• Tunnel and Underground Space
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• v.24 no.1
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• pp.67-80
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• 2014

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.

• Tunnel and Underground Space
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• v.17 no.5
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• pp.389-410
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• 2007

For moderately jointed to massive rock masses, the failure and deformation behaviors around an excavated opening are absolutely influenced by the initial rock stress and strength of in-situ rock mass. The localized and progressive brittle failure around an opening does not mean whole collapse of an excavated opening. But, for many cases, it may induce temporary stopping of excavation works and reexamination of the current supporting system, which can result in delay of the entire construction works and additional construction cost. In this paper, the characteristics of the brittle failure around an opening with stress level and tunnel shape was studied by the biaxial compressive test using scaled specimen and by the numerical simulation with $PFC^{2D}$. The biaxial test results were well coincided with the stress induced failure patterns around the excavated openings observed and monitored in the in-situ condition. For the circular part of the opening wall, the stress induced cracks initially occurred at the wall surface in the direction of the minimum principal stress and contributed to the localized notch shaped failure region having a certain range of angle. But for the corner and straight part of the opening wall, the cracks initiated at sharp corners were connected and coalesced each other and with existing micro cracks. Further they resulted in a big notch shaped failure region connecting two sharp corners.

• Journal of Korean Tunnelling and Underground Space Association
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• v.9 no.4
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• pp.403-413
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• 2007

As rock bolts become one of the main support systems in tunnels and underground structures, the integrity of the rock bolts affects the safety of these types of structures. The purpose of this study is the evaluation of rock bolt integrity using Fourier and wavelet transforms of the guided ultrasonic waves. After five rock bolt specimens with various defect ratios are embedded into a large scale concrete block, guided waves are generated by a PZT (lead zirconate titanate) element and measured by an acoustic emission (AE) sensor. The captured signals are analyzed in the frequency domain using the Fourier transform, and in the time-frequency domain using the wavelet transform based on a Gabor wavelet. The spectrum obtained from the Fourier transform shows that a portion of high frequency contents increases with increase in the defect ratio. Peak values in the time-frequency domain represent the interval of travel time of each echo. The energy velocities of the guided waves increase with the defect ratio. This study shows that the spectrum ratio and the energy velocity may be indicators fur the evaluation of rock bolt integrity.

• 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.

• Tunnel and Underground Space
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• v.28 no.6
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• pp.620-634
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• 2018

The NTR(New Tubular Roof) method was used to secure the stability of the tunnel and minimize the subsidence of the road. The tunnel was constructed at about 7.5 meters deep below the highway. with a width of about 21 meters. Following the NTR method, 13 steel pipes with a diameter of 2.3 meters were digged and pushed in longitudinally along the tunnel profile and cut out sides of pipes to connect to adjacent pipes, then filled the inside of pipes and the connected space between pipes with concrete to complete the lining of the tunnel to be excavated. As the steel pipes were digged in sequentially, the area of relaxation was connected to each other and behaves like a gradually widening tunnel. When the steel pipes were digged in to the widest points of the tunnel, the settlement rate of the road surface was increasing to the maximum as 2.2 mm and the total settlement until the lining construction was approximately 7.7 mm. After that, by excavating a tunnel inside the pre-installed lining, an additional settlement of about 4.3 mm was occurred, resulting in the total settlement of about 11.8 mm after completing of tunnel construction.

• Journal of Korean Tunnelling and Underground Space Association
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• v.9 no.2
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• pp.133-141
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• 2007

The excavation damaged zone (EDZ) is an area around an excavation where in situ rock mass properties, stress condition, displacement, groundwater flow conditions have been altered due to the processes induced by the excavation. Various studies have been carried out on EDZ, but most studies have focused on the mechanical bahavior of EDZ by in situ experiment. Even though the EDZ could potentially form a high permeable pathway of groundwater flow, only a few studies were performed on the analysis of groundwater flow in EDZ. In this study, the 'hydraulic EDZ' was defined as the rock zone adjacent to the excavation where the hydraulic aperture has been changed due to the excavation by using H-M coupling analysis. Fundamental principles of distinct element method (DEM) were used in the analysis. In the same groundwater level, the behavior of hydraulic aperture near the cavern was analyzed for different stress ratios, initial apertures, fracture angles and fracture spacings by using a two-dimensional DEM program. We evaluate the excavation induced hydraulic aperture change. Using the results of the study, hydraulic EDZ was defined as an elliptical shape model perpendicular to the joint.

