[논문]MODELING THE HYDRAULIC CHARACTERISTICS OF A FRACTURED ROCK MASS WITH CORRELATED FRACTURE LENGTH AND APERTURE: APPLICATION IN THE UNDERGROUND RESEARCH TUNNEL AT KAERI

Bang, Sang-Hyuk; Jeon, Seok-Won; Kwon, Sang-Ki

doi:10.5516/net.02.2011.026

[국내논문] MODELING THE HYDRAULIC CHARACTERISTICS OF A FRACTURED ROCK MASS WITH CORRELATED FRACTURE LENGTH AND APERTURE: APPLICATION IN THE UNDERGROUND RESEARCH TUNNEL AT KAERI 원문보기

Nuclear engineering and technology : an international journal of the Korean Nuclear Society, v.44 no.6, 2012년, pp.639 - 652

Bang, Sang-Hyuk (Department of Energy Systems Engineering, Seoul National University) , Jeon, Seok-Won (Department of Energy Systems Engineering, Seoul National University) , Kwon, Sang-Ki (Department of Energy Resources Engineering, Inha University)

Abstract ▼ AI-Helper

A three-dimensional discrete fracture network model was developed in order to simulate the hydraulic characteristics of a granitic rock mass at Korea Atomic Energy Research Institute (KAERI) Underground Research Tunnel (KURT). The model used a three-dimensional discrete fracture network (DFN), assuming a correlation between the length and aperture of the fractures, and a trapezoid flow path in the fractures. These assumptions that previous studies have not considered could make the developed model more practical and reasonable. The geologic and hydraulic data of the fractures were obtained in the rock mass at the KURT. Then, these data were applied to the developed fracture discrete network model. The model was applied in estimating the representative elementary volume (REV), the equivalent hydraulic conductivity tensors, and the amount of groundwater inflow into the tunnel. The developed discrete fracture network model can determine the REV size for the rock mass with respect to the hydraulic behavior and estimate the groundwater flow into the tunnel at the KURT. Therefore, the assumptions that the fracture length is correlated to the fracture aperture and the flow in a fracture occurs in a trapezoid shape appear to be effective in the DFN analysis used to estimate the hydraulic behavior of the fractured rock mass.

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제안 방법

The DFN model is widely used to simulate the hydraulic characteristics of fractured crystalline rock masses. The DFN approach relies on stochastic realizations of the fracture systems using Monte Carlo simulations in order to improve the reliability of the DFN models, thereby providing improved evaluations of the effects of the size of the sampling domain, the existence of a representative elementary volume (REV), and the overall hydraulic properties [1].
The objective of this paper is to develop a 3D DFN model that can simulate the hydraulic characteristics of a fractured rock mass, considering the correlation between the distribution functions of the fracture aperture and length, and assuming that the flow occurs through 2D trapezoidal channels in order to create a more practical and reasonable model. This model is then applied to an analysis of the KURT constructed by KAERI.
Disc-shaped fractures are assumed in the 3D DFN model in this study due to their advantages in formulating mathematical expressions. In order to describe a fracture, it is necessary to determine its diameter (d), center position (x, y, z), direction vector (l, m, n), and aperture (a).
The observation data were used to validate the numerical simulations. In order to estimate the amount of groundwater flow into KURT, a DFN model was created with a cube with a side length of 200 m and a tunnel with the same dimensions as KURT. Cubic samples with sizes of 16, 21, 26, 31, 36, 41, 46, 66, and 86 m were selected from the center of the cube in order to investigate the effect of the DFN side length on the estimates of the groundwater flow into the tunnel.
In order to estimate the amount of groundwater flow into KURT, a DFN model was created with a cube with a side length of 200 m and a tunnel with the same dimensions as KURT. Cubic samples with sizes of 16, 21, 26, 31, 36, 41, 46, 66, and 86 m were selected from the center of the cube in order to investigate the effect of the DFN side length on the estimates of the groundwater flow into the tunnel. For the above cube sizes, the distances from the tunnel side to the side boundary were 5.
0 m, respectively. It was necessary to conduct this investigation in order to determine the region of influence of the effective region of the fracture length. For each sample, the tunnel was placed horizontally at the center of the cube (Fig.
A numerical model was developed to simulate the hydraulic behaviour of KURT; it was based on a threedimensional discrete fracture network assuming a correlation between the fracture length and aperture, as well as a trapezoidal flow path in the fractures in order to produce more effective flow simulations in a fractured rock mass. The DFN model that assumes the equivalent linear pipe fluid flow is capable of representing the characteristics of the fluid flow in a fractured rock mass as a result of the model simplicity through the application of hydraulic and geometric parameters.

대상 데이터

Vertical and inclined boreholes were drilled in order to obtain the geological, mechanical, hydraulic, and geotechnical characteristics of the rock mass at the site. KURT, which has a maximum depth of 80 m from the surface, length of 255 m (access tunnel, 180 m; research modules, 75 m), and dimensions of 6 6 m in a U shape, was excavated in a rock mass consisting of gneissic granite and granitite rock. Fifteen boreholes at the site provide the geologic and hydraulic data that are collected via a televiewer and packer tests.

데이터처리

The fracture diameters were derived using a negative exponential distribution function based on the results of the field mapping at KURT.

이론/모형

The objective of this paper is to develop a 3D DFN model that can simulate the hydraulic characteristics of a fractured rock mass, considering the correlation between the distribution functions of the fracture aperture and length, and assuming that the flow occurs through 2D trapezoidal channels in order to create a more practical and reasonable model. This model is then applied to an analysis of the KURT constructed by KAERI.

성능/효과

The obtained results of the hydraulic conductivity K_xx, K_yy, K_zz, K_xy, K_yz, and K_zx were plotted as a function of the DFN model size, including the minimum, maximum, mean, and CV values (Fig. 7). With an increasing DFN side length, the difference between the minimum and maximum values of the hydraulic conductivity K_xx, K_yy, and K_zz, and the corresponding CV value, decreases.
5 m, the hydraulic conductivity K_xx demonstrated a slight change and the CV fell below 15%. In this study, it was assumed that the REV value is the DFN side length when the CV value is below 15%. Therefore, the REV value was 17.
Furthermore, the assumptions that the fracture length is correlated to the fracture aperture and the flow in a fracture occurs in a trapezoidal shape appear to be effective in the DFN analysis estimating the hydraulic behaviour of the fractured rock mass. The validity of the conclusion drawn in this study is restricted to the assumptions that the fracture lengths are negatively exponentially distributed and the apertures of fractures are log-normally distributed. Thus, the impact of other possible distributions of fracture lengths and apertures, such as the power law and gamma distributions, are not examined.

후속연구

Moreover, depending on the site specific conditions, the assumption that the fracture length and aperture are not correlated can be more able to estimate the hydraulic characteristics of a rock mass. Further study is planned for these cases in the future.

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