• Journal of Korean Tunnelling and Underground Space Association
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• v.2 no.2
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• pp.50-61
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• 2000

The ground around the portal of a tunnel is the most typical part showing the 3-dimensional mechanical behavior in the tunnel. The portal slope is constructed at the weathered soft rock-mass, and remains as a potential sliding mass. The slope failure around the tunnel portal may happen drastically and induce the great disaster; hence, for the permanent stability several special techniques are required. To solve this problem, the ground around the tunnel portal may be excavated in the arch shape to develop the arching effect in horizontal direction. With the arch-type portal slope, one can reduce considerably the excavation mass and the damage of environments. This approach has not been attempted yet due to the lack of understanding and the well-defined analyzing method, so the retaining wall type portal is more universal. The 3-dimensional finite element analyses were carried out to prove that the arch type is more advantageous in safety and cost than the right angle type. The influence of the tunnel construction sequence and the strength of the rock-mass on the slope stability was investigated by focusing on the maximum shear strain in the slope, and the yield zone at the tunnel face.

• Journal of Korean Tunnelling and Underground Space Association
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• v.19 no.6
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• pp.959-972
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• 2017

In evaluation of blast-induced vibration, peak particle velocity (PPV) is generally calculated by using attenuation relation curve. Calculated velocity is compared with the value in legal requirements or the standards to determine the stability. Attenuation relation curve varies depending on frequency of test blasting, geological structure of the site and blasting condition, so it is difficult to predict accurately using such an equation. Since PPV is response value from the ground, direct evaluation of the structure is impractical. Because of such a limit, engineers tend to use the commercial numerical analysis program in evaluating the stability of the structure more accurately. However, when simulate the explosion process using existing numerical analysis program, it's never easy to accurately simulate the complex conditions (fracture, crushing, cracks and plastic deformation) around blasting hole. For simulating such a process, the range for modelling will be limited due to the maximum node count and it requires extended calculation time as well. Thus, this study is intended to simulate the elastic energy after fractured zone only, instead of simulating the complex conditions of the rock that results from the blast, and the analysis of response characteristics of the velocity depending on shape and size of the fractured zone was conducted. As a result, difference in velocity and attenuation character were calculated depending on fractured zone around the blast source appeared. Propagation of vibration tended to spread spherically as it is distanced farther from the blast source.

• Journal of the Korean Geotechnical Society
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• v.22 no.5
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• pp.83-91
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• 2006

Recently, the intensive rainstorm possibly induced by global warming plays a key role on the instability of railway adjacent slopes. The instability of slopes results as covering and loss of railway lines induced by slided soil mass. According to the site investigation on the failed slopes triggered by rainfall, low types of slope failure were observed: shallow, intermediate, gully erosion, and soil-rock interface failures. The observation reveals the different characteristics of slope failure depending on the thickness of soil layer, morphological features of slope, etc. Based upon the observations, flume tests were conducted to analyze the sliding mechanism of each failure. The variables of flume test are soil layer thickness, rainfall intensity, and morphology of slope under the constant condition of the percentage of fine, initial soil moisture content, slope angle and compaction energy. Test results show that shallow failure was mostly observed from the surface of the slope and caused by the soil erosion; in addition, compared to the other types of failure, the occurrence of initial erosion is late, however, the development of erosion is fast. In gully erosion failure, the collected water from the water catchment area helps erosion of the upper soil layer and transfer of residual corestone, which impedes the erosion process once the upper soil layers are eroded and corestone are exposed. The soil-rock interface failure shows the most fast initial erosion process among the failure types. Interestingly, the common feature observed from the different types of failure was the occurrence of the initial deformation near the toe of slopes which implies the existence of surbsurface flow along the downslope direction.

• Tunnel and Underground Space
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• v.22 no.5
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• pp.346-353
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• 2012

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.

• The Journal of Engineering Geology
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• v.21 no.2
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• pp.147-156
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• 2011

We conducted laboratory experiments to determine the physical and mechanical properties, and the failure behaviors, of cements for use as grouting material in a $CO_2$-injection borehole. Samples with lour different ratios of water to cement mass (0.4, 1, 2, and 3) were tested. The analyzed properties (porosity, sonic velocity, modulus, and compressive and tensile strengths) varied systematically as a function of the ratio of water to cement (w/c), showing a sharp change between w/c ratios of 0.4 and 1. Triaxial compression tests revealed a clear boundary between brittle and ductile failure depending on the w/c ratio and confining pressure. The present results can be utilized as input parameters for numerical models to understand the initial deformation and failure behavior of grouting cements in a $CO_2$-injection borehole.

• 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.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.14 no.4
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• pp.515-523
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• 2001

In this paper results of a linear structural analysis for design and dimensioning of disposal containers for spent pressurized water reactor nuclear fuel and spent Canadian deuterium and uranium reactor nuclear fuel are presented. The container structure studied here is a solid structure with a cast insert and a corrosion resistant outer shell, which is designed for the spent nuclear fuel disposal in a deep repository. An evenly distributed load of hydrostatic pressure from the groundwater and large swelling pressure from the bentonite buffer are applied on the container. Hence, the container must be designed to endure these large pressure loads. In this study, the array type of inner baskets and thicknesses of outer shell and lid/bottom are attempted to be determined through a linear static structural analysis.

• Journal of Korean Tunnelling and Underground Space Association
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• v.5 no.4
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• pp.323-336
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• 2003

Recently, the development of tunnel reinforcement method has been required relating to the shallow tunnelling in soft ground. In this study, the improvement method on tunnel stability is proposed by evaluating the efficiency of face reinforcement which enables to control extrusion of advance core, however, it is often neglected in urban tunnelling under the poor ground conditions. Systematic pre-confinement ahead of the face improves the tunnel stability, subsequently, displacement of the crown and surface settlement can be restrained by proper method. 3-dimensional numerical analysis including horizontal reinforcement modelling on a face is applied to estimate the behaviour of a tunnel in relation to the ground and reinforcement conditions. Consequently, extrusion at the face decreases significantly after using the horizontal reinforcement and the effect of reinforcement is much increased in case of applying the supplemental reinforcement ahead of the face together. Especially, confinement effect around the tunnel and the core is proved by means of the core reinforcement in poor ground conditions.

• Tunnel and Underground Space
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• v.10 no.2
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• pp.173-179
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• 2000

The influence of the plastic flow rules on the elasto-plastic behaviour of a discrete joint element was investigated by performing the numerical direct shear tests under both constant normal displacement and normal displacement conditions. The finite interface elements obeying Plesha’s joint constitutive law was used to allow the relative motion of the rock blocks on the joint surface. Realistic results were obtained in the tests adopting the non-associated flow rule, while the associated flow rule overestimated the joint dilation. To overcome the computational drawbacks coming from the non-symmetric element stiffness matrix in the conventional non-associated plasticity, the symmetric formulation of the tangential stiffness matrix for a non-associated joint element was proposed. The symmetric elasto-plastic matrix it derived by assuming an imaginary equivalent joint with associated flow rule which shows the same plastic response as that of original Joint with non-associated flow rule. The validity of the formulation was confirmed through the numerical direct shear tests under constant normal stress condition.

• Tunnel and Underground Space
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• v.22 no.3
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• pp.196-204
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• 2012

Quantification of roughness plays an important role in modeling strength deformability and fluid flow behaviors of rock joints. A procedure was suggested to simulate joint roughness, and characteristics of the roughness was investigated in this study. Stationary fractional Brownian profiles with known input values of the fractal parameter and other profile properties were generated based on random midpoint displacement method. Also, a procedure to simulate three dimensional roughness surface was suggested using the random midpoint displacement method. Selected statistical roughness parameters were calculated for the generated self-affine profiles to investigate the attribute of roughness. Obtained results show that statistical parameters applied in this study were able to consider correlation structure and amplitude of the profiles. However, effect of data density should be tackled to use statistical parameters for roughness quantification.