반사 탄성파를 이용한 터널막장 전방의 3차원적 파쇄대 예측 및 암반 물성치 평가
3-D Analysis of Fractured Zones and Assessment of Rock Properties Ahead of Tunnel Face using Seismic Reflection
탄성파이론 터널막장 암반물성치;
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Since the recent trend in tunnel construction is toward larger and longer, the demand to improve the reliability and speed of tunnelling technology and operations is increasing. Accordingly, a geophysical exploration technology is more frequently utilized during underground construction to achieve above demand. Since site investigation through tunnel line is very restricted because of above-ground structures, sub-surface structures, utilities, deep construction depth, etc., the recent trend of geological investigation for tunnel construction is toward surveys performed inside of the tunnel during construction. The most frequently used geological investigation technique during tunnel construction is the tunnel seismic prediction (TSP hereafter) technique. This is the technique that utilizes the VSP (vertical seismic prediction) used in petroleum exploration, and the geological condition ahead of tunnel face can be obtained in advance by this technique. However, results of the TSP technique are generally restricted to 2-dimensional estimation of reflector plane, and rock properties obtained from the TSP are based on the assumption that material is homogeneous and isotropic. However, in most cases, discontinuity orientation is far from perpendicular to tunnel axis, and many rocks exhibit material anisotropy caused by anisotropic stresses as well as preferential alignment of minerals or cracks. The goals of this study are to propose a more general 3-dimensional migration technique and assessment method of obtaining anisotropic rock properties ahead of tunnel face in order to improve the applicability of the TSP technique. In the first part of this dissertation, the two 3-dimensional data processing techniques to predict the fractured zone ahead of tunnel face by the tunnel seismic survey were proposed so that the geometric formation of the fractured zone could be estimated, i.e., the angle between tunnel axis and discontinuity plane, and the dip. The first 3-dimensional data processing was developed based on the principle of ellipsoid. The input data for the 3-dimensional migration can be obtained from the 2-dimensional tunnel seismic prediction (TSP) test where the TSP test should be performed in each sidewall of a tunnel. In addition, the second 3-dimensional migration technique was proposed based on the concept of wave travel plane. This technique can be applied when the TSP is operated only in one sidewall of a tunnel while the receivers are installed in both walls. A numerical simulation using the finite element method was performed to demonstrate the applicability of the proposed migration techniques. First of all, the influence factors affecting 3-dimensional numerical analysis, such as mesh size, time step, pressure history, etc., were assessed. Then, a numerical analysis to simulate the tunnel seismic exploration using the pre-determined influence factors was performed. The 3-dimensional migration was conducted using the reflected waves obtained from the numerical simulation and the result was compared with the geometry of the modelled fractured zone. The proposed migration technique could have been verified through this numerical simulation. New migration technique was applied to an in-situ tunnelling site. Several TSP tests were carried out in both of the twin tunnels where several fault zones were existing. The 3-dimensional migration was performed using measured TSP data and its results were compared with the geological investigation results that were monitored during tunnel construction. This comparison revealed that the proposed migration technique could reconstruct the discontinuity planes reasonably well. In the second part of this dissertation, the relationship between anisotropic elastic properties and seismic velocities of an otherwise isotropic material containing preferentially oriented microcracks was derived by using energy conservation principles. Based on this relationship, an assessment method of anisotropic rock properties using the velocity ratio obtained from the seismic survey was proposed. The stored energy of a cracked elastic material is composed of that of the homogeneous, isotropic and intact portion of the material, and that in cracks. Both of elastic properties and seismic velocities of this material are therefore dependent on average crack length, orientation and density. The elastic properties could then be estimated from the ratio of wave velocities depending on the effective crack density and Poisson's ratio when waves are propagating in principal directions. From this study, it was found that elastic properties of a material that exhibits velocity anisotropy were significantly different from those estimated assuming the material is homogeneous and isotropic. Therefore, utilizing the proposed methodology, anisotropic rock properties can be reasonably estimated using the seismic velocity profiles obtained from the TSP test and the discontinuity orientation monitored in tunnel face. The reassessed anisotropic rock properties can be used to reevaluate the tunnel face stability during tunnel construction.