• Journal of Korean Tunnelling and Underground Space Association
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• v.16 no.1
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• pp.91-104
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• 2014

An Excavation Damaged Zone (EDZ), in which rock properties are permanently changed due to blasting impact or stress redistribution, can influence the behavior and stability of structures. In this study, the mechanical stability of an underground opening was simulated by using FLAC, which is a two-dimensional modeling code, with a consideration of EDZ. A sensitivity analysis was also carried out with fractional factorial design. From the modeling, it was found that the behavior and the stability of an underground tunnel are strongly dependent on the existence of the EDZ. The sensitivity analysis showed that the key parameters affecting the factor of safety around the tunnel are in-situ stress ratio, depth, cohesion, reduction ratio, internal friction angle, and height and width of the tunnel. It is necessary to consider the EDZ, which can significantly affect mechanical stability in tunnel design.

• Geomechanics and Engineering
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• v.14 no.1
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• pp.9-17
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• 2018

Brazilian splitting tests, uniaxial compression tests and triaxial compression tests are carried out on the coal samples cored from Shanxi group $II_1$ coal seam of Jiaozuo coal mine, Henan province, China, to obtain their property parameters. Considering the bedding has notable effect on the property parameter of coal, the samples with different bedding angles are prepared. The effects of bedding on the anisotropic characteristics of the coal seam are investigated. A geological geomechanical model is built based on the geology characteristics of the Jiaozuo coal mine target reservoir to study the effects of bedding on the fracture propagations during hydraulic fracturing. The effects of injection pressure, well completion method, in-situ stress difference coefficient, and fracturing fluid displacement on the fracture propagations are investigated. Results show bedding has notable effects on the property parameters of coal, which is the key factor affecting the anisotropy of coal. The hydraulic cracks trends to bifurcate and swerve at the bedding due to its low strength. Induced fractures are produced easily at the locations around the bedding. The bedding is beneficial to form a complicated fracture network. Experimental and numerical simulations can help to understand the effects of bedding on hydraulic fracturing in coalbed methane reservoirs.

• Proceedings of the Korean Geotechical Society Conference
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• 2008.10a
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• pp.1115-1123
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• 2008

When constructing projects such as road embankments, bridge approaches, dikes or buildings on soft, compressible soils, significant settlements may occur due to the consolidation of these soils under the superimposed loads. The compressibility of the soil skeleton of a soft clay is influenced by such factors as structure and fabric, stress path, temperature and loading rate. Although it is possible to determine appropriate relations and the corresponding material parameters in the laboratory, it is well known that sample disturbance due to stress release, temperature change and moisture content change can have a profound effect on the compressibility of a clay. The early research of Tezaghi and Casagrande has had a lasting influence on our interpretation of consolidation data. The 24 hour, incremental load, oedometer test has become, more or less, the standard procedure for determining the one-dimensional, stress-strain behavior of clays. An important notion relates to the interpretation of the data is the ore-consolidation pressure ${\sigma}_p$, which is located approximately at the break in the slope on the curve. From a practical point of view, this pressure is usually viewed as corresponding to the maximum past effective stress supported by the soil. Researchers have shown, however, that the value of ${\sigma}_p$ depends on the test procedure. furthermore, owing to sampling disturbance, the results of the laboratory consolidation test must be corrected to better capture the in-situ compressibility characteristics. The corrections apply, strictly speaking, to soils where the relation between strain and effective stress is time independent. An important assumption in Terzaghi's one-dimensional theory of consolidation is that the soil skeleton behaves elastically. On the other hand, Buisman recognized that creep deformations in settlement analysis can be important. this has led to extensions to Terzaghi's theory by various investigators, including the applicant and coworkers. The main object of this study is to suggestion the modified compression index value to predict settlements by back calculating the $C_c$ from different numerical models, which are giving best prediction settlements for multi layers including very thick soft clay.

• Geophysics and Geophysical Exploration
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• v.8 no.1
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• pp.1-6
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• 2005

Analogue physical modelling using granular materials (i.e., sandbox experiments) has been applied with great success to a number of geological problems at various scales. Such physical experiments can also be simulated numerically with the Discrete Element Method (DEM). In this study, we apply the DEM simulation to the collision between the Indian subcontinent and the Eurasian Plate, one of the most significant current tectonic processes in the Earth. DEM simulation has been applied to various kinds of dynamic modelling, not only in structural geology but also in soil mechanics, rock mechanics, and the like. As the target of the investigation is assumed to be an assembly of many tiny particles, DEM simulation makes it possible to treat an object with large and discontinuous deformations. However, in DEM simulations, we often encounter difficulties when we examine the validity of the input parameters, since little is known about the relationship between the input parameters for each particle and the properties of the whole assembly. Therefore, in our previous studies (Yamada et al.,2002a,2002b,2002c), we were obliged to tune the input parameters by trial and error. To overcome these difficulties, we introduce a numerical biaxial test with the DEM simulation. Using the results of this numerical test, we examine the validity of the input parameters used in the collision model. The resulting collision model is quite similar to the real deformation observed in eastern Asia, and compares well with GPS data and in-situ stress data in eastern Asia.

• The Journal of Engineering Geology
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• v.15 no.3
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• pp.309-323
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• 2005

Dynamic stiffness of subsurface materials is one of the most important geotechnical parameters in predicting deformational behavior of structures as well as practicing seismic design and performance evaluation. In an effort to measure the parameter efficiently and accurately, various borehole seismic testing techniques have been, thus, developed and used during past several decades. Moreover, recent rush of underground-space projects and increasing size of structures put more stress on reliable site investigation techniques in estimating stiffness of rock mass. In this paper, a new technique called 'in-hole seismic test' has been implemented to measure the dynamic stiffness of rock masses at subsurface foundations and tunnel-faces. The reliance of in-hole seismic test was evaluated by comparing the testing results at several rock sites with those of other borehole techniques and the technique tunnls out to be an efficient and accurate in-situ testing technique.

• Journal of the korean society of oceanography
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• v.39 no.1
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• pp.46-56
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• 2004

Spectral attenuation of light and upwelling radiance were measured in the western coast of Korea on board the R/V Inchon 888 of the Korean National Fisheries Research and Development Institute(NFRDI) during four seasons. The goal of these efforts was to determine the spatial and temporal distribution of the inherent and apparent optical properties of the water, and the factors that control their distribution. Our data indicate that while stratification of the water column, phytoplankton, and wind stress determined the vertical distribution of the optical parameters offshore, it was the tidal current and sediment type that controlled both the vertical and horizontal distribution in the coastal areas. These findings led to the development of a model that estimates the spectral attenuation of light with respect to depth and time for the Yellow Sea. The model integrates water leaving radiance from satellites, sediment types, current vectors, sigma-t, bathymetry, and in situ optical measurements in a learning algorithm capable of extracting optical properties with only knowledge of the environmental conditions of the Yellow Sea. The performance of the model decreases with increase in depth. The mean absolute percentage error (MAPE) of the model is 2% for the upper five meters, 8-10% between 6 and 50 meters, and 15% below 51 meters.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.33 no.2
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• pp.631-642
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• 2013

The standard CPT(Cone Penetration Test), which can be easily performed to investigate in-situ soil engineering properties, has been widely used. CPT are also widely being utilized in centrifuge model tests. In this study, a miniature cone with 10mm diameter was developed and its applicability in the centrifuge was evaluated. The developed miniature cone was equipped with a four degree-of-freedom in-flight robot. A series of cone penetration tests was performed under four centrifuge acceleration levels. As results, the cone resistances measured at the same confining stress within shallow penetration depth were affected by the centrifugal accelerations. The critical depth was proportional to the cone diameter and relative density. Cone resistances results below the critical depth and soil parameters obtained from the laboratory tests were compared with those by previously proposed empirical relations.

• Geophysics and Geophysical Exploration
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• v.7 no.3
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• pp.174-183
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• 2004

The knowledge of rock strength is important in assessing wellbore stability problems, effective sanding, and the estimation of in situ stress field. Numerous empirical equations that relate unconfined compressive strength of sedimentary rocks (sandstone, shale, and limestone, and dolomite) to physical properties (such as velocity, elastic modulus, and porosity) are collected and reviewed. These equations can be used to estimate rock strength from parameters measurable with geophysical well logs. Their ability to fit laboratory-measured strength and physical property data that were compiled from the literature is reviewed. While some equations work reasonably well (for example, some strength-porosity relationships for sandstone and shale), rock strength variations with individual physical property measurements scatter considerably, indicating that most of the empirical equations are not sufficiently generic to fit all the data published on rock strength and physical properties. This emphasizes the importance of local calibration before one utilizes any of the empirical relationships presented. Nonetheless, some reasonable correlations can be found between geophysical properties and rock strength that can be useful for applications related to wellhole stability where haying a lower bound estimate of in situ rock strength is especially useful.

• Journal of Korean Tunnelling and Underground Space Association
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• v.15 no.6
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• pp.637-652
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• 2013

After studying the characteristics of three different evaluation methods of rock load; namely theoretical method, empirical method and numerical method, there were too many limitations for them to be applied on tunnels. Therefore, in this research paper, the method based on numerical analysis is selected to use as this method is the most reasonable one since it considers all parameters that are necessary for rock load estimations, and it also considers the interaction between ground and tunnel support. The critical shear strain concept formulated by Sakurai (1981) was used in order to measure exact rock load values based on numerical analysis. Evaluation on a Level 1 rock load height, which is depicted by the stable region in the graph shows that rock load is not affecting between ground grade 1~3, and it was evaluated that the fourth and fifth grades show less values of rock load height which led to the conclusion of a more economical design of concrete lining.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.14 no.5
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• pp.1243-1251
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• 1994

The implicit finite difference program was developed to evaluate the relationship between time and consolidation ratio within the zone of vertical drain effective radius. In the evaluation, the excess pore water pressure was considered to dissipate in two directions, namely, vertical and radial flow direction. To calculate subsoil stress increments in the soil due to multi-step embanking, the foundation soil was assumed to be an isotropic and homogeneous elastic medium and the initial excess pore water pressure was estimated by using Skempton's parameters whose condition is plane strain and elastic phase of pore pressure response within the soft ground. Regarding to the settlement estimation, immediate and primary consolidation settlements were calculated. The secondary or delayed consolidation settlement was not considered. Numerically calculated excess pore water pressure and settlements were similar to the measured data in situ. Thus, this method can be used to predict the time-consolidation ratio of each layer treated by vertical drain method.