• Journal of Korean Tunnelling and Underground Space Association
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• v.22 no.3
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• pp.261-275
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• 2020

When excavating a small cross-section tunnel in a fault fracture zone using the shield TBM method, there is a high possibility of excessive convergence and collapse. Appropriate ground reinforcement is required to minimize construction cost loss and trouble due to a fault fracture zone. In this study, the optimal reinforcement area was suggested and the surrounding ground behavior was investigated through numerical analysis using MIDAS GTS NX (Ver. 280). For the parameters, the width of the fault fracture zone, the existence of fault gouge, and the groundwater level and depth of cover were applied. As a result, when there is not fault gouge, the convergence and ground settlement are satisfied the standard when applying ground reinforcement by up to 0.5D. And, due to the high permeability coefficient, it is judged that it is necessary to apply 0.5D reinforcement. There is a fault gouge, it was possible to secure stability when applying ground reinforcement between the entire fault fracture zone from the top of the tunnel to 0.5D. And, because the groundwater discharge occurred within the standard value due to the fault gouge, reinforcement was unnecessary.

• Journal of Korean Tunnelling and Underground Space Association
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• v.2 no.3
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• pp.37-45
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• 2000

In recent years two reflection methods, i.e. GPR and seismic Impact-Echo, are usually performed to obtain the information about tunnel lining structures composed of concrete lining, shotcrete, water barrier, and voids at the back of lining. However, they do not lead to a desirable resolution sufficient for the inspection of tunnel safety, due to many problems of interest including primarily (1) inner thin layers of lining structure itself in comparison with the wavelength of source wavelets, (2) dominant unwanted surface wave arrivals, (3) inadequate measuring strategy. In this sense, seismic physical modeling is a useful tool, with the use of the full information about the known physical model, to handle such problems, especially to study problems of wave propagation in such fine structures that are not amenable to theory and field works as well. Thus, this paper deals with various results of seismic physical modeling to enable to show a possibility of detecting the inner layer boundaries of tunnel lining structures. To this end, a physical model analogous to a lining structure was built up, measured and processed in the same way as performed in regular reflection surveys. The evaluated seismic section gives a clear picture of the lining structure, that will open up more consistent direction of research into the development of an efficient measuring and processing technology.

• Tunnel and Underground Space
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• v.28 no.3
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• pp.258-276
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• 2018

The accurate and quantitative ground information on the hydraulic conductivity characteristics of rock mass is one of the key factors for evaluation of the hydro-geological behaviour of rock mass around an excavated opening under high water pressure. For tunnel and rock structures in seabed, where the sea acts as an infinite source of water, its importance become greater with increasing construction depth below sea level. In this study, to improve the problems related with poor system configuration and incorrect data acquisition of previous hydraulic packer testing equipment, we newly developed an integrated main frame and 30 bar level waterproof downhole sonde apparatus, which were optimized for deep hydraulic packer test in seabed rock mass. Integration of individual test equipment into one frame allows safe and efficient field testing work on a narrow offshore drilling platform. For the integrated type main frame, it is possible to make precise stepwise control of downhole net injection pressure at intervals of $2.0kg_f/cm^2$ or less with dual hydraulic oil volume controller. To ensure the system performance and the operational stability of the prototype mainframe and downhole sonde apparatus, the field feasibility tests were completed in two research boreholes, and using the developed apparatus, the REV(Representative Elementary Volume) scale deep hydraulic packer tests were successfully carried out at a borehole located in the basalt region, Jeju. In this paper, the characteristics of the new testing apparatus are briefly introduced and also some results from the laboratory and in-situ performance tests are shown.

• Tunnel and Underground Space
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• v.15 no.2 s.55
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• pp.157-167
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• 2005

Current initial rock stress state is one of the key factors required to evaluate the stability and failure around an excavated opening and its importance increases as the construction depth become deeper and the scale of the rock structure become larger. In this paper, the study was performed to evaluate the characteristics of the regional stress state at Chuncheon-Yanggu mountainous region, the East-North part of Kyeonggi Massif. Forty nine field stress measurements in 9 boreholes were conducted at the depth from 20 m to 290 m by hydraulic fracturing method. The fracturing tracing works were carried out by acoustic televiewer scanning. The study results revealed that the different intial rock stress states presented at different formation rock type and the excessive horizontal stress state with stress ratio(K) close to 3.0 was measured at the depth of 200 m and deeper in the intrusive unite body of the study area. The results from the investigation of excessive horizontal stress and its effect on failure mode showed that there exist several points where the localized excessive horizontal stresses are big enough to potentially induce brittle failures around the future openings greater than 100 m in depth within the granite body of the study area.