• Tunnel and Underground Space
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• v.33 no.1
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• pp.57-69
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• 2023

In this study, we simulated induced seismicity in the Groningen natural gas reservoir using 2D hydro-mechanical coupled discrete element modelling (DEM). The code used is PFC2D (Particle Flow Code 2D), a commercial software developed by Itasca, and in order to apply to this study we further developed 1)initialization of inhomogeneous reservoir pressure distribution, 2)a non-linear pressure-time history boundary condition, 3)local stress field monitoring logic. We generated a 2D reservoir model with a size of 40 × 50 km² and a complex fault system, and simulated years of pressure depletion with a time range between 1960 and 2020. We simulated fault system failure induced by pressure depletion and reproduced the spatiotemporal distribution of induced seismicity and assessed its failure mechanism. Also, we estimated the ground subsidence distribution and confirmed its similarity to the field measurements in the Groningen region. Through this study, we confirm the feasibility of the presented 2D hydro-mechanical coupled DEM in simulating the deformation of a complex fault system by hydro-mechanical coupled processes.

• Tunnel and Underground Space
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• v.33 no.3
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• pp.189-207
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• 2023

In the present study, the thermoshearing experiment on a rough rock fracture were modeled using a three-dimensional grain-based distinct element model (GBDEM). The experiment was conducted by the Korea Institute of Construction Technology to investigate the progressive shear failure of fracture under the influence of thermal stress in a critical stress state. The numerical model employs an assembly of multiple polyhedral grains and their interfaces to represent the rock sample, and calculates the coupled thermo-mechanical behavior of the grains (blocks) and the interfaces (contacts) using 3DEC, a DEM code. The primary focus was on simulating the temperature evolution, generation of thermal stress, and shear and normal displacements of the fracture. Two fracture models, namely the mated fracture model and the unmated fracture model, were constructed based on the degree of surface matedness, and their respective behaviors were compared and analyzed. By leveraging the advantage of the DEM, the contact area between the fracture surfaces was continuously monitored during the simulation, enabling an examination of its influence on shear behavior. The numerical results demonstrated distinct differences depending on the degree of the surface matedness at the initial stage. In the mated fracture model, where the surfaces were in almost full contact, the characteristic stages of peak stress and residual stress commonly observed in shear behavior of natural rock joints were reasonably replicated, despite exhibiting discrepancies with the experimental results. The analysis of contact area variation over time confirmed that our numerical model effectively simulated the abrupt normal dilation and shear slip, stress softening phenomenon, and transition to the residual state that occur during the peak stress stage. The unmated fracture model, which closely resembled the experimental specimen, showed qualitative agreement with the experimental observations, including heat transfer characteristics, the progressive shear failure process induced by heating, and the increase in thermal stress. However, there were some mismatches between the numerical and experimental results regarding the onset of fracture slip and the magnitudes of fracture stress and displacement. This research was conducted as part of DECOVALEX-2023 Task G, and we expect the numerical model to be enhanced through continued collaboration with other research teams and validated in further studies.

• Journal of Korean Tunnelling and Underground Space Association
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• v.25 no.6
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• pp.509-523
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• 2023

The high-level nuclear waste (HLW) repository is exposed to complex environmental conditions consisting of high temperature, high humidity, and radiation, resulting in structural deterioration. Therefore, structural health monitoring is essential, and piezo sensors are used to detect cracks and estimate strength. However, since the monitoring sensors installed in the disposal tunnel and disposal container cannot be replaced or removed, the quantitative life of the monitoring sensor and its suitability must be assessed. In this study, the life of a piezo sensor for monitoring was assessed using an accelerated life test (ALT). The failure mode and mechanism of the piezo sensor under high temperature conditions were determined, and temperature stress's influence on the piezo sensor's life was analyzed. ALT was conducted on temperature stress and the relationship between temperature stress and piezo sensor life was suggested. The life of the piezo sensor was assessed using the Weibull probability distribution and the Arrhenius acceleration model. The suggested relationship can be used in multiple stress ALT designs for more precise life assessment.

• Journal of the Korean Geosynthetics Society
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• v.16 no.3
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• pp.11-19
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• 2017

A series of pullout tests to compression type anchors is conducted. The test is carried out on a couple of steel cables installed in sandy soil with 60% of relative density. The test is performed with 6 different bonded lengths, which are 1, 2, 3, 4, 5, 6 times longer than the initial bonded length (Lc =30 mm). A numerical analysis with the same condition as the test is also performed to compare each other. Finally, those results are compared with theoretical result by Oosterbaan and Gifford (1972). The result shows that the ultimate pullout capacity appears to increase with an increase of bonded length, and that the results of test, numerical analysis and theoretical approach have a good agreement in the ultimate pullout capacity at failure.

• Journal of the Korean Society for Railway
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• v.8 no.4
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• pp.348-353
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• 2005

In designing the structures of railway rolling stocks, deterministic methods associated with the concept of a safety factor have been traditionally used. The deterministic approaches based on the mean values of applied loads and material properties have been used as safety verification for the design of rolling-stock car body structures. The uncertainties in the applied loading for the high speed train and the strength of new materials in the rolling stocks require the application of probabilistic approaches to ensure fatigue safety in the desired system. Pressure loadings acting on the car body when the train passes through tunnels show reflected pressure waves for high-speed trains and they may cause a fatigue failure in vehicle bodies. Use of new material technology as body structures also introduces uncertainties in the material strength. A probabilistic approach is more adaptable in designing reliable structures when the pressure waves from the tunnels pounds and new material technology is adopted. In this paper, it is proposed that a fatigue design and assessment method based on a reliability which deals with the loading variations on a railway vehicle due to the pressure reflected in tunnels and the strength variations of material. Equation for the fatigue reliability index has been formulated to calculate the reliability assessment of a vehicle body under fluctuating pressure loadings in a tunnel. Considered in this formulation are the pressure distribution characteristics, the fatigue strength distribution characteristics, and the concept of stress-transfer functions due to the pressure loading.

• Tunnel and Underground Space
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• v.14 no.6 s.53
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• pp.391-398
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• 2004

An algebraic algorithm for predicting the joint trace distribution on the cut-face of rock slope based on the orientations and the locations of joints investigated in the borehole has been developed. Joint trace prediction is manipulated by utilizing the three dimensional plane equations of both joint planes and projection face, and the extent of trace within the projection area is calculated by considering the persistence of each joint plane. Joint trace prediction method is efficiently applied for analyzing the stability and the adequacy of support design of Gimhae Naesam cut-slope, which is structurally unstable due to slumping. Structural characteristics of rock mass is investigated by performing DOM drilling and the potential rock mass sliding inside slope face is analyzed by examining the orientations of joint planes which can induce the slope failure. Also, the efficiency of anchor support design is evaluated by considering the joint trace distribution on the anchor installation area and its sliding potential.

• Tunnel and Underground Space
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• v.14 no.6 s.53
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• pp.411-422
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• 2004

Sinkhole subsidence occurs over abandoned mine workings and can be detrimental to human lives, damage to properties and other surface structures. In this study, simulation of sinkhole development process is performed using special finite element procedure. Especially, creation of mine voids due to roof falls and generation of goaf from broken rocks are simulated using active-passive-active finite elements. An active or solid element can be made passive or void once the tensile failure criterion is satisfied in the specified sinkhole formation zone. Upon completion of sinkhole development process, these passive elements in again be made active to simulate goal region. Several finite element models are analyzed to evaluate the relationships between sinkhole formation with width of gallery. depth of mine, roof condition and bulking factor of roof rocks. This study demonstrates that the concept of passive elements in numerical analysis can be used effectively for analyzing sinkhole formation or roof fall phenomenon in general.

• Tunnel and Underground Space
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• v.13 no.1
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• pp.64-75
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• 2003

This study presents a newly suggested test method of Mode II fracture toughness measurement called "Punch Through Shear Test" which was originally proposed by Backers and Stephansson in 2001. The purpose of this study is to check the validity of the suggested testing method by performing Mode II fracture toughness tests for Daejeon Granite. In addition, the optimal specimen geometry for the testing and the relation between Mode II fracture toughness and confining pressure were also investigated. Fractured surface was observed to be very smooth with lots of rock debris which came off fracture surface which obviously implies that the surface was sheared off. This confirms that Mode II fracturing actually occurred. In addition, numerical analyses including continuum analysis, particle flow code analysis and crack propagation simulations were performed. Results of these numerical analyses indicated that the cracks occurred in the specimen were predominantly in Mode II and these cracks led to failure of the test specimen. From this investigation, it can be concluded that the newly suggested "Punch Through Shear Test" method provides a reliable means of determining the Mode II fracture toughness. fracture toughness.

• Tunnel and Underground Space
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• v.9 no.4
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• pp.315-327
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• 1999

Rock fracture mechanics has been widely applied to blasting, hydraulic fracturing, rock slope and many other practical problems in rock engineering. But a measuring method for the fracture toughness of rock, one of the mort important parameters in fracture mechanics as an intrinsic property of rock, has not been yet well established. To obtain mode I rock fracture toughness, the more favorable disc-typed specimens such as CCNBD, SCB, chevron-notched SCB and BDT were used in this study. Rock fracture toughness under mixed-mode and mode II conditions was measured by using the STCA applied to the CCNBD specimen. Size effects such as specimen thickness, diameter and notch length on fracture toughness were investigated. From the mixed-mode results, fracture envelops were obtained by applying various regression curves. The mixed-mode results were also compared with three mixed-mode failure criteria. In each fracture toughness test, acoustic emission was measured to get the data for determining the load levels of different crack propagation patterns.

• Tunnel and Underground Space
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• v.28 no.3
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• pp.277-291
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• 2018

LSTM (Long Short-Term Memory) algorithm which is a kind of recurrent neural network was used to establish a model to predict the stress-strain curve of an material under uniaxial compression. The model was established from the stress-strain data from uniaxial compression tests of silica-gypsum specimens. After training the model, it can predict the behavior of the material up to the failure state by using an early stage of stress-strain curve whose stress is very low. Because the LSTM neural network predict a value by using the previous state of data and proceed forward step by step, a higher error was found at the prediction of higher stress state due to the accumulation of error. However, this model generally predict the stress-strain curve with high accuracy. The accuracy of both LSTM and tangential prediction models increased with increased length of input data, while a difference in performance between them decreased as the amount of input data increased. LSTM model showed relatively superior performance to the tangential prediction when only few input data was given, which enhanced the necessity for application of the model.