under heating and cooling conditions must be considered in the modeling of thermal–mechanical coupled problems. More specifically, the Youngs modulus obtained from the tensile test should be used in case of cooling environments.

Management and concept of the monitoring system considering the characteristics of subsea tunnels

In order to ensure the safety of the subsea tunnel during its construction and operation, unlike the underground structures on land, the special monitoring system is essential which considers the characteristics of subsea tunnels in addition to conventional stress and displacement measurements applied to existing land tunnels. Therefore, the concept applied to NATM is reorganized to evaluate the stability of subsea tunnels. And the observation system for making a monitoring plan, the critical strain theory for tunnel safety management and MS monitoring methods for detecting the local failure and crack initiation of rock and supports, are introduced. Finally, the scheme of monitoring and management for subsea tunnels by using these methods is suggested.

Modelling the effect of ice swelling in the rock mass around an LNG underground storage cavern using FRACOD

Underground storage of liquefied natural gas (LNG) has many advantages over the conventional above-ground LNG storage due to its safety and energy efficiency. One of the key technical challenges for the underground storage of LNG is to understand the behaviour of the rock mass in the vicinity of the LNG cavern and hence to take measures to prevent leakage caused by possible rock fracturing in response to the cooling of the rock mass. With the extremely low temperature of the LNG ( − 162°C) in the storage cavern, sub-zero temperatures will be induced in the surrounding rock mass, which on one hand may cause tensile stress due to thermo-mechanical effect and on the other will cause the formation of ice in pores and cracks of the saturated rock mass. Ice formation is likely to cause compressive stress in the rock mass due to its expansion in volume. The combined effect of rock cooling and ice swelling is complicated as cooling tends to create open fractures whereas ice swelling tends to cause compression and the closure of these discontinuities. Understanding this effect is crucially important as it is related to the integrity of the surrounding rock mass for leakage prevention. This study presents recent developments of the thermal-mechanical coupling and ice swelling functions in FRACOD, a numerical code designed to predict rock fracturing processes in fractured rock masses. The new functions enable us to investigate the complicated response of an in situ rock mass to the excavation of LNG cavern and the storage of low temperature LNG in a most realistic way. The pilot LNG storage cavern experiment at Daejeon Korea is used as a case study site, and the measured temperature in the surrounding rock mass are used to validate the numerical model and the modelling method. The ice swelling effect was modelled by introducing a large number of random cracks in the rock mass, which have certain hydraulic apertures to hold water. When the local rock temperature is below zero, the aperture water will become ice and cause crack expansion. It has been found in the study that ice swelling has a major effect on the displacement and stress distribution in the rock mass. It causes a compression zone around the LNG cavern, and effectively prevents tensile fracturing and fluid leakage.

Mechanical Properties of a Lining System under Cyclic Loading Conditions in Underground Lined Rock Cavern for Compressed Air Energy Storage

In a material, micro-cracks can be progressively occurred, propagated and finally lead to failure when it is subjected to cyclic or periodic loading less than its ultimate strength. This phenomenon, fatigue, is usually considered in a metal, alloy and structures under repeated loading conditions. In underground structures, a static creep behavior rather than a dynamic fatigue behavior is mostly considered. However, when compressed air is stored in a rock cavern, an inner pressure is periodically changed due to repeated in- and-out process of compressed air. Therefore mechanical properties of surrounding rock mass and an inner lining system under cyclic loading/unloading conditions should be investigated. In this study, considering an underground lined rock cavern for compressed air energy storage (CAES), the mechanical properties of a lining system, that is, concrete lining and plug under periodic loading/unloading conditions were characterized through cyclic bending tests and shear tests. From these tests, the stability of the plug was evaluated and the S-N line of the concrete lining was obtained.

Development and Application of Mode II Fracture Toughness Test Method Using Rock Core Specimen

Rock fracture mechanics has been widely applied to various fields of rock and civil engineering. But most researches covered mode I behavior, though mode II behavior is dominant in rock engineering. Until now, there is only one ISRM suggested method for mode II toughness of rock. A new SCC (Short Core in Compression) mode II toughness test method was developed considering 1) application of confining pressure, 2) easiness of notch creation, 3) utilization of existing equipment, 4) simple test procedure. The stress intensity factors were determined by 3D finite element method considering line and distributed loading conditions. The tests with granite specimens were carried out using MTS 815 rock test system with a loading rate of 0.002 mm/s. The mean value of mode II fracture toughness of granite showed 2.33  . Mode I toughness of the same granite was 1.12  , determined by Brazilian disk test and     . The smooth fracture surface with rock powder formation also supported mode II behavior of SCC method. The SCC method can be used for the determination of mode II fracture toughness of rocks based on the current results.

Mathematical Understanding of the Saint-Venant Approximation in Analysis of a Transverse Isotropy

All five independent elastic constants of a transversely isotropic rock sometimes need to be determined from a single specimen. Saint-Venant approximation has been widely used for a long time in the analysis of single specimen test. This paper has proven how this empirical equation can be mathematically transformed into a form of the apparent Young’s modulus based on theory of elasticity. The transformed equation is a monotonous function on anisotropic angle and can be useful in the analysis of the in-situ stress measurement in an anisotropic rock mass. The estimations of data in literatures have shown that the measured values of G2 are uniform on anisotropic angles and smaller than that of Saint-Venant’s case. This decrement may be caused by sliding of the interface of strata and the decrement rate is inferred to relate well with the combination of bonding condition of strata and strength of rock material. Accumulation of these kinds of studies in the future enables to define the decrement and to determine elastic constants of a transversely isotropic rock from a single specimen from modifying Saint-Venant approximation.

Estimation of the Characteristics of Delayed Failure and Long-term Strength of Granite by Brazilian Disc Test

Long-term stability and delayed failure of granite were evaluated through the laboratory test based on Wilkins method and Brazilian disc test (BDT) which yields tensile strength, mode I fracture toughness and subcritical crack growth parameters. Then, the long-term strength of granite was estimated by using analytical models and long-term stability of compressed air-energy storage (CAES) pilot cavern pressurized up to 5 ~ 6 MPa was evaluated using numerical code, FRACOD with the determined subcritical crack growth parameters. The results of test and analyses showed that the subcritical crack growth index, n was determined as 29.39 and the inner pressure of 5 ~ 6 MPa had an insignificant effect on the long-term stability of pilot cavern. It was also found that the measurement and analysis of acoustic emission events can describe the accumulation of damage due to subcritical crack growth quantitatively. That is, AE monitoring can provide the current status of rock under loading if we make an identical installation condition in the field with that of the laboratory test.

Analysis of Whole Tunnel Stability by Using Rock Mass Classification and Mohr-Coulomb Analytical Solution

Finite element or difference methods are applied to the analysis of the tunnel stability and they provide detailed behaviour of analyzed tunnel sections but it is rather inefficient to analyze all the section of tunnel by using these methods. In this study, the authors suggest a new stability analysis method for whole tunnel to provide an efficient and easy way to understand the behaviour of whole tunnel by using an analytical solution with the assumption of equivalent circular tunnel. The mechanical behaviour, radial strain and plastic zone radius of whole tunnel were analyzed and appropriate support pressure to maintain the displacement within the allowable limit was suggested after the application of this method to the tunnel. Consequently, it was confirmed that this method can provide quick analysis of the whole tunnel stability and the quantitative information for subsequent measures such as selection of tunnel sections for detailed numerical analysis, set up of the monitoring plan, and so on.

Experimental Study on the Elastic Constants of A Transversely Isotropic Rock by Multi-Specimen Compression Tests Report 2 - Statistical Evaluation and Determination of True Values of Elastic Constants

Abstract Multi-specimen uniaxial compression test has been carried out in order to find the method to determine the five independent elastic constants from a single standard specimen of a transversely isotropic rock. Total 35 specimens of 7 different angles from a large block of rhyolite presenting the flow structure obviously are used in tests. This second report is to focus on the statistical evaluation of measured strains and analyzed elastic constants. And the determination of their true or near-true values is discussed. As the result of RSD analysis, it turns out that the reliability of measured strains is sufficiently obtained and Saint-Venant approximation is well applicable except 15 degree angled specimen in tests. RSD is decreasing on the increase of the angle of anisotropy. This tendency may be caused not only by the decreasing of the deviation of measured strains, but also by the better applicability of Saint-Venant approximation on the increase of angle. It can be concluded that the analyzed values are considered the near-true ones of five independent constants on the high reliability. But the variation of the apparent Young’s modulus expected by these values is not proved to match the measured tendency. It is inferred that the factor to decrease the apparent Young’s modulus and/or to increase the shear strain, is present in the test or in the nature of the anisotropy in consideration of this inconsistency.