This study discusses the average Young's modulus arising out of the inhomogeneity and the difference of frequency distribution of material in the rock mass. Numerical analyses of tunnel excavation in composite rock mass with statistically distributed homogeneous materials were carried out. The frequency distribution of materials in the rock mass was carried out using two patterns, which the each volume rate of all material properties is triangle distribution and square distribution. The displacements of extensometer measurement on numerical simulation of tunnel excavation were collected. Back analysis assuming the homogeneous rock mass was then carried out. The Young's modulus and initial stress on tunnel excavation area was calculated by using back analysis.

In this study, the Young's modulus from back analysis was compared with the average Young's modulus of the original numerical model. From this result, the Young's modulus calculated by back analysis is more widely distributed than the average Young's modulus of the original numerical model. It was clear that the inhomogeneity in rock mass affects the size of area of excavation, especially, if homogeneous size in the rock mass is greater than approximately 10% for a tunnel diameter, the inhomogeneity in the rock mass must be considered in the numerical analysis.

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