• Tunnel and Underground Space
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• v.23 no.3
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• pp.241-259
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• 2013

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.

• The Journal of the Acoustical Society of Korea
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• v.40 no.4
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• pp.320-329
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• 2021

In this paper, an empirical model of air bubble size to be applied to an air masker for reduction of underwater radiation noise is presented. The proposed model improves the divergence problem under the low-speed flow condition of the existing model derived using Rayleigh's jet instability model and simple continuity condition by introducing a jet flow velocity of air. The jet flow velocity of air is estimated using the bubble size where the liquid is quiescent. In a medium without flow, the size of the bubble is estimated by an empirical method where bubble formation regime is divided into a laminar-flow range, a transition range, and a turbulent-flow range based on the Reynolds number of the injected air. The proposed bubble size model is confirmed to be in good agreement with the Computational Fluid Dynamics (CFD) analysis result and the experimental results of the existing literature. Using the acoustic inversion method, the air bubble population is estimated from the insertion loss measured during the air injection experiment of the air- masker model in a large cavitation tunnel. The results of the experiments and the bubble size model are compared in the paper.

• Journal of Korean Tunnelling and Underground Space Association
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• v.21 no.4
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• pp.573-585
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• 2019

The objective of this study is to propose the drainage system that has been improved the workability, waterproofing and drainage performance to treat the leakage from the cement concrete structures in underground. It is improved that the pipe for conveying ground leak in the existing drainage system had the problem in workability and waterproof. The drainage systems with the improved pipe for conveying ground leak were constructed in conventional concrete lining tunnels to evaluate the workability, waterproofing and drainage. The waterproof and the drainage performance of the drainage system was evaluated by injecting 1,000 ml of red water in the back of the drainage system at 3 weeks, 6 weeks, 9 weeks, 11 weeks, 14 weeks, 17 weeks and 23 weeks. During 6 months of field performance test, the average daily temperature of the tunnel site was measured from $-12.4^{\circ}C$ to $19.7^{\circ}C$. The daily minimum temperature was $-17.2^{\circ}C$ and the daily maximum temperature was $26.7^{\circ}C$. There was no problem in waterproof and drainage performance on the pipe for conveying ground leak and the drainage system during 6 months for field performance test. It is concluded that the improved drainage system can be applied to various cement concrete underground structures where leakage occurs, and has little seasonal effect.

• Tunnel and Underground Space
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• v.29 no.3
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• pp.197-213
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• 2019

We simulated the fault reactivation experiment conducted at 'Main Fault' intersecting the low permeability clay formations of Mont Terri Underground Research Laboratory in Switzerland using TOUGH-FLAC simulator. The fluid flow along a fault was modelled with solid elements and governed by Darcy's law with the cubic law in TOUGH2, whereas the mechanical behavior of a single fault was represented by creating interface elements between two separating rock blocks in FLAC3D. We formulate the hydro-mechanical coupling relation of hydraulic aperture to consider the elastic fracture opening and failure-induced dilation for reproducing the abrupt changes in injection flow rate and monitoring pressure at fracture opening pressure. A parametric study was conducted to examine the effects of in-situ stress condition and fault deformation and strength parameters and to find the optimal parameter set to reproduce the field observations. In the best matching simulation, the fracture opening pressure and variations of injection flow rate and monitoring pressure showed good agreement with field experiment results, which suggests the capability of the numerical model to reasonably capture the fracture opening and propagation process. The model overestimated the fault displacement in shear direction and the range of reactivated zone, which was attributed to the progressive shear failures along the fault at high injection pressure. In the field experiment results, however, fracture tensile opening seems the dominant mechanism affecting the hydraulic aperture increase.

• Tunnel and Underground Space
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• v.28 no.6
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• pp.596-608
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• 2018

As urban infrastructure is aging, the possibility of accidents due to the failures or breakdowns of infrastructure increases. Especially, aging underground infrastructures like sewer pipes, waterworks, and subway have a potential to cause an urban ground sink. Urban ground sink is defined just as a local and erratic collapse occurred by underground cavity due to soil erosion or soil loss, which is separated from a sinkhole in soluble bedrock such as limestone. The conventional measurements such as differential settlement gauge, inclinometer or earth pressure gauge have a shortcoming just to provide point measurements with short coverage. Therefore, these methods are not adequate for monitoring of an erratic subsidence caused by underground cavity due to soil erosion or soil loss which occurring at unspecified time and location. Therefore, an alternative technology is required to detect a change of underground physical condition in real time. In this study, the feasibility of a novel magnetic resonance based monitoring method is investigated through laboratory tests, where the changes of path loss (S21) were measured under various testing conditions: media including air, water, and soil, resonant frequency, impedance, and distances between transmitter (TX) and receiver (RX). Theoretically, the transfer characteristic of magnetic field is known to be independent of the density of the medium. However, the results of the test showed the meaningful differences in the path loss (S21) under the different conditions of medium. And it is found that the reflection coefficient showed the more distinct differences over the testing conditions than the path loss. In particular, input reflection coefficient (S11) is more distinguishable than output reflection coefficient (S22).

• Journal of Korean Tunnelling and Underground Space Association
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• v.21 no.4
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• pp.455-478
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• 2019

In this paper, we conducted experimental investigation on the field applicability through the verification of reinforcement effect of the steel pipe reinforcement grouting using high strength steel pipe. SGT275 (formerly known as STK400) steel pipe is generally applied to the traditional steel pipe reinforcement grouting method. However, the analysis of tunnel collapse cases applying the steel pipe reinforcement grouting shows that there are cases where the excessive bending and breakage of steel pipe occur. One of the reasons causing these collapses is the lack of steel pipe stiffness responding to the loosening load of tunnels caused by excavation. The strength of steel pipe has increased due to the recent development of high strength steel pipe (SGT550). However, since research on the reinforcement method considering strength increase is insufficient, there is a need for research on this. Therefore, in this study, we conducted experiments on the tensile and bending strength based on various conditions between high strength steel pipe, and carried out basic research on effective field application depending on the strength difference of steel pipe through the conventional design method. In particular, we verified the reinforcement effect of high strength steel pipe through the measurement results of deformed shape and stress of steel pipe arising from excavation after constructing high strength steel pipe and general steel pipe at actual sites. The research results show that high strength steel pipe has excellent bending strength and the reinforcement effect of reinforced grouting. Further, it is expected that high strength steel pipe will have an arching effect thanks to strength increase.

• Tunnel and Underground Space
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• v.29 no.4
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• pp.243-261
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• 2019

Microseismic monitoring technologies have been recognized for its superiority over traditional methods and are used in domestic and overseas underground mines. However, the complex gangway layout of underground mines in Korea and the mixed structure of excavated space and rock masses make it difficult to estimate the microseismic propagation and to determine the arrival time of microseismic wave. In this paper, experimental studies were carried out to determine the source location according to various arrival picking methods and dimensional conditions. The arrival picking methods used were FTC (First Threshold Cross), Picking window, AIC (Akaike Information Criterion), and 2-D and 3-D source generation experiments were performed, respectively, under the 2-D sensor array. In each experiment, source location algorithm used iterative method and genetic algorithm. The iterative method was effective when the sensor array and source generation were the same dimension, but it was not suitable to apply when the source generation was higher dimension. On the other hand, in case of source location using RCGA, the higher dimensional source location could be determined, but it took longer time to calculate. The accuracy of the arrival picking methods differed according to the source location algorithms, but picking window method showed high accuracy in overall.

• Explosives and Blasting
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• v.36 no.4
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• pp.1-8
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• 2018

This numerical study was intended to evaluate the applicability of the half charge blasting to mining and tunnelling. The half charge blasting is a method that two separate rounds are sequentially blasted for the rock burdens in which long blast holes have already been drilled at one operation. The aim of the method is to decrease the construction cost and period in mining and tunnelling projects as well as to increase the blasting efficiency. Several numerical analyses were conducted by using the Euler-Lagrange solver on ANSYS AUTODYN to identify the effects of the suggested method on the blasting results in underground excavations. The overall performance of the suggested method was also compared to an ordinary blasting method. The analysis model was comprised of the Eulerian parts (explosive, air, and stemming materials) and the Lagrangian parts (rock material). As a result, it was found that, owing to the air decks formed in the bottom parts of the long blast holes, the first round of the suggested method presented a higher shock pressure and particle velocities in the vicinity of the blast holes compared to the ordinary blasting method.

• Tunnel and Underground Space
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• v.31 no.5
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• pp.333-359
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• 2021

Gases such as hydrogen and radon can be generated around the canister in high-level radioactive waste disposal systems due to several reasons including the corrosion of metal materials. When the gas generation rate exceeds the gas diffusion rate in the low-permeability bentonite buffer, the gas phase will form and accumulate in the engineered barrier system. If the gas pressure exceeds the gas entry pressure, gas can migrate into the bentonite buffer, resulting in pathway dilation flow and advective flow. Because a sudden occurrence of dilation flow can cause radionuclide leakage out of the engineered barrier of the radioactive waste disposal system, it is necessary to understand the gas migration behavior in the bentonite buffer to quantitatively evaluate the long-term safety of the engineered barrier. Experimental research investigating the characteristics of gas migration in saturated bentonite and research developing numerical models capable of simulating such behaviors are being actively conducted worldwide. In this technical note, previous gas injection experiments and the numerical models proposed to verify such behaviors are introduced, and the future challenges necessary for the investigation of gas migration are summarized.

• Tunnel and Underground Space
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• v.31 no.4
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• pp.243-269
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• 2021

Since the early 2010s, the social importance of research and practical projects targeting deep geological disposal of high-level nuclear waste, underground CO₂ storage and characterization of deep subsurface by borehole investigation has been increasing. In this regard, there is also a significant increase in the need for in situ test technology to obtain quantitative and reliable information on the hydraulic characteristics of deep rock mass. Through years of research and development, we have independently set up Deep borehole Hydraulic Test System (DHTS) based on the key apparatuses designed and made with our own technology. Using this system, high precision constant pressure injection tests were successfully completed at the two 1 km boreholes located in Mesozoic granite and sedimentary rock regions, Gyeongju. During the field tests, it was possible to measure very low flow rate below 0.01 l/min with micro flow rate injection/control module. In this paper, the major characteristics of DHTS are introduced and also some results obtained from the high precision field tests under the deep and low permeable rock mass environment are briefly discussed